Conformational stability, r0 structural parameters, ab initio calculations, and vibrational assignment for silacyclopentane

Abstract

The infrared spectra (3500–50 cm −1) of the gas and solid and the Raman spectrum (3200–30 cm −1) of liquid silacyclopentane, c-C 4H 8SiH 2, have been recorded. Additionally the infrared spectrum (3200–400 cm −1) of liquid xenon solutions has been recorded at −65 and −95 °C. In all of the physical states only one conformer was detected which is the twisted C 2 form. The conformational energetics have been calculated with the Møller–Plesset perturbation method to the second order, (MP2(full)) as well as with density functional theory by the B3LYP method utilizing a variety of basis sets. All of these calculations predict only one stable conformer i.e. the C 2 form with an average barrier to planarity of 2558 cm −1 (30.60 kJ/mol) from the MP2 calculations and a significantly lower value of 1918 cm −1 (22.95 kJ/mol) from the DFT calculations with neither calculations being significantly effect by inclusion of diffuse functions. From the isolated SiH frequency from the SiHD isotopomer the SiH distance was calculated to be 1.486(3) Å. By utilizing the previously reported microwave rotational constants for three isotopomers ( 28Si, 29Si, and 28Si-d 2) combined with the structural parameters predicted from the MP2(full)/6-311 + G(d,p) calculations, adjusted r 0 structural parameters have been obtained. The determined heavy atom distances are: r 0(SiC) = 1.890(5); r 0(C 2C 4) and (C 3C 5) = 1.547(5); r 0(C 4C 5) = 1. 535(5) Å and the angles in degrees: ∠CSiC = 95.9(5)°; ∠SiCC = 103.3(5)°; ∠CCC = 107.9(5)° with the two dihedral angles ∠SiCCC = −38.3(3)° and ∠CCCC = 52.4(3)°. A complete vibrational assignment is given for the twisted C 2 conformer for the normal species and the Si-d 2 isotopomer which are supported by normal coordinate calculations utilizing scaled force constants from ab initio MP2(full)/6-31G(d) calculations. The results of these spectroscopic and theoretical studies are discussed and compared to the corresponding results for some similar molecules.