The conformations, phase transitions and vibrational spectra of bicyclohexyl

Abstract

The i.r. spectra of bicyclohexyl as a liquid under atmospheric pressure and as three solid crystalline phases obtained under high pressure and ambient temperature were recorded. Low temperature i.r. spectra of the amorphous and of two crystalline phases were obtained. Raman spectra of the liquid, including semiquantitative polarization data, and of two crystalline phases at low temperatures were obtained. Bicyclohexyl apparently exists in the ee conformation as a mixture of anti ( C 2 h symmetry) and gauche ( C 2 symmetry) around the CC pivot bond. In the high pressure crystal formed at 1–2 kbar (phase I HP) both conformers were present and one series of dichroic measurements of a single crystal revealed complete order. At 2–8 kbar (phase II HP) only the anti conformer, and above 15 kbar (phase III HP) only the gauche conformer were present in the crystals; the three phases I HP, II HP and III HP were reversibly formed by pressure variations. By cooling, the phases II LT and III LT containing anti and gauche conformers, respectively, were reversibly formed, but phase I LT was not observed with certainty. An additional low temperature phase believed to be metastable was formed by annealing the amorphous solid from 90 K to ca 130 K. The differential thermal analysis (DTA) data suggested variations in the phase transition temperatures and enthalpies depending upon the thermal history of the sample. A nearly complete assignment of the i.r. and Raman spectra to anti and gauche (both of ee) conformers were made, supported by a force constant calculation.