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Raman and infrared spectra, conformational stability, barriers to internal rotation and ab initio calculations of trifluoroacetyl isothiocyanate
Author(s) -
Durig J. R.,
Guirgis Gamil A.,
Krutules K. A.
Publication year - 1995
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.1250260612
Subject(s) - conformational isomerism , raman spectroscopy , chemistry , infrared , basis set , ab initio quantum chemistry methods , infrared spectroscopy , ab initio , crystallography , analytical chemistry (journal) , computational chemistry , molecule , density functional theory , organic chemistry , physics , optics
The Raman (3100‐10 cm −1 ) and infrared (3100‐30 cm −1 ) spectra of trifluoroacetyl isothiocyanate [CF 3 C(O)NCS] were recorded for the gas and solid. Additionally, the Raman spectrum of the liquid was recorded and qualitative depolarization values were obtained. The observed bands are assigned on the basis of the more stable cis conformer (isothiocyanate group cis to the carbonyl bond) and the less stable trans conformer in the vapor state. From the variable‐temperature studies of the infrared spectrum of the sample dissolved in liquified xenon, two conformer pairs were used to determine a Δ H value of 104 ± 9 cm −1 (270 ± 26 cal mol −1 ). From the temperature dependence of the Raman spectrum of the liquid, two conformer pairs were used to determine experimentally a Δ H value of 90 ± 31 cm −1 (258 ± 88 cal mol −1 ), but for this physical state the trans conformer is the more stable form. In the annealed crystalline solid, only the trans conformer remains. A complete vibrational assignment is proposed for both conformers based on infrared band contours, Raman depolarization data, group wavenumbers and normal coordinate calculations. The experimental conformational stability, barriers to internal rotation and fundamental vibrational wavenumbers are compared with those obtained from the ab initio calculations employing the RHF/6‐31G * basis set and to the corresponding quantities obtained for some similar molecules.