Knowledge Bank

Rotational spectra of several isotopomers of the $Ne-H2S$ van der Waals dimer were measured in the frequency range from 4 to 22 GHz, using a pulsed molecular beam Fourier transform microwave spectrometer. Two $K=0$ progressions were recorded for the symmetrical isotopomers (with $H2S/D2S$). This doubling is attributed to an internal rotation motion of the $H2S$ moiety within the complex. The two states can be correlated to the $000$ and $101$ internal rotor states of free $H2S$ and $D2S$. Only one $K=0$ progression was measured for $Ne-DSH$. The excited internal rotor state is no longer metastable since the symmetry constraints no longer apply. An anomalous isotope effect observed in $Ar-H2S$, where the substitution of hydrogen by deuterium causes an increase in the ground state B rotational $constant,a$ was not observed here, in agreement with the ab initio study by $Dykstraandco-workers.b$ Nuclear quadrupole hyperfine structure was resolved or partially resolved for isotopomers containing $\mathrm{<mrow\; data-mjx-texclass="ORD">33</mrow>}S$ and $D$, respectively, and the corresponding quadrupole coupling constants were determined. These were used to derive information about the intermolecular dynamics. The results are compared with those of $Ar-H2S$ and $Ar-H2O$.