Structure and Dynamics of the Methane-Propane van der Waals Complex.

Microwave transitions in the region 7-26 GHz were measured for the methane-propane van der Waals complex. The nearly free internal rotation of methane within the complex gives rise to three states that do not relax even in a 5 K supersonic expansion. Eighteen lines have been assigned to the lowest state and are well fitted to a semirigid rotor model, with rotational constants A = 7553.8229 (24) MHz, B = 2483.9200 (8) MHz, and C = 2041.8692 (5) MHz, and six distortion constants. The structure has the methane positioned above the plane defined by the propane carbon atoms with a center-of-mass van der Waals bond distance of 3.98 Å. This is significantly larger than the equilibrium value of 3.71 Å found with ab initio calculations done at the CCSD(T)-F12a/aug-cc-pVTZ level of theory. Further calculations encompassing a large range of angular orientations of the methane subunit indicate that angular motion produces a large zero-point contribution to the energy, which not only lowers the effective barrier to internal rotation of the methane but also increases the radial distance between subunits. Therefore, although in the lowest energy structure the methane can get close to the propane by interdigitating the hydrogens atoms, the zero-point energy effectively flattens out the potential so that the hydrogens become less restricting.

Microwave spectra and structure of the argon-cyclopentanone and neon-cyclopentanone van der Waals complexes.

The rotational spectra of cyclopentanone and its van der Waals complexes with argon and neon have been observed with a Balle-Flygare type pulsed jet Fourier transform microwave spectrometer in the 6 to 20 GHz region. This work improves the rotational constants and quartic centrifugal distortion constants for cyclopentanone and its five (13)C and the (18)O isotopologues. The argon-(12)C5H8(16)O van der Waals complex has rotational constants of A = 2611.6688, B = 1112.30298, and C = 971.31969 MHz. The (20)Ne-(12)C5H8(16)O complex has rotational constants of A = 2728.8120, B = 1736.5882, and C = 1440.4681 MHz. In addition, the five unique, singly substituted (13)C and (18)O isotopologues of the argon complex are reported. The five single-substituted (13)C of the (20)Ne complex and the (22)Ne-(12)C5H8(16)O complex are reported. The rare gases are in van der Waals contact with the carbonyl α carbon and nearly in contact with the hydrogen on ß and γ carbons toward the back of the ring.

Determination of the structure of cyclopentene oxide and the argon-cyclopentene oxide van der Waals complex.

Rotational spectra of cyclopentene oxide and the argon-cyclopentene oxide van der Waals complex were studied using pulsed-jet Fabry-Perot Fourier transform microwave (FTMW) spectroscopy. Spectra of the parent along with those of the (13)C and (18)O singly substituted isotopologues, in natural abundance, of the monomer and of the complex were measured in the frequency region of 5-26.5 GHz. The complete heavy atom substitution structure was determined for the monomer and complex. The boat structure for cyclopentene oxide was confirmed with naturally abundant (13)C and (18)O isotopes. For the argon cyclopentene oxide complex, both a and b-type transitions were observed and the rotational constants for the all-(12)C (16)O isotopologue were determined to be A = 3268.254(2), B = 993.345(1), and C = 950.430(9) MHz. The r(0) coordinates of the argon in the principal axis system of cyclopentene oxide are a = 0.27, b = 0.42, and c = 3.91 A, such that the argon is exo to the boat of the ring and on the opposite side of the ring from the oxygen and is 0.42 A off to the side and 0.27 A from the center of mass toward the back end of the ring (again away from the oxygen). Large amplitude van der Waals bending vibrations require an averaging model to account for differences between the observed complex and monomer planar moments of inertia.

Fourier transform microwave spectroscopy of monobromogermylene (HGeBr and DGeBr), a heavy atom carbene analog.

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.

Microwave spectra and ab initio studies of Ar-propane and Ne-propane complexes: structure and dynamics.

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.

Microwave observation of the "recently found" polar OCS dimer.

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.