MILLIMETER WAVE SPECTROSCOPY OF THE INTERNAL ROTATION BANDS OF He-HCN AND He-DCN.

$^{a}$S. Drucker, F. Tao, and W. Klemperer, J. Phys. Chem. 99, 2646(1995). $^{b}$R. R. Toczylowski, F. Doloresco, and S. M. Cybulski, J. Chem. Phys. 114, 851(2001).Author Institution: Department of Chemistry, Faculty of Sciences, Kyushu University; Institute for Molecular Science, Faculty of Sciences, Kyushu UniversityMillimeter-wave absorption spectroscopy combined with a pulsed-jet expansion technique was applied to the measurement of the internal-rotation band of the He-HCN complex in the frequency region of $95-125$ GHz. In total 13 rovibrational lines, split into nitrogen nuclear hyperfine structure, were observed for the fundamental internal-rotation band, $j = 1 - 0$. The observed transition frequencies and the previous MBER $data^{a}$ were analyzed to yield an empirical intermolecular potential energy surface, which was obtained by improving the recent ab initio potential $surface.^{b}$ The surface obtained has a global minimum in the linear configuration $(He{\cdots}H-C-N)$ with a well depth of $30.2 cm^{-1}$, and the saddle point located in the anti-linear configuration $(H-C-N{\cdots}He)$ is higher in energy than the global minimum by $8.92 cm^{-1}$. The distance $R_{m}$ from the He atom to the center of mass of HCN along the minimum energy path shows a large angular dependence; $R_{m}$ is $4.169 {\AA}$ and $4.039 {\AA}$ in the linear and anti-linear forms, respectively, and has a minimum value of $3.528 {\AA}$ in a T-shaped configuration. The He-DCN was also measured and analyzed. The empirical potential energy surfaces are compaired with the ab initio potential energy $surface^{b}$ for both isotopic species