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Conformational equilibria for 5‐fluoro‐, 5‐chloro‐ and 5‐bromo‐ trans ‐penta‐1,3‐diene: the effect of the halogen substituent
Author(s) -
Marques Cristina M. C.,
RibeiroClaro Paulo J. A.,
TeixeiraDias José J. C.
Publication year - 1999
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/(sici)1097-4555(199902)30:2<133::aid-jrs367>3.0.co;2-7
Subject(s) - substituent , chemistry , raman spectroscopy , molecule , ab initio , population , ab initio quantum chemistry methods , computational chemistry , crystallography , stereochemistry , organic chemistry , physics , demography , sociology , optics
The results of ab initio SCF MO calculations at theHartree–Fock (3–21G and 6–31G*) andDFT levels (B3LYP/6–31G*,B3LYP/6–311+G**) for5‐fluoro‐ (5FP), 5‐chloro‐(5ClP) and 5‐bromo‐ trans ‐penta‐1,3‐diene (5BrP) moleculesand vibrational spectra (Raman and FTIR) of thecorresponding liquid phases are reported and discussed. For theisolated systems, ab initio calculations at theB3LYP/6–311+G** level yield the anti ‐ skew configuration (rotations around theC2—C3 and C4—C5 bonds, respectively) as the moststable form for all the molecules considered in this study. The Ramanspectra of the liquid samples show pairs of bands whosetemperature‐dependent intensities are ascribed toconformational equilibria. From the plot of logarithm of intensityratio vs . inverse temperature, Δ H values of−2±0.8, 1±0.3 and 1.5±0.2 kJ mol −1 were derived for 5FP, 5ClP and 5BrP, respectively,corresponding to anti ‐ skew → anti ‐ syn equilibria. Considering the Boltzmannpopulations at room temperature, the skew form is clearly thedominant one in all cases, except for 5FP in the liquid phase, whereit shares approximately 50% of the population with theslightly more populated syn form. In addition, for 5ClP and5BrP, the skew form is practically the only form to occur inthe isolated molecule situation. In all cases, passing from theisolated molecules to the liquid phase has the effect of increasingthe population of the syn form, a trend which is clearly moreimportant for 5FP, where the latter form becomes barely dominant atroom temperature. Copyright © 1999 John Wiley & Sons, Ltd.