The molecular structure and conformation of (trifluorosilylmethyl)cyclopropane as determined by gas-phase electron diffraction and ab initio calculations

The molecular structure and conformational stability of (trifluorosilymethyl)cyclopropane (TFSMCP), C3H5-CH2-SiF3, with respect to the C-ring-CH2 and the Si-CH2 axes have been studied by gas-phase electron diffraction and ab initio calculations using the basis sets 3-21G*, 4-21G* and 6-31G**. The electron diffraction conformational analysis with respect to the C-ring-C bond has provided an anticlinal:synperiplanar conformational composition of 75(12):25 (12). A reasonable explanation for the preference of the anticlinal conformation is provided by the sigma-pi hyperconjugation effect. The occurrence of the syn conformer is most likely the result of attractive interactions between the fluorine atoms and the ring moiety. The major bond distances (r(alpha), Angstrom) and angles (angle a, deg) for the anticlinal conformer obtained from the least-squares refinements with uncertainties estimated at 3 sigma are: r(C-H)= 1.095(8), r(C-ring-C) = 1.548 (12), r(Si-C) -1.837 (5), r(Si-F)= 1.593 (i); angle F6SiC = 111.3 (2.4), angle SiCC = 115.0(1.2), angle C-C(ring plane)= 129.5 (1.7), and the dihedral torsional angle tau(SiCCX)=115.7 (2.9) (X is the center of the ring). All bonded distances in the syn conformer were assumed to be equal to the corresponding distances in the anticlinal form. Only the following angles for the syn conformer were refined: angle F-7(8)SiC = 115.9 (6.4), angle SiCC = 123.4 (3.7), angle C-C-(ring plane)= 124.6 (5.3). The potential barriers to internal rotation about the Si-C and the C-C-ring axes have been calculated and compared to the corresponding barriers in various methyl cyclopropyl derivatives. It has been postulated that there is a peculiar similarity between the allyl and methyl cyclopropyl systems. Numerous derivatives of both systems have been calculated to demonstrate this intriguing resemblance. (C) 1997 Elsevier Science B.V.