Can you differentiate ‘A’ from ‘B’ using 1H NMR in each pair?

To be NMR active any nucleus must have a spinquantum number, different from zero (I≠0)

As in 1H, the spin quantum number (I) of 13C is 1/2

Two possible orientations of the nuclear spin in an external magnetic field.

Which is of low natural abundance and less sensitive than 1H

Only one possible orientation under B0

Only 13C nuclei are NMR active

Sensitivity is proportional to g3

13C/1H signal intensity ratio is then (0.2514)3 = 0.0159

DE = hgB0/2p

Boltezman’s excess for 13C is less than that of 1H

n = gB0/2p g13C = 0.25 X g1H DE = hgB0/2p

B0 (Tesla) 1H (MHz) 13C (MHz)

1.4 60 15

2.35 100 25

4.7 200 50

14 600 150

19 800 200

Signal intensity is proportional to the number of resonating nuclei

Limitations:-Small NMR tube size (usually 5 mm), -Solubility of sample, -amount of sample available, -high concentration results in line broadening

Sensitivity can be increased slightly by lowering the temperature (relative population of the lower energy level is increased)

Problems:-decrease in solubility of sample at lower temperature-Increase in viscosity results in line broadening

High magnetic field results in increased sensitivity

DE = hgB0/2p

Sensitivity can be enhanced by the use of strongermagnetic field

2.35T, 4.7T, 7.05T, 9.4T, 11.74T, 14.10T, 16.2, 18.4T

Increasing order of sensitivity

Repeated recording allows the accumulation of thousands of spectra by digital computer

Absorption signals are always positive, signals arising from noise vary in intensity and their signMost noise signals will cancel each other out.This process is known as the CAT (computer-averaged transients)

S/N = n1/2, S is intensity of signal, N is the intensity of noisen is the number of scans.

Recording 100 spectra of a sample will increase the signal to noise ratio 10 times

Continuous Wave (CW) NMR Spectrometer

solution of the sample in a uniform 5 mm glass tube is orientedbetween the poles of a powerful magnet, and is spun to average any magnetic field variations,as well as tube imperfections. Radio frequency radiation of appropriate energy is broadcast into the sample from an antenna coil (colored red). A receiver coil surrounds the sample tube, and emission of absorbed rf energy is monitored by dedicated electronic devices and a computer. An NMR spectrum is acquired by varying or sweeping the magnetic field over a small range while observing the rf signal from the sample (constant rf). An equally effective technique is to vary the frequency of therf radiation while holding the external field constant.

A CW nmr spectrometer functions by irradiating each set ofdistinct nuclei in turn, independently. For a high resolution spectrum this must be done slowly,

Suppose the smallest line spacing (resolution)we would like to resolve is 1 Hz

The uncertainty principle tells us:

DEDt ~ hIf Dn = 1, we need to perform the measurement during a time interval of 1 sWe spend around 1 s measuring 1 Hz Sweep rate is 1Hz/s

hDnDt ~ h DnDt ~ 1

Typical width of 13C NMR spectrum is 250 ppmwhich on 100 MHz is 25,000 Hz

A complete scan requires 25,000 s (417 min or 7 hrs)100 scan requires 700 hrs, 30 days!!

Solution is Pulsed Fourier Transform Spectroscopy

n = gB0/2p

w0 = n2p = gB0

(in cycles per second, Hz)

(in radians per second)The frequency ωo is called the Larmor frequency.

If rf energy having a frequency matching the Larmor frequency is introduced at a right angle of the external field (e.g. along the x-axis), the precessing nucleus will absorb energy and themagnetic moment will flip to its I = -1/2 state.

(Revisiting the NMR Phenomenon)Pulsed Fourier Transform Spectroscopy

The energy difference between nuclear spin states is small and the +1/2 and -1/2 states are nearly equally populated. in a field of 2.34 T the excess population of the lower energy state is only six nuclei per million. the numerical excess in the lower energy state is sufficient for selective and sensitive spectroscopic measurements.

slight excess of +1/2 spin states precess randomly in alignmentwith the external field and a smaller population of -1/2 spin states precess randomly in an opposite alignment.

An overall net magnetization therefore lies along the z-axis.

changes in net macroscopic magnetization occurs as energy is introduced by rf irradiation at right angles to the external field. The net magnetization shifts away from the z-axis and toward the y-axis.After irradiation the nuclear spins return to equilibrium in aprocess called relaxation.Short and strong rf irradiation is called PULSE.

Pulsed Fourier Transform Spectroscopy:

hDnDt ~ h

From uncertainty principle:

If the irradiation is applied for a time Dt,the normally monochromic irradiation is uncertainin frequency by about 1/Dt if we only apply it for Dt s

DnDt ~ 1DEDt ~ h

Allows the possibility of extracting the complete frequency response in ONE GO

Measuring all frequencies simultaneouslyinstead of one after the other.

Dt 1ms, corresponds to Dn = 1/10-3 s = 1000 Hz

By exposing the sample to a very short (10 to 100 μsec), relatively strong (about 10,000 times that used for a CW spectrometer) burst of rf energy (pulse)along the x-axis,. all of the nuclei in the sample areexcited simultaneously.The overlapping resonance signals generated as the excited nuclei relax are collected by a computer and subjected to a Fourier transform mathematical analysis.

The rf signal emitted by the sample decays exponentially, and is called a Free Induction Decay (FID).

FT

FID signal collected after one pulse, may be stored and averaged with theFID's from many other identical pulses prior to the Fourier Transform (FT),

spectra from low abundance isotopes, such as 13C can thus be analyzed

CH3-CH2-OH

13C peaks are not integrated

CH3 CH3

OAmXn

1H,13C coupled spectrum

1H,13C decoupled spectrum

Multiplicity of signals

1J C,H= 125.5 Hz

2J C,H= 5.5 Hz

1H,13C coupled spectrum

Signal overlapping

O

O

CH3

O

O CH3

HH

H H

HH

HH

Diethyl phthalate

123

4 56

1H,13C decoupled spectrum

The intensities of quaternary carbon signals are always much lower than protonated carbon atoms.

Distinct peaks for different sets of Equivalent carbon atoms

O

O

CH3

O

O CH3

HH

H H

HH

HH

Diethyl phthalate

123

4 56

FACTORS AFFECTING CHEMICAL SHIFT IN 13C NMR

si = sdia + spara + sN

si magnetic shielding constantsdia diamagnetic shielding constant accountsfor local electrons around the nucleus induced by B0

sdia ∞ r-1

The diamagnetic term decreases with distance rbetween nucleus and circulating electrons.

s electrons will cause stronger shieldingthan p electrons

sdia is dominant in hydrogen with only s electrons

For 13C, the paramagnetic term spara predominates.It opposes sdia and is deshielding

DE = average electronic excitation energy

= inverse cube of the distance between 2p electron and the nucleus

spara increases with a decrease in DE and theinverse cube of r

QAA number of electrons in p orbitalSQAA multiple bond contribution

sN is anisotropic effect (most important in triple bonds)

OCH2CH3

OCH2CH3

O

O

n

pss

Increasing order of p electron density at carboncauses electron repulsionThe bonding orbital expands, r increasesspara decreasesCarbons are more shielded.

Hybridization of CarbonThe hybridization of carbon determines to a greater extent the range within which its 13C signal is found.

sp3 carbons resonate at highest field (-10 to 80 ppm) followed by sp carbons (60 to 95 ppm), sN , anisotropic effect is shielding for sp carbon- sp2 hybridized centres are low field (100-150 ppm)Carbonyl (160-220 ppm)

The hybridization effect in 13C NMR thus parallels the effect in 1H NMR

Inductive EffectElectronegativity of substituents that are attached at the carbon atom changes the resonance frequency

The effect is transmitted through bonds.

X

g

g

Effect of halogen substituents on chemical shift in n-pentane derivatives

These are additive parameters to values where X=H

The -effect increases with increasing electronegativityof the substituent (F, Cl, Br).

The effect of iodine (-7.4 ppm) is an exceptionto the pattern.

The bond polarization due to electronegative substituents should propagate along the carbon chain.However there is no correlation between the substituent electronegativities and the observed chemical shift of -and g-carbon resonance.Inductive effect decreases with inverse cube of distance and the effect should be smaller at - and g-positions.The shielding of g-position is called g-gauche effect

The effect increases with multiple substitution.This is attributed to increased diamagnetic shieldingcaused by the large number of electrons introduced by heavy atoms.

Z is the atomic numberr is the internuclear distanceme is the electron masse is the electron charge

The up-field chemical shift produced by iodine and bromine atoms is called ‘heavy atom effect’.

Steric Effect (g-gauche effect) Steric interactions arise from an overlapping of van der-Waals radii of substituents which are closely spaced. The carbon atom is always shielded if substituents are introduced at the g-position.

This is observed in both acyclic and cyclic systems.

Because of the necessity of the confirmation, this is called the g-gauche steric effect.

gauche

The nonbonding interaction causes a polarization of the C-H bonds increasing the electron densities atcarbon atoms (up-field shift).

CH3CH3

22.7

35.6

H

CH3

H HH 20.130.6

Due to steric interactions, the chemical shift of the g-carbon atoms will move up-field.

endo, endo

exo, exo

endo, exo

g-effect is additive

Sterically induced up-field shift is dtrans > dcis at theallylic carbon in cis and trans alkenes

CH3 CH3 CH3CH3

HH

CH3

CH3

CH3

H CH3

CH3

CH3

CH3CH4

methane primary secondary tertiary quaternary

-2.3 5.7 15.4 24.3 31.4

Increasing order of substitution by more electronwithdrawing group

causes a successive down-field shift

Electron deficiency at a carbon causes drastic de-shielding

248 ppm

If the positive charge is dispersed, electron deficient carbon will be less de-shielded

128.5 ppm

OMe..

:H

HH

OMe

-

:+

H

HH

OMe

-

+

129.7

134.3129.0

OH

Mesomeric Effect (Resonance Effect)OMe

159.9121.4

128.1

125.3

136.7192.0

54.8

Ipso

Mesomeric Effect (Resonance Effect)

Electron releasing substituents increase electron densityat ortho and para positions (shield)meta not affected

Electron withdrawing substituents decrease electron densityat ortho and para positions (de-shield)meta not affected

a,b-Unsaturated Carbonyl

-carbon is de-shielded

Decrease in bond order and results in shielding of the central carbon atoms

O

The carbonyl carbon of a,b-unsaturated carbonylcompounds are shielded by about 10 ppm

166.4

Steric repulsion between alkyl groups may oppose conjugation and cancel the up-field shift characteristic of conjugated state.

Ф is tortional angle between phenyl and carbonyl bonding

Steric Effect on Conjugation

si = sdia + spara + sN

sN neighbouring anisotropy term

Important in 1H NMR

7.3-5.6 = 1.7 ppm in a 10 ppm scale

17% difference

128.5-127.0=1.5 ppm in a 200 ppm scale

0.75% differenceNOT as important in 13C NMR

127.0 128.5

H 5.6 ppm H 7.3 ppm

What would you use to differentiate alkenes from aromatics?1H NMR

Intra-molecular Hydrogen bonding,carbonyl is more polarised,carbonyl carbon become more positive(De-shielded)

Intramolecular Hydrogen Bonding

chelation

1H NMR is of 10 ppm scale

13C NMR is of 200 ppm scale

Knowledge of the relationship between chemical shiftand molecular structure is more important in 13C NMR than in 1H NMR.

Empirical rules with substituent constants orincrements are available,to help predict resonance positions of carbon atoms.

ALKANESCarbon is sp3 hybridized Alkanes resonate between -10 to 80 ppm

As the number of carbon substituents is increasedthe chemical shift move downfield.

After analyses of a large number of hydrocarbons, empirical rule have been developed

dk is the chemical shift of carbon of interest-2.3 chemical shift of CH4nkl number of carbon atoms at position lwith respect to atom kAl additive shift parameterSkl steric correction parameter

1

2

3

4

5

6

7

1,7

2,6

4

3,5

CDCl3 (t)

mC =2nI+1

I for D is 1, n=1

m =(2X1X1)+1= 3

Substituents have influence on chemical shift.-effect correlates with substituent electronegativity-effect is much smaller and is always de-shieldingbut no-direct relationship with electronegativity.g-effect is shielding due to steric g-gauche interaction.

To calculate the value of chemical shift position in substituted alkanes:1)Calculate the value in the parent hydrocarbon.2)Shift and steric correction parameters are added.

SUBSTITUTED ALKANES

CDCl3

OH1

2

3

4

5

6

7

8

(RH) chemical shift of the parent hydrocarbon

(parent hydrocarbon)

CYCLOALKANES

-3.8 ppm

Torsional strain because of eclipsed hydrogen atomsAnisotropic effect, ring strain cause shielding

Hax

Heq1

3

56

2

4 Hax

Heq1

3

5

6

24ring

flipCH3

13

56

2

4CH3

1

3

5

6

24ring

flip

The ring inversion can be frozen out at lower temperature (-100ºC)The two different conformers can be observed separately by NMR spectroscopy.The shift parameters of substituents can be determined for each conformation

27.60 ppm

sp2 hybridized centres are low field (100-150 ppm)

ALKENESspara is important for alkenesp- p* transition occursDE is small

Electronic nature and the number of substituents attached to alkene carbon can affect the resonance frequency.Shift parameters of alkyl groups determine chemical shift position.

nkl number of carbon atoms at position l with respect to atom kAl additive shift parameterSkl steric correction parameter

+’,’+1.3

107.8

CYCLOALKENES

AROMATIC COMPOUNDSAnisotropy is important in 1H NMR Spectroscopy.In 1H NMR Spectroscopy, aromatic protons resonate at lower field (1-2 ppm) as compared to olefinic protons.1H NMR is the best criteria for aromaticity.Aromatic carbons appear in the same region asthose of olefinic carbons.Generally between 100 and 150 ppm.On attachment of electron-withdrawing and electron-releasing substituents, the shift may extend,to 90-180 ppm.

1)Mesomeric effect (resonance)2)Inductive effect3)Field effect (arising from through-space polarization

of p-system by the electric field of a substituent4)Steric (g) effect on the ortho-carbon nuclei.5)Anisotropic effect of triple bonds (alkynyl and cyano)on ipso carbon6)Heavy atom effect which is shielding.

Factors affecting the chemical shiftposition of aromatic compounds:

Mesomeric Effect (Resonance Effect)

Electron releasing substituents increase electron densityat ortho and para positions (shield)meta not affected

Electron withdrawing substituents decrease electron densityat ortho and para positions (de-shield)meta not affected

128.5 ppm

CH3128.5+9.3

128.5-3.0

128.5+0.7

128.5-0.1

Ar-CH3

Hyperconjugation(shielding at ortho & para)-effect (de-shielding) at ortho

ipso

para

orthometa

O CH3

H128.5+31.4

128.5+1.0

128.5–14.4

128.5–7.8

OCH3

ipso

meta

para

ortho

Resonance(shielding at ortho & para)

g-effect(shielding at ortho)

128.5+8.2

128.5+5.8

128.5+0.5

128.5+0.5

CHO

Resonance(de-shielding at ortho & para)

OH

H

Field effect(shielding at ortho)

ipso

para

orthometa

N+ OO

HH128.5+20.6

128.5+6.2

128.5+1.3

128.5–4.3

High electron density at oxygen will force the bonding electron density towards the carbon atom andshielding the ortho-carbon atoms (field effect).

Heavy atom effect

I 128.5-32.3

128.5-0.4

128.5+2.6

128.5+9.9

Polyaromatic Compounds

An increase in conjugation will decrease the p-bond order of a double bond,will result in a shift towards low frequency

Aldehydes and ketones resonate at low field(between 195-220 ppm) due to structure B.

Carboxylic acids and derivatives (including amides) appear at higher field (160-180 ppm) due to structure B.

CARBONYL COMPOUNDSAbsorb in the characteristic region of 150-220 ppm.Substituents at carbonyl influence resonances.

O199.0

C+

O

Electron density at the carbonyl carbon has increased,shift towards higher field due to resonance

The introduction of a second double bondcauses further shift towards higher field

C=O in ketones appear at lower field than aldehydes and -carbons results in a shift towards lower fieldg-carbon results in a shift towards high field.

High

Properties of Some Deuterated NMR Solvents

Solvent B.P. °C Residual1H signal (δ)

13C signal (δ)

acetone-d6 55.5 2.05 ppm 205.4 & 30.5 ppm

acetonitrile-d3 80.7 1.95 ppm 117.8 & 1.3 ppm

benzene-d6 79.1 7.16 ppm 128.5 ppm

chloroform-d 60.9 7.27 ppm 77.0 ppm

cyclohexane-d12 78.0 1.38 ppm 26.4 ppm

dichloromethane-d2 40.0 5.32 ppm 53.8 ppm

dimethylsulfoxide-d6 190 2.50 ppm 39.5 ppm

nitromethane-d3 100 4.33 ppm 62.8 ppm

pyridine-d5 114 7.19, 7.55 & 8.71 ppm 150, 135.5 & 123.5 ppm

tetrahydrofuran-d8 65.0 1.73 & 3.58 ppm 67.4 & 25.2 ppm

Spin-Spin Coupling in 13C NMR13C -13C coupling is not important because:The probability of having two 13C atomsnext to each other is only 0.01%

1H -13C coupling is what is normally considered.

AmXn

m= n+1

Coupling can be through: One bond (1JC,H),two bonds (2JC,H) or three bonds (3JC,H)

Empirical relationship between JCH and fractional s-character (denoted by s)

Four bond (4JC,H) coupling can also occur provided zigzag arrangement(‘W-coupling’)

For sp3 carbon 1JCH is between 125-150 HzFor sp2 carbon 1JCH is between 150-170 HzFor sp carbon 1JCH is between 200-250 Hz

For sp2 carbon 2JCH is 20-50 HzFor sp carbon 2JCH is 40-65 Hz

For sp3 carbon 2JCH is -6 to -4 Hz