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- (5ClP) and 5-bromo-trans -penta-1,3-diene (5BrP) molecules and vibrational 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 around the C2—C3 and C4—C5 bonds, respectively) as the most stable form for all the molecules considered in this study. The Raman spectra of the liquid samples show pairs of bands whose temperature-dependent intensities are ascribed to conformational equilibria. From the plot of logarithm of intensity ratio vs. inverse temperature, ΔH 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 Boltzmann populations at room temperature, the skew form is clearly the dominant one in all cases, except for 5FP in the liquid phase, where it shares approximately 50% of the population with the slightly more populated syn form. In addition, for 5ClP and 5BrP, the skew form is practically the only form to occur in the isolated molecule situation. In all cases, passing from the isolated molecules to the liquid phase has the effect of increasing the population of the syn form, a trend which is clearly more important for 5FP, where the latter form becomes barely dominant at room temperature.