Nearly free internal rotation in Ar–CH3Cl

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Abstract
Rotational spectra of Ar–CH3Cl, for both Cl isotopes, have been observed, at 4 kHz resolution, using a pulsed nozzle Fourier transform microwave spectrometer. The observed spectra are consistent with a T‐shaped complex in which the methyl group is undergoing nearly free internal rotation. Analysis of the ground (A) internal rotor state spectrum for Ar–CH335Cl using an asymmetrical top Hamiltonian produces the following spectroscopic constants (in MHz); A=13 633.020(14), B=1593.5683(79), C=1420.4572(52), ΔJ=0.012 16(14), ΔJK =0.1381(48), δJ =0.000 96(17), HJJK=−0.000 58(25), eQqaa =34.895(30), eQqbb =−72.185(25), and eQqcc =37.290(35). A combined analysis of the ground and excited (E) internal rotor states places an upper bound of 20 cm−1 on the threefold barrier to internal rotation. The Coriolis interactions in the E state also allow the determination of ‖eQqab‖ for Ar–CH335Cl as 13.0(3) MHz. The symmetry axis of the CH3Cl subunit is nearly perpendicular (∼82°) to the line joining the centers of mass of the two binding partners. The isotopic data indicate that the Cl end of the methyl chloride is tilted toward the argon. The distance between the centers of mass of the two subunits is 3.7826 Å for Ar–CH335Cl and 3.7839 Å for Ar–CH337Cl implying an Ar–Cl distance of 3.750 Å. Centrifugal distortion analysis yields a weak bond stretching force constant of 0.0157 mdyn/Å and stretching frequency of 34.6 cm−1 for Ar–CH335Cl. The results from this investigation are related to previous lower resolution microwave studies on Ar–CH3Cl.