Rotational spectrum of acetonitrile CH 3C 14N and CH 3C 15N in a doubly excited degenerate vibrational state

The rotational spectrum of the acetonitrile molecules CH ₃C ¹⁴N and CH ₃C ¹⁵N in the degenerate vibrational state v₈ = 2 has been studied for the transitions J = 0 → 1, 2 → 3, 3 → 4, 4 → 5, 5 → 6, 6 → 7, and 7 → 8. The assignment of the quantum numbers K and l for these spectra has been obtained by means of a graphic procedure which can be generalized for application to other types of spectrum. This assignment and analysis of the measurements with a least-squares procedure has made it possible to obtain a new value for the anharmonicity constant x_l8l8 of approximately 165,000 MHz for the two molecules, a value which is quite different from that previously obtained from infrared measurements. This new value may be explained by the presence of an accidentally strong resonance which is revealed in these measurements. Good agreement is achieved between the experimental results and the theoretical formulation of the frequency expressions.