The rotation - torsion - vibration interaction in the normal and 15N isotopic species of Acetyl Cyanide is studied in the rotational spectrum of ground, first excited state of methyl torsion and first excited state of CCN in plane bending vibration. With respect to a previous -work [1] a more detailed check of a model with five degrees of freedom, comprising three for the overall rotation and two for the two lowest vibrations was possible. Potential parameters were fitted simultaneously to the splittings of the rotational transitions in the ground, excited torsional and excited vibrational states for the normal and 15N isotopic species of the molecule. The coefficients V3 and V6 of the Fourier expansion of the hindering potential for the torsion and two interaction constants V3c′ and F3c″ for the torsion and in plane CCN bending vibration were determined, apart from the harmonic force constant k2q for the vibration, which is obtained from the measured infrared data of the normal species. Using these results, the (E-A) splittings of the rotational transitions could be nicely reproduced but not the absolute frequencies of the rotational transitions.
