Planar 1,3λ⁴δ²,2,4-benzodithiadiazine and its nonplanar 5,6,7,8-tetrafluoro derivative: Gas-phase structures studied by electron diffraction and ab initio calculations

The gas-phase molecular structures of 1,3λ⁴δ²,2,4-benzodithiadiazine and 5,6,7,8-tetrafluoro-1,3λ⁴δ ²,2,4-benzodithiadiazine have been investigated by ab initio calculations and electron diffraction using the SARACEN method of structural analysis. Important structural parameters (r_h1 structure) for the parent compound were found to be: < r(S=N) > 1.546(3), r(S-N) 1.697(5), r(C-S) 1.784(5), and r(C-N) 1.393(6) Å. For the tetrafluoro derivative, these are (r_h1 structure): < r(S=N) > 1.552(3), r(S-N) 1.723(8), r(C-S) 1.812(9), and r(C-N) 1.396(7) Å. Furthermore, the GED experiment (Gas Electron Diffraction) quite convincingly demonstrates the nonplanarity of the former and the planarity of the latter in agreement with DFT calculations; but the results contradict calculations at the MP2 level. The effect of the fluorine atoms on the conformations of the molecules is discussed.