Empirical force field calculations on methylammonium chloride and its deuterated derivatives

A valence force field for the methylammonium ion is presented. The force field was determined from a modified least squares fit of the observed Raman frequencies of methylammonium chloride and three of its deuterated derivatives. In addition, a Urey-Bradley force field was tested; it was found to be impossible to obtain a good fit to the observed frequencies without the inclusion of a non-bonded hydrogen-hydrogen interaction in the potenial energy. The reason is that the unmodified Urey-Bradley force field does not include any bend-bend interactions, which are necessary in order to reproduce the rocking modes. (orig.)