The structure and vibrational frequencies of crystalline HCl trihydrate

The crystal of hydrogen chloride trihydrate has been studied from theoretical and experimental points of view. This species may be relevant in atmospheric processes. A DFT study of the crystal using the program siesta has been performed. The geometrical structure has been refined and the vibrational modes of the crystal have been calculated. The crystal has been formed in the laboratory by exposure of amorphous water ice to excess pressure of HCl, followed by annealing and further cooling to 87 K. Reflection-absorption infrared spectra were recorded at this temperature. The experimental spectrum is assigned by comparison with the theoretical prediction, with the help of the atomic Cartesian displacements per normal mode. The results are compared with previous investigations on the HCl trihydrate and also with theoretical works on the Zundel ion H5O2+, which is present in the atomic arrangement of the crystal of HCl·3(H2O).