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Raman and infrared spectra, conformational stability, barriers to internal rotation, vibrational assignment and ab initio calculations of trans ‐1‐bromobut‐2‐ene
Author(s) -
Durig J. R.,
Costner T. G.,
Goodson Burt
Publication year - 1993
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.1250241102
Subject(s) - conformational isomerism , raman spectroscopy , enthalpy , chemistry , basis set , ab initio , ab initio quantum chemistry methods , infrared , infrared spectroscopy , computational chemistry , density functional theory , molecule , thermodynamics , physics , organic chemistry , optics
Raman (3200–10 cm −1 ) and infrared (3200–20 cm −1 ) spectra were recorded for the fluid and solid phases of trans ‐1‐bromobut‐2‐ene (crotyl bromide), trans ‐CH 3 CHCHCH 2 BR, and a complete vibrational assignment is proposed. The fundamental asymmetric torsion for the more stable gauche conformer was observed in the far‐infrared spectrum of the gas as a hybrid band centered at 75.0 cm −1 . The enthalpy difference between the gauche and syn conformers is estimated to be 434 ± 111 cm −1 (1.24 ± 0.32 kcal mol −1 ) in the gas phase from the relative intensities of the Raman lines utilizing the differences in the Raman activities from the ab initio calculations with the RHF/STO‐3G* basis set. The enthalpy difference was determined experimentally for the liquid from the relative intensities of the C–Br stretches as a function of temperature. The determined value is 370 ± 77 cm −1 (1.06 ± 0.22 kcal mol −1 ) with the gauche conformer the more stable form. A reasonable range of values was obtained for the coefficients of the potential function governing internal rotation about the CC bond. The three‐fold barrier governing internal rotation of the CH 3 group was determined from the far‐infrared spectrum of the gas. All of these data are compared to the corresponding quantities obtained from ab initio Hartree–Fock gradient calculations employing the RHF/STO‐3G* and RHF/LANL1DZ basis sets. Additionally, complete equilibrium geometries were determined for both rotamers. The results are discussed and compared with the corresponding quantities for some similar molecules.