The silabenzenes: Structure, properties, and aromaticity

The electronic structure and properties of the silabenzenes series have been investigated using basis sets of spdf quality and many-body perturbation theory, hybrid density functional theory, and coupled cluster methods. Basic measures of aromatic character derived from structure, molecular orbitals, isodesmic and homodesmotic bond separation reactions, and a variety of magnetic criteria (magnetic isotropic and anisotropic susceptibilities, magnetic susceptibility exaltations, NICS) are considered. Energetic criteria suggest that 1,3,5-trisilabenzene and, to a lesser extent, 1,3-disilabenzene and its complement 1,2,3,5-tetrasilabenzene enjoy conspicuous stabilization. However, by magnetic criteria, these systems are among the least aromatic of the family: population and bond order analyses reveal that they derive part of their stability from ionic contributions to the bonding. Within their isomer series, 1,2-disilabenzene, 1,2,3-trisilabenzene, and 1,2,3,4-tetrasilabelizene are the most aromatic using magnetic criteria: overall, "magnetic aromaticity" decreases with increasing number of Si atoms. The different magnetic aromaticity criteria are fairly consistent within an isomer series: over the complete set of silabenzenes, the magnetic susceptibility exaltations correlate fairly well with the magnetic susceptibility anisotropies. Second-order Jahn-Teller effects cause deviations from planarity to occur in all systems with at least four silicon ring atoms, except for 1,2,4,5-tetrasilabenzene. The relative energetics (isomers, deviation from planarity) at our highest level of theory, CCSD(T)/cc-pVTZ, are better reproduced by the B3LYP/cc-pVTZ density functional method than by any of the less accurate wave function methods (HF, MP2, CCSD) considered. In general, the use of high levels of theory with large basis sets removes some ambiguities in previously reported studies.