FTIR study of the conformational dynamics in the solid phases of fluorocyclohexane and bromocyclohexane

Abstract

The infrared spectra of the solid phases of fluorocyclohexane (FCH) and bromocyclohexane (BCH) have been investigated at temperatures between 77 and 286 K and between 77 and 216 K, respectively. The solid films were formed by cooling liquid films and by condensing vapors of FCH or BCH onto a cold window. During annealing of the deposited film of the FCH a metastable phase, which was interpreted to be the undercooled plastic phase, was observed in the temperature interval between 100 and 143 K; in this phase both axial (a) and equatorial (e) conformers are present with similar concentrations. Near 143 K, an irreversible transition from the undercooled plastic phase to an anisotropic phase was observed. In this phase a non-negligible and reproducible amount of the a-conformer remains. Using dissolution experiments in liquid krypton, the concentration ratio of the conformer (c_e/c_a)_solid in the anisotropic phase was determined to be 17(1). At 188 K the reversible phase transition from the anisotropic to the plastic phase was observed. The infrared spectra of the plastic phase of FCH in the temperature interval between 188 and 283 K indicate the existence of a conformational equilibrium e ⇆ a. The similarity of the spectra suggests this phase is identical to the phase observed between 77 and 143 K. The standard enthalpy difference ΔH° between the conformers in this phase is found to be 1.16(5) kJ mol⁻¹, with the e-conformer being more stable. During the annealing of the deposited BCH film a new metastable, anisotropic phase, crystal I, was observed in the temperature interval between 77 and 160 K. At ca. 160 K an irreversible phase transition from crystal I to another anisotropic crystalline phase, crystal II, takes place. In both crystalline phases only the e-conformer is present. From the experiments it was concluded that the conformational mobility dominates the formation of the anisotropic phase of FCH and of the lower temperature anisotropic phase I of BCH.