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Spectra and structure of small‐ring compounds. LXII —Raman and infrared spectra, conformational stability, vibrational assignment and ab initio calculations of cyclopropylmethyltrifluorosilane
Author(s) -
Little T. S.,
Vaughn C. A.,
Zhu Xiang,
Dakkouri M.,
Durig J. R.
Publication year - 1994
Publication title -
journal of raman spectroscopy
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.748
H-Index - 110
eISSN - 1097-4555
pISSN - 0377-0486
DOI - 10.1002/jrs.1250250902
Subject(s) - conformational isomerism , raman spectroscopy , chemistry , enthalpy , ab initio quantum chemistry methods , ab initio , infrared spectroscopy , infrared , spectral line , crystallography , potential energy , analytical chemistry (journal) , molecule , atomic physics , thermodynamics , organic chemistry , physics , astronomy , optics
The Raman (3500–20 cm −1 ) and infrared (3500–50 cm −1 ) spectra of gaseous and solid cyclo‐propylmethyltrifluorosilane, c‐C 3 H 5 CH 2 SiF 3 , were recorded. Additionally, the Raman spectrum of the liquid was obtained along with qualitative depolarization values. The spectra of the gas and liquid were interpreted on the basis of the predominance of a conformation having a gauche structure and this form exists exclusively in the solid. Spectral evidence to support the existence of a higher energy cis conformer where the SiF 3 group is cis to the cyclopropyl ring is observed in the Raman spectra of the gas and liquid. From studies of the Raman spectra at various temperatures, the conformational enthalpy difference was determined to be 490 ± 122 cm −1 (1.4 ± 0.35 kcal mol −1 ) and 548 ± 110 cm −1 (1.57 ± 0.32 kcal mol −1 ) (1 kcal = 4.184 kJ) for the gas and liquid, respectively. The cyclopropyl torsion was observed as a weak infrared band centered at 77 cm −1 in the spectrum of the gas. A complete assignment of the normal vibrational modes is provided. Ab initio calculations utilizing the RHF/6‐31G* and MP2/6‐31G* basis sets are consistent with the gauche rotamer lying 554 cm −1 (1.6 kcal mol −1 ) and 296 cm −1 (0.85 kcal mol −1 ), respectively, lower in energy than the cis conformation. It is estimated that there is only 4–10% of the higher energy conformer present in the vapor at ambient temperature. Optimized structural parameters were obtained for both conformers with both basis sets and the results should provide good starting parameters for the analysis of the electron diffraction data. The force field obtained with the RHF/6–31G* basis set was used to perform a normal coordinate analysis. These results are compared with corresponding quantities for some similar molecules.