CYCLIC ARYLENEAZACHALKOGENS: SYNTHESIS, VIBRATIONAL SPECTRA, AND ?-ELECTRON STRUCTURES

Article abstract of DOI:10.1007/BF00480877

It is shown that a convenient general method for the synthesis of benzo-2,1,3-thiadiazole, its Se analog, their 4,5,6,7-tetrafluoro derivatives, and naphtho<1,8-c,d><1.2.6>thiadiazine is the reaction of the corresponding arylenediamines with chalcogen tetrahalides.A complete interpretation of the vibrational spectra of the synthesized compounds was obtained using the experimental shifts of the bands caused by the introduction of the (15)N and (77)Se isotopes into the compositions of the molecules and calculations of the frequencies and forms of the normal vibrations within the stretching-force field approximation.The peculiarities of the ?-electron structures of the molecules of these substances were established by means of an analysis of the force constants of the CC, CN, NS, and NSe bonds and quantum-chemical calculations by the MNDO method: 1) The group ? orbitals of the NXN fragments (X = S, Se) interact effectively with the group ? orbitals of the carbocycles; 2) the p AO of nitrogen and sulfur make the principal contribution to the ? MO of different symmetries (a₂ and b₁, respectively), as a consequence of which their ? overlapping with one another is small, and the chemical bond within the limits of the NSN group is best described in the form of : resonance; the same is also valid for the NSeN group; 3) on the whole, the ? MO display a tendency for localization in individual molecular fragments.All of these factors decrease the aromatic character of the molecules.