Infrared Spectrum of Crystalline CH2I2. Crystal Structure and Phase Transition

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.