Recently Afshari et al. reported on the detection of a new infrared band which was assigned to the "long-anticipated polar isomer of the OCS dimer" [J. Chem. Phys. 126, 071102 (2007)]. The authors report here the microwave confirmation of their results. The lowest energy, nonpolar isomer of (OCS)2 has long been known from IR spectroscopy, while the polar form has only been deduced from qualitative beam refocusing experiments. The higher energy, polar isomer of (OCS)2 has been produced by high pressure expansion of dilute OCS in helium. A surprisingly strong microwave spectrum of Cs (OCS)2 has been observed and assigned.

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Microwave spectra in the 7-26 MHz region have been measured for the van der Waals complexes, Ar-CH3CH2CH3, Ar-(13)CH3CH2CH3, 20Ne-CH3CH2CH3, and 22Ne-CH3CH2CH3. Both a- and c-type transitions are observed for the Ar-propane complex. The c-type transitions are much stronger indicating that the small dipole moment of the propane (0.0848 D) is aligned perpendicular to the van der Waals bond axis. While the 42 transition lines observed for the primary argon complex are well fitted to a semirigid rotor Hamiltonian, the neon complexes exhibit splittings in the rotational transitions which we attribute to an internal rotation of the propane around its a inertial axis. Only c-type transitions are observed for both neon complexes, and these are found to occur between the tunneling states, indicating that internal motion involves an inversion of the dipole moment of the propane. The difference in energy between the two tunneling states within the ground vibrational state is 48.52 MHz for 20Ne-CH3CH2CH3 and 42.09 MHz for 22Ne-CH3CH2CH3. The Kraitchman substitution coordinates of the complexes show that the rare gas is oriented above the plane of the propane carbons, but shifted away from the methylene carbon, more so in Ne propane than in Ar propane. The distance between the rare gas atom and the center of mass of the propane, Rcm, is 3.823 A for Ar-propane and 3.696 A for Ne-propane. Ab initio calculations are done to map out segments of the intermolecular potential. The global minimum has the rare gas almost directly above the center of mass of the propane, and there are three local minima with the rare gas in the plane of the carbon atoms. Barriers between the minima are also calculated and support the experimental results which suggest that the tunneling path involves a rotation of the propane subunit. The path with the lowest effective barrier is through a C2v symmetric configuration in which the methyl groups are oriented toward the rare gas. Calculating the potential curve for this one-dimensional model and then calculating the energy levels for this potential roughly reproduces the spectral splittings in Ne-propane and explains the lack of splittings in Ar-propane.

Eight isotopologues of HGeBr and nine of DGeBr have been studied in natural abundance by pulsed-jet Fourier transform microwave spectroscopy. The reactive germylene species were produced in an electric discharge at the exit of a pulsed molecular beam valve using precursor mixtures of H(3)GeBr or D(3)GeBr in high pressure neon. In the 5-25 GHz operating range of the spectrometer, only a-type transitions were observed; K = 0 transitions for HGeBr and K = 0 and 1 transitions for DGeBr. From the observed transitions, an improved molecular geometry has been determined and nuclear quadruple constants for Ge and Br have been determined. The Townes-Dailey model has been extended to obtain the electron densities of the 4p orbitals on the germanium and bromine atoms from the quadruple coupling constants. These results are discussed in terms of qualitative molecular orbital theory.