Conformational equilibria for 5-fluoro-, 5-chloro- and 5-bromo-trans-penta-1,3-diene: the effect of the halogen substituent

The results of ab initio SCF MO calculations at the Hartree-Fock (3-21G and 6-31G*) and DFT levels (B3LYP/6-31G*, B3LYP/6-311+G**) for 5-fluoro- (5FP) , 5-chloro- (5CIP) and 5-bromo-trans-penta-1,3-diene (5BrP) molecules and v ibrational spectra (Raman and FTIR) of the corresponding Liquid phases are reported and discussed. For the isolated systems, ab initio calculations at the B3LYP/6-311+G** level yield the anti-skew configuration (rotations aro und the C2-C3 and C4-C5 bonds, respectively) as the most stable form for al l the molecules considered in this study. The Raman spectra of the liquid s amples show pairs of bands whose temperature-dependent intensities are ascr ibed to conformational equilibria. From the plot of logarithm of intensity ratio vs. inverse temperature, LIH values of - 2 +/- 0.8, 1 :+/- 0.3 and 1. 5 +/- 0.2 kJ mol(-1) were derived for 5FP, 5CIP and 5BrP, respectively, cor responding to anti-skew -, anti-syn equilibria. Considering the Boltzmann p opulations at room temperature, the skew form is clearly the dominant one i n all cases, except for 5FP in the liquid phase, where it shares approximat ely 50% of the population with the slightly more populated syn form. In add ition, for 5CIP and 5BrP, the skew form is practically the only form to occ ur in the isolated molecule situation. In all cases, passing from the isola ted molecules to the liquid phase has the effect of increasing the populati on of the syn form, a trend which is clearly more important for 5FP, where the latter form becomes barely dominant at room temperature. Copyright (C) 1999 John Wiley & Sons, Ltd.