Integration of 1H NMR
Information about the total
number of hydrogen atoms
Signal Areas
‹ Relative
areas of signals are proportional to
the number of hydrogens giving rise to each
signal.
‹ All
modern NMR spectrometers
electronically integrate and record the area
of each signal.
1
2
Peak splitting in 1H NMR
Information about the number of
nearby hydrogen atoms
Typical 1H splitting
ethylbenzene
3
Origins of Signal Splitting
‹ Spin-½
nucleii are influenced by neighboring
spin-½ nucleii. This is a magnetic effect.
‹ When the chemical shift of one nucleus is
influenced by the spin of another, the two
are said to be coupled.
‹ Consider nonequivalent hydrogens Ha and
Hb on adjacent carbons:
Ha Hb
C
C
Origins of Signal Splitting
B0
Hb
Hb reduces the field
felt by Ha; Ha signal is
shifted upfield.
upfield
Hb
Hb increases the field
felt by Ha; Ha signal is
shifted downfield.
downfield
Ha
4
Origins of Signal Splitting
Origins of Signal Splitting
‹ Observed
signal splitting patterns for an H
with 0, 1, 2, and 3 equivalent neighboring
hydrogens
Structure
Spin States of H b
Signal of H a
Ha
C
C
Ha Hb
C
1
1
C
5
Origins of Signal Splitting
Ha Hb
C
C
1
2
1
Hb
Ha Hb
C
C
Hb
1
3
3
1
Hb
The (n+1) rule
‹
‹
Peak: the units into which an NMR signal is split;
doublet, triplet, quartet, etc.
(n + 1) rule: the NMR signal of a set of equivalent
spin-½ nucleii is split into (n + 1) peaks by a
nonequivalent set of n equivalent neighboring spin½ nucleii
6
The (n+1) rule
ethylbenzene
The (n+1) rule
3-pentanone
7
The (n+1) rule
1,1-dichloroethane
Signal Splitting (n + 1)
n = 1. Their signal
is split into (1 + 1) or
2 peaks ; a doublet
Cl
CH 3 -CH-Cl
n = 3. Its signal
is split into (3 + 1)
or 4 peaks; a quartet
predict the number of 1H-NMR
signals and the splitting pattern of each
‹ Problem:
O
(a) CH 3 CCH 2 CH 3
O
(b) CH 3 CH 2 CCH 2 CH 3
O
(c) CH 3 CCH(CH 3 ) 2
8
Coupling Constants
‹
Coupling constant (J): the distance between peaks
in an NMR multiplet, expressed in hertz
J is a quantitative measure of the magnetic
interaction of nuclei whose spins are coupled
Ha
Ha
HaHb
Hb
-C-CHb
6-8 Hz
8-14 Hz
Ha
Ha
C
C
Ha
Hb
C
C
Hb
11-18 Hz
0-5 Hz
5-10 Hz
C
C
Hb
0-5 Hz
Splitting in vinyl acetate
9
300-MHz 1H-NMR
spectrum of vinyl acetate
60-MHz 1H-NMR
spectrum of vinyl acetate
10