Chapter 14
NMR Spectroscopy
Organic Chemistry
4th Edition
Paula Yurkanis Bruice
Irene Lee
Case Western Reserve University
Cleveland, OH
©2004, Prentice Hall
Nuclear Magnetic Resonance (NMR)
Spectroscopy
Identify the carbon–hydrogen framework of an organic
compound
Certain nuclei such as 1H, 13C, 19F, and 31P have
allowed spin states of +1/2 and –1/2; this property allows
them to be studied by NMR
The spin state of a nucleus is affected by an applied
magnetic field
The energy difference between the two spin states
depends on the strength of the magnetic field
absorb DE
a-spin states
b-spin states
release DE
Signals detected by NMR
An NMR Spectrometer
The electrons surrounding a nucleus affect the effective
magnetic field sensed by the nucleus
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
The Chemical Shift
The reference point of an NMR spectrum is defined by
the position of TMS (zero ppm)
HC
3
H3C
Si
CH3
H3C
The chemical shift is a measure of how far the signal is
from the reference signal
The common scale for chemical shifts = d
d=
distance downfield from TMS (Hz)
operating frequency of the spectrometer (MHz)
1H
NMR spectrum of
1-bromo-2,2-dimethylpropane
The chemical shift is independent of the operating
frequency of the spectrometer
Electron withdrawal causes NMR signals to appear at
higher frequency (at larger d values)
Characteristic Values of
Chemical Shifts
1H
NMR spectrum of
1-bromo-2,2-dimethylpropane
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 absolute number
Diamagnetic Anisotropy
The p electrons are less tightly held by the nuclei than
are s electrons; they are more free to move in response
to a magnetic field
Causes unusual chemical shifts for hydrogen bonded to
carbons that form p bonds
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
1H
NMR Spectrum of
1,1-Dichloroethane
The ways in which the magnetic fields of three protons
can be aligned
Splitting is observed if the protons are separated by more
than three s bonds
Long-range coupling occurs when the protons are
separated by more than three bonds and one of the
bonds is a double or a triple bond
More Examples of 1H NMR Spectra
The three vinylic protons are at relatively high frequency
because of diamagnetic anisotropy
The Difference between a Quartet
and a Doublet of Doublets
The signals for the Hc, Hd, and He protons overlap
The signals for the Ha, Hb, and Hc protons do not overlap
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
The trans coupling constant is greater than the cis
coupling constant
A Splitting Diagram for
a Doublet of Doublets
A Splitting Diagram for
a Quartet of Triplets
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 when the
coupling constants for the two sets are different
When the coupling constants are similar, the N + 1 rule
can be applied to both sets simultaneously
The three methyl protons are chemically equivalent due
to rotation about the C–C bond
We see one signal for the methyl group in the 1H NMR
spectrum
1H
NMR spectra of cyclohexane-d11 at various
temperatures
axial
equatorial
H
H
H
H
equatorial
the rate of
chair–chair
conversion is
temperature
dependent
axial
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
dry ethanol
ethanol with acid
To observe well-defined splitting patterns, the difference
in the chemical shifts (in Hz) must be 10 times the
coupling 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
DEPT 13C NMR distinguish among 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-pentanone
indicates coupling between protons and the carbon to
which they are attached