Rotational Spectroscopy as a Tool to Study Vibration–Rotation Interaction: Investigations of 13 CH 3 CN and CH 3 13 CN up to v 8 = 2 and a Search for v 8 = 2 Transitions toward Sagittarius B2(N)

Methyl cyanide, CH 3 CN, 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 13 CH 3 CN and CH 3 13 CN in their v 8 = 1 lowest excited vibrational states ( E vib ≈520 K) are quite prominent in Sagittarius B2(N). In order to be able to search for transitions of the next higher vibrational state v 8 = 2, we recorded spectra of samples enriched in 13 CH 3 CN and CH 3 13 CN up to v 8 = 2 in the 35–1091 GHz region and reinvestigated existing spectra of CH 3 CN in its natural isotopic composition between 1085 and 1200 GHz. Perturbations caused by near-degeneracies in K = 4 of v 8 = 2 0 and K = 2 of v 8 = 2 –2 yielded accurate information on the energy spacing of 22.93 and 21.79 cm –1 between the l components of 13 CH 3 CN and CH 3 13 CN, respectively. Fermi-type interaction between K = 13 and 14 of v 8 = 1 –1 and v 8 = 2 +2 probe the energy differences between the two states of both isotopomers. In addition, a Δ K ± 2, Δ l ∓ 1 interaction between the ground vibrational state of 13 CH 3 CN and v 8 = 1 +1 provides information on their energy spacing. Furthermore, we obtained improved or extended ground-state rotational transition frequencies of 13 CH 3 13 CN and extensive data for 13 CH 3 C 15 N and CH 3 13 C 15 N. Finally, we report the results of our search for transitions of 13 CH 3 CN and CH 3 13 CN in their v 8 = 2 states toward Sagittarius B2(N).