## MILLIMETER WAVE SPECTRA OF THE INTERNAL ROTATION EXCITED STATES OF ($o$)H$_2$-H$_2$O AND ($o$)H$_2$-D$_2$O

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Ohio State University##### Abstract:

H$_2$-H$_2$O is a weakly bound complex and it has a various states according to the internal rotation for both H$_2$ and H$_2$O moieties. In our previous study,, \textbf{RD08} (2001).} we have reported the pure rotational transitions of the (o)H$_2$ complex in the ground H$_2$O rotational state, $0_{00}(\Sigma)$, for both H$_2$-H$_2$O and H$_2$-D$_2$O, where ($o$)H$_2$ ($j_{\rm H2}$ =1) is rotating perpendicular to the intermolecular axis to give the projection of $j_{\rm H2}$ to the axis $k_{\rm H2}$ to be zero ($i.e.$ $\Sigma$ state). In the present study, we have observed the rotational transitions for the $0_{00}$ ($\Pi$) states in the millimeter-wave region up to 220 GHz, where the ($o$)H$_2$ is rotating around the intermolecular axis to give the projection $k_{\rm H2}$ to be one ($i.e.$ $\Pi$ state). The center of mass bond lengths derived from the observed rotational constants for $0_{00} (\Pi)$ are longer by 5 $\%$ than those for $0_{00} (\Sigma)$, while force constants for the intermolecular stretching for $0_{00} (\Pi)$ derived from centrifugal distortion constants are smaller by 23 $\%$ than those for $0_{00} (\Sigma)$, suggesting the $\Pi$ and $\Sigma$ substates have quite different structures. The recent theoretical calculation \textbf{134}, 044314 (2011).} indicates that for $0_{00}(\Sigma)$, ($o$)H$_2$ is bound to the oxygen site of H$_2$O, while for the $0_{00} (\Pi)$ state, ($o$)H$_2$ to the hydrogen site of H$_2$O, and the $0_{00}(\Sigma)$ state is by 14 cm$^{-1}$ more stable than the $0_{00} (\Pi)$ state. Observed molecular constants for $0_{00}(\Sigma)$ and $(\Pi)$ are consistent with the structures given by the theoretical calculation. We also observed the rotational spectrum in the $1_{01} (\Sigma)$ and $(\Pi)$ states, where $\Sigma$ and $\Pi$ correspond to the rotation of H$_2$O perpendicular and parallel to the intermolecular axis and ($o$)H$_2$ is calculated to be bound to the oxygen site of H$_2$O. The energy difference between the $1_{01} (\Sigma)$ and $(\Pi)$ states will be discussed due to the Criolis interaction between these substates.

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Author Institution: Department of Chemistry, Faculty of Science, Kyushu University, Hakozaki, Higashiku, Fukuoka, 812-8581 JAPAN

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