Proton NMR Spectroscopy

Proton NMR Spectroscopy

The NMR Phenomenon

• Most nuclei possess an intrinsic angular momentum, P.

• Any spinning charged particle generates a magnetic field.

P = [I(I+1)]1/2 h/2where spin quantum #

I = 0, 1/2, 1, 3/2, 2, …

Which nuclei have a “spin”?• If mass # and atomic # are both even, I = 0 and the

nucleus has no spin. e.g. Carbon-12, Oxygen-16

• For each nucleus with a spin, the # of allowed spin states can be quantized:

• For a nucleus with I, there are 2I + 1 allowed spin states.

1H, 13C, 19F, 31P all have I = 1/2E = h/2)Bo

Spin states split in the presence of B0

no field applied field

E

+1/2 parallel

-1/2 antiparallel

Bo

When a nucleus aligned with a magnetic field, B0, absorbs radiation frequency (Rf), it can change spin orientation to a higher energy spin state. By relaxing back to the parallel (+1/2) spin state, the nucleus is said to be in resonance. Hence,

NMR

Presence of Magnetic Field

NMR instruments typically have a constant Rf and a variable B0.

A proton should absorb Rf of 60 MHz in a field of 14,093 Gauss (1.4093 T).

Each unique probe nucleus (1H perhaps) will come into resonance at a slightly different - and a very small percentage of - the Rf.

All protons come into resonance between 0 and 12/1,000,000 (0 – 12 ppm) of the Rf.

Energy Difference (E) Between Two Different

Spin States of a Nucleus With I=1/2

+1/2

-1/2

E 400 MHz300 MHz200 MHz100 MHz

23,500 47,000 70,500 104,000

parallel

antiparallel

inc. magnetic field strength, Gauss

B0

What Does an NMR Spectrum Tell You?

• # of chemically unique H’s in the molecule # of signals

• The types of H’s that are present e.g. aromatic, vinyl, aldehyde …

chemical shift• The number of each chemically unique H

integration• The H’s proximity to eachother

spin-spin splitting

How many signals in the Proton NMR Spectrum?

CH3CH2CH2CH3

OCH3

CH3CH2OCCH2CH3

O

CH3

CH3

CH3CH2CH2CH3

OCH3

CH3CH2OCCH2CH3

O

CH3

CH3

2 1 4

4 4 2

© 2013 Pearson Education, Inc. Chapter 13 12

Shielded Protons

• A naked proton will absorb at 70,459 gauss.• A shielded proton will not absorb at 70,459 gauss so the

magnetic field must be increased slightly to achieve resonance.

Chemical Shift

• The variations in the positions of NMR absorptions, arising from electronic shielding and deshielding, are called chemical shifts.

• The chemical shift (in ppm) is independent of the spectrometer used.

• Most common scale is the (delta) scale.

Typical 1H NMR Scale is 0-10 ppm

The Scale

Tetramethylsilane (TMS)

CH3SiCH3

CH3

CH3

TMS

Arbitrarily assigned a chemical shift

of 0.00

© 2013 Pearson Education, Inc. Chapter 13 22

Electronegative Atoms

• More electronegative atoms deshield more and give larger shift values.

• Additional electronegative atoms cause an increase in chemical shift.

© 2013 Pearson Education, Inc. Chapter 13 23

Location of Electronegative Atoms

• The deshielding effect of an electronegative substituent drops off rapidly with distance.

Chemical Shift Ranges, ppm

Diamagnetic AnisotropyShielding and Deshielding

Deshielding in Alkenes

Shielding in Alkynes

Chemical Shift is Affected by Electron Density Around Nucleus

Increased electron density

Decreased electron density Deshields nucleus

upfield shiftShields nucleus

downfield shift

from chemistry.msu.edu

© 2013 Pearson Education, Inc. Chapter 13 31

Number of Signals

• Methyl tert-butyl ether has two types of protons, giving two NMR signals.

• Chemically equivalent hydrogens have the same chemical shift. All the methyl groups of the tert-butyl group are equivalent, and they produce only one signal.

© 2013 Pearson Education, Inc. Chapter 13 32

Intensity of Signals: Integration

• The amount the integral trace rises is proportional to the area of that peak.

• The integration will have a trace for the tert-butyl hydrogens that is three times as large as the trace for the methyl hydrogens. The relative area for methyl and tert-butyl hydrogens is 1:3.

© 2013 Pearson Education, Inc. Chapter 13 33

tert-Butyl Acetoacetate

• The spectrum of tert-butyl acetoacetate has only three signals. The most shielded protons are the methyl groups of the tert-butyl. The most deshielded signal is the methylene (CH2) because it is in between two carbonyl groups.

Toluene at Higher Field

Integral TraceHow many protons give rise to each signal?

Spin-Spin Splitting

The Doublet in 1H NMR

C C

HH

B0

a b

Ha splits into a 1:1 doublet peak

Hb is parallel or anti-parallel to B0

Ha is coupled to Hb

Hb in 1,1,2-Tribromoethane

The Triplet in 1H NMR

C C

H

H

H

B0

a b

Ha splits into a 1:2:1 triplet peak

Hb can both be parallel, anti-parallel

Ha is coupled to Hb and Hb

b

or one parallel and one anti-parallel

Ha in 1,1,2-Tribromoethane

1,1,2-Tribromoethane

The Quartet in 1HMR

B0

C

H

C

H

H

H

proton splits into n+1

n = # adjacent H'squartet 1:3:3:1

shieldeddeshielded

Chemical Shift

The N + 1 Rule

If a proton is coupling to N equivalent protons, (on adjacent atoms) it is split into N + 1 peaks.

• Equivalent protons do not split each other.

• Protons bonded to the same carbon will split each other if they are nonequivalent.

• Protons on adjacent carbons normally will split each other.

• Protons separated by four or more bonds will rarely split each other.

Spin-Spin Splitting Distance

Predict Splitting

ClCH2CH2Br

ClCH2CH2Cl

CH3CHCl2

CH3CH2NO2

CHCl

CH3

CH3

CH3 C

CH3

CH3

CH2Br

1,1-Dichloroethane

Ethyl benzene

Methyl Isopropyl Ketone

1-Nitropropane

CH2CH2CH3O2N

Ethyl, propyl and isopropyl groups are common. Learn to recognize them from their splitting patterns.

2-Methyl-1-propanol

HOCH2CH

CH3

CH3

Propose a structure for the compound of molecular formula C4H10O whose proton NMR spectrum follows.

Solved Problem 3

Solution

Solved Problem 3 (Continued)Solution (Continued)

Para Nitrotoluene

C-13 NMR Spectroscopy

C-13 chemical shifts

One signal for each chemically unique carbon

Coupling in C-13 NMR

methine group

B0

the doublet in C-13 NMR

C splits into a 1:1 doublet peak

H is parallel or anti-parallel to B0

C is coupled to H

C

H

methylene group

C

H

HB0 a

the triplet in C-13 NMR

C splits into a 1:2:1 triplet peak

Ha & Hb can both be parallel, anti-parallel

C is coupled to Ha and Hb

b

or one parallel and one anti-parallel

B0

C

H

H

H

the quartet in C-13 NMR

carbon splits into n+1

n = # attached H'squartet 1:3:3:1

shieldeddeshielded

methyl group

Butanone - Coupled and Decoupled

1,2,2-Trichloropropane1H and 13C NMR Spectra

Coupled C-13 NMR Spectrum

HC CCH2CH2CH2CH3

CH2CO2H

SPA

CH3CH2OCCH2CCH3

O O

coupled spectrum