THE VIBRATIONAL SPECTRA AND VALENCE FORCE FIELD OF METHYL AMMONIUM CHLORIDE

Abstract

$^{1}$N. Meinander, C. Bergetrom, S. Force and F. Stenman, J. Raman Spectroac. 8(1979) 265,; N. Meinander, S. Forss and G. Bergatrom, J. Raman Spectroac. 11(1981) 155.; N. Meinander, Thesis, University of Helanki, Report Series in Physics HU-p-D24 (1981) $^{2}$T. Sundius and N. Meinander, J. Mol. Struct, Theochem. 76 (1981) 227. $^{3}$I. A. Oxton, O. Knop, and J.L. Dunean, Srtuct. 38 (1977)25.The Raman and TR spectra of the three crystalline modifications of methyl ammonium chloride, $\alpha-, \beta- and \gamma-MAC$, and the deuterated derivatives have been $recorded.^{1}$ The correlation field and crystal field splitting of the vibrational bands have been investigated with isotopic solid solutions. An effective valence force field for the methyl ammonium ion $(MA^{+})$ has been constructed, which reproduces the observed vibrational frequency of the isotopically isolated $MA^{+}$-ions in $\beta-MAC$, The $NH_{3}$ -hydrogen atoms are shown to be all inequivalent. The observed splitting of the $vibrational^{3}$ bands of $MA^{+}ions$ with a partly deuterated methyl or ammonium group, caused by the dependence of the vibrational energy levels on the orientation of this group in the lattice, is also reproduced by the force field. Differences in the effective force fields for the $MA^{+}$-ion, constructed from the observed vibrational frequencies of $\alpha-MAC^{2}, \beta-MAC$ and $(MA)_{2}PtCl_{6}$ respectively, can be related to the varying strength of the N-H...X hydrogen bonding to the anions in the different lattices