Infrared Spectrum of Crystalline CH2I2. Crystal Structure and Phase Transition

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Abstract
The infrared spectra of two crystalline modifications, I and II of CH2I2, have been measured at various temperatures ranging from 3° to −196°C between 4000 and 400 cm−1. The transition between the two crystal forms I→II has been demonstrated to be monotropic. The absence of the A2 fundamental in Form I allows a C2v site symmetry to be established, while for Form II the activity of the A2 fundamental and the observed factor‐group splitting for A1, B1, and B2 fundamentals is consistent only with trivial site symmetry. Among the multiplicity of factor groups consistent with the observed crystal spectra, a choice is possible using packing considerations. Orthorhombic, (D2h5) or (D2h13), and monoclinic (C2h5) structure are proposed for Forms I and II, respectively, which are consistent with the ir crystal spectra and densities of the two crystalline solids.