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 (rh1 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 (rh1 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.