Quintet state electron spin resonance spectra of pyridyldinitrenes

A number of 2,4- and 2,6-diazidopyridine derivatives were photolyzed under frozen matrix conditions (77 K), and their X-band electron spin resonance spectra (ESR) investigated. Typically, both mononitrene and dinitrene spectral features were observed. Cyano substituents reduced or eliminated spectral intensities, especially of the dinitrenes. The 2,4-dinitrenes were readily distinguished from 2,6-dinitrenes, since the major ESR resonance of the former typically occurred at about 3000 G, while that of the latter typically occurred at about 3300 G. Spectral line shape simulations suggest that the 2,6-dinitrenes have consistently larger zero-field splitting (zfs) than the related 2,4-dinitrenes. The 2,4-dinitrenes are estimated to have zfs of \D/hc\(Q) similar to 0.21-0.24 cm(-1) and \E/hc\(Q) similar to 0.03-0.04 cm(-1), and the 2,6-dinitrenes to have zfs of \D/hc\(Q) similar to 0.24-0.27 cm(-1) and \E/hc\(Q) similar to 0.040-0.05 cm(-1). The difference in spectral behavior is attributed to perturbation of spin density distributions and geometry-influenced interactions between the nitrene units in the 2,4- vs the 2,6-connectivities. Other substitutions on the pyridine had only small effects on the ESR spectra, although the photoefficiency of nitrene and dinitrene production was affected.