Nuclear magnetic resonance proton nmr

Nuclear Magnetic Resonance

Spectroscopy - PMR

1

2

Introduction

Nuclear spin and magnetic movement

Theory and principle

Applied field and precession

Precessional frequency

Width of absorption line in NMR

Shielding and Deshielding

Reference standardApril 10, 2023 3M.M.C.P.

CONTENTS

Chemical shift

Factor affecting chemical shift

Interpretation of PMR

Instrumentation of NMR

Splitting of the signals

Spin-spin coupling

Intensities of Multiplet Peaks

Spin DecouplingApril 10, 2023 4M.M.C.P.

NMR spectroscopy is different from the interaction of electromagnetic radiation with matter.

In this spectroscopy the sample is subjected simultaneously with two magnetic field, One is a stationary and another is varying at same radio frequency.

The particular combination of these two field energy is absorbed by sample and signal is obtained when electromagnetic field is provided to the nucleus of sample. The nucleus start to spin around the nuclear axies and generate an another magnetic field. And particular combination of this two field the energy is absorbed by nucleus this technique is called as a NMR spectroscopy.

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INTRODUCTION

This transition of nucleus occurs in radio frequency region. The radio waves are considered for lowest energy and this energy is just sufficient to affect the nuclear spin of the atom in a molecule. Hence, this is a most fundamental part of NMR spectroscopy.

In general, the study of radio frequency radiation by nuclei is called nuclear magnetic resonance.

The method of NMR was first developed by E.M. Purcell and Felix Bloch (1946).

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In synthetic organic and organometallic chemistry, solution-state NMR means a 300-500 MHz NMR spectrometer, high-precision glass sample tubes, 2 ml of deuterated solvent (typically fully deuterated chloroform, acetone, benzene, or dichlorobenzene), several milligrams of pure sample, and a reference substance, NMR experiments with several hours of spectrometer time and data interpretation.

The structures of new compounds with molecular weights up to 2000 Da can be determined, especially when analyzed along with results from NMR databases and mass spectroscopy.

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It is a well known fact that spectra given by all forms of spectroscopy may be described in terms of the following three important factors.

1. Frequency of spectral lines or bands.2. Intensity of spectral lines or bands.3. Shape of spectral lines or bands.

All above properties depends on the molecular parameters of the system. In case of the NMR these molecular parameters are found to be:

1. Shielding constant of nuclei.2. Coupling constant of nuclei.3. Lifetime of energy level.

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NUCLEAR SPIN AND MAGNETIC MOMENT

Magnetic field

Nucleus axis

Nucleus

Fig: Spinning of Nucleus

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All nuclei carry a charge. In some nuclei this charge spins on the nuclear axis and this circulation of nuclear charge generates a magnetic dipole along the axies.

The nuclei of atoms are composed of protons and neutrons. Like electrons, these particle also have the properties to spin on their own axis and each of them possesses angular momentum1/2(h/2π) in accordance with the quantum theory. The net resultant of the angular momentum of all nuclear particles is called nuclear spin.

For a nucleus having a spin quantum number I, these are(2I +1) spin states.

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Two properties of nuclear particles which are important in understanding of NMR spectroscopy are:

• The net spin associated with the proton and neutron. • The distribution of positive charge.

The net spin number or spin quantum number I of a particular nucleus can be obtained by adding spin numbers of individual proton and neutron of ½ each, assuming that neutrons cancel only neutrons and protons cancel only protons, because of pairing or spinning in opposite directions.

The spin number I have values 0,1/2, 1, 3/2, 5/2 and so forth. If I=0 that represent no spin.

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PRINCIPLE FOR NUCLEAR SPIN

If the sum of protons and neutrons is even, I is zero or integral (0,1,2,3 …..)

If the sum of proton and neutrons is odd, I is a half integral (1/2, 3/2, 5/2….)

If the both protons and neutrons are even numbered, I is zero.

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17

35Cl,

17

16O

•The NMR is mostly consult with nucleus spin quantum no. (I)= ½ . The proton having a I = ½ when place in external magnetic field (Ho) it’s start to spin around the nuclear axis and generate a another magnetic field.•According to quantum mechanics there are 2I + 1 so two spin stage + ½ and - ½ for the proton.

I = ½

EI = - ½

I = + ½

E2

E1

HoSpin state of proton

THEORY AND PRINCIPLE

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• When a charge particle place in magnetic field. It’s start to revel and therefore it’s pusses angular movement due to generation of another magnetic field. The charge particle with nucleus spin has magnitude and direction. Both this property is describe by the factor called as magnetic movement (µ).

•So, when the proton take place in magnetic field . It has two spin steps + ½ and - ½ so, there are two energy level for spin steps + ½ & -½.

E1 = + ½ µ Ho ………………….1E2 = - ½ µ Ho ……………….….2

where, Ho = magnetic field strength. µ = magnetic movement

ΔE = E1 – E2 …………………....3

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ΔE = µ Ho …………………………4by Boher’s frequency eq. we can write

ΔE = hv ……………………….5 v= frequencyfrom the eq. 4 &5 hv = µ Ho ………………………… 6

so, µ Ho ………………………….7 h This is a basic eq. in NMR spectroscopy.

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v =

1.41 Tesla = 60 MHz

2.35 Tesla = 100 MHz

7.05 Tesla = 300 MHz

ᵦ ααβ

αα

ᵦ

Nuclear magnetic movement withNo magnetic field

Applied magnetic field (Ho)

Ho

APPLIED FIELD AND PRECESSION

Spinning nuclei-magnetic moments

Some elements have isotopes with nuclei that behave as though they were spinning about an axis much like the earth. The spinning of charge particle generates a magnetic field. As a consequence, the spinning nuclei behave as though they were tiny bar magnets having a north and a south pole.

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•Since a nucleus or an electron bears a charge, its spin gives rise to a magnetic field that is analogous to the field produce when an electric current is passed through a coil of wire. The resulting magnetic dipole (µ) is oriented along the axis of spin and has a value that is characteristic for each kind of particle.

∆E = µᵦHo

I= - ½

18

0energy

No field

Applied field Ho

E= µᵦHo

E= µᵦHoI= + ½

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PRECESSION

PRECESSIONAL MOTION

•Because the proton is behaving as a spinning magnet, it can align itself either with or opposed to an external magnetic field. It can also move a characteristic way under the influence of the external magnet.

•Considering the behavior of a spinning top, the top has a spinning motion around its axis. It also performs a slower waltz like motion in which the spinning axis of top moves slowly around the vertical. Thus is called “precessional motion” and the top is said to be precessing around the vertical axis of the gravitational force of the earth.

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•Precession arises due to the interaction of spin and the gravitational force acting downwards. This is the reason why only a spinning top will precess; where as a static top will topple over.

•Since the proton is a spinning magnet, it will precess around the axis of an applied external magnetic field. It will precess in two main orientations.

•Aligned or parallel with the field-low energy.

•Opposed or anti parallel to the field-high energy.

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PRECESSIONAL FREQUENCYThe precessional frequency of the nucleus is directly proportional to the strength of external field and also depends on the nature of the nuclear magnet.

Magnetic nuclei different atoms have different characteristic precessional frequency. according to Larmor precessional theory ω = γH0 ………………1 where, ω= Larmor precessional frequency. ω = 2 πV ………………2 2 πV = γH0 ………………..3 ………………….4 V α H0 ..................................5

Where, γ = is gyromagnetic ratio =

V =H0

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Intrinsic magnetic dipole momentum

Spin angular momentum

2 πγ

ENERGY TRANSITIONS

A proton when kept in an external magnetic field will precess and can take one of the two orientations with respect to the axis of the external field. Either aligned or opposed.

If a proton is precessing in the aligned orientation it can absorbed energy and pass into the opposed orientation and vice versa by losing energy.

If we irradiate the precessing nuclei with a beam of radio frequency, the low energy nuclei may absorb this energy and move to a higher energy state.

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The precessing proton will absorb energy from the radio frequency source, if the precessing frequency same as the frequency of the radio frequency beam.

When this occurs, the nucleus and radio frequency beam are said to be resonance, hence the term “ nuclear magnetic resonance”.

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WIDTH OF ABSORPTION LINES IN NMR

The separation of two absorption lines depends on how close they are to each other and the absorption line width.

The width of the absorption line is affected by a number of factors, only some of which we can control.

These are the factors:

I.The homogeneous fieldII.Relaxation timeIII.Magic angle NMRIV.Other source of line broadening

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1. THE HOMOGENEOUS FIELD

The most important factor controlling the absorption line width is the applied magnetic field H0.

It is very important that this field be constant over all parts of the sample, which may be 1-2 inch long. If it is not, H0 is different parts of the sample and therefore v, the frequency of the absorbed radiation, will vary in different parts of sample.

This variation results in a wide absorption line. For qualitative or quantitative analysis a wide absorption line is very undesirable, since we may get overlap between neighbouring peaks.April 10, 2023 25M.M.C.P.

2. RELAXATION TIMEThe second important feature that influences the absorption line width is the length of time that an excited nucleus stays in the exited state. ΔE Δt = constant where, ΔE is the uncertain in the value of E Δt is the length of time a nucleus of time a nucleus spends in the excited state.

Since ΔE Δt is a constant, when Δt is small, ΔE is large. But we know that E = hv and that h is constant.

Therefore any variation in E will result in a variation in v. If E is not an exact number but varies over the range E+ ΔE, then v will not be exact but vary over the corresponding range v + Δv. Then we have E + ΔE = h(v + Δv)

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We can summarize this relationship by saying that when Δt is small, ΔE is large and therefore Δv is large. If Δv is large, then the frequency range over which absorption takes place is wide and a wide absorption line results.

There are two principle modes of relaxation,

1. Longitudinal relaxation /spin lattice2. Transverse relaxation /spin spin

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A. LONGITUDINAL RELAXATION When the nucleus loses its excitation energy to the surrounding molecules, the system becomes warm as the energy is changed to heat.

This process is quite fast when the molecules are able to move quickly. This is the state of affairs in liquid.

The excitation energy becomes dispersed throughout the whole system of molecules in which the sample finds it self. No radiation energy appears, no other nuclei become exited. Instead, as numerous nuclei lose their energy in this fashion, the temperature of the sample goes up. This process is called longitudinal relaxation T1.

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B. TRANSVERSE RELAXATION

An excited nucleus may transfer its energy to an unexcited nucleus of a similar molecules that is nearby.

In this process, the nearby unexcited nucleus becomes excited a the previously excited nucleus become unexcited.

There is no net change in energy of the system, but the length of the time that one nucleus stays excited is shortened. This process, which is called transverse relaxation T2.

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3. MAGIC ANGLE NMR

A problem with the examination of solids that the nuclei can be frozen in space and cannot freely line up in magnetic field.

The NMR signals generated are dependent among other things, on the orientation of the nuclei. The randomly oriented nuclei therefore give broad band spectra which are not very useful analytically.

It can be shown that when one rotates a solid sample such that its axis of rotation 54.70 to the direction of the applied magnetic field, the broadening caused by random nuclear orientations tends to be average out, resulting in narrower spectra.

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This is more useful analytically because it allow better resolution and therefor better measurement of chemical shift and spin-spin splitting. In turn, this is very informative of the functional group and their positions relative to each other in the sample molecule.

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4. OTHER SOURCES OF LINE BROADENING

Any process of deactivating, or relaxing, an excited molecule results in a decrease in a lifetime of the excited state. This is turn causes line broadening.

Other causes of deactivation include 1. The presence of ions2. Paramagnetic molecules 3. Nuclei with quadrupole moment

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1.INDUCED MAGNETIC FIELD 1.INDUCED MAGNETIC FIELD :-:-

In the applied magnetic field, the valence electrons around the nucleus are cause to circulates and they generates their own secondary magnetic field is known as induced magnetic field.induced magnetic field.

2.SHIELDING:-2.SHIELDING:-The circulation of electron around the protons itself generates field in a such way that , it oppose the applied field.The field felt by the protons is thus diminished and the proton is said to be shieldedshielded and the absorption said to be upfield.upfield.

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SHIELDING AND DESHIELDING

DESHIELDINGDESHIELDING:-

If the induced magnetic field reinforced the applied magnetic field ,then the field felt by the proton is augmented augmented and the proton is said to be deshieldeddeshielded and the absorption is known as downfield.downfield.

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Compare with naked proton, a shielded proton required higher applied field strength than the deshielded protons.

Shifts in the position of NMR absorption arising from shielding and deshielding by electron, due to different chemical environments around protons are called chemical shift.chemical shift.

Generally chemical shift measured from the signal of reference standard such as TMS

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The extent of shielding is represented in terms of shielding parameter α. When absorption occurs, the field H felt by the proton is represented as, H = HH = H0 0 (1 - (1 - α α )………………. 1 )………………. 1

where, H0 = applied field strength.

Greater value of α, greater will be the value of applied strength which has to be applied to get the effective field required for absorption and vice versa. ………………. 2

From 1 and 2

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HH00

It is clear that the proton with different electronic environments or with shielding parameter can be brought into resonance in two ways

1. The strength of external field is kept steady and the radio frequency is constantly varied

2. The radio frequency is kept steady and strength of the applied field is constantly varied.

Clearly at constant radio frequency, shielding shift the absorption upfield in the molecules where these is a spherical distribution of electrons around the proton, It is called positive shielding.

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CHARACTERISTICS:-CHARACTERISTICS:-

Chemical inertness

Magnetically neutral

Gives single sharp peak

Easily recognizable peak

Miscible with wide range of solvents

Volatility –to facilitate recovery from valuable samples

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REFERENCE STANDARDS

1.TMS(Tetra Methyl Silan):- 1.TMS(Tetra Methyl Silan):- It is generally used as internal standard for measuring the position of 1H,13C, 29Si in NMR

TMS at 0.5%0.5% concentration is used normally

TMS has 12 protons which are uniformly shielded because of highly electro-positive highly electro-positive nature of silicon at centre

Hence this 12 protons gives single sharp peak at ooδδ which require maximum magnetic field than protons of the most organic compounds

It is chemically inert and miscible with large range of solvents

Highly volatile and easily removed to get back the sampleApril 10, 2023 40M.M.C.P.

It does not take part in intermolecular association with the sample

It’s all protons are magnetically equivalent

TMS can be used as an external reference external reference also.

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2.Sodium salts of 3-(trimethyl silyl)propane 2.Sodium salts of 3-(trimethyl silyl)propane sulphonate:-sulphonate:-

It is a water soluble compound. It is used as internal standard for running PMR spectra of water soluble substances in the duterium oxide solvent.

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““Chemical shift is the difference between the absorption position of the sample proton and the absorption position of reference standard”

Variations of the positions of NMR absorptions due to the electronic shielding and deshielding.

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CHEMICAL SHIFTCHEMICAL SHIFT

Chemical Shifts….• Measured in parts per million (ppm).

• It is the ratio of shift downfield from TMS (Hz) to total spectrometer frequency (MHz).

• The chemical shift is independent of the operating frequency of the spectrometer.

• Same value for 60, 100, or 300 MHz machine.

• Common scale used is the delta (δ) scale.

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MEASUREMENT OF CHEMICAL SHIFTMEASUREMENT OF CHEMICAL SHIFT

Each proton in a molecule has slightly different chemical environment and consequently has a slightly different amount of electronic shielding, which results in a slightly different resonance frequency. These differences in resonance frequency are very small.

For example, the difference between the resonance frequencies of the protons in Chloromethane and in Fluromethane is only 60 MHz when the applied field is 1.41 Tesla.

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Since the radiation used to induce proton spin transitions at that magnetic field strength is of a frequency near 60 MHz, the difference between Chloromethane and Fluoromethane represents a change in frequency of only slightly, not more than one part per million.

It is very difficult to measure the exact resonance frequencies to that precision. Hence instead of measurement of the exact radio frequency of any proton, a reference compound is placed in the solution of the substance to be measured and the resonance frequency of each proton in the sample is measured relative to the resonance frequency of the protons of the substance.

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The standard reference substance used universally is TETRAMETHYLSILANE (TMS), the standard reference is also known as Internal Standard.

Chemical Shift, ppm = Shift from TMS in HzSpectrometer frequency (MHz)

Eg: for CH3Br protons, chemical shift from TMS = 162Hz in a 60 MHz instrument and 270Hz in a 100 MHz instrument. Calculate value.

= 162Hz / 60 MHz = 270 / 100 = 2.7ppm

Hence, Value remains same irrespective of the spectrometer.

X 101066

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Chemical shift is measure in three major spectra.Delta(δ)Tau scale(τ) Hertz (Hz)

5 4 3 2 1 0

Up field shielding

Down field shielding

δ scale

5 6 7 8 9 10 Τ scale

1000 800 400 100 HZ

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Each δ unit is 1 ppm difference from TMS 60Hz and 300Hz

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1 ppm = 60 Hz = 6 ͯ 10-5

MHz

1 ppm = 300 Hz = 3 ͯ 10 MHz-4

CHART FOR DIFFERENT TYPES OF CHART FOR DIFFERENT TYPES OF PROTON CHEMICAL SHIFT VALUESPROTON CHEMICAL SHIFT VALUES

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BASIC CONCEPTS….BASIC CONCEPTS….

1) The chemical shift or position of line in NMR spectrum gives information on molecular environment of the nuclei from which it arises.

2) The chemical shift of nuclei in the different molecules are similar. If the molecular magnetic environment are similar.

3) The intensity of lines gives directly the relative number of magnetically active nuclei undergoing the different chemical shift.

4) The chemical shift is used for the identification of functional groups and as an aid in determining structural arrangement of groups.

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5)Greater is the deshielding of proton higher will be the value of delta. Greater is the shielding of proton lower will be the value of delta.

6)Electron withdrawing substituents like halogens which deshielded the protons. Electron releasing substituents like alkyl groups which shielded the protons.

7)The delta unit is independent of shield strength. Chemical shift position measured in the Hz are field dependent.

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• Electronegativity Effects.

• Van der Waal’s Deshielding.

• Hydrogen Bonding.

• Magnetic Anisotropy.

• Concentration, Tempareture and Solvent Effect.

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FACTORS INFLUENCING CHEMICAL SHIFT

ELECTRONEGATIVITY EFFECTSELECTRONEGATIVITY EFFECTS :- :-

• The chemical shift simply increase as the electronegativity of the attached element increases.

• Following table illustrates this relationship for several compounds of the type CH3X.

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• Multiple substituents have a stronger effect than a single substituent. The influence of the substituent drops off rapidly with distance, an electronegative element having little effect on protons that are more than three carbons distant. This effect is illustrated in the following table.

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Electronegative substituents attached to a carbon atom reduces that valence electron density around the protons attached to that carbon due to their electron withdrawing effects.

Electronegative substituents on carbon reduce the diamagnetic shielding in the neighborhood of the attached protons because they reduce the electron density around those protons.

The greater the electronegativity of the substituents, the more deshielding of protons and hence the greater is the Chemical Shift of those protons.

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HYDROGEN BONDINGHYDROGEN BONDING :- :-

Hydrogen atom exhibit property of hydrogen bonding in a compound which absorbs at a low field in comparison to the one which does not shows hydrogen bonding.

Hydrogen bonded proton being attached to a highly electronegative atom will have smaller electron density around it. less shielded resonance will occurs downfield and downfield shift depends up on the strength of hydrogen bonding.

Intramolecular and Intermolecular hydrogen bonding can be easily distinguished as the latter does not show any shift in absorption due to change in concentration.

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In case of phenols. Absorption occurs between 4-8 but if the concentration is decrease and volume of carbon tetrachloride is increase then absorption of OH proton occurs upfield, Exchangeable Hydrogen: protons that exhibit hydrogen bonding (

eg. Hydroxyl or amino protons ) show resonance over a wide range. These protons are usually found to attached to a heteroatom.

The more hydrogen bonding that takes place, the more deshielded proton becomes.

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MAGNETIC ANISOTROPYMAGNETIC ANISOTROPY :- :-

Circulation of electrons, especially the π electrons near by nuclei generates an induced field which can either oppose or reinforced the applied field at proton, depending upon location of proton or space occupied by the protons.

In case of alkynes, shielding occurs but in case of alkenes, benzene and aldehydes deshielding takes place.

The occurrence of shielding and deshielding can be determined by the location of proton in the space and so this effect is known as space effect.

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• There are some types of protons whose chemical shifts are not easily explained by simple consideration of the electronegativity of the attached groups.

• For example, when benzene is placed in magnetic field, the π electrons in the aromatic ring system are induced to circulate around the ring. This circulation is called as Ring current. The moving electrons generate a magnetic field which influence the shielding of the benzene hydrogens.

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Diamagnetic anisotropy in Benzene

Circulating π electron

Secondary magneticfield generated by circulating π electronswhich deshields aromatic protons

Applied field B0

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The benzene hydrogens are said to be deshielded by the diamagnetic anisotropy of the ring.

In electromagnetic terminology; an Isotropic field is one of either uniform density or spherically symmetric distribution.

Anisotropic field is nonuniform. In case of benzene labile electrons in the ring interact with the applied field and thus rendered it anisotropic.

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Thus a proton attached to a benzene ring is influenced by three magnetic fields:

1)The strong magnetic field applied by the electromagnets of the NMR spectrophotometer.

2)Weak magnetic field due to shielding by the valence electrons around the proton.

3)Anisotropy generated by the ring-system π electrons.

So anisotropic effect gives the benzene protons at higher resonance δ value.

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• All groups in a molecule that have π electrons generate secondary anisotropic fields.

• In Acetylene the magnetic field generated by induced circulation of the π electrons has a geometry such that the acetylenic hydrogens are shielded. Hence acetylenic hydrogens have resonance at higher field.

Diamagnetic anisotropy in Acetylene

π

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VAN DER WAAL’S DESHIELDING :-VAN DER WAAL’S DESHIELDING :-

In the overcrowded molecules. It is possible that some proton may be occupying stearic hindered position.

Clearly electron cloud of bulky group or hindering group will tend to repel the electron cloud surrounding the proton and such proton will shielded and will resonate at slightly higher value of than expected in the absence of this effect.

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CONCENTRATION, TEMPERATURE AND CONCENTRATION, TEMPERATURE AND SOLVENT EFFECT :-SOLVENT EFFECT :-

In ccl4 and cdcl3 chemical shift of proton attached to carbon is independent of concentration and temperature, while proton of -OH, –NH2, –SH groups exhibits a substantial conc. and temperature effects due to the hydrogen bonding

The intermolecular hydrogen bonding is less affected than intramolecular bonding by concentration change

Both type of hydrogen bonding affected by the temperature variation

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NMR spectrum of a substance gives very valuable information about its molecular structure. This information is gathered as follows :

(1)The number signalsnumber signals in PMR spectrum tell us how many kinds of protons in different chemical environments are present in structure under examination(2)The position of signal position of signal tell us about the electronic environment of each kind of proton (3)The intensities of different signals intensities of different signals tell us about the relative number of protons of different kind(4)The splitting of signals splitting of signals tell us about environment of the absorbing protons with respect to the environments of neighboring protons

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INTERPRETATION OF PMR SPECTRAINTERPRETATION OF PMR SPECTRA

Triplet = Triplet = δδ-1.7,3H (CH-1.7,3H (CH33 ) ) Quartet= Quartet= δδ-3.4, 2H (CH-3.4, 2H (CH22 ) )

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Doublet = Doublet = δδ-2.5,3H (CH-2.5,3H (CH33 ) ) Quartet= Quartet= δδ-5.8,1H (CH )-5.8,1H (CH )

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Triplet = Triplet = δδ-1.2,3H (CH-1.2,3H (CH33 ) ) Quartet= Quartet= δδ-3.6, 2H (CH-3.6, 2H (CH22 ) )

Singlet=Singlet= δ δ-4.8, 1H (OH) -4.8, 1H (OH) April 10, 2023 71M.M.C.P.

Doublet = Doublet = δδ-2.2,3H (CH-2.2,3H (CH33 ) ) Quartet= Quartet= δδ-9.8,1H (CHO )-9.8,1H (CHO )

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Singlet=Singlet= δ δ-2.3, 3H (CH-2.3, 3H (CH33)) Doublet = Doublet = δδ-7.4,2H (CH-7.4,2H (CH33 ) ) Doublet = Doublet = δδ-8.2,2H (NO-8.2,2H (NO22 ) )

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INSTRUMENTATION OF NMR

1. Conventional/Continuous NMR spectrophotometer Minimal type. Multiple type. Wide line. Or It can also be classified as a. Single coil spectrophotometer b. Two coil spectrophotometer

2. Pulsed Fourier transforms NMR spectrophotometer

CLASSIFICATION OF THE NMR SPECTROPHOTOMETERS

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COMPONENTS OF THE SPECTROPHOTOMETER

Basically NMR instrumentation involves the following units.

1.A magnet to separate the nuclear spin energy state.

2. Two RF channels, one for the field/frequency stabilization and one to supply RF irradiating energy.

3. A sample probe containing coils for coupling the sample with the RF field; it consists of Sample holder, RF oscillator, Sweep generator and RF receiver.

4. A detector to process the NMR signals.

5. A recorder to display the spectrum.

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•It is used to supply the principal part of the field Ho, which determines the Larmer frequency of any nucleus.

•The stronger the magnetic field, the better the line separation of chemically shifted nuclei on the frequency scale.

•The relative populations of the lower energy spin level increases with the increasing field, leading to a corresponding increase in the sensitivity of the NMR experiment.

MAGNETS

FEATURES:1. It should give homogeneous magnetic field i.e.; the strength and direction of

the magnetic field should be constant over longer periods.

2. The strength of the field should be very high at least 20,000 gaus.

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TYPES OF MAGNETS:

1. PERMANENT magnets2. ELECTRO magnets and 3. SUPER CONDUCTING magnets

MAGNETIC COILS

It is not easy or convenient to vary the magnetic field of large stable magnets, however this problem can be overcome by superimposing a small variable magnetic field on the main field. Using a pair of Helmholtz coils on the pole faces of the permanent magnet does this. These coils induce a magnetic field that can be varied by varying the current flowing through them.

The small magnetic field is produced in the same direction as the main field and is added to it. The sample is exposed to both fields, which appear one field to the nucleus.

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THE PROBE UNITIt is a sensing element of the spectrophotometer system. It is inserted between the pole faces of the magnet in X-Y plane of the magnet air gap an adjustable probe holder.

So the sample in NMR experiment experiences the combined effect of two magnetic fields ie Ho and RF (EMR).

The usual NMR sample cell is generally made up of the glass, which is strong and cheap. It consist of a 5 mm outer diameter and 7.5 cm long glass tube containing 0.4 ml of liquid. The sample tube in NMR is held vertically between the poles faces of the magnet. The probe contains a sample holder, sweep source and detector coils, with the reference cell. The detector and receiver coils are orientated at 90 to each other. The sample probe rotates the sample tube at a 30-40 revolutions on the longitudinal axis. Each part of the sample tube experiences the same time average the field.

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THE RADIOFREQUENCY GENERATOR

Using an RF oscillator creates the radio frequency radiation, required to induce transition in the nuclei of the sample from the ground state to excited states.

The source is highly stable crystal controlled oscillator. It is mounted at the right angles to the path of the field of wound around the sample tube perpendicular to the magnetic field to get maximum interaction with the sample. The oscillator irradiates the sample with RF radiation.

Radio frequencies are generated by the electronic multiplication of natural frequency of a quartz crystal contained in a thermo stated block.

In order to generate radiofrequency radiation, RFO is used. To achieve the maximum interaction of the RFradiation with the sample, the coil of oscillator is wound around the sample container.

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The RFO coil is installed perpendicular (90 ºC) to the applied magnetic field and transmits radio waves of fixed frequency such as 60,100,200 or 300 MHz to a small coil that energies the sample in the probe.

This is done so that the applied RF field should not change the effective magnetic field in the process of irradiation.

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SWEEP GENERATOR

Resonance

This can achieved by two methods

•Frequency sweep method

If the applied magnetic field is kept constant, the precession frequency is fixed. In order to bring about resonance, the frequency of the RF field should be changed so that it is becomes equal to the resonance frequency.

83

Thus resonance condition is reached by the holding the applied magnetic field Ho constant and scanning the Rf transmitter through the frequencies, until the various nuclei come to resonance in turn as their precessional frequency matched by the scanning source.

April 10, 2023 M.M.C.P.

•Field sweep method

•There is a relationship between the resonance frequency of the nucleus and the strength of the magnetic field in which the sample is placed.

•If the RF radiation is constant, in order to bring their resonance, the precession of the nucleus is to be changed by changing the applied magnetic field.

•Generally the field sweep method is regarded as better because it is easier to vary the magnetic field than the RF radiation so as to bring about resonance in nuclei.

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•Practically it is not very easy to vary the magnetic field of a large stable magnet. This is technical problem is solved by superimposing a small variable magnetic field on the main field.

•Helmholtz coils

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RADIO FREQUENCY RECEIVER OR DETECTOR

A few turns of wire is wound around the sample tube lightly. The receiver coil is perpendicular to both the external magnetic and radiofrequency transmitter coil. When RF radiation is passed through the magnetised sample, resonance occurs which cause the current voltage across the coil to drop. This electrical signal is small and is usually amplified before recording.

Detection of NMR.

When the radiofrequency radiation is passed through the magnetised sample two phenomena namely absorption and dispersion may occur. The absorption of either signal will enable the resonance frequency to be determined. It is found that the interpretation of absorption spectrum is easier as compared to the dispersion spectrum. The detector should be capable of separating absorption signal from dispersion signals.

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THERE ARE TWO WAYS OF DETECTING THE NMR PHENOMENA 1. Radio frequency bridge (single coil detection) 2. Nuclear detection (crossed coil detection)

SINGLE COIL METHOD

Single coil probe has one coil that not only supplies the RF radiation to the sample but also serves as part of the detector circuit for the NMR absorption signal. To detect the resonance absorption and to separate the NMR signal from the imposed RF field, a RF bridge is used.

At the fixed frequency the current flowing through the coils wrapped around the pole pieces of the magnet is varied. At the resonance there is a imbalance generated in this coil by virtue of the developing magnetization of the sample and this out of balance is detected in RF circuit.This technique is widely used in modern NMR spectrophotometer.

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CROSSED COIL PROBES

Nuclear induction has two coils, one for the irradiating the sample and second coil mounted orthogonally for the signal detection.

The irradiating coil oriented with its axis perpendicular to the magnetic field (i.e. along the x-axis). The detector coil is wound around the sample tube with its axis is the (y-axis) perpendicular to the both Ho (z-axis).

The RF current in the first coil wound around the x-axis excites the nuclei. The nuclei induction in the second coil wound around the y-axis is detected. The number of turns in the coil determines the particular frequency involved.

The RF detector can be tuned to detect either a signal in the absorption mode or in the dispersion mode. Phase sensitive detector is used which helping the operator to select the phase of the signal to be detected.

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THE CONTINUOUS –WAVE (CW) INSTRUMENT

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WORKINGIn the CW spectrometers the spectra can be recorded either with field sweep or frequency sweep.

Keeping the frequency constant, while the magnetic field is varied, (swept) is technically easier than holding the magnetic field constant and varying the frequency.

The sample (0.5 mg) is dissolved in a solvent containing no interfering protons usually CCl4 or CdCl3 0.5 ml and a small amount of TMS is added to serve as

an internal reference.

The sample cell is a small cylindrical glass tube that is suspended in the gap between the faces of the pole pieces of the magnet. The sample cell is rotated around its axis to ensure that all parts of the solution experience a relatively uniform magnetic field. This increases the resolution of the spectrum.

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Also in the magnetic gap, the radio frequency oscillator coil is installed perpendicular (90˚) to the applied magnetic field.

This coil supplies the electromagnetic energy used to change the spin orientations of the protons.

Detector coil is arranged perpendicular to the RF oscillator coil. As the magnetic field strength is increased, the precessional frequencies of all the nucleus increases (a peak or series of peaks)As the magnetic field strength is increased linearly, a pen travels from left to the right on a recording chart.

As each chemically distinct type of proton comes into resonance, it is record as a peak on the chart. The peak δ=0 ppm is due to the internal reference compound TMS.

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Instruments which vary the magnetic field in a continuos fashion scanning from the downfield end to upfield end of the spectrum, are called continuous wave instruments.

Because the chemical shifts of the peaks in this spectrum are calculated from the frequency differences from the TMS, this type of spectrum is said to be frequency domain spectrum.

Since highly shielded protons precess more slowly than relatively deshielded protons. Hence highly shielded protons appear to the right of the chart, and less shielded or dishelded protons appear to the left.

The region of the chart to the left is sometimes said to be downfield and that to the right is said to be upfield.

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Peaks generated by a CW instrument have ringing. Ringing occurs because the excited nuclei do not have time to relax back to their equilibrium state. And pen of the instrument have advanced to a new position. Ringing is most noticeable when a peak is a sharp singlet.

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TYPES OF CONTINUOUS –WAVE (CW) INSTRUMENT

1. Minimal-type NMR spectrometer

This basic instrument often utilizes a permanent of 14, 21 or 23 K gaus field strength and RF fields of 60, 90 or 100 MHz respectively.

Each frequency needed for the selected magnetic nuclei is synthesized from a suitable harmonic of a 15 MHz crystal oscillator and mixed with the output of an appropriate low frequency incremental oscillator.

The minimal type has,

1. Stressed reliability 2. Ease of operation 3. High performance 4. Low cost

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2. Multipurpose NMR spectrometers

These instruments are designed primarily for research, high performance, expensive and versatility better than minimal type.

The high precision comes through the use of homonuclear and heteronuclear lock systems and frequency synthesizers.

They are also characterized by high intrinsic sensitivity and the ability to study a variety of nuclei.

The strength of the magnetic field is quite important since sensitivity, resolution and the separation of chemically shifted peaks increase as the field strength increases.

These instruments uses RF field of 220,300 or even 500MHz.

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3. Wild-line CW NMR spectrometer

The wild line NMR spectrometer uses a frequency synthesizer to generate the RF field and a permanent magnet or a compact lightweight electromagnet.Slowly varying scan voltages are directly injected in the regulator for the magnet power supply for the electromagnet. Sample probe temperatures may be varied over the range 170 to 2000 ºC. Sample tubes are 15-18mm in outer diameter. The std magnetic field is 9.4 K gaus for protons and 10 K gaus for F19;the RF field is 40 MHz.Instruments are also available in which RF applied field is continuously adjustable over a basic frequency range of 300 Hz to 31MHz usually in steps of 10 Hz. For signal detection a sweep unit generates audio-modulation voltages that have selectable frequencies of 20,40,80,200 and 400 MHz.The output is amplified for simultaneous application to the probe modulation coils and to the oscilloscope.

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THE PULSED FOURIER TRANSFORM (FT ) INSTRUMENT

•The continuous wave type of NMR spectrometer operates by exciting the nuclei of the isotope under observation one type at a time.

•In the case of H1 nuclei each distinct type of proton (phenyl, vinyl, methyl and so on) is excited individually and its resonance peak is observed and recorded, independently of all the others. As we look at first one type of hydrogen and then another scanning until all of the types have come into resonance.

•An alternative approach common to modern sophisticated instrument is to use a powerful but short burst of energy called a pulse that excites all of the magnetic nuclei in the molecule simultaneously and all the signals are collected at the same time with a computer.

•In an organic molecule for instance all of the H1 nuclei are induced to undergo resonance at the same time.

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•When the pulse is discontinued the excited nuclei begin to lose their excitation energy and return to the original state or relax. As each excited nucleus relaxes it emits EMR.

•Since the molecule contains many different nuclei many different frequencies of EMR are emitted simultaneously. This emission is called a free-induction decay (FID) signal.

•The intensity of FID decays with the time as all of the frequencies emitted and can be quite complex. We usually extract individual frequencies due to different nuclei by using a computer and a mathematical method called a Fourier-transform analysis.

•The Fourier transform breaks the FID into its separate since or cosine wave components. This procedure is too complex to be carried out by eye or by hand; it requires a computer.

•The pulse actually contains a range of frequencies centered about the hydrogen in the molecule at once this signal burst of energy.

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ADVANTAGES OF FT-NMR

FT-NMR is more sensitive and can measure weaker signals.

The pulsed FT-NMR is much faster (seconds instead of min) as compared to continuous wave NMR.

FT-NMR can be obtained with less than 0.5 mg of compound. This is important in the biological chemistry, where only μg quantities of the material may be available.

The FT method also gives improved spectra for sparingly soluble compounds.

Pulsed FT-NMR is therefore especially suitable for the examination of nuclei that are magnetic or very dilute samples.

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COMPONENTS OF FT-NMR

A simplified form of the block diagram showing the instrument components of a typical Fourier transform NMR spectrometer.

The central component of the instrument is a highly stable magnet in which the sample is placed.

The sample is surrounded by the transmitter/receiver coil.

A crystal controlled frequency synthesizer having an output frequency of Vc produces radio-frequency radiation.

This signal passes into a pulse switch and power amplifier, which creates an intense and reproducible pulse of RF current in the transmitter coil.

Resulting signal is picked up by the same coil which now serves a as receiver.

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The detector circuitry produced the difference between the nuclear signals Vn and the crystal oscillator output Vc which leads to the low frequency time-domain signal as shown in the fig.

This signal is digitalized and collected in the memory of the computer for analysis by a Fourier transform program and other data analysis software.

The output from this program is plotted giving a frequency domain spectrum.

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The signal is then amplified and transmitted to a phase sensitive detector.

SAMPLE HANDLING TECHNIQUES IN NMR SPECTROSCOPY

The sample is placed in the probe, which contains the transmitter and receiver coils and a spinner to spin the tube about its vertical axis in order to average out field in homogeneities. In the electromagnet, the tube spins at right angles to the Z axis, which is horizontal, where as in the superconducting magnet, the tube fits in the bore.A routine sample for proton NMR on a scanning 60 MHz instrument consists about 5 – 20mg of the sample in about 0.4ml of the solvent in a 5mm glass tube.500MHz instrument consists about less than 1μg of the sample of modest molecular weight in a microtube.

IDEAL SAMPLE SIZEFor continuous wave spectra – less than 50mg.For FT spectra 1 – 10mg

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IDEAL SOLVENTS

InertNon polarLow boiling pointInexpensiveShould contain no protons

COMMONLY USED SOLVENTS

CCl4

CdCl3

DMSOD2OCd3OD

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• Each signal in an NMR spectrum represents one kind or one set of protons in a molecule.

• It is found that in certain molecules, a single peak (singlet) is not observed, but instead, a multiplet (groups of peaks) is observed.

SPLITTING OF THE SIGNALSSPLITTING OF THE SIGNALS

April 10, 2023 107M.M.C.P.

E.g. A molecule of CH3CH2Br, ethyl bromide.

April 10, 2023 108M.M.C.P.

SPIN-SPIN COUPLING

• The interaction between two or more protons, most often through the bonds, results in splitting of the spectral lines.

• It is related to the number of possible combinations of the spin orientations of the neighboring protons.

• The magnitude of the spin coupling interaction between protons in general decreases as the number of bonds between the coupled nuclei increases.

April 10, 2023 109M.M.C.P.

Consider a molecule of ethyl bromide (CH3-CH2-Br).the spin of two protons (-CH2-) can couple with the adjacent methyl group (-CH3-) in three different ways relative to the external field . The three different ways of alignment are ;

Thus a triplet of peaks results with the intensity ratio of 1 : 2 : 1 which corresponds to the distribution ratio of alignment .

April 10, 2023 110M.M.C.P.

Similarly the spin of three protons (CH3-) can couple with the adjacent methylene group (-CH2-) in four different ways relative to the external field

Thus a quartet of peaks results with an intensity ratio of 1:3:3:1 which corresponds to the distribution ratio of all the alignment.

April 10, 2023 111M.M.C.P.

• The relative intensities of the individual lines of a multiplet corresponds to the lines in the binomial expression .

• If n=1, then (1+x)n = 1 + x.

• If n=2, then (1+ x )2 = 1+2x + x2, thus the lines of triplet have relative intensities 1: 2 :1.

• If n=3, then ( 1 + x )3 = 1 +3X + 3X + X3, the lines of quartet have relative intensities 1 : 3: 3 : 1.

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Often a group of hydrogen's will appear as a multiplet rather than as a single peak.

Multiplets are named as follows:

Singlet QuintetDoublet Sextet SeptetTriplet OctetQuartet Nonet

This happens because of interaction with neighboring hydrogens and is called,

SPIN-SPIN SPLITTING.

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C CH

Cl

Cl H

H

Cl

integral = 2

integral = 1

triplet doublet

1,1,2-Trichloroethane1,1,2-TrichloroethaneThe two kinds of hydrogens do not appear as single peaks,rather there is a “triplet” and a “doublet”.

The sub peaks are due tospin-spin splitting and are predicted by the n+1 rule.

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nn + 1 RULE + 1 RULE

April 10, 2023 115M.M.C.P.

1,1,2-Trichloroethane1,1,2-Trichloroethane

C CH

Cl

Cl H

H

Cl

integral = 2

integral = 1

Where do these multiplets come from ? ….. interaction with neighbors

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C C

H H

H

C C

H H

H

two neighborsn+1 = 3triplet

one neighborn+1 = 2doublet

singletdoublettripletquartetquintetsextetseptet

MULTIPLETSthis hydrogen’s peakis split by its two neighbors

these hydrogens aresplit by their singleneighbor

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EXCEPTIONS TO THE n+1 RULEEXCEPTIONS TO THE n+1 RULE

IMPORTANT !

Protons that are equivalent by symmetryusually do not split one another

CH CHX Y CH2 CH2X Y

no splitting if x=y no splitting if x=y

1)

2) Protons in the same group usually do not split one another

C

H

H

H or C

H

HApril 10, 2023

118M.M.C.P.

3) The n+1 rule applies principally to protons in aliphatic (saturated) chains or on saturated rings.

Con….Con….

CH2CH2CH2CH2CH3

CH3

Hor

but does not apply (in the simple way shown here) to protons on double bonds or on benzene rings.

CH3

H

H

H

CH3

NONO NONO

YESYES YESYES

April 10, 2023 119M.M.C.P.

INTENSITIES OF INTENSITIES OF MULTIPLET PEAKSMULTIPLET PEAKS

PASCAL’S TRIANGLE

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1 2 1

PASCAL’S TRIANGLEPASCAL’S TRIANGLE

11 1

1 3 3 11 4 6 4 1

1 5 10 10 5 11 6 15 20 15 6 1

1 7 21 35 35 21 7 1

singlet

doublet

triplet

quartet

quintet

sextet

septet

octet

The interiorentries arethe sums ofthe two numbersimmediatelyabove.

Intensities ofmultiplet peaks

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M.M.C.P.

The simple rule to find the multiplicity of the signal from a group of protons, is to count the number of neighbours (n) & add 1. That is (n+1) .

No coupled hydrogen

One coupled hydrogen

Two coupled hydrogen

Three coupled hydrogen

CC –C – C –H C

HC- C – C –H C

HH - C –C-H C

H

H - C – C- H

H

A singlet

A doublet

A triplet

A quartet

J

J J

JJ

J

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THE ORIGIN OF THE ORIGIN OF SPIN-SPIN SPLITTINGSPIN-SPIN SPLITTING

HOW IT HAPPENS ?

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C C

H H

C C

H HA A

upfielddownfield

Bo

THE CHEMICAL SHIFT OF PROTON HTHE CHEMICAL SHIFT OF PROTON HAA IS IS

AFFECTED BY THE SPIN OF ITS NEIGHBOURSAFFECTED BY THE SPIN OF ITS NEIGHBOURS

50 % ofmolecules

50 % ofmolecules

At any given time about half of the molecules in solution willhave spin +1/2 and the other half will have spin -1/2.

aligned with Bo opposed to Bo

neighbor aligned neighbor opposed

+1/2 -1/2

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C C

H H

C C

H H

one neighbor n+1 = 2 doublet


SPIN ARRANGEMENTSSPIN ARRANGEMENTS

The resonance positions (splitting) of a given hydrogen is affected by the possible spins of its neighbor.

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C C

H H

H

C C

H H

H

two neighbors n+1 = 3 triplet



methylene spinsmethine spins

April 10, 2023 126M.M.C.P.

three neighbors n+1 = 4 quartet

two neighbors n+1 = 3 triplet


C C

H H

H

H

H

C C

H H

H

H

H

methyl spins methylene spinsApril 10, 2023127M.M.C.P.

128

NMR Spectroscopy

NOMENCLATURE

• The spacing between the two adjacent peaks of a multiplet is referred to as the J or coupling constant

• The value of J for a given coupling is constant, regardless of the field strength or operating frequency of the instrument

• Coupling between nuclei of the same type is referred to as homonuclear coupling

• Coupling between dissimilar nuclei is referred to as heteronuclear coupling

• The magnitude of this effect is dependent on the number of bonds intervening between two nuclei – in general it is a distance effect, where one-bond couplings

would be the strongest

Advanced Spin-spin Coupling

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NMR Spectroscopy

Con….There are many variations of the subscripts and superscripts associated with J

constants

In general, the superscript numeral to the left of J is the number of intervening bonds through which the coupling is taking place3J is a coupling constant operating through three bonds

Subscripts to the right of J can be used to show the type of coupling, such as HH for homonuclear between protons or HC for heteronuclear between a carbon and proton

Often, this subscript will be used to define the various J-constants within a complex multiplet: J1, J2, J3, etc. or JAB, JBC, JAC]

Although J values are referred to as positive numbers, they may in actuality be positive or negative


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NMR Spectroscopy

MECHANISM OF COUPLING• The most coherent theory of how spin information is transferred from one

nucleus to another is the Dirac vector model

• In this model, there is an energetic relationship between the spin of the electrons and the spin of the nuclei

• An electron near the nucleus has the lowest energy of interaction if its spin is opposite to that of the nucleus


Nuclear spin electron spin

Energy

Nuclear spin electron spin

April 10, 2023 130M.M.C.P.

NMR Spectroscopy

MECHANISM OF COUPLING – ONE BOND COUPLINGS, 1J• Here, a single bond (two electrons) joins two spin-active nuclei – such as 13C-

1H

• The bonding electrons will tend to avoid one another, if one is near the 13C nucleus the other will be near the 1H nucleus

• By the Pauli principle, these electrons must be opposite in spin

• The Dirac model then predicts that the most stable condition between the two nuclei must be one in which they too are opposite in spin:


13C spin 1H spin

electrons opposite in spin

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NMR Spectroscopy

MECHANISM OF COUPLING – ONE BOND COUPLINGS, 1J

• These alignments can be used for any heteronuclear pair of spin-active nuclei – 13P-13C, etc.

• When two nuclei prefer an opposed alignment, as in this example, the J is positive

• If the two nuclei have parallel spins, the J will be negative (remember spin information is transferred through the electrons!)


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NMR Spectroscopy

MECHANISM OF COUPLING – ONE BOND COUPLINGS, 1J• The Dirac model predicts the observed spin-spin coupling for the methine

13C-1H system

• It is important to note that the electron spins must be opposite


13C1H

13C nuclear resonance

13C1H

Dirac model favored ground state

13C1H

13C1H

Dirac model less-favored ground state

Excited state is of lower energy

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NMR Spectroscopy

MECHANISM OF COUPLING – ONE BOND COUPLINGS, 1J• It is these two upper energy states, and the two DEs that generated them that

result in the doublet for an undecoupled methine in a 13C spectrum


13C1H

13C1H

13C1H

13C1H

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NMR Spectroscopy

MECHANISM OF COUPLING – TWO BOND COUPLINGS, 2J• As the bond angle H-C-H decreases, the amount of electronic interaction

between the two orbitals increases, the electronic spin correlations also increase, and J becomes larger. They are sometimes called geminal coupling, because the two nuclei that interact are attached to the same central atom(Latin gemini = “twins”)


H

H

H-C-H 109o

2JHH = 12-18 Hz

H

H

H-C-H 118o

2JHH = 5 Hz

H

HH-C-H 120o

2JHH = 0-3 Hz

In general:

JHH

90 100 110 120

40

20

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NMR Spectroscopy

MECHANISM OF COUPLING – TWO BOND COUPLINGS, 2J

• Variations in J also result from ring size

• As ring size decreases, the C-C-C bond angle decreases, the resulting H-C-H bond angle increases, – J becomes smaller


H

H

H

H

HH H

HH H

CH

H

2JHH (Hz) = 3 5 9 11 13 9 to 15

April 10, 2023 136M.M.C.P.

NMR Spectroscopy

MECHANISM OF COUPLING – THREE BOND COUPLINGS, 3J

• These couplings are the one most common to introductory studies in NMR, and are observed as the coupling through a C-C bond between two C-H bonds - vicinal coupling.

• Observe the two possible spin intra C-C cations:


April 10, 2023 137M.M.C.P.

+1/2

+1/2

+1/2

-1/2

NMR SpectroscopyAdvanced Spin-spin Coupling

0o dihedral angle

MECHANISM OF COUPLING – THREE BOND COUPLINGS, 3J

Observe that the orbitals must overlap for this communication to take place

The magnitude of the interaction, it can readily be observed, is greatest when the orbitals are at angles of 0o and 180o to one another:

0o dihedral angle 180o dihedral angle

April 10, 2023 138M.M.C.P.Maximum overlap

NMR SpectroscopyAdvanced Spin-spin Coupling

MECHANISM OF COUPLING – THREE BOND COUPLINGS, 3J8. Examples of this effect in operation:

3Jdiaxial = 10-14 Hz

180

H

H

H H

3Jdiequitorial = 4-5 Hz

60

H

H

3Jaxial-eq. = 4-5 Hz

60

April 10, 2023 139M.M.C.P.

NMR Spectroscopy

MECHANISM OF COUPLING – LONG RANGE COUPLINGS, ≥4J

• the greater the number of intervening bonds the greater the reduction in opportunity for orbital overlap – long range couplings are uncommon

• In cases where a rigid structural feature preserves these overlaps, however, long range couplings are observed


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NMR Spectroscopy

MECHANISM OF COUPLING – LONG RANGE COUPLINGS, ≥4J

• Examples include the meta- and para- protons to the observed proton on an aromatic ring and acetylenic systems:


H

H

C C C C

H H

5J = 0-1 Jz Hz 3J = 7-10 Hz 4J = 1-3 Hz 5J = 0-1 Hz

H

H

April 10, 2023 141M.M.C.P.

HH

ortho meta para

NMR Spectroscopy

MECHANISM OF COUPLING – LONG RANGE COUPLINGS, ≥4J• Rigid aliphatic ring systems exhibit a specialized case of long range coupling – W-

coupling – 4JW

• The more heavily strained the ring system, the less “flexing” can occur, and the ability to transmit spin information is preserved


H H O

H

H HH

4J = 0-1 4J = 3 4J = 7 Hz

April 10, 2023 142M.M.C.P.

SPIN DECOUPLING

• It is a powerful tool for simplifying a spectrum & is of great value to organic chemists working with complex molecules. It helps in the identification of coupled protons in spectra that are too complex for detailed analysis.

• This technique involves the irradiation of a proton or a group of equivalent proton with sufficiently intense radio frequency energy to eliminate completely the observed coupling of the neighboring protons.

• The simplification of the complex spectrum for easy interpretation is done by,

1) By using an instrument with a more powerful homogeneous magnetic field, e.g. a 100 MHz instrument in preference to 60

MHz instruments. 2) By spin- spin decoupling techniques.

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ISOTOPE EXCHANGE• Deuterium (2H or D ), the heavy isotope of hydrogen, has been used

extensively in proton NMR spectroscopy for two reasones. First it is easily introduced into a molecule. Second, the presence of deuterium in a molecule is not detected in the proton NMR spectrum.

• Deuterium has a much smaller magnetic dipole moment than hydrogen & therefore, it absorbs at different field strengths. In case of ethylbromide the deuterium replaces the methyl hydrogens & the following changes occurs.

Br-CH2-CH3

Br-CH2-CH2D

Br-CH2-CHD2

Br-CH2-CD3

2H 3H

2H

2H

2H

1H

2H

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SHIFT REAGENTS

• Lanthanide series of elements are used as shift reagents. A lanthanide ion can increase its co-ordination number by interacting with unshared electrons. As a result the NMR spectrum of the comp. that contains a group possessing unshared pair of electron undergoes change & large chemical shift as a difference in peaks is observed.

• All the shift reagents are mild Lewis acids. Shift reagent separates NMR signals those normally overlap. Thus it gives more simplified spectrum.

• Shift reagent are paramagnetic, so large chemical shift take place.

• Shift reagents is normally used in non polar solvents like CdCl3, CCl4 etc.

• Shift reagents, provide a useful technique for spreading out proton NMR absorption patterns which normally overlap, without increasing the strength of the applied magnetic field.

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• In the proton NMR spectrum of n – hexanol, the high field triplet is distorted which represents the absorption of a methyl group adjacent to a - CH2 – group. The low field broad multiplet is due to the methylene group adjacent to the hydroxyl group. The proton of the remaining methylene groups are all burried in the methylene envelope between δ 1.2 & 1.8 .

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• When the same spectrum is recorded after addition of a soluble europium (III) complex, that is the shift reagent , the spectrum is spread out over a wider range of frequencies. So that it is now simplified almost to first order. In the spectrum OH absorption signal is shifted too far to be.

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COMPARISIONS BETWEEN 13C-NMR & 1H-NMR

13C-NMR 1H-NMR

1. Pulse Fourier Technique is used 1. Continuous wave method is followed.

2. Very fast. 2. Slow process.

3. No peak overlapping observed 3. Peak overlapping observed in case of

in the spectrum. complex samples.

4. Sweep generator & sweep coil 4. Required.

are not required in the NMR

instrument.

5. Chemical shift range is wide 5. δ range is very narrow (δ 0-15).

(δ 0-200).

6. Wide band RF is applied rather 6. Tuned to one frequency.

than tuned to a precise frequency.

7. Work on frequency sweep 7. Works on either field sweep

technique. or frequency sweep techniques.

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QUESTIONS :- 2o marks:-

1. (a) Explain the basic principles involved in NMR spectroscopy.

(b) Write an account of NMR spectra. How its interpretation ? Explain

with examples. (Sep’07)(Apr’08).

1o marks:-

1. Write a note on splitting of signals in NMR spectra. (May’10).

2. Briefly indicate the functions of various units of NMR spectrometer. (Apr’08).

3. Explain shielding & deshielding effect in NMR spectroscopy. (Apr’08).

4. What is chemical shift ? Explain the factors affecting chemical shift. (Apr’08).

April 10, 2023 M.M.C.P. 150

5 marks:-

1. Explain chemical shifts in NMR. (‘03)

2. Explain advantages and applications of FT NMR. (‘97)

Con….

REFERENCES :-1. Sharma YR. Elementary organic spectroscopy principles and

chemical applications. 1st ed. S. Chand and Company ltd; New Delhi :2008.

2. Chatwal GR, Anand SK. Instrumental methods of chemical analysis. 1st ed. Himalaya Publishing house; Mumbai: 2004.

3. Jag Mohan. Organic spectroscopy principles and applications. 1st ed. Narosa publishing House; New Delhi: 2001.

4. Sharma BK. Instrumental methods of chemical analysis. 24th ed. Goel Publishing house; Meerut: 2005.

5. S. Ravi Shankar. Text book of pharmaceutical analysis. 3rd ed. Rx publication; Tirunelveli: 2006.

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6. O.V.K. Reddy. Pharmaceutical analysis. Pulse publication; Hyderabad.

7. Willey. Handbook of spectroscopy. 2003.

8. Pavia, Lampman, Kriz. Introduction of spectroscopy. 3ed edition.

9. Skoog DA, West DM. principle of instrumental analysis. 2ed edition.

10.Willard HH, Merritt LL, Dean JA, Settle FA. Instrumental methods of analysis. Jr CBS publishing and distributors, 7 th edition.

11. Kasture AV, Mahadik KR, More HN, Wadodkar SG. Pharmaceutical analysis. Nirali Prakashan. 17th edition 2008.

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12. Silverstein R.M, Webmaster F.A, Spectrometric identification of organic compounds, 6th edition.148-150.

13. Kemp W. organic spectroscopy. 3rd edition.1996.

14. www.google.co.in

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154

Any Questions……..?????

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Education

Nuclear magnetic resonance proton nmr