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 B0.

• Nuclei aligned with the magnetic field are lower in energy than those aligned against the field

• The nuclei aligned with the magnetic field can be flipped to align against it if the right amount of energy is added (DE)

• The amount of energy required depends on the strength of the external magnetic field

NMR Spectrometer• Schematic diagram of a nuclear magnetic

resonance spectrometer.

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

Chemical EquivalenceHow many signals in 1H NMR spectrum?

O OO

O

CH3

Number of Equivalent Protons

O

OOO

6 4

CH3

1 3 5 2 4

Homotopic H’s– Homotopic Hydrogens

• Hydrogens are chemically equivalent or homotopic if replacing each one in turn by the same group would lead to an identical compound

Enantiotopic H’s• If replacement of each of two

hydrogens by some group leads to enantiomers, those hydrogens are enantiotopic

Diastereotopic H’s• If replacement of each of two hydrogens

by some group leads to diastereomers, the hydrogens are diastereotopic

– Diastereotopic hydrogens have different chemical shifts and will give different signals

Vinyl Protons

Typical 1H NMR Scale is 0-10 ppm

The Scale

Tetramethylsilane (TMS)

CH3SiCH3

CH3

CH3TMS

Arbitrarily assigned a chemical shift of 0.00

Chemical Shift Ranges, ppm

Diamagnetic AnisotropyShielding and Deshielding

Deshielding in Alkenes

Shielding in Alkynes

Integration

CH3

CH3

CH3

Br

H H

Methyl t-butyl ether (MTBE)

Toluene at Higher Field

Splitting patterns in aromatic groups can be confusing

A monosubstituted aromatic ring can appear as an apparent singlet or a complex pattern of

peaks

Integral Trace

Signal Splitting; the (n + 1) Rule

• Peak:Peak: The units into which an NMR signal is split; doublet, triplet, quartet, multiplet, etc.

• Signal splitting:Signal splitting: Splitting of an NMR signal into a set of peaks by the influence of neighboring nonequivalent hydrogens.

• ((nn + 1) rule: + 1) rule: If a hydrogen has n hydrogens nonequivalent to it but equivalent among themselves on the same or adjacent atom(s), its 1H-NMR signal is split into (n + 1) peaks.

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

CH

HH

proton splits into n+1

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

shieldeddeshielded

Chemical Shift

Summary of Signal Splitting• The origins of signal splitting patterns. Each arrow

represents an Hb nuclear spin orientation.

1,1-Dichloroethane

Ethyl benzene

CH3CH2OCH3

Equivalent Protons do not Couple

Pascal’s Triangle

Signal Splitting (n + 1)

ProblemProblem: Predict the number of 1H-NMR signals and the splitting pattern of each.

CH3CCH2CH3O

CH3CH2CCH2CH3O

CH3CCH(CH3)2O

(a)

(b)

(c)

Differentiate using 1H NMR

Methyl Isopropyl Ketone

1-Nitropropane

Para Nitrotoluene

Coupling Constants (J values)

Bromoethane

Coupling Constants– An important factor in vicinal coupling is the angle

between the C-H sigma bonds and whether or not it is fixed.

– Coupling is a maximum when is 0° and 180°; it is a minimum when is 90°.

Physical Basis for (n + 1) Rule• Coupling of nuclear spins is mediated through

intervening bonds.– H atoms with more than three bonds between them

generally do not exhibit coupling.– For H atoms three bonds apart, the coupling is called

vicinal coupling.

Physical Basis for (n + 1) Rule

• Coupling that arises when Hb is split by two different nonequivalent H atoms, Ha and Hc.

More Complex Splitting Patterns

– Complex coupling that arises when Hb is split by Ha and two equivalent atoms Hc.

More Complex Splitting Patterns– Since the angle between C-H bond determines the extent of

coupling, bond rotation is a key parameter.– In molecules with free rotation about C-C sigma bonds, H

atoms bonded to the same carbon in CH3 and CH2 groups are equivalent.

– If there is restricted rotation, as in alkenes and cyclic structures, H atoms bonded to the same carbon may not be equivalent.

– Nonequivalent H on the same carbon will couple and cause signal splitting.

– This type of coupling is called geminal couplinggeminal coupling.

More Complex Splitting Patterns

– In ethyl propenoate, an unsymmetrical terminal alkene, the three vinylic hydrogens are nonequivalent.

More Complex Splitting Patterns– Tree diagram for the complex coupling seen for the

three alkenyl H atoms in ethyl propenoate.

More Complex Splitting Patterns– Cyclic structures often have restricted rotation about

their C-C bonds and have constrained conformations.– As a result, two H atoms on a CH2 group can be

nonequivalent, leading to complex splitting.

More Complex Splitting Patterns– A tree diagram for the complex coupling seen for the

vinyl group and the oxirane ring H atoms of 2-methyl-2-vinyloxirane.

More Complex Splitting Patterns

• Complex coupling in flexible molecules.– Coupling in molecules with unrestricted bond rotation often

gives only m + n + I peaks.– That is, the number of peaks for a signal is the number of

adjacent hydrogens + 1, no matter how many different sets of equivalent H atoms that represents.

– The explanation is that bond rotation averages the coupling constants throughout molecules with freely rotation bonds and tends to make them similar; for example in the 6- to 8-Hz range for H atoms on freely rotating sp3 hybridized C atoms.

More Complex Splitting Patterns– Simplification of signal splitting occurs when

coupling constants are the same.

More Complex Splitting Patterns

– Peak overlap occurs in the spectrum of 1-chloro-3-iodopropane.

– Hc should show 9 peaks, but because Jab and Jbc are so similar, only 4 + 1 = 5 peaks are distinguishable.

Styrene

Ha splitting in Styrene“Tree” Diagram

C CH

HHa

b

c

In the system below, Hb is split by two different sets of hydrogens : Ha and Hc

– Theortically Hb could be split into a triplet of quartets (12 peaks) but this complexity is rarely seen in aliphatic systems

0.750.50

60 MHz

100 MHz

300 MHz

300 240 180 120 60 0 Hz

300 240 180 120 60 0 Hz

300 240 180 120 60 0 Hz

Why go to a higher field strength?

0.75 (t, 2H, J=10)0.50 (t, 2H, J=10)

60 MHz

100 MHz

300 MHz

300 240 180 120 60 0 Hz

300 240 180 120 60 0 Hz

300 240 180 120 60 0 Hz

Homonuclear Decoupling

Allyl Bromide

CH2=CHCH2Br

CH2=CHCH2Br

irradiate

1-Phenyl-1,2-dihydroxyethane

C

H

OH

C

H

H

OH

a b

c

1-Phenyl-1,2-dihydroxyethane

Irradiate at 4.8