Causes of Nonplanarity in Fluorinated 1,3λ 4 δ 2 ,2,4‐Benzodithiadiazines: Gas‐Phase Electron Diffraction, Ab initio and DFT Structures

The gas‐phase molecular structures of 6,8‐difluoro‐1,3λ 4 δ 2 ,2,4‐benzodithiadiazine and 5,6,7‐trifluoro‐1,3λ 4 δ 2 ,2,4‐benzodithiadiazine have been determined using quantum chemical calculations and electron diffraction via the SARACEN method of structural analysis. Of particular interest was the planarity or nonplanarity of the heterocyclic fragments of the molecules. It was shown that the difluoro compound is planar with respect to the heterocyclic fragment [folding angle: 0.0(5)° (figures in parenthesis indicate e.s.d.s in the final digits)] and that the trifluoro compound probably deviates from planarity to a small extent [folding angle: 4.0(−30;+3)°; the large uncertainty in the value of this angle is due to the low‐frequency, large‐amplitude vibrational motion associated with the folding of the heterocyclic fragment]. Pertinent structural parameters from the electron diffraction data ( r h 1 structure) of the difluoro compound are av [ r (S=N)] = 155.7(7) pm, r (S−N) = 175.8(5) pm, r (C−S) = 176.6(8) pm and r (C−N) = 141.6(2) pm. For the trifluoro compound the equivalent structural parameters are av [ r (S=N)] = 155.5(7) pm, r (S−N) = 170.7(6) pm, r (C−S) = 180.6(8) pm and r (C−N) = 140.6(2) pm. The gas‐phase electron diffraction structures have been compared with results from ab initio and DFT calculations and also with experimental solid‐state structures. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)