The infra-red spectrum, structure, and normal vibrations of the guanidinium ion

The infra-red spectrum of guanidinium chloride and iodide and their perdeutero derivatives have been obtained in the region 3600 to 400 cm ^-1. From the present infra-red data and previously determined Raman data an assignment has been made for the spectroscopically active fundamental vibration frequencies of the guanidinium ion which is in good agreement with the requirements of the product rule. The infra-red results for the particularly symmetrical iodide crystals are consistent with a D_3h all-planar structure for the ion slightly distorted by crystal forces corresponding to the site symmetry C_3v. The normal vibrations of the C(NH ₂)₃⁺ and C(ND₂)₃ ⁺ ions have been calculated as a ten-body problem on the basis of a Urey-Bradley type of force field. A reasonable set of force constants is obtained so as to give the best fit with the observed frequencies. The distribution of the potential energy among symmetry co-ordinates has been calculated and the contributions of the latter to each normal vibration has been discussed. Close analogies between the spectra of urea (which is known to be an all-planar molecule) and of the guanidinium ion and the amides, R . CONH ₂, support the assumption of planar structures for the latter two species.