Variable-temperature and -pressure studies of the vibrational spectra and phase transition in quadricyclane

The variable-temperature and -pressure vibrational spectra of the solid phases of quadricyclane have been measured. Factor group analysis has been used to predict possible space group symmetries of the ordered low-temperature phase, and the pressure dependences of the frequencies have been employed to make partial assignments of the vibrational modes. The order-disorder transition occurs at 9.3 +/- 0.5 kbar on compression and at about 7.0 kbar on decompression. The differences observed in the transition temperature, pressure, and entropy change for quadricyclane are compared with the data obtained for several other molecules with cycloheptyl skeletons.