Nucleus independent chemical shift evaluation of the aromaticity of pentafulvene and its exocyclic Si, Ge, and Sn derivatives

Abstract

Nucleus independent chemical shift (NICS) values, the negative isotropic shielding calculated at the ring centers, were used to evaluate the aromatic character of a series of substituted pentafulvene analogs. NICSs were calculated for pentafulvene 3a, its exocyclic Si, Ge, and Sn analogs 3b– d, and their 7,8-difluoro derivatives 4a– d. Calculations on cyclopentadiene, 1, and the aromatic cyclopentadienyl anion, 2, were also carried out for comparison. Geometries were optimized using the B3-LYP method while NICS values were calculated at both the HF and B3-LYP levels using the gauge invariant atomic orbital (GIAO) approach. All calculations used a triple split basis set with two sets of polarization functions (tsV+2P). Calculated NICS values suggest 3a– d and 4a exhibit little or no aromaticity. However, the 7,8-difluorosilyl- and 7,8-difluorogermyl derivatives ( 4b and 4c) appear to exhibit a degree of aromaticity, which might be due to π-donation from fluorine to silicon (germanium) permitting some net π-donation from Si and Ge to the cyclopentadiene ring. The SnF 2 derivative 4d, in contrast, does not appear to exhibit aromaticity.