MOLECULAR-STRUCTURE AND PSEUDOROTATION IN 3,3-DIMETHYL-3-SILATETRAHYDROTHIOPHENE FROM A JOINT GAS-PHASE ELECTRON-DIFFRACTION AND AB-INITIO MOLECULAR-ORBITAL STUDY

The molecular structure of 3,3-dimethyl-3-silatetrahydrothiophene has been reinvestigated utilizing a joint electron diffraction/ab initio a pproach. Two local minima with T-1(5) and T-4(5) conformations of the ring could not be distinguished on the basis of electron-diffraction d ata alone. However, the model with the five-membered ring having a dis torted T-1(5) form was in agreement with the ab initio calculations an d had a slightly lower R-factor than the T-4(5) form. Molecular mechan ics calculations suggest that the conformation is determined mainly by angular and torsional strains in the molecule, with the angular strai n predominating. The barrier to pseudorotation was estimated to be 9.6 kJ mol(-1) from the HF/6-311++G* ab initio calculations including co rrection for the zero-point vibrational energy. Differences between C- S, Si-C bond lengths and between exocyclic C-Si-C angles obtained from the MP2(FC)/6-311++G* ab initio calculations were used as constraint s in the electron diffraction structure analysis. Two different method s were used to calculate the shrinkage corrections from the force fiel d scaled to fit the experimental frequencies. The following values (I, bond lengths and angle(alpha) angles with total errors) were found fo r the main parameters in the molecule: r(S-C)(mean) 1.829 +/- 0.005 An gstrom; r(Si-C)(mean) 1.883 +/- 0.006 Angstrom; r(C-C) 1.541 +/- 0.007 Angstrom; r(CH)(mean) 1.112 +/- 0.003 Angstrom; angle(C-S-C)(ring) 98 .6 +/- 0.8 degrees; angle (C-Si-C)(ring) 97.7 +/- 0.6 degrees; ring pu ckering amplitude, q(0), 10.7 +/- 0.6 degrees; pseudorotation phase an gle, f, 168.1 +/- 4.4 degrees (corresponds to a distorted T-1(5) form of the ring).