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

Abstract

The molecular structure and conformational stability of (trifluorosilylmethyl)cyclopropane (TFSMCP), C 3H 5CH 2SiF 3, with respect to the C ringCH 2 and the SiCH 2 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 σ-π 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 a , Å) and angles (∠a, deg) for the anticlinal conformer obtained from the least-squares refinements with uncertainties estimated at 3σ 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 (1); ∠F 6SiC = 111.3 (2.4), ∠SiCC = 115.0 (1.2), ∠CC(ring plane) = 129.5 (1.7), and the dihedral torsional angle τ 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: ∠F 7(8)SiC = 115.9 (6.4), ∠SiCC = 123.4 (3.7), ∠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.