Substituted benzene cations formed by radiolysis: an electron spin resonance study

Abstract

Exposure of dilute solutions of substituted benzenes in trichlorofluoromethane to ⁶⁰Co γ-rays at 77 K gave the corresponding radical cations, which were detected and characterised by e.s.r. spectroscopy. In general, the e.s.r. results show that the SOMO is that predicted by theory and found from photoelectron spectroscopic studies, but several interesting facets have emerged. For the monosubstituted cations, (Ph–X)⁺, where X =CHO or CMeO, the SOMO is largely confined to the benzene ring, with no evidence for cations with a SOMO largely confined to the n oxygen orbital. For styrene and substituted styrene cations there is considerable delocalisation on to the olefin moiety (ca. 30%). Nitrobenzene cations have a nodal plane through the nitro group with zero spin density on nitrogen (a₂), but they rearrange on annealing to give (RONO)⁺ cations with the unpaired electron strongly confined to the nitro group. For the disubstituted cations, the effect of a methyl group para to a nitro group is of interest since the alternative SOMO is selected (b₁), giving a high spin density on the methyl group (A_Me=20 G). However, delocalisation on to the nitro group remains small. The effect of para-cyano groups on the cations of halogenobenzenes is to increase the extent of delocalisation on to halogen, but no hyperfine splitting from ¹⁴N is detectable. The effect of a para-nitro group is to increase the spin density on the halogen still further.

Benzyl derivatives, (PhCH₂X)⁺, are of interest because of the range of conformations available. When X =Me, the methyl group lies close to the plane of the ring, giving maximum σ–π interaction with the methylene protons [A(¹H)=29 G]. Curiously, for p-diethylbenzene cations, the major species seems to have the two methyl groups well removed from the nodal plane. Halogen substituents (X =Cl, Br) also give variable conformations, the in-plane site being favoured for chlorine, and the maximum out-of-plane site for bromine. However, the cation of p-nitrobenzyl chloride has a high spin density on chlorine. As with benzaldehyde, the SOMO for phenylacetaldehyde cations is clearly confined to the benzene ring rather than being the n oxygen orbital. In this case, the CHO group sits close to the plane of the ring, whereas the CMeO group is well removed from this plane.