Post-resonance Raman and theoretical studies on 1,3,2,4-benzodithiadiazines, formally anti-aromatic compounds

DFT calculations on 1,3,2,4-benzodithiadiazine (1) and its 5-F (2), 6-Cl (3) and 7-CH3 (4) derivatives showed these compounds to have essentially non-planar molecular conformations. The vibrational (IR and Raman) spectra of 1–4 were measured and assigned on the basis of the results of the theoretical calculations. NSN anti-symmetric and symmetric stretching modes were found at ∽1220–1230 and ∽965–975 cm−1, respectively. Resonance Raman spectra of 1–4 were obtained with the 514.5, 496.5, 488.0, 476.5 and 457.9 nm excitation lines of an Argon laser, and a post-resonance Raman effect was observed. The Raman excitation profiles revealed a weak but definite π-interaction of the carbocyclic and heterocyclic parts of the molecules which became stronger on going from 1 to 2. Comments on the possible existence of antiaromaticity for the title compounds in a planar conformation are given.