Alkyl cations electronic spectra

The radical cation of cysteine, Cys, serves as an example of the first and third point.This amino acid contains two potential donor sites, sulphur and nitrogen, so electron transfer to an excited sensitizer could result in a sulphur-centred or a nitrogen-centred radical cation. Decarboxylation of Cys to give an a-amino alkyl radical occurs on a timescale slightly faster than 1 ns, so a direct observation by EPR is not feasible. However, the CIDNP spectrum with 4-carboxybenzophenone CB as the sensitizer exhibits polarizations of the starting amino acid, which can only be the product of reverse electron transfer of pairs Cys " CB but obviously not in any way the product of the radical pairs formed by the decarboxylation. Concentrating on the cysteine polarizations thus avoids all complication due to the rich secondary chemistry. ... 

Merocyanines (5) undergo a red shift of their electronic absorption maximum on micellization, the degree of which increases with the hydrophobicity of the alkyl group.This implies solubilization in a water-poor region in both cationic and anionic co-micelles. Pyridinium iodide (6) has a charge-transfer absorption band in its u.v. spectrum, but only above the c.m.c., and the frequency suggests a polarity in the environment of the head-group similar to that of ethanol. ... 

ESR properties of RSSR cations were reviewed by Bassindale and Hey [1]. Typical g values for the acyclic species with smaller alkyl groups are g, = 2.035, g2 = 2.018, and = 2.003, with = 2.018. The fact that the value of g is close to the free spin value indicates that the cation radicals have relaxed from the orthogonal parent structures to planar structures [1]. The seven line spectrum, which is observed for MeSSMe [1,98] indicates coupling to six equivalent protons, which would not be possible if the radical center were localized on one sulfur. Theoretical calculations indicate a spin population of 0.48 on each S [98]. Other evidence for localization of the unpaired electron in a ti orbital involving both S atoms of disulfide cations comes from an analysis of the anisotropic g tensors and the splittings in symmetrical alkyl disulfides [ 1 ]. 

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