2,3-Dibromobutanes, 2,3-dibromopentanes, and isopropyl bromide—Conformations, vibrational assignments, and force constants

Abstract

Infrared spectra of meso- and dl-2,3-dibromobutane, threo- and erythro-2,3-dibromopentane, and isopropyl bromide have been measured in the region 1500-290 cm −1 for these compounds in the liquid and solid states. The spectra of condensed films of the 2,3-dibromo compounds were modified upon annealing due to the interconversion of rotational isomers. Specific rotational isomers were identified. Almost all of the bands observed for 2,3-dibromobutanes and isopropyl bromide were assigned. The analysis was aided by a normal coordinate calculation, constrained by the assumption of force constant transferability. The refined valence force field and the potential energy distributions of the fundamentals are listed. The stable conformation of meso- and dl-2,3-dibromobutane and of threo- and erythro-2,3-dibromopentane in their crystalline states as well as the relative stabilities of the rotational isomers of meso- and dl-2,3-dibromobutane in the liquid state are reported.