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

Abstract
The electronic absorption spectrum of gaseous s-tetrazine (H2C2N4) 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 1Agground state and in the A~ 1B3u 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.