Organic Chemistry

Section 14.12 Coupling Constants 551

a. f. k.

b. g. l.

c. h. m.

d. i. n.

e. j. o.

14.12 Coupling Constants

The distance, in hertz, between two adjacent peaks of a split NMR signal is called the
coupling constant(denoted by J). The coupling constant for being split by is
denoted by The signals of coupled protons (protons that split each other’s signal)
have the same coupling constant; in other words, (Figure 14.20). Coupling
constants are useful in analyzing complex NMR spectra because protons on adjacent
carbons can be identified by identical coupling constants.
The magnitude of a coupling constant is independent of the operating frequency of
the spectrometer—the same coupling constant is obtained from a 300-MHz instrument
as from a 600-MHz instrument. The magnitude of a coupling constant is a measure of
how strongly the nuclear spins of the coupled protons influence each other. It, there-
fore, depends on the number and type of bonds that connect the coupled protons, as
well as the geometric relationship of the protons. Characteristic coupling constants are
shown in Table 14.3; they range from 0 to 15 Hz.
The coupling constant for two nonequivalent hydrogens on the same carbon is
often too small to be observed (Figure 14.17), but it is large for nonequivalent hydrogens
bonded to adjacent carbons. Apparently, the interaction between the hydrogens is
strongly affected by the intervening electrons. We have seen that electrons also allow
long-range coupling—that is, coupling through four or more bonds (Section 14.10).

p p

sp^2

sp^2

Jab=Jba

Jab.

Ha Hb

CH 3 CCH 2 COCH 3 O

O O

CC

HCl

Cl H

CH 3 CHCHCl 2

Cl

CH 3 CCH 2 CH 3

Br

CH 3

CC

HCl

H Cl

O O CH 3 CH 2 OCH 2 Cl

CH 3 OCH 2 CH 2 CH 2 Br CH 3 CH 2 OCH 2 CH 3 CH 3 OCH 2 CH 2 CH 2 OCH 3

CH 3 CHCH 2 CH

CH 3 O

CC

HCl

H H

BrCH 2 CH 2 Br

frequency

CH 3 CHCl 2

ab

Jba

Jba

Jab

Jba >Figure 14.20

The and protons of

1,1-dichloroethane are coupled

protons, so their signals have the

same coupling constant, Jab=Jba.

Ha Hb