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O. BAUM, M. KOERBER, S. SCHLEMMER, T. F. GIESEN, I. Physikalisches Institut, Universität zu Köln, 50937 Köln, Germany; S. N. YURCHENKO, TU Dresden, Institut für Physikalische Chemie und Elektrochemie, 01062 Dresden, Germany; W. THIEL, MPI für Kohlenforschung, 45470 Mülheim an der Ruhr, Germany; P. JENSEN, FB C -- Theoretische Chemie, Bergische Universität, 42097 Wuppertal, Germany; K. M. T. YAMADA, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba-West, 305-8569, Japan.

%\vspace-0.7cm \hspace*-.5cm \epsfigfile=enantiomers.eps,

%\endwrapfigure 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.

%homo-nuclear species, HSSH

%and HOOH

%With two different rotor moieties, SH and OH, HSOH represents the

%molecule. \indent HSOH can be considered as a link between the well-known molecules HSSH and HOOH. For these two species

a simple model to explain the alternation of the torsional splittings with the rotational quantum number K_a has been proposed by Hougen\footnoteJ.T. Hougen and B. DeKoven, J. Mol. Spectrosc. 98 (1983) 375; J.T. Hougen, Can. J. Phys. 62 (1984) 1392.

%energy-level structure resulting from the torsional motion has been

%Winnewisser and K.M.T. Yamada, Vib. Spectrosc. 1 (1991)

% J. Mol. Spectrosc. 159 (1993) 507.

%XYYX (with X=H,D and Y=S,O) have two equivalent equilibrium

%enantiomers. Tunneling between the two corresponding potential

%about the heavy atoms bond YY, causes rotational transitions to

%HSSH and HOOH. In case of these two molecules it was found, that the

%^a,b

%a theoretical model introduced by Hougen^,\footnoteJ.~T.~Hougen. Can. J. Phys. 62

%is of lower symmetry than the C₂ molecules HSSH and HOOH, hence,

%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\footnoteR.I.~Ovsyannikov, V.V.~Melnikov, W.~Thiel, P.~Jensen, O.~Baum, T.F.~Giesen, S.N.~Yurchenko, J.Chem.Phys. 129 (2008)~154314^,\footnoteK.M.T.~Yamada, G.~Winnewisser, P.~Jensen, J. Mol. Struct. 695--696, (2004) 323^,\footnoteK.M.T.~Yamada, P.~Jensen, S.~Ross, O.~Baum, T.F.~Giesen, S.~Schlemmer, 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.

%CCSD(T)/cc-pCVTZ)\footnoteG.~Winnewisser, F.~Lewen,

% Chem. Eur. J. 9 (2003) 5501--5510

%K_a=1 0, K_a=2 1, andK_a=3 2all

%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.

%related to state mixing, i.e. the torsional tunneling effect

%C₁, of the molecule. Hence, it indicates that in the symmetric

%vanishes accidentally for HSOH in the ground vibrational state.