Physical Chemistry , 1st ed.

hydrogen atoms, and ultimately only the hydrogen atoms bonded to adjacent

carbon atoms will participate in spin-spin coupling. (Hydrogens that are at-

tached to the same carbon do couple, but selection rules are such that transi-

tions to such coupled states are not observed. Therefore, the only coupling that

is observed occurs between hydrogens on differentcarbons.) For nuclei having

spin I^12 (which includes^1 H and^13 C, the two most prevalent nuclei studied

by NMR), the presence ofnnuclei induces a splitting of an absorption into

n 1 individual absorptions. This splitting occurs whether the nuclei on

adjacent carbons are equivalent or not.

The split NMR peaks also have a characteristic intensity pattern. A double

peak consists of two peaks of roughly equal intensities. Three peaks have in-

tensity ratios of roughly 1 2 1. A quartet has intensities of roughly 1 3  3 1, and

so forth. The relative intensity patterns are caused by the overlapping of ab-

sorptions that are split by spin-spin coupling, as illustrated in Figure 16.19.

578 CHAPTER 16 Introduction to Magnetic Spectroscopy

Resulting

spectrum

Coupled to the

third hydrogen

Coupled to the

first hydrogen

Coupled to the

second hydrogen

Coupled to the

first hydrogen

Uncoupled

signal

Coupled to the

second hydrogen

(b)

Resulting

spectrum

Coupled to the

other hydrogen

Uncoupled

signal

(a)

Resulting

spectrum

Uncoupled

signal

(c)

Coupled to the

third hydrogen

Coupled to the

first hydrogen

Coupled to the

second hydrogen

Coupled to the

fourth hydrogen

Resulting

spectrum

Uncoupled

signal

(d)
Figure 16.19 The nuclei ofnhydrogens on
adjacent atoms of C or O in a molecule couple
with the proton NMR signal and split the ab-
sorption into n 1 peaks. These peaks have char-
acteristic intensity ratios, as illustrated for a hy-
drogen having (a) 1, (b) 2, (c) 3, and (d) 4
neighbors.