Low-Frequency Vibrational Spectra of Molecular Crystals. XXII: Cyclobutane-d_o and Cyclobutane-d₈

Abstract

The Raman spectra (0-300 cm^–1) of the two crystal modifications of cyclobutane-and cyclobutane-rf8 were recorded at various temperatures. The infrared spectra (30 to 300 cm^–1) were also recorded at various temperatures. Lattice modes were observed at 109, 106, 93, 46 and 33 cm^–1 for cyclobutane in crystal phase II at 20°K. From the shift factors with deuteration, it is concluded that the highest three bands are librational modes whereas the two bands of lowest frequency are optical translations. The number and activity of the lattice modes are consistent with the previously proposed crystal symmetry of C_cea (D²² _2h) with two molecules per primitive cell for phase II. The ring-puckering vibration is observed at 255 cm^–1 and shifts to 201 cm^–1 with deuteration. The temperature dependency of these bands is discussed and it is shown that the ring-puckering vibration is intimately involved in the phase transition II → I.