Rotational spectrum, structure and modeling of the OCS–CS2 van der Waals dimer

Abstract

The rotational spectrum of a 1 : 1 weakly bound complex between OCS and CS₂ has been measured by Fourier-transform microwave spectroscopy, giving ground state rotational constants of A = 2369.6942(9) MHz, B = 994.4467(6) MHz and C = 700.5137(3) MHz for the normal isotopic species. The experimental dipole moment components are μ_a = 0.2893(4) D and μ_b = 0.6364(27) D, with μ_total = 0.6991(25) D. The rotational constants and dipole moment components are consistent with a structure of C_s symmetry, in which the CS₂ and OCS monomers are aligned almost parallel to one another, with a center of mass separation of 3.8017(2) Å. This structure is in good agreement with the lowest energy geometry obtained from an ab initio calculation at the MP2/6-311++G(2d,2p) level which predicts rotational constants of A = 2322 MHz, B = 1036 MHz and C = 716 MHz and dipole moment components of μ_a = 0.32 D and μ_b = 0.69 D. Semi-empirical modeling using the ORIENT program gives similarly good agreement, although the predicted rotational constants and dipole moment are a little further from the experimental results (A = 2458 MHz, B = 1027 MHz and C = 725 MHz and μ_a = 0.34 D and μ_b = 0.68 D).