Single-crystal Raman spectroscopy of ‘square-planar’ and ‘tetrahedral’ CuCl42– ions, of the ZnCl42– Ion, and of CuCl2,2H2O

Abstract

The Raman spectrum of CuCl₂,2H₂O is assigned unambiguously from single-crystal and deuteriation studies. In the case of (CH₃NH₃)₂CuCl₄ the assignment of the main bands from single-crystal studies is again unambiguous, but for the species containing ‘tetrahedral’ ions (Cs₂CuCl₄ and Cs₂ZnCl₄) only tentative assignments can be given in terms of the molecular ions. Discussions of the position of fundamental vibrations of ‘long’ and ‘short’ Cu—Cl bonds in the absence of adequate analysis are meaningless. The following points arise from this study: (a) for CuCl₂,2H₂O ‘short’ bond stretching modes occur at 216 and 237 cm.^–1 while the ‘long’ bond stretching mode occurs (as a libration of linear Cl—Cu—Cl) at 112 cm.^–1. (b) For (CH₃NH₃)₂CuCl₄ the motion involving stretching of the ‘long’ Cu—Cl bond involves simultaneous compression of a ‘short’ bond. This mode is also largely coupled with the ‘terminal’ Cu—Cl stretch, with ‘in phase’ frequencies 248 and 286 cm.^–1(a_g of the free planar ion) and ‘out of phase’ at 174 and 182 cm.^–1(b_1g or b_2g of the free ion according to choice of x and y axes). (c) In Cs₂CuCl₄ and Cs₂ZnCl₄ the considerable distortions of the ions from regular tetrahedral are exhibited in terms of splitting of degenerate modes, the f₂ modes of T_d symmetry both being split over a range of 29 cm.^–1 for Cs₂CuCl₄, and over a range of 17 and 13 cm.^–1 respectively for Cs₂ZnCl₄. It must be noted that various assumptions were necessary to reach the conclusions of the last sentence.