Page 987 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

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SECTION 11.1
Generation and
Characterization of Free
Radicals

No interacting One interacting Two interacting
hydrogen; one hydrogen; two hydrogens. three
absorption line absorption lines absorption lines.
Fig. 11.1. Hyperfine splitting in ESR spectra.

signal into a doublet. Interaction with three equivalent hydrogens, as in a methyl group,
14
gives splitting into four lines. This splitting is illustrated in Figure 11.1. Nitrogen ( N)
with I = 1 splits each energy level into three lines. Neither 12 C nor 16 O has a nuclear
magnetic moment, and just as they cause no signal splitting in NMR spectra, they
have no effect on the multiplicity in ESR spectra.
A great deal of structural information can be obtained by analysis of the hyperfine
splitting pattern of a free radical. If we limit our discussion for the moment to radicals
without heteroatoms, the number of lines indicates the number of interacting hydrogens,
and the magnitude of the splitting, given by the hyperfine splitting constant a,is
a measure of the unpaired electron density in the hydrogen 1s orbital. For planar
conjugated systems in which the unpaired electron resides in a -orbital system, the
relationship between electron spin density and the splitting constant is given by the
8
McConnell equation :
a = Q (11.3)

where a is the hyperfine coupling constant for a proton, Q is a proportionality constant
(about 23 G), and is the spin density on the carbon to which the hydrogen is attached.
For example, taking Q = 23 0G, the hyperfine splitting in the benzene radical anion
can be readily calculated by taking = 1/6, because the one unpaired electron must
be distributed equally among the six carbon atoms. The calculated value of a = 3 83
is in good agreement with the observed value. The spectrum (Figure 11.2a) consists
9
of seven lines separated by a coupling constant of 3.75 G. Note that EPR spectra,
unlike NMR and IR spectra, are displayed as the derivative of absorption rather than
as absorption.
The ESR spectrum of the ethyl radical shown in Figure 11.2b is readily interpreted,
and the results are of interest with respect to the distribution of unpaired electron
density in the molecule. 10 The 12-line spectrum is a triplet of quartets resulting from
unequal coupling of the electron spin to the - and ß-hydrogens. The two coupling
constants, a = 22 4G and a = 26 9G, imply extensive delocalization of spin density

ß
through the bonds.
8
H. M. McConnell, J. Chem. Phys., 24, 764 (1956).
9 J. R. Bolton, Mol. Phys., 6, 219 (1963).
10
R. W. Fessenden and R. M. Shuler, J. Chem. Phys., 33, 935 (1960);J. Phys. Chem., 39, 2147 (1963).