Aromatic character of tria- and pentafulvene and their exocyclic Si, Ge, and Sn derivatives. An ab initio study

The structures and dipole moments have been calculated for both methylenecy clopropene (triafulvene) and pentafulvene and their exocyclic Si, Ge, and S n analogues 1a-d and 2a-d, respectively. Ab initio calculations employing t he HF, DFT; and MP2 methods, each using split valence plus polarization and triple split valence with two sets of polarization functions basis sets, h ave been performed. The results of these six levels of theory on each of th e eight molecules were compared, and the aromatic character of these system s was examined. The structures 1a-d exhibited pronounced bond alternation i n the ring suggesting the existence of only weak cyclopropenyl cation-like resonance contribution. However, very large dipole moments were predicted i n the direction that would be expected from an aromaticity contribution. Si nce Si, Ge, and Sn are less electronegative than C, the large dipole moment s in the direction of the exocyclic heteroatom were surprising. The most st riking finding was that, while la is planar, 4-sila-, 4-germa-, and 4-stann atriafulvene were nonplanar with trans-bent structures in which the exocycl ic double bond is bent slightly out of the cyclopropene ring plane and out of the HXH plane (X = Si, Ge, Sn) by large angles. Thus, the Si, Ge, and Sn atoms exhibit pyramidal geometries. The dipole moment of pentafulvene, 2a, (oriented toward the ring) was substantially smaller than that of its Si, Ge, and Sn analogues, 2b-d, The large dipoles of 2b-d were due to greater c harge separation resulting from the lower electronegativities of Si, Ge, an d Sn versus that of C. The cyclopentadienyl rings exhibited strong bond alt ernation indicating only a modest cyclopentadienyl anion-like contribution in 2b-d.