RESULTADOS Y DISCUSIONES.

Electron spin resonance spectra are characterised by three parameters: 2 the g -fa c to r, the hyperfine s p l i t t i n g constants, and the linewidths. A close study of these parameters and of t h e ir temperature dependencies enables much de ta ile d structural information about the p a r tic u la r radical to be gleaned.

9 .3 .1 q-Factors

In a magnetic f i e l d an unpaired electron in a free radical possesses, in addition to i t s spin angular momentum, a small amount of unquenched o rb ita l angular momentum as a re s u lt of s p in -o rb it coupling. This causes the electron to have a s lig h t ly d if fe r e n t e f f e c t iv e magnetic moment from th a t which a free electron would possess (g 2.00232). The experimentally-measured isotrop ic g -fa c to r of a polyatomic free radical as defined by the resonance condition [equation ( 9 .3 ) ] w i ll thus deviate s lig h t ly from the spin-only value.

165

Hence, fo r a given operating frequency, radicals with d if fe r e n t g-factors resonate at d if f e r e n t applied f i e l d strengths. The difference in the g -fa c to r fo r a radical and th a t fo r the free electron is analogous to the chemical s h i f t in NMR spectroscopy. Differences in g-values are small, e.g. g-values fo r CM3* and HOCH2 are 2.0026 and 2.0033, re sp ec tiv e ly , but are nevertheless s ig n ific a n t and can give valuable information about the structure of a r a d ic a l.

In th is work g-values were determined by measurement of the microwave frequency (using an E . I . P . Autohet microwave counter, model 331) and the magnetic f i e l d at the centre of the spectrum (using a Varian NMR gaussmeter). The difference in f i e l d between the gaussmeter probe and the sample was determined by measuring the g-value of the pyrene radical anion, which is accurately known to be 2.002710,3 generated by the reduction of pyrene with sodium in THF. The unknown g-value was calculated using the resonance condition shown in equation ( 9 . 3 ) .

9 .3 .2 Hyperfine S p littin g Constants

These are by f a r the most useful c h a rac te ris tic s of ESR spectra, both fo r determining the id e n t it y and also the detailed structure of the radical under study. Hyperfine coupling arises from in te ra c tio n between the unpaired electron and neighbouring magnetic nuclei (^H, ^ B , ^"^N, ^^0, e t c , ) present in the r a d ic a l. The in te ra c tio n with n equivalent nuclei of spin I results in { 2 n l + 1) lin e s and the distance between each of these lines is (to f i r s t - o r d e r ) equal to the hyperfine s p l i t t i n g constant. Since has no magnetic moment, proton hyperfine couplings dominate ESR

spectra of neutral and ionic hydrocarbon ra d ic a ls . The in teraction of the unpaired electron with n equivalent protons ( / = 1/2) gives {n + 1) lines and, furthermore, the r e la t iv e in te n s itie s of these lines are given by the c o e ffic ie n ts of the binomial expansion of ( 1 + %)", which can be found re a d ily from Pascal's t r ia n g l e . Although the natural abundance of { I = 1/2) is only ca. 1.1%, other elements have non-zero spin isotopes which are present in high abundance. These include ( / = 3) ca. 19.8%, { / = 3/2) ca. 80.2%, and

( I = 1) 99.6%.

9 .4 Origins o f Hvoerfine S p littin g

Anisotropic hyperfine s p l i t t i n g , which arises from magnetic d ip o lar in te ra c tio n s , is only important in the solid state or in viscous media and w i l l not be considered. In solution, such in teractions are averaged to zero by rapid tumbling of the ra d ic a ls . Isotropic hyperfine s p l i t t i n g only results i f the unpaired electron has a f i n i t e p ro b a b ility of being at the magnetic nucleus in question. This is usually referred to as the Fermi contact in te r a c tio n . Thus, coupling might be expected to be observable only when the singly occupied molecular o r b ita l (SOMO) has some s-character, since only then w i ll there be a f i n i t e electron density a t the nucleus. For n -ra d ic a ls , no s p l i t t i n g would be expected, since the unpaired electron is in a n -o rb ita l which has a node in the molecular plane which contains a l l the magnetic n u clei. Experimentally, i t is found tha t though s p l i t t i n g fo r electrons in o r b it a ls with s-character can be very large (506 G fo r the hydrogen atom), there is nevertheless also some s p l i t t i n g fo r n -ra d ic a ls .

167

9 .4 .1 g-Proton S p lit t in g s *

Hyperfine in te ra c tio n of th is type can be best il l u s t r a t e d by the methyl radical H 3 C " . Figure 9.1 shows schematically the spin

a b

Figure 9.1 The spin polarisation mechanism for the methyl radical.

p o la ris atio n mechanism, which is responsible fo r the g-H and g-^^C s p l i t t in g s . For the two possible arrangements of electron spins about the trigonal carbon, th a t shown in Figure 9.1a is the more probable (Hund's r u le s ) . The electrons in the o bond are not p e rfe c tly paired due to an exchange in te r a c tio n , which causes the a electron with the same spin as the electron in the SOMO to come closer to th is electron than the a electron of opposite spin [Figure 9 .1 b ]. The value of a(Ha) w i l l thus be negative while w i ll be p o s itiv e .

McConnell 4 has shown fo r n -radicals tha t the unpaired spin