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The infrared spectra of crystalline CH2Cl2 and CD2Cl2 were measured at temperatures ranging from 5 to 125°K and at 85°K, respectively, between 5000 and 200 cm−1. Splitting into two or more components was observed for several fundamentals. The mixed-crystal technique was used to prove the origin of the observed splittings, and the isotopic shifts for 37Cl and 13C as well as the factor-group splittings were identified. These data, together with polarization measurements on polycrystalline-oriented films, relative intensity measurements, and other independent data, such as crystal density and Raman spectrum, were used to determine the crystal structure. The data accumulated are consistent with an orthormbic D2h14(Pbcn) unit cell with four molecules located on C2 sites.

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 D2h 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-ds. IntroductionSeveral low temperature infrared and Raman studies on molecular crystals have been carried out in recent years with the aim of determining the crystal structure when X-ray data were not available. In order to restrict the choice of the possible structures compatible with the spectral data, simple closest-packing arguments and, when available, the comparison between calculated and observed density, have often been used.A different approach to this problem concerns the calculation of the structure using the structural information obtained from the analysis of the crystal spectra. At any given temperature and pressure the most stable structure is governed by the arrangement of molecules that has the lowest free energy. Assuming the zero point lattice energy to be comparable for a number of possible close-packed structures, one can reasonably assume that the most probable structure is the one with the lowest calculated lattice energy.Cyclobutane offers an attractive possibility for this kind of investigation since: (i) the molecular geometry has been widely investigated; (ii) the molecule possesses only C and H atoms; (iii) infrared spectra in polarized E. CASTELLUCCI, M. G. MIGLIORINI AND P. MANZELLI 433 light of oriented crystals have been obtained, which are of help in deriving structural information. Point (ii) is very important for calculating the minimum energy crystal structure since different sets of semi-empirical atom-atom potential parameters exist in the literature which have been widely tested on a number of experimental crystal properties and structures of aromatic and aliphatic hydrocarbons (Williams, 1967). It is widely accepted that cyclobutane has a puckered equilibrium configuration with D2a symmetry and a dihedral angle of about 32 ° . The evidence for this has been given by electron diffraction experiments (Almenningen, Bastiansen & Skancke, 1961), by an unpublished vibrational analysis of Lord and Rea quoted by Lord & Nakagawa (L & N) (1963), by relative intensity measurements of CH2 rocking vibrat...

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We tender very special thanks to a referee for his remark that an antiphase vector t ± ~, 2,0 should result in a discontinuity of the fringes: on reexamining our photographs we actually observed the phenomenon in both crystals of the twin.