The near-ultraviolet absorption spectrum of diimide: a re-examination

The absorption spectra of N2H2 and N2D2 in the region 3000–4300 Å have been re-examined with somewhat higher resolution than was used in earlier work. The vibrational assignments of the principal progressions are unchanged except for the revision of the vibrational numbering suggested by the calculations of Perić, Buenker, and Peyerimhoff in 1977. The transition is electronically forbidden (1Bg–1Ag) but is made vibronically allowed by excitation of one quantum of the antisymmetric stretching vibration ν4′(bu). Weaker progressions are induced by the torsional vibration ν4′(au) and the antisymmetric H—N—N bending vibration ν6′(bu). Agreement between experiment and theory is generally satisfactory though some significant discrepancies in Franck–Condon factors have been noted.