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Chapter 14 NMR Spectroscopy. Organic Chemistry 6 th Edition Paula Yurkanis Bruice. Nuclear Magnetic Resonance (NMR) Spectroscopy. Identify the carbon–hydrogen framework of an organic compound. Certain nuclei, such as 1 H, 13 C, 15 N, 19 F, and 31 P, have

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slide1

Chapter 14

NMR Spectroscopy

Organic Chemistry

6th Edition

Paula Yurkanis Bruice

slide2

Nuclear Magnetic Resonance (NMR) Spectroscopy

Identify the carbon–hydrogen framework of an organic

compound

Certain nuclei, such as 1H, 13C, 15N, 19F, and 31P, have

non-zero value for their spin quantum number; this

property allows them to be studied by NMR

slide4

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

slide5

absorb DE

a-spin states

b-spin states

release DE

Signals detected by NMR

slide6

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

slide7

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

Beffective = Bo – Blocal

slide8

Chemically equivalent protons: protons in the same chemical environment

Each set of chemically equivalent protons in a compound

gives rise to a signal in an 1H NMR spectrum of that

compound:

slide10

The Chemical Shift

The common scale for chemical shifts = d

distance downfield from TMS (Hz)

d =

operating frequency of the spectrometer (MHz)

The reference point of an NMR spectrum is defined by

the position of TMS (zero ppm):

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

from the reference signal

slide11

1H NMR spectrum of

1-bromo-2,2-dimethylpropane

The greater the chemical shift, the higher the frequency

The chemical shift is independent of the operating

frequency of the spectrometer

slide13

Protons in electron-poor environments show signals at

high frequencies

Electron withdrawal causes NMR signals to appear at

higher frequency (at larger d values):

slide17

Diamagnetic Anisotropy

The unusual chemical shifts associated with hydrogens

bonded to carbons that form p bonds:

Thep electrons are freer to move than the s electrons in

response to a magnetic field

slide18

The protons show signals at higher frequencies because

they sense a larger effective magnetic field:

benzene

slide20

The alkyne proton shows a signal at a lower frequency

than it would if the p electrons did not induce a magnetic

field:

alkyne

slide21

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:

slide22

Integration Line

The area under each signal is proportional to the number

of protons that give rise to that signal

The height of each integration step is proportional to the

area under a specific signal

The integration tells us the relative number of protons

that give rise to each signal, not the absolute number

slide24

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
slide25

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 triplet

b: a quartet

c: a singlet

slide31

Splitting is observed if the protons are separated by no

more than three s bonds:

Long-range coupling occurs over systems, such as benzene

slide32

Triplet: two neighboring protons

Quintet: four neighboring protons

More Examples of 1H NMR Spectra

slide33

Doublet: one neighboring proton

Sextet: five neighboring protons

Septet: six neighboring protons

Triplets: two neighboring protons

slide35

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:

slide36

The signals for the Ha, Hb, and Hc protons do not overlap

because of the strong electron-withdrawing property of

the nitro group:

slide37

Coupling Constants

The coupling constant (J) is the distance between two

adjacent peaks of a split NMR signal in hertz:

Coupled protons have the same coupling constant

slide39

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

slide40

A Splitting Diagram for

a Doublet of Doublets

complex splitting
Complex Splitting

JAC = JAB

Triplet

JAC > JAB

Doublet of doublets

slide43

A Splitting Diagram for

a Quartet of Triplets

slide46

The Difference between a Quartet

and a Doublet of Doublets

Methylene has three neighbors, appears as a quartet

Doublet

Doublet

slide47

When two different sets of protons split a signal, the

multiplicity of the signal is determined by using the N + 1

rule 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

slide48

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

slide51

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 NMR

spectrum

slide52

1H NMR spectra of cyclohexane-d11 at various

temperatures:

axial

equatorial

equatorial

axial

The rate of

chair–chair

conversion is

temperature

dependent

slide53

Protons Bonded to

Oxygen and Nitrogen

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

These protons can undergo proton exchange

They always appear as broad signals

slide54

pure ethanol

ethanol with acid

slide55

A 60-MHz 1H

NMR spectrum

A 300-MHz 1H

NMR spectrum

slide56

To observe well-defined splitting patterns, the difference

in the chemical shifts (in Hz) must be 10 times the

coupling constant values

slide57

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.
slide62

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

slide64

The COSY spectrum identifies protons that are coupled:

Cross peaks indicate pairs of protons that are coupled

slide66

The HETCOR spectrum of 2-methyl-3-pentanone

indicates coupling between protons and the carbon to

which they are attached:

unknown identification using spectroscopy
Unknown Identification Using Spectroscopy

Example 1: 13C-NMR of C5H9Br

slide70

33.4

24.1

174.4

Solvent:

Example 2: 13C-NMR of C6H10O4

slide71

2.21

1.50

11.97

Example 2: 1H-NMR of C6H10O4