Theoretical study of the vibrational structure of the 1(n,π*) transition in diimide: potential curves and Franck–Condon analysis

Abstract

Ab initio CI potential curves are reported for the ground and 1(n,π*) excited states of diimide for each of the six possible internal coordinates. These results are then used to obtain vibrational wavefunctions and frequencies for both states, which in turn are combined with electronic transition moment data to allow a Franck–Condon analysis of the band structure of the (dipole-forbidden) n–π* absorption system. This procedure allows one to reproduce the main features of the observed spectra of N2H2 and N2D2 and indicates that the majority of the vibrational transitions seen are vibronically induced via the antisymmetric NH stretching mode v5. The calculations are in essential agreement with the earlier experimental interpretation of the vibrational structure of this transition in terms of progressions in the symmetric bending (v2) and NN stretching (v3) frequencies, except that they indicate that the previous v2′ numbering should be altered by three units. According to this interpretation the isotope shift for the vibronic origin is 672 cm−1 compared with the corresponding calculated value of 666 cm−1. It is argued that several other weaker transitions seen experimentally arise via a different inducement mechanism, namely the torsion (v4) mode, and as such are only observed in energy regions where v5-induced transitions cannot occur.