Chapter 14 NMR Spectroscopy

© 2011 Pearson Education, Inc.1

Chapter 14

NMR Spectroscopy

Organic Chemistry 6th Edition

Paula Yurkanis Bruice


Nuclear Magnetic Resonance (NMR) Spectroscopy

Identify the carbon–hydrogen framework of an organiccompound

Certain nuclei, such as 1H, 13C, 15N, 19F, and 31P, havenon-zero value for their spin quantum number; this property allows them to be studied by NMR


The spin state of a nucleus is affected by an appliedmagnetic field:


The energy difference between the spin states increases with the strength of the applied magnetic field:


-spin states -spin states

absorb E

release E

Signals detected by NMR


An NMR Spectrometer

In pulsed Fourier transform (FT) spectrometers, the magnetic field is held constant, and a radio frequency (rf) pulse of short duration excites all the protons simultaneously


The electrons surrounding a nucleus decrease the effective magnetic field sensed by the nucleus:

Beffective = Bo – Blocal


Chemically equivalent protons: protons in the same chemical environment

Each set of chemically equivalent protons in a compoundgives rise to a signal in an 1H NMR spectrum of that compound:


The Chemical ShiftThe reference point of an NMR spectrum is defined bythe position of TMS (zero ppm):

The chemical shift is a measure of how far the signal isfrom the reference signal

The common scale for chemical shifts =

=distance downfield from TMS (Hz)

operating frequency of the spectrometer (MHz)


1H NMR spectrum of 1-bromo-2,2-dimethylpropane

The greater the chemical shift, the higher the frequencyThe chemical shift is independent of the operating frequency of the spectrometer


Electron withdrawal causes NMR signals to appear athigher frequency (at larger values):

Protons in electron-poor environments show signals at high frequencies


Characteristic Values of Chemical Shifts


Diamagnetic Anisotropy

The unusual chemical shifts associated with hydrogens bonded to carbons that form bonds:

The electrons are freer to move than the electrons in response to a magnetic field


The protons show signals at higher frequencies because they sense a larger effective magnetic field:

benzene


The alkene and aldehyde protons also show signals at higher frequencies:

alkene aldehyde


The alkyne proton shows a signal at a lower frequency than it would if the electrons did not induce a magnetic field:

alkyne


1H NMR spectrum of 1-bromo-2,2-dimethylpropane

The area under each signal is proportional to the number of protons giving rise to the signal:


The area under each signal is proportional to the numberof protons that give rise to that signal

The height of each integration step is proportional to thearea under a specific signal

The integration tells us the relative number of protonsthat give rise to each signal, not the absolute number

Integration Line


Splitting of the Signals

• An 1H NMR signal is split into N + 1 peaks, where N is the number of equivalent protons bonded to adjacent carbons

• Coupled protons split each other’s signal

• The number of peaks in a signal is called the multiplicity of the signal

• The splitting of signals, caused by spin–spin coupling, occurs when different kinds of protons are close to one another


It is not the number of protons giving rise to a signal that determines the multiplicity of the signal

It is the number of protons bonded to the immediately adjacent carbons that determines the multiplicity

a: a tripletb: a quartetc: a singlet


Equivalent protons do not split each other’s signal:


The ways in which the magnetic fields of three protonscan be aligned:


Splitting is observed if the protons are separated by no more than three bonds:

Long-range coupling occurs over systems, such as benzene


More Examples of 1H NMR SpectraTriplet: two neighboring protons

Quintet: four neighboring protons


Doublet: one neighboring proton

Septet: six neighboring protonsTriplets: two

neighboring protons

Sextet: five neighboring protons


The three vinylic protons are at relatively high frequencybecause of diamagnetic anisotropy


The signals for the Hc, Hd, and He protons overlap because the electronic effect of an ethyl substituent is similar to that of a hydrogen:


The signals for the Ha, Hb, and Hc protons do not overlapbecause of the strong electron-withdrawing property of the nitro group:


Coupling ConstantsThe coupling constant (J) is the distance between twoadjacent peaks of a split NMR signal in hertz:

Coupled protons have the same coupling constant


Summary

1. The number of chemical shifts specify the number of proton environments in the compound

2. The chemical shift values specify the nature of the chemical environment: alkyl, alkene, etc.

3. The integration values specify the relative number of protons

4. The splitting specifies the number of neighboring protons

5. The coupling constants specify the orientation of the coupled protons


A Splitting Diagram for a Doublet of Doublets


Complex Splitting

Ha

JAC

JAB

JAC = JAB

Triplet

Ha

JAC

JAB

JAC > JAB

Doublet of doublets


The trans coupling constant is greater than the cis coupling constant:


A Splitting Diagram for a Quartet of Triplets


Why is the signal for Ha a quintet rather than a triplet of triplet?


The Difference between a Quartet and a Doublet of Doublets

Methylene has three neighbors, appears as a quartet

Doublet Doublet


When two different sets of protons split a signal, themultiplicity of the signal is determined by using the N + 1rule separately for each set of the hydrogens, as long as the coupling constants for the two sets are different

When the coupling constants are similar, the multiplicity of a signal can be determined by treating both sets of adjacent hydrogens as though they were equivalent


Replacing one of the enantiotopic hydrogens by a deuterium or any other atom or group other than CH3 or OH forms a chiral molecule:

prochiral carbon

Ha is the pro-R-hydrogen, whereas Hb is the pro-S-hydrogen; and they are chemically equivalent


Diastereotopic hydrogens have different chemical shifts:


Diastereotopic hydrogens are not chemically equivalent:


The three methyl protons are chemically equivalent because of rotation about the C—C bond:

We see one signal for the methyl group in the 1H NMRspectrum


1H NMR spectra of cyclohexane-d11 at various temperatures:

H

HH

H

axial

equatorial

axial

equatorial

The rate of chair–chair

conversion istemperaturedependent


Protons Bonded to Oxygen and Nitrogen

These protons can undergo proton exchange

They always appear as broad signals

The greater the extent of the hydrogen bond, the greater the chemical shift


pure ethanol

ethanol with acid


A 60-MHz 1H NMR spectrum

A 300-MHz 1H NMR spectrum


To observe well-defined splitting patterns, the differencein the chemical shifts (in Hz) must be 10 times thecoupling constant values


13C NMR Spectroscopy

• The number of signals reflects the number of different kinds of carbons in a compound.

• The overall intensity of a 13C signal is about 6400 times less than the intensity of an 1H signal.

• The chemical shift ranges over 220 ppm.

• The reference compound is TMS.


Proton-Decoupled 13C NMR of 2-Butanol


Proton-Coupled 13C NMR of 2-Butanol


The intensity of a signal is somewhat related to the number of carbons giving rise to it

Carbons that are not attached to hydrogens give very small signals


DEPT 13C NMR distinguishes CH3, CH2, and CH groups:


The COSY spectrum identifies protons that are coupled:

Cross peaks indicate pairs of protons that are coupled


COSY Spectrum of 1-Nitropropane


The HETCOR spectrum of 2-methyl-3-pentanoneindicates coupling between protons and the carbon to which they are attached:


Unknown Identification Using Spectroscopy

Example 1: 13C-NMR of C5H9Br


Example 1: 1H-NMR of C5H9Br


Example 1: IR of C5H9Br

Answer:


Example 2: 13C-NMR of C6H10O4

24.133.4

174.4

Solvent:


Example 2: 1H-NMR of C6H10O4

11.97

1.502.21


Example 2: IR of C6H10O4

Answer:

Chapter 14 NMR Spectroscopy

Documents

Transcript of Chapter 14 NMR Spectroscopy