Normal coordinate treatment and molecular constants for fluoro-, chloro- and bromo-forms

By a least-square iteration process and following the analytical criteria recently proposed by A ldous and M ills the recalculation of the quadratic potential energy function for fluoro-, chloro- and bromo-forms in a modified valence force field approximation was carried out, adopting actual interbond angles. The assignment of ν 2 fundamental for CF 3 H necessary to obtain convergence was found to be different from that used in previous calculations. The values of stretch—bend interaction constants which lead to a well-conditioned convergence suggest that the hybrid orbital force field approximation is applicable for the present molecules. Coriolis coupling constants, rotation distorsion constants, mean-square amplitudes of vibration and thermodynamic properties were also calculated.