Structural and conformational analysis of 1-monofluorosilacyclobutane and 1-monochlorosilacyclobutane. A gas-phase electron diffraction and ab initioinvestigation

The structures of numerous 1,1-disubstituted silacyclobutanes have been inv estigated thoroughly, but none of the monosubstituted representatives have been studied as yet. In the present work the geometric structures and confo rmational equilibria of 1-monofluorosilacyclobutane (MFSCB) and 1-monochlor osilacyclobutane (MCSCB) have been investigated by means of gas-phase elect ron diffraction and ab initio calculations. The conformational analysis rev eals that both molecules are present with both equatorial and axial conform ations. Consistent with one another, the electron diffraction data and the ab initio results show that the equatorial conformers of MFSCB and MCSCB ar e more stable than the higher energy axial forms. The experiments demonstra te that the equatorial conformers of MFSCB and MCSCB are lower in energy th an the axial conformers by 4.30 (0.21) kJ/mol (corresponding to eq : ax = 8 5 : 15 (5)) and 3.92 (0.23) kJ/mol, (eq : ax = 83 : 17 (6)), respectively. For comparison, ab initio calculations at the MP/6-31G(d,p) level predict e nergy differences of 6.04 and 3.43 kJ/mol, respectively, in favor of the eq uatorial forms of MFSCB and MCSCB. During the structural refinement it was assumed that all of the structural parameters except the puckering angle th eta for both the equatorial and axial conformers are equal. This assumption was supported by the ab initio calculations. The major (r(a)) bond distanc es and bond angles which were obtained from the final refinement of the exp erimental data are (with uncertainties of 3 sigma) for MFSC: r(Si-C) = 1.85 5(1) Angstrom, r(Si-F) = 1.592(2) Angstrom, r(C-H)= 1.089(3) Angstrom, angl e (H-Si-F)= 106.8(6)degrees, angle (C-Si-C)= 80.8(6)degrees, angle (C-C-C)= 98.6(19)degrees and the puckering angles theta (eq) = 37.4(20)degrees and theta (ax) = 23.5(70)degrees. For MCSCB the following structural parameters were obtained: r(Si-C)= 1.864(2) Angstrom, r(Si-Cl)= 2.059(3) Angstrom, r( Si-H)= 1.470(12) Angstrom, r(C-C)= 1.591(5) Angstrom, r(C-H)= 1.112(4) Angs trom, angle (H-Si-Cl)= 106.0(6)degrees, angle (C-Si-C)= 80.7(14)degrees, an gle (C-C-C) = 98.7(22)degrees. The puckering angles were found to be theta (eq) = 34.2(25)degrees and theta (eq) = 21.5(50)degrees. The observed simul taneous reduction of the Si-C and the Si-F(Cl) bonds can be examined by ele ctrostatic arguments and other concepts such as bond polarity, negative hyp erconjugation and carbon(2p pi)-silicon(3p pi) orbital overlap. By applying various ab initio methods such as HF/6-31G(d,p), MP2/6-31G(d,p) and DFT/b3 pw91/6-31G(p) the structures and conformations of mono- and dihalogenated s ilacyclobutanes of the type (CH2)(3)SiY (Y = HF, HCl, HBr, HI, H-2, F-2 Cl- 2, Br-2 and IZ) have been investigated. Our results show that there is a re gular increase of the preferability of the equatorial conformer with the in crease of the electronegativity of the halogen atom. This finding is consis tent with the correlation which was postulated earlier by Jonvik and Boggs [4-7] for monosubstituted cyclobutanes. In order to gain more insight regar ding the influence of the electronegativity of the substituent on the degre e of strain in silacyclobutanes and for the purpose of comparison of the st ructures of different mono- and dihalogenated acyclic silanes and silacycli c compounds of larger size, as silacyclopentane and silacyclohexane have be en also computed. (C) 2001 Elsevier Science B.V. All rights reserved.