MOLECULAR STRUCTURE A combined ab initio and gas-phase electron diffraction investigation of the molecular structure and conformation of (silylmethyl)cyclopropane

The molecular structure and conformational composition of (silylmethyl)cyclopropane (SMCP), C3Hs-CH2-SiH3, with respect to the Cring-CH 2 and the CH2-Si axes have been studied by gas-phase electron diffraction and ab initio calculations using a 6-31 G* basis set. The conformational analysis, with respect to the Cring-C bond, has shown that the gauche (skew) form is the predominant conformation (> 98%) in the gas phase. A reasonable explanation for the preference of the gauche conformation is provided by the a-Tr hyperconjugation effect. Repulsive through-space interactions are most probably responsible for the cis conformer, being higher in energy and thus less favorable. The major bond distances (ra, A. -I) and angles (°) for the skew conformer obtained from the least-squares refinements with uncertainties estimated at 3~r are r(C H ) = 1.120(4), r(Cl-Ce)ring = 1.510(10), r(C2 C3)ring = 1.490(12), r(Cring C) = 1.540(10), r (Si -C)= 1.876(2); ZHSiC = 111.0(2.0), ZSiCC = 112.1(1.0), ZC C(ring plane)= 122.7(0.5), and the dihedral torsional angle %iccx = 119.0(2.0) (X is the center of the ring).