The molecular packing of solid II cyclobutane by means of spectroscopic data and potential-energy calculations

The molecular packing of solid II cyclobutane has been calculated starting from the knowledge of the site symmetry (point group D₂) and of two possible symmetries of the primitive unit cell (D₂ and D_2h factor group symmetry), derived from analysis of the infrared and Raman spectra. Crystal potential energy has been calculated for 13 orthorhombic space groups using the packing program written by Williams. The calculation was based on a pairwise potential of Buckingam type, widely tested on a large number of hydrocarbon crystal structures and properties. The energy was calculated as a function of the unit cell parameters (length of the cell edges), considering the molecules as rigid bodies and leaving the crystal symmetry unchanged. The results show the packing corresponding to space group Ccca to have the lowest calculated potential energy. The spectroscopic results allowed some changes to be made in the previous assignment of the fundamental frequencies of cyclobutane and cyclobutane-d₈.