Infrared and Raman Spectra, Conformational Stability, ab Initio Calculations, and Vibrational Assignments for Cyclopropylchlorosilane

Rotational spectrum, structure, and quadrupole coupling of cyclopropylchloromethyldifluorosilane

Cyclopropylchloromethyldifluorosilane, c-C3H5SiF2CH2Cl, has been synthesized, and its rotational spectrum has been recorded by chirped-pulse Fourier transform microwave spectroscopy. The spectral analysis of several isotopologues indicates the presence of two distinct conformations in the free-jet expansion, which are interconvertible through a rotation of the chloromethyl group. A partial substitution structure is presented for the lower energy conformation and is compared to the equilibrium structure obtained from quantum chemical calculations. Additionally, the presence of the chlorine nucleus leads to the rotational transitions splitting into multiple hyperfine components and χaa, a measure of the electric field gradient along the a axis, is unusually small at merely +0.1393(73) MHz. Various common ab initio and density functional theory methods fail to predict good quadrupole coupling constants (in the principal axis system) that adequately reproduce the observed hyperfine splitting, although diagonalizing the quadrupole coupling tensor from the principal axis system into a nucleus-centered axis system reveals that, overall, these methods calculate reasonably the electric field gradient about the chlorine nucleus. Finally, a total of nine electric dipole forbidden, quadrupole allowed transitions are observed in the rotational spectra of the parent species of the higher energy conformation and the 37Cl isotopologue of the lower energy conformation. These include those of x-type (no change in parity of Ka or Kc), which, to our knowledge, is the first time such transitions have been observed in a chlorine-containing molecule.

Extension of the density functional derived scaled quantum mechanical force field procedure

The density functional derived scaled quantum mechanical (SQM) force field method of Rauhut and Pulay [J. Phys. Chem. 99 (1995) 3093] has been extended. The original procedure (employing B3-LYP/6-31G* computations and 11 transferable scale factors for the different kinds of internal coordinates) was capable to reproduce the vibrational fundamentals of 31 simple organic (H, C, N, O) molecules with a total mean deviation of about 13 cm(-1). The present Density Functional Theory based SQM force field method is an extension of the original one: with the help of 20 transferable scale factors can reproduce the fundamentals of 20 inorganic, organic and organosilicon molecules containing nonmetallic first and second-row atoms with a total mean deviation of 10.8 cm(-1). The transferability and reliability of the new set of scale factors are demonstrated on the examples of the a priori SQM vibrational spectra of cis and gauche-cyclopropylchlorosilane.

Spectra and structure of silicon containing compounds. XXVII. Raman and infrared spectra, conformational stability, vibrational assignment and ab initio calculations of vinyldichlorosilane

The infrared (3200-30 cm(-1) spectra of gaseous and solid and the Raman spectra of liquid (3200-30 cm(-1), with quantitative depolarization values, and solid vinyldichlorosilane, CH2=CHSiHCl2, have been recorded. Both the gauche and the cis conformers have been identified in the fluid phases. Variable temperature (105-150 degrees C) studies of the infrared spectra of the sample dissolved in liquid krypton have been carried out. From these data the enthalpy difference has been determined to be 20 +/- 5 cm(-1) (235 +/- 59 J mol(-1) with the gauche conformer the more stable rotamer. It was not possible to obtain a single conformer in the solid even with repeated annealing of the sample. The experimental enthalpy difference is in agreement with the prediction from MP2/6-311 + G(2d,2p) ab initio calculations with full electron correlation. However, when smaller basis sets, i.e. 6-31G(d) and 6-311 + G(d,p) were utilized the cis conformer was predicted to be the more stable form. Complete vibrational assignments are proposed for both conformers based on infrared contours, relative infrared and Raman intensities, depolarization values and group frequencies, which are supported by normal coordinate calculations utilizing the force constants from ab initio MP2/6-31G(d) calculations. From the frequencies of the Si-H stretches, the Si-H bond distance of 1.474 A has been determined for both the gauche and the cis conformers. Complete equilibrium geometries have been determined for both rotamers by ab initio calculations employing the 6-31G(d), 6-311 + G(d,p) and 6-311 + (2d,2p) basis sets at level of Hartree-Fock (RHF) and/or Moller Plesset to the second order (MP2) with full electron correlation. The potential energy terms for the conformer interconversion have been obtained from the MP2/6-31G(d) calculations. The results are discussed and compared with those obtained for some similar molecules.