NMR Spectroscopy (Part 1)

8/4/2019 NMR Spectroscopy (Part 1)

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NMR SPECTROSCOPYPART IBASIC PRINCIPLES OF NMR

SAMPLE PREPARATIONSCHEME OF NMR INSTRUMENT

Argamino, C.; Buenaseda, A.; Gajigan,A.

CHEM 127.1Maam Fatima


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Basic Principles of NMR


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Introduction

NMR - identify the carbonhydrogen framework of anorganic compound

In many atoms (such as 12C) spins are paired such

that the nucleus of the atom has no overall spin. atoms (such as 1H,13C 15N, 19F, 31P.) the nucleus does

possess an overall spin

12C = no spin = no magnetic dipole


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NMR phenomenon

absence of an applied magnetic field

the nuclear spins are randomly

applied magnetic field

the nuclei twist and turn to align themselves withor against the field


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NMR phenomenon

The energy difference (E) between the and

states depends on the strength of the appliedmagnetic field (Bo)

Bo E

Radiation required isin the radiofrequency(rf) region of theelectromagneticspectrum and iscalled rf radiation


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NMR phenomenon

Sample - subjected to a pulseof radiation (equal to E)

Nuclei promoted

Nuclei undergo relaxation emit electromagnetic signals

whose frequency depends on

The NMR spectrometer detects

these signals

displays a plot of signalfrequency versus intensityanNMR spectrum


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NMR phenomenon

Resonance flipping back and forth of nucleibetween spin states]

the magnetic field is proportional to theoperating frequency (MHz).

if the spectrometer has a more powerful magnet,

it must have a higher operating frequency

v = frequency = gyromagnetic ratio

(depends on the magnetic

moment of the particular kindof nucleus)


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Fourier Transform (FT)spectrometer

Dispersive IR

viewing each componentfrequency sequentially

Fourier Transform IR

all frequencies are examinedsimultaneously

Radiation sourceinterferometer detector


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Individual protons absorbfrequency

Protons relax

produce a complex signal freeinduction decay (FID) E

intensity of the signal decays asthe nuclei lose the energy

computer converts the

intensity-versus-time datainto intensity- versus-frequency information in amathematical operation knownas a Fourier transform


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Michelson interferometer

allows somewavelengths to passthrough but blocks othersdue to wave interference

is modified for each new

data point by moving oneof the mirrors


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Typical Layout of FTIRspectrometer


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Shielding

protons do not experience the same applied magneticfield

cloud of electrons partly shieldsthe nucleus

Protons in electron-rich environments

shielded and appear at lower frequencies

Protons in electron-poor environments less shielded and appear at higher frequencies


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Number of Signals


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Chemical Shift

position at which a signal occurs in an NMRspectrum

a measure of how far the signal is from the

reference signalDoes not depend on operating frequency


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Relative positions of signals

The closer the protons are to the electron-withdrawing group, the less they are shieldedfrom the applied magnetic field, so the higher

the frequency (i.e., the farther downfield)


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Characteristic Values of ChemicalShift


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Integration of NMR Signals

the area under each signal is proportional tothe number of protons that gives rise to thesignal

1.6 : 7.0= 1 : 4:4= 2 : 8.8

= 2 : 9


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Splitting of Signals

Splitting is caused by protons bonded toadjacent (i.e., directly attached) carbons.

splitting of a signal is described by N + 1 rule

where N is the number of equivalentprotons bondedto adjacent carbons


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Hb split into a quartet by Ha, spilt into triplet by Hc

Peaks depend on J (overlap is possible)


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diff coupling constants = use (N + 1) separately

similar J = use (N + 1) as if the adjacent H are

equivalent


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Coupling constant

The distance (Hz) between two adjacent peaksof a split NMR signal

useful in analyzing complex NMR spectra because

protons on adjacent carbons can be identified byidentical coupling constants


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Resolution of HNMR Spectra

To observe separate signals with clean

splitting patterns, the difference in thechemical shifts of two adjacent protons (in Hz)

must be at least 10 times the value of thecoupling constant (J).


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13C NMR

13C NMR requires Fourier transform technique because the signals obtained from a single scan

are weak to be distinguished from backgroundelectronic noise.

13C are weak since they exist in low amount(isotope)

area under a 13C NMR signal is not

proportional to the number of atoms Carbon splitting - rare ; carbon-proton coupling

possible


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13C NMR

Proton coupled 13C NMR splitting is observed


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SaMPLE PREPARATION


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SAMPLE PREPARATION

Sample tubes accurate dimensions

keep tubes free from dust,grease etc

never be cleaned withchromic acid tubes can be rinsed with

distilled water and thenacetone

blow nitrogen or air throughthe tube, with a pipette,while warming it gently for afew minutes or to leaveunder vacuum for somehours


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SAMPLE PREPARATION

To obtain high resolution NMRspectra it is necessary that yourNMR sample is free ofsuspended material

will increase the line width of theSpectrum

broad spectral lines reducespectral resolution and no amountof shimming can correct for this

Suspended material can easilybe removed from an NMRsample by constructing a filterusing cotton wool as a filtering

agent


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SAMPLE PREPARATION

The solute of interest can thenbe dissolved in a separateglass vial using deuteratedsolvent (e.g. CDCl3, DMSO-

D8, CD3OD, etc.) required tomake your NMR sample

After the solute has beendissolved it can be transferred

directly to a NMR tube bypassing the solution throughthe cotton filter


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SAMPLE PREPARATION

the sample volume can beadjusted by adding theremaining solvent to theNMR tube so that a finalsample volume of ~700 Lor a sample height of ~55mm is reached

followed by vigorousshaking of the sample toeffectively mix its content


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SAMPLE PREPARATION

Sample Volume

best results will beobtained using a sample

volume of 0.6 - 0.8 mlfor a 5mm NMR tube

Smaller volumes can beused but will require

much more shimming too long are also more

difficult to shim and arewaste costly solvent


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SAMPLE PREPARATION

Sample Quality

Solid particles distort themagnetic field homogeneitybecause the magnetic

susceptibility of a particle isdifferent from that of asolution

A sample containingsuspended particles thushas a field homogeneitydistortion around everysingle particle.

causes broad lines andindistinct spectra that cannotbe corrected.


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SAMPLE PREPARATION

Sample Solvent

contain deuteriuminstead of hydrogen

The NMR signal fromthe deuterium nuclei iscalled the NMR lockand is used by the

spectrometer forstabilization.

Solvents should bekept dry and free from

impurities.


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SAMPLE PREPARATION

Sample Solvent

Deuterium have the same properties of the "light"solvents with the added bonus of having a

substantially reduced proton signal (somesolvents are > 99.99% deuterated).

Deuterium is a spin 1 nucleus and hence has areasonably short T1 compared to 1H permitting

the use of rapid pulse-acquire measurement.


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SAMPLE PREPARATION

Air sensitiveAn inert atmosphere or

vacuum

Special NMR tubes arecommercially available forsealing samples undervacuum or an inertatmosphere

alternative to attach a glass tube to the top

of a regular NMR tube with aconstriction

Care must be taken to maintainconcentricity.


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SAMPLE PREPARATION

High Pressure Sample

can be sealed in thickwalled NMR tubes

alternative to use shortsealed 4 mm

tubes placed in a 5 mmtube

A tiny piece of tissue paperin the bottom of the outertube makes it less likely tobreak


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SAMPLE PREPARATION

Solid and Semi-Solid

Rotor

Special Inserts Teflon (white)

and Kel-F(transparent)

Caps


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SAMPLE PREPARATION

Solid and Semi-Solid

Red pepperwashingwith D2O,cutting tosize andinsertinginto the

rotor

leafcutting to size,rolling up andinserting into the

rotor


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INSTRUMENTATION

on nuous ave


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on nuous aveinstruments

Similar in principle tooptical spectrometers.

The sample is held ina strong magneticfield, while frequency

of the source is slowlyscanned (in someinstruments, thereverse is done).


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Due to the lowermaintenance andoperating cost of cw

instruments, they arestill commonly usedfor routine 1H NMRspectroscopy at 60

MHz.


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our er rans orm


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our er rans orminstruments

All frequencies in aspectrum areirradiated

simultaneously with aradio frequency pulse.Following the pulse,the nuclei return to

thermal equilibrium.


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A timedomain emissionsignal is recorded by

the instrument as thenuclei relax.


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A frequency domainspectrum is obtainedby Fourier

transformation.


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The Pulse

Range of radiation:

F 1/t

Where:

F-signal frequencey

1/t-bandwidth

t-pulse duration


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Documents

NMR Spectroscopy (Part 1)