FOURIER-TRANSFORM MICROWAVE AND MILLIMETERWAVE SPECTROSCOPY OF THE H2-HCN MOLECULAR COMPLEX

Fourier-Transform microwave (FTMW) spectroscopy has been applied to observe the $J$ = 1 - 0 rotational transitions of the H$_{2}$-HCN/DCN complexes containing both the $para$-H$_{2}$ ($I_{rm H2}$=0) and $ortho$-H$_{2}$ ($I_{rm H2}$=1) moleculefootnote{M.~Ishiguro~et al., textit{Chem.~Phys.~Lett.} textbf{554}, 33 (2012).}. Rotational spectra of H$_{2}$-HCN/DCN up to $J$ = 5 - 4 were also observed in the millimeter-wave (MMW) region below 180 GHzfootnote{M.~Ishiguro~et al., textit{J.~Chem.~Phys.} textbf{115}, 5155 (2001).}. Observed FTMW lines for H$_{2}$-HCN/DCN split into hyperfine components due to the nuclear quadrupole interaction of N and D nuclei. For the $ortho$-H$_{2}$ species, the hyperfine splitting due to the magnetic interaction between the hydrogen nuclear spin of $ortho$-H$_{2}$ part ($j_{rm H2}$=1, $I_{rm H2}$=1) was also observed, but not for the $para$-H$_{2}$ species ($j_{rm H2}$=0, $I_{rm H2}$=0). From the observed nuclear spin-spin coupling constants of $ortho$-H$_{2}$ species, $d$ = 21.90(47) and 24.66(68) kHz for HCN and DCN complexes, respectively, the average values of $$ = 0.380(8) and 0.439(10) were derived indicating the nearly free rotation of H$_{2}$ in the complex with $j_{rm H2}$= 1 and $k_{rm H2}$= 0. The nuclear quadrupole interaction constants due to N and D nuclei show that the HCN/DCN part executes a floppy motion with a large mean square amplitude of about 29/25 and 33/30 degree in the $para$ and $ortho$ species, respectively. From the observed rotational constants, the center-of-mass distances of H$_{2}$ and HCN/DCN were derived to be 3.9617(5)/4.00356 (43) AA ~ for the $ortho$ species and 4.1589(13)/4.1596 (36) AA ~for the $para$ species. The isotope effect on rotational constants confirmed the totally different configurations in the $ortho$ and $para$ species: H$_{2}$ is attached to the H/D end of HCN/DCN for the $para$ species, while to the N end for the $ortho$ species, as suggested by IR spectroscopy and theoretical study