## 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$_{3u}$ 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 $\widehat{\mathrm{NCN}} = 124^\circ\cdot6; \quad r(\mathrm{C-N}) = 1\cdot338 \overset{\circ}{\mathrm A}; \quad r(\mathrm{N-N}) = 1\cdot330 \overset{\circ}{\mathrm A}; \quad r(\mathrm{C-H}) = 1\cdot07 \pm 0\cdot02 \overset{\circ}{\mathrm A}.$ On electronic excitation the N-N bond lengths are reduced by 0$\cdot$11 $\overset{\circ}{\mathrm A}$ and the separation of the carbon atoms increases by 0$\cdot$10 $\overset{\circ}{\mathrm A}$. It does not seem possible to understand the considerable change of geometry on excitation on the basis of simple Huckel $\pi$-electron theory.