Knowledge Bank

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, $000(\Sigma )$, for both H$2$-H$2$O and H$2$-D$2$O, where ($o$)H$2$ ($jH2$ =1) is rotating perpendicular to the intermolecular axis to give the projection of $jH2$ to the axis $kH2$ to be zero ($i.e.$ $\Sigma$ state). In the present study, we have observed the rotational transitions for the $000$ ($\Pi$) 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 $kH2$ to be one ($i.e.$ $\Pi$ state). The center of mass bond lengths derived from the observed rotational constants for $000(\Pi )$ are longer by 5 $\%$ than those for $000(\Sigma )$, while force constants for the intermolecular stretching for $000(\Pi )$ derived from centrifugal distortion constants are smaller by 23 $\%$ than those for $000(\Sigma )$, suggesting the $\Pi$ and $\Sigma$ substates have quite different structures. The recent theoretical calculation \textbf{134}, 044314 (2011).} indicates that for $000(\Sigma )$, ($o$)H$2$ is bound to the oxygen site of H$2$O, while for the $000(\Pi )$ state, ($o$)H$2$ to the hydrogen site of H$2$O, and the $000(\Sigma )$ state is by 14 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ more stable than the $000(\Pi )$ state. Observed molecular constants for $000(\Sigma )$ and $(\Pi )$ are consistent with the structures given by the theoretical calculation. We also observed the rotational spectrum in the $101(\Sigma )$ 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 $101(\Sigma )$ and $(\Pi )$ states will be discussed due to the Criolis interaction between these substates.