MODELS FOR INTRAMOLECULAR EXCHANGE IN ORGANIC PI-CONJUGATED OPEN-SHELL SYSTEMS - A COMPARISON OF 3 NON-KEKULE BIPHENYLDINITRENES

The important effect of connectivity upon the nature of intramolecular exchange coupling of unpaired electrons by the biphenyl group was investigated by cryogenic matrix photolysis of diazide precursors expected to yield biphenyl-3,4'-dinitrene (1), biphenyl-3,3'-dinitrene (2), and biphenyl-4,4'-dinitrene (3). System 1 was predicted by parity-based qualitative models to exhibit a high-spin ground state. By Curie law analysis of its electron spin resonance (ESR) spectrum at cryogenic temperatures in frozen 2-methyltetrahydrofuran solution, 1 was found to be a quintet ground-state dinitrene with zero field splitting (zfs) parameters of \D/hc\ = 0.153 cm-1, \E/hc\ = 0.003 cm-1. System 2 was predicted to be disjoint with nearly-degenerate high-spin and low-spin states. Cryogenic frozen solution ESR spectroscopy showed no ESR-active dinitrene 2 upon extended photolysis of its appropriate diazide precursor, although the related mononitrene with zfs of \D/hc\ = 0.996 cm-1 was clearly visible. We presume that system 2 was produced in this experiment, but that it acts either as a singlet ground-state dinitrene or as a pair of isolated mononitrenes. A biradical ESR spectrum was found for photolysis of the diazide precursor to 3, with zfs of \D/hc\ = 0.189 cm-1 and \E/hc\ < 0.003 cm-1. The thermal dependence of this biradical spectrum shows it to be an excited state, consistent with quinonoidal dinitrene 3 having a singlet ground state and a triplet excited state ca. 0.6 kcal/mol higher in energy. The results for dinitrenes 1-3 were compared to related work on dicarbene molecules described by others, and it was found that the dinitrenes and dicarbenes exhibit similar electron exchange coupling behavior.