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

Nucleus independent chemical shift (NICS) values, the negative isotropic sh ielding calculated at the ring centers, were used to evaluate the aromatic character of a series of substituted pentafalvene analogs. NICSs were calcu lated for pentafulvene 3a, its exocyclic Si, Ge, and Sn analogs 3b-d, and t heir 7,8-difluoro derivatives 4a-d. Calculations on cyclopentadiene, 1, and the aromatic cyclopentadienyl anion, 2, were also carried out for comparis on. Geometries were optimized using the B3-LYP method while NICS values wer e calculated at both the BF and B3-LYP levels using the gauge invariant ato mic 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-dif luorosilyl- and 7,8-difluorogermyl derivatives (4b and 4c) appear to exhibi t a degree of aromaticity, which might be due to pi -donation from fluorine to silicon (germanium) permitting some net pi -donation from Si and Ge to the cyclopentadiene ring. The SnF2 derivative 4d, in contrast, does not app ear to exhibit aromaticity. (C) 2001 Elsevier Science B.V. All rights reser ved.