Carbo-siloles, Part 1: A Theoretical Study.

The ring carbo-mer of silole, termed as carbo-silole, was investigated at the DFT level of theory (B3PW91/6-31G**) and compared to its carbo-cyclopentadiene and carbo-cyclopentadienone analogues. The electronic delocalization of these species and their parent molecules is analyzed indirectly from their optimized geometry, and directly from their canonical and NBO molecular orbitals by comparison of the HOMO-LUMO gaps. All criteria indicate that the π electron systems of carbo-silole and carbo-cyclopentadiene are delocalized to a similar extent, in particular through a π - σ*AH2 conjugation (A = C, Si). The cyclic component of the delocalization, i.e., aromaticity, has been more directly appraised by calculation of NICS values at the center of the rings: the latter definitely confirm that in contrast to carbo-cyclopentadienone (NICS = -8.1 ppm), both carbo-silole (NICS = -1.1 ppm) and carbo-cyclopentadiene (NICS = +2.4 ppm) are non-aromatic. The electron affinity is significantly increased by carbo-merization of silole (from 0.25 to 1.95 eV), suggesting that the carbo-silole ring might be a relevant unit for the design of functional materials. Studies of the carbo-silolylium cation show that its aromaticity is comparable to that of the long studied carbo-cyclopentadienyl analogue. [ABSTRACT FROM AUTHOR]