3a,11b-Dimethyl-1-phenyl-1,3a,3b,4,7,11b-hexahydroquinolino[2,1-c:4,3-d] dioxazolo-2,6-dione molecular ions eliminate methyl radicals in a selective manner. This effect increases when slow reacting ions are examined, showing that the ionized molecule at low energy states contains a similar population of the rotational levels as the precursor, the spatial disposition of the phenyl group being almost unchanged by rotation. The results, obtained by exact mass measurements, m.i.k.e. and combined scan analysis, deuterium labelling, and X-ray stereochemical studies, indicate that the high selectivity is due to stereochemical effects on the single-bond dissociation which is controlled by delocalization of the MO of the fragment ions; loss of the 3a-methyl produces greater stabilization of the corresponding fragment than 11b-methyl elimination.
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