An ESR and ab-initio MO study of πCH2–n0 orbital crossover in cycloalkylketone radical cations

The cyclobutanone and cyclopentanone radical cations were generated in a halocarbon matrix by ionizing radiation at 77 K. The ESR spectrum of a cyclobutanone radical cation consisted of a triple-quintet hf structure with ca. 0.9 mT (2H) and 2.4 mT (4H) splittings. Whereas, both cyclopentanone and 2,2,5,5-tetradeuteriocyclopentanone radical cations gave an identical triplet with 1.5 mT (2H) splitting. Based on the 1H hf splittings, it was concluded that the SOMO in cycloalkylketone radical cations was changed from a πCH2 oribital to a n0 (and σC–C) orbital (i.e., πCH2–n0 orbital crossover) when the ring size increased from a four-member to a five-member.