Yaqian Liu

Microwave and ab initio studies of rare gas-methane van der Waals complexes

Rotational spectra of the weakly bound Kr-methane van der Waals complex were recorded using a pulsed molecular beam Fourier transform microwave spectrometer in the range from 3.5 to 18 GHz. Spectra of 25 isotopomers of Kr-methane were assigned and analyzed. For isotopomers containing CH4, 13CH4, and CD4, two sets of transitions with K = 0 and one with K = 1 were recorded, correlating to the j = 0, 1, and 2 rotational levels of free methane, respectively (j is the rotational angular momentum quantum number of the methane monomer). For isotopomers containing CH3D and CHD3, two K = 0 components were recorded, correlating to the j(k) = 0(0) and 1(1) rotational levels of free methane (k corresponds to the projection of j onto the C3 axis of CH3D and CHD3). The obtained spectroscopic results were used to derive van der Waals bond distance R, van der Waals stretching frequency nu(s), and the corresponding stretching force constant k(s). Nuclear spin statistical weights of individual states were obtained from molecular symmetry group analyses and were compared with the observed relative transition intensities. The tentatively assigned j = 2 transitions were more intense than predicted from symmetry considerations. This is attributed to a relatively large effective dipole moment of this state, supported by ab initio dipole moment calculations. Ab initio potential energy calculations of Kr-CH4 and Ar-CH4 were done at the coupled cluster level of theory, with single and double excitations and perturbative inclusion of triple excitations, using the aug-cc-pVTZ basis set supplemented with bond functions. The theoretical results show that the angular dynamics of the dimer does not change significantly when the binding partner of methane changes from Ar to Kr. The dipole moment of Ar-CH4 was calculated at various configurations, providing a qualitative explanation for the unsuccessful spectral searches for rotational transitions of Ar-CH4.

Microwave investigation of the CO-CH4 van der Waals complex.

Rotational spectra of eight isotopomers of the weakly bound van der Waals complex CO-CH4 were recorded in the frequency range from 4 to 19 GHz using a pulsed molecular beam Fourier transform microwave spectrometer. For the isotopomers containing methane monomers of Td symmetry, namely, 12C16O-12CH4, 12C16O-13CH4, 12C16O-12CD4, 13C16O-12CH4, and 13C18O-12CH4, three rotational progressions were observed that correlate to the jm=0, 1, and 2 rotational levels of free methane. For those containing partially deuterated methane monomers with C3V symmetry, namely, 12C16O-12CH3D and 12C16O-12CHD3, only two progressions were recorded, correlating to the jk=0(0) and 1(1) rotational levels of free CH3D and CHD3, respectively. The van der Waals bond distance R, intermolecular stretching frequency nus, and the corresponding stretching force constant ks were derived from the obtained spectroscopic results. The results obtained for the jm=0 ground state are compared to the previous infrared and millimeter wave data. A 17O nuclear quadrupole coupling constant was determined from the resolved hyperfine structure of 13C17O-12CH4 and was used to obtain angular information about the carbon monoxide subunit. A Coriolis interaction was deduced from the irregular spectral pattern involving levels with jm=1. Qualitative information about the extent of the perturbation was obtained from a comparison of spectroscopic constants of different isotopomers.

Rotational spectra and internal dynamics of Ne—H2S

Rotational spectra of 15 isotopomers of the Ne-H2S van der Waals complex were measured in the frequency range 4–22 GHz using a pulsed molecular beam Fourier transform spectrometer. Two K = 0 progressions were observed for each of the symmetric isotopomers (with H2S or D2S). This doubling is attributed to an internal rotation motion of the H2S subunit within the complex. These two states can be correlated with the 000 and 101 rotational states of free H2S and D2S. By contrast, symmetry constraints no longer apply to isotopomers with DHS. The excited internal rotor state is no longer metastable, and only one K = 0 progression could be observed. The rotational constants obtained were compared with those of Ar-H2S and Ar—H2O. The ground state rotational constant remained almost constant upon substitution of H with D, showing an unusual isotope effect, similarly to a previous observation in Ar-H2S (GUTOWSKY, H. S., EMILSSON, T., and ARUNAN, E., 1997, J. chem. Phys., 106, 5309). This behaviour is in agreement with the ab initio study by OLIVEIRA, G. D., and DYKSTRA, C. E., 1999, J. chem. Phys., 110, 289. An approximate substitution analysis was carried out to deduce structural information from the ground state rotational constants. Nuclear quadrupole hyperfine structures were observed and resolved or partially resolved for isotopomers containing 33S and D, respectively, and the corresponding nuclear quadrupole coupling constants were determined. These were used to derive information about the internal dynamics of the dimer. Different sensitivities of the quadrupole coupling constants of D and 33S to the extent of out-of-plane motion were revealed.

Microwave and ab initio studies of the internal rotation of ethylene in the Ar-ethylene and Ne-ethylene van der Waals complexes

Rotational spectra of the weakly bound complexes Ar-ethylene and Ne-ethylene were measured with a pulsed molecular beam Fourier transform microwave spectrometer in the range from 3.5 to 26 GHz. Spectra of five isotopomers of Ar-ethylene, namely Ar–C2H4, Ar–13C2H4, Ar–C2D4, Ar-trans-1,2-C2D2H2, and Ar-cis-1,2-C2D2H2, and of eight isotopomers of Ne-ethylene, namely 20Ne–C2H4, 20Ne–C2D4, 20Ne-trans-1,2-C2D2H2, 20Ne-cis-1,2-C2D2H2, 22Ne–C2H4, 22Ne–C2D4, 22Ne-trans-1,2-C2D2H2, and 22Ne-cis-1,2-C2D2H2, were assigned and analyzed. The spectra are in accord with T-shaped, planar structures, where the rage gas atoms are located on the b-principal inertial axis of the ethylene monomer. For isotopomers containing C2H4, 13C2H4, C2D4, and trans-1,2-C2D2H2, all observed transitions are doubled due to an internal rotation motion of the ethylene subunit within the complexes. The observed transition intensities are in agreement with nuclear spin statistical weights obtained from molecular symmetry group analyses under the...

Microwave spectroscopic investigation of the Ne–acetylene van der Waals dimer

Rotational spectra of eight isotopomers of the Ne–acetylene van der Waals dimer were measured in the frequency range of 5 to 23 GHz, using a pulsed molecular beam Fourier transform microwave spectrometer. Transitions within both the Σ0 ground state and the Σ1 first excited van der Waals bending state were measured for the isotopomers containing HCCH and H13C13CH, respectively. For the Ne–DCCD isotopomers, Σ0 and Π1 transitions were recorded, indicating a reversal of the energy level ordering upon deuterium substitution. The observed transitions indicate a Coriolis perturbation of the Σ1 and Π1 energy levels. The Σ1/Π1 state is no longer metastable for the Ne–DCCH isotopomers, so only ground state transitions were recorded. Nuclear quadrupole hyperfine structures were observed and partially resolved for isotopomers containing deuterium nuclei. The measured transition frequencies were compared to the values obtained by Bemish et al. from an ab initio potential energy surface using nuclear dynamical calculations. [R. J. Bemish, R. E. Miller, R. Moszynski, T. G. Heijmen, T. Korona, P. E. S. Wormer and A. van der Avoird, J. Chem. Phys., 1998, 109, 8968].