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

摘要

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 CH_4,~(13)CH_4,and CD_4,two sets of transitions with K=Q and one with K=1 were recorded,correlating to the 7 = 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 CH_3D and CHD_3,two K=0 components were recorded,correlating to the j_k=0_0 and 1 rotational levels of free methane (k corresponds to the projection of j onto the C_3 axis of CH_3D and CHD_3).The obtained spectroscopic results were used to derive van der Waals bond distance R,van der Waals stretching frequency v_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 7=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-CH_4 and Ar-CH_4 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-CH_4 was calculated at various configurations,providing a qualitative explanation for the unsuccessful spectral searches for rotational transitions of Ar-CH_4.