Rotational Spectroscopy as a Tool to Study Vibration–Rotation Interaction: Investigations of 13CH3CN and CH313CN up to v8 = 2 and a Search for v8 = 2 Transitions toward Sagittarius B2(N)

Methyl cyanide, CH3CN, is present in diverse regions in space, in particular in the warm parts of star-forming regions where it is a common molecule. Rotational transitions of 13CH3CN and CH313CN in their v8 = 1 lowest excited vibrational states (Evib ≈520 K) are quite prominent in Sagittarius B2(N). In order to be able to search for transitions of the next higher vibrational state v8 = 2, we recorded spectra of samples enriched in 13CH3CN and CH313CN up to v8 = 2 in the 35–1091 GHz region and reinvestigated existing spectra of CH3CN in its natural isotopic composition between 1085 and 1200 GHz. Perturbations caused by near-degeneracies in K = 4 of v8 = 20 and K = 2 of v8 = 2–2 yielded accurate information on the energy spacing of 22.93 and 21.79 cm–1 between the l components of 13CH3CN and CH313CN, respectively. Fermi-type interaction between K = 13 and 14 of v8 = 1–1 and v8 = 2+2 probe the energy differences between the two states of both isotopomers. In addition, a ΔK ± 2, Δl ∓ 1 interaction between the ground vibrational state of 13CH3CN and v8 = 1+1 provides information on their energy spacing. Furthermore, we obtained improved or extended ground-state rotational transition frequencies of 13CH313CN and extensive data for 13CH3C15N and CH313C15N. Finally, we report the results of our search for transitions of 13CH3CN and CH313CN in their v8 = 2 states toward Sagittarius B2(N).