Methyl cianide: Spectroscopic studies of isotopically substituted species,and the harmonic potential function

The infrared gas-phase spectra of CH₃CN, ¹³CH₃CN, CH₃¹³CN, CH₃C¹⁵N, CD₃CN, and CD₃¹³CN have been studied in detail, in order to determine accurately the fundamental vibration frequency displacements on heavy isotopic substitution. A number of important Fermi resonances have been identified, and treated quantitatively. The unperturbed fundamental frequencies and heavy isotopic displacements form a self-consistent set of data, which, together with Coriolis zeta and centrifugal distortion constants, enable the harmonic potential function of methyl cyanide to be determined with only one constraint. A comparison between the latter and results from an ab initio calculation reveals disagreement in the values of two interaction constants, which seem well outside our experimental error. Infrared frequencies in crystalline films of CD₃CN and CD₃¹³CN at 78 K are also reported.