Vibrational Spectra and Molecular Structure of Some Halogenated Methyl Cyanides

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Abstract
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≡N stretch, C–C≡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.