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%\begin{wrapfigure}[4]{r}{3cm} %\vspace{-0.7cm} \hspace*{-.5cm} \epsfig{file=enantiomers.eps, %width=3cm} %\end{wrapfigure} In this paper we present highly accurate spectral data of oxadisulfane, HSOH, in the region of 1.1--1.3~THz. The simple skew chain molecule HSOH is an asymmetric rotor close to the limiting case of a symmetric prolate top molecule. Therefore the pure rotational spectra of this molecule appear very simple at first glance. However, if the spectra are inspected in detail, the molecule manifests its peculiarities.\ \\ %HSOH can be considered as a link between the two well known %homo-nuclear species, HSSH % andK.~M.~T.~{Yamada}, {\em Vib Spectrosc.} {\bf 1}, (1991) 263} %and HOOH %, K.~M.~T.~{Yamada}, and G.~{Winnewisser}. {\it J. Mol. Spectrosc.} {\bf 159} (1993) 507--520}. %With two different rotor moieties, SH and OH, HSOH represents the %most general case of a four-atomic, internally rotating chain %molecule. \indent HSOH can be considered as a link between the well-known molecules HSSH and HOOH. For these two species %, which have $C_{2}$ symmetry at equilibrium, a simple model to explain the alternation of the torsional splittings with the rotational quantum number $K_a$ has been proposed by Hougen, {\em J. Mol. Spectrosc.} {\bf 98} (1983) 375; {J.T. Hougen}, {\em Can. J. Phys.} {\bf 62} (1984) 1392}. %, and the %energy-level structure resulting from the torsional motion has been %the subject of detailed experimental study, {\em Vib. Spectrosc.} {\bf 1} (1991) %263}$^,$, %{\em J. Mol. Spectrosc.} {\bf 159} (1993) 507}. %Molecules of type %$XYYX$ (with $X$=H,D and $Y$=S,O) have two equivalent equilibrium %conformations which are the most simple representatives of molecular %enantiomers. Tunneling between the two corresponding potential %energy minima, which is classically an internal rotation (torsion) %about the heavy atoms bond $YY$, causes rotational transitions to %split into doublets. This tunneling splitting is well studied for %HSSH and HOOH. In case of these two molecules it was found, that the %size of the splitting alternates with $K_a$ modulo two %$^{a,b}$ %. This phenomenon has been described in %a theoretical model introduced by Hougen and %B.~M.~{DeKoven}. {\it J. Mol. Spectrosc.} {\bf 98} (1983) %375--391}$^,$. {\it Can. J. Phys.} {\bf 62} %(1984) 1392}, which is in good agreement with the experiment. HSOH %is of lower symmetry than the $C_2$ molecules HSSH and HOOH, hence, %the tunneling splitting is found to be a more complicated function %of $K_a$ HSOH obviously has lower symmetry than HSSH and HOOH and therefore the observed variation of the torsional splittings with the rotational quantum number $K_a$ cannot be explained by the Hougen model. The new data allow to calculate the experimental tunneling splitting of energy levels up to $K_a$=7 for the first time. The obtained results are essential to test novel models, V.V.~{Melnikov}, W.~{Thiel}, P.~{Jensen}, O.~{Baum}, T.F.~{Giesen}, S.N.~{Yurchenko}, {\em J.Chem.Phys.} {\bf 129} (2008)~154314}$^,$, G.~{Winnewisser}, P.~{Jensen}, {\em J. Mol. Struct.} {\bf 695--696}, (2004) 323}$^,$, P.~{Jensen}, S.~{Ross}, O.~{Baum}, T.F.~{Giesen}, S.~{Schlemmer}, {\em J. Mol. Struct.} (2009) accepted} on torsional tunneling splitting in HSOH. \ \\ \indent In case of $K''_a<3$ the HSOH molecule displays a dominating perpendicular-type spectrum in the vibrational ground state with strong $c$- and somewhat weaker accompanying $b$-type transitions, as can be understood from theoretical values of the dipole-moment components. %($\mu_a=0.044$~D, $\mu_b=0.77$~D, and $\mu_c=1.43$~D; %CCSD(T)/cc-pCVTZ), F.~{Lewen}, %S.~{Thorwirth}, M.~{Behnke}, J.~{Hahn}, J.~{Gauss}, E.~{Herbst}. %{\it Chem. Eur. J.} {\bf 9} (2003) 5501--5510} %Transitions with $K''_a<3$ %$K_a=1\leftarrow 0$, $K_a=2\leftarrow 1$, and$K_a=3\leftarrow 2$all %exhibit strong $c$- and somewhat weaker %$b$-type transitions. In contrary, transitions with $K_a''\ge 3$ display only $c$- but no $b$-type transitions. The absence of $b$-type transitions is completely unexpected and yet not well understood. %Preliminary results show the observed intensity anomalies to be %related to state mixing, {\it i.e.} the torsional tunneling effect %mixes the state of different $K_c$-levels due to the low symmetry, %$C_1$, of the molecule. Hence, it indicates that in the symmetric %top limit ($K_a\ge 3$) the intensity of $b$-type transitions %vanishes accidentally for HSOH in the ground vibrational state.


G.~{WinnewisserG.~{Pelz{J.T. Hougen and B. DeKoven{\sc G. %Winnewisser and K.M.T. Yamada{\sc G. Pelz, K.M.T. Yamada, and G. WinnewisserJ.~T.~{HougenJ.~T.~{HougenR.I.~{OvsyannikovK.M.T.~{YamadaK.M.T.~{YamadaG.~{Winnewisser
Author Institution: I. Physikalisches Institut, Universitat zu Koln, 50937 Koln, Germany; TU Dresden, Institut fur Physikalische Chemie und Elektrochemie, 01062 Dresden, Germany; MPI fur Kohlenforschung, 45470 Mulheim an der Ruhr, Germany; FB C -- Theoretische Chemie, Bergische Universitat; 42097 Wuppertal, Germany; National Institute of Advanced Industrial Science and; Technology (AIST), Tsukuba-West, 305-8569, Japan