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An ab initio implementation of the vibronic coupling theory of Ñafie and Freedman is described and applied to the computation of vibrational circular dichroism (VCD) intensities of (S)-2-deuteriooxirane, (S,S)-2,3-dideuteriooxirane, and (5)-2,2,3-trideuteriooxirane. The theory is found to reproduce the published spectral features of (S,S)-2,3-dideuteriooxirane very well if the basis set incorporates derivatives of the normal set of basis functions. A suitable basis set, based on the standard 6-3 lg set and designated 6-31g(-2s,2p,2pH), is proposed. The 6-31g(-2s,2p,2pH) basis set is found to yield a geometry for oxirane that is closer to the microwave structure than obtained by optimization with the comparably sized 6-31G** set. Uniform scaling or limited nonuniform scaling of the 6-3 lg(-2s,2p,2pH) force constants permit excellent agreement (<1% error) with observed vibrational frequencies of oxirane and tetradeuteriooxirane. Quite satisfactory agreement with the measured IR intensities for these two compounds is also obtained. Scaling of the force constants in a nonuniform manner has a slightly adverse effect on calculated IR intensities, suggesting that the ab initio normal modes may be preferable. The APT theory

The equilibrium geometry of tetrasilabicyclo[ 1.1 .O]butane, as determined by full gradient optimization at the RHF level of theory by using 6-31G* basis set, has C2, symmetry. The two Si-Si bond lengths are 2.304 and 2.370 A, the latter being the bond connecting the bridgehead atoms. The dihedral angle between the two rings is 122.9O. The corresponding bonds of the transition structure for ring inversion, which has D2,, symmetry, are 2.295 and 2.805 A, respectively. The barrier to ring inversion is calculated to be 75 kJ mol-' after correction for correlation energy differences up to MP3 and inclusion of zero-point vibrational energies. The bond connecting the bridgehead atoms is not broken but changes from a bent u bond at the equilibrium geometry to a a bond in the transition structure. By contrast, bicyclo[l.l.O]butane suffers bond rupture during inversion, the transition structure being most probably a triplet diradical. The inversion barrier, estimated to be about 300 kJ mol-', is higher than that required for the sigmatropic rearrangement to butadiene. The origin of the differences between the C and Si analogues is discussed.Interest in the properties and preparation of strain decyclic polysilanes has increased substantially in recent years. Examples of the preparation and characterization of cyclotrisilanes, both symmetrically' and unsymmetrically2 substituted, have appeared. More recently, the preparation and characterization of a spiro-[2.2]pentasilane has been r e p~r t e d ,~ and the first example of the tetrasilabicyclo [ 1.1 .O] butane system, 1,3-di-tert-butyl-2,2,4,4tetrakis(2,6-diethylphenyl)tetrasilabicyclo[ 1.1 .O] butane (1) has H \ Ar AT \ / 10" H 3 1 I Ar = 2 6-diethylphenyli 2 1984, 777-778.