The 5515 Å electronic transition and the geometric structure of s -tetrazine

Abstract

The electronic absorption spectrum of gaseous s -tetrazine (H 2 C 2 N 4 ) in the visible region has been examined under high resolution, and rotational analyses of the J -structure have been performed for bands of six isotopic species. The results have permitted the determination of the molecular structure of s -tetrazine, both in the 1 A g ground state and in the A ~ 1 B 3 u excited state. The s -tetrazine molecule in its ground state is not as distorted from the hexagonal con­formation as previous X-ray data suggest: the principal parameters are NĈN = 124°.6; r(C—N) = 1.338Å; r (N—N) = 1.330Å; r (C—H) = 1.07 ± 0-02 Å.. On electronic excitation the N—N bond lengths are reduced by 0.11A and the separation of the carbon atoms increases by 0.10Å. It does not seem possible to understand the con­siderable change of geometry on excitation on the basis of simple Hiickel π-electron theory.