The silabenzenes: Structure, properties, and aromaticity

The electronic structure and properties of the silabenzenes series have bee n investigated using basis sets of spdf quality and many-body perturbation theory, hybrid density functional theory, and coupled cluster methods. Basi c measures of aromatic character derived from structure, molecular orbitals , isodesmic and homodesmotic bond separation reactions, and a variety of ma gnetic criteria (magnetic isotropic and anisotropic susceptibilities, magne tic susceptibility exaltations, NICS) are considered. Energetic criteria su ggest 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 the ir isomer series, 1,2-disilabenzene, 1,2,3-trisilabenzene, and 1,2,3,4-tetr asilabelizene are the most aromatic using magnetic criteria: overall, "magn etic aromaticity" decreases with increasing number of Si atoms. The differe nt magnetic aromaticity criteria are fairly consistent within an isomer ser ies: over the complete set of silabenzenes, the magnetic susceptibility exa ltations correlate fairly well with the magnetic susceptibility anisotropie s. Second-order Jahn-Teller effects cause deviations from planarity to occu r in all systems with at least four silicon ring atoms, except for 1,2,4,5- tetrasilabenzene. The relative energetics (isomers, deviation from planarit y) at our highest level of theory, CCSD(T)/cc-pVTZ, are better reproduced b y the B3LYP/cc-pVTZ density functional method than by any of the less accur ate wave function methods (HF, MP2, CCSD) considered. In general, the use o f high levels of theory with large basis sets removes some ambiguities in p reviously reported studies.