Structures of diazabenzene cations formed by radiolysis. An electron spin resonance study
Exposure of 1,2-, 1,3- and 1,4-diazabenzene in dilute solution in trichlorofluoromethane at 77 K to 60Co γ-rays gives the corresponding radical cations, characterised by their e.s.r. spectra. The 1,2 derivative has a SOMO (semi-occupied molecular orbital) largely confined to the two nitrogen n orbitals, with a 2s-orbital contribution of approximately twice that observed for pyridyl cations. This suggests that the extent of distortion for these radicals is far less than that for pyridyl cations. In contrast, the 1,4-diazabenzene cation has a considerably reduced 14N isotropic coupling to two equivalent nitrogen nuclei, and hence a reduced 2s-orbital contribution, suggesting more extensive flattening at both nitrogen atoms.
In marked contrast, the pyrimidine cation has a SOMO largely localised on one of the nitrogen atoms in a non-bonding orbital closely resembling that for the pyridine cation. This is interpreted in terms of a preferred asymmetric distortion which lowers this SOMO below that of the expected symmetrical SOMO for this cation.