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)

Abstract

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$${}^{13}$CN in their $v_8=1$ lowest excited vibrational states ($E_{\mathrm{vib}}\approx 520$ K) are quite prominent in Sagittarius B2(N). In order to be able to search for transitions of the next higher vibrational state $v_8=2$, we recorded spectra of samples enriched in ${}^{13}$CH${}_3$CN and CH${}_3$${}^{13}$CN up to $v_8=2$ in the 35 to 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$${}^{13}$CN, respectively. Fermi-type interaction between $K=13$ and 14 of $v_8=1^{-1}$ and $v_8=2^{+2}$ probe the energy differences between the two states of both isotopomers. In addition, a $\hat{I}”K\pm 2$, $\hat{I}”l\mp 1$ interaction between the ground vibrational state of ${}^{13}$CH${}_3$CN and $v_8=1^{+1}$ provides information on their energy spacing. Furthermore, we obtained improved or extended ground state rotational transition frequencies of ${}^{13}$CH${}_3$${}^{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).

in press at ACS Earth and Space Chemistry; 18 pages, 5 Tables, 10 Figures