The molecular structure of 1,1-dichlorosilacyclopentane as obtained from gas-phase electron diffraction and ab initio calculations

Abstract

The molecular structure of 1,1-dichlorosilacyclopentane (DCSCP) has been investigated by means of gas-phase electron diffraction and quantum mechanical calculation. We applied both a pseudorotation model to account for the dynamic and large amplitude motion in DCSCP, and a one-conformer model of C 1 symmetry. Using the computational results we analyzed the dependency of the ring geometrical parameters and vibrational mean amplitudes on the phase angle φ. The joint electron diffraction and ab initio study has led to the following r a structural parameters of DCSCP ( C 1 conformer): r(Si–Cl) = 2.047(2) Å, r(Si–C) = 1.867(4) Å, average r(C–C) ring = 1.548(4) Å, average r(C–H) = 1.103(7) Å, <(C–Si–C) = 97.4(6)°, <(Cl–Si–Cl) = 104.8(10)°, and effective phase angle φ = 74.8(58)°. The puckering amplitude for the five-membered ring was determined to be q = 0.480(24) Å. The quantum mechanical calculations were performed by utilizing the UHF, B3LYP, and MP2 methods in combination with basis sets 6-311++G(2df,2pd), 6-311++G(df,pd), 6-311++G(p,d), 6-311+G(d,p), 6-311G(d,p) and Dunning double and triple zeta (with and without augmentation). All these methods have consistently shown that the C 2 conformer is more stable than the C s symmetric form. For all calculations we used the MOLPRO and Gaussian03 packages. NBO and AIM analyses were also carried out to explore the bond/anti-bond hyperconjugative interactions and the topological properties of the charge density distribution in DCSCP. NBO scheme including second-order perturbation analysis has shown that the major orbital stabilizing interactions are between the chlorine lone pair ( n π) Cl and the low-lying σ Si – C2 ∗ and σ Si – C5 ∗ antibonding orbitals. It was found that remote σ Si–C → σ C – H ∗ interactions are stabilized by 4.4 kcal mol −1 and contribute to the stabilization of the C 2 conformer in DCSCP. Deletion analysis was performed using various deletion algorithms like NOSTAR, NOVIC, NOGEM (see text). The stabilization energy for the C 2 conformer resulting from the difference of the non-Lewis energy E(NL) C 2 – E(NL) Cs was 7.1 kcal mol −1. Similar analyses using the same quantum mechanical procedures mentioned above have been conducted to study silacyclopentane (SCP), 1,1-difluorosilacyclopentane (DFSCP), and 1,1-dibromosilacyclopentane (DBSCP).