Cyclic aryleneazachalcogens: Synthesis, vibrational spectra, and π-electron structures

Abstract

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 ¹⁵N and ⁷⁷Se 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 π-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 π orbitals of the NXN fragments (X=S, Se) interact effectively with the group π orbitals of the carbocycles; 2) the p AO of nitrogen and sulfur make the principal contribution to the π MO of different symmetries (a₂ and b₁, respectively), as a consequence of which their π overlapping with one another is small, and the chemical bond within the limits of the NSN group is best described in the form of $( - \mathop {\rm N}\limits^ - - \mathop {\text{S}}\limits^{\text{ + }} - {\text{N}} - {\text{)}}\begin{array}{*{20}c} \leftarrow & \to \\ \end{array} ( - {\text{N}} = \mathop {\text{S}}\limits^{\text{ + }} - \mathop {\rm N}\limits^ - - {\text{)}}$; resonance; the same is also valid for the NSeN group; 3) on the whole, the π MO display a tendency for localization in individual molecular fragments. All of these factors decrease the aromatic character of the molecules.