10min:
MILLIMETER-WAVE SPECTROSCOPY OF THE HCN-H2 CLUSTER.

M. ISHIGURO AND T. TANAKA, Department of Chemistry, Faculty of Science, Kyushu University 33, Hakozaki, Higashiku, Fukuoka 812-8581, Japan; C. J. WHITHAM, K. HARADA AND K. TANAKA, Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan.

Millimeter-wave absorption spectroscopy combined with a pulsed-jet expansion technique was applied to the measurement of the rotational and rovibrational transitions of the HCN-H2 cluster in the frequency region of 75-150 GHz. So far, four rotational lines for the ground Sigma0 state of the HCN-( o -H2) cluster split into hyperfine structure due to the nitrogen nucleus were observed. Rotational constant B0 = 12899.718(20) MHz and centrifugal distortion constant D0 = 12.2470(16) MHz were derived together with its higher constants. The hyperfine constants determined eqQ = -2.830(33) MHz which is smaller than that of HCN molecule means a large amplitude motion of HCN of < theta> = 31.1 degree in the ground linear form. The bond length between HCN and H2 parts is derived to be 3.90 Å.

Some lines belonging to the Sigma1 - Sigma0 van der Waals bending band of HCN-( o -H2) were also observed. The Sigma1 - Sigma0 van der Waals bending frequency 136.831 GHz of HCN-( o -H2) is larger than that of He-HCN 98.70 GHz, but much smaller than that of Ar-HCN 164.89 GHz. The hyperfine constant in the Sigma1 state indicate the cluster has T-shape in the excited state of the vdW bending mode.

A search of the rotational lines of the ground Sigma0 state of HCN-( p -H2) and the Pi0 state of HCN-( o -H2) are now in progress as well as the vdW mode rovibrational lines for both HCN-( o -H2) and -( p -H2) clusters.