Knowledge Bank

The series of halogenated methyl cyanides: $CCl3CN,CCl2FCN,CClF2CN$, and $CF3CN$, presents the interesting case where the substituent, X, in the $CX3$ group is progressively replaced by an atom of greater electronegativity. The present communication reports the infrared and Raman spectra and vibrational assignments for the fundamental bands of the molecules in this series. Comparison of the results clearly shows regularities in certain fundamentals ($C\equiv N$ stretch, $C-C\equiv N$ symmetric and antisymmetric bends, $C-F$ symmetric and antisymmetric stretches) which may be attributed to the changes in the force field caused by increasing electronegativity of the $CX3$ group. Force constants are calculated for $CX3CN$ molecules belonging to the $C3v$ point group by the Wilson FG matrix method, the potential function containing all possible second degree terms. The limits of the invariancy of the force constants are examined in this series.