Microwave spectrum and structure of the lowest A – internal rotor state of Ar⋅⋅⋅CH₃I

The microwave spectrum of the lowest A-internal rotor state of Ar⋅⋅⋅CH₃I has been observed by chirped-pulse and conventional cavity Fourier transform microwave spectroscopy. Using the observed spectroscopic constants, the system is determined to be an asymmetric rotor with an Ar⋅⋅⋅I distance of 4.1941(14) Å an Ar⋅⋅⋅C distance of 3.874(16) Å and an Ar⋅⋅⋅I–C angle of 66.5(4)°. Additionally, MP2 and M06-2X calculations were performed with MIDI! and SDB-aug-cc-PVTZ basis sets on iodine and a cc-PVTZ basis set on all other atoms. The MP2 calculations gave better agreement with the experimentally determined intermolecular separation, while the M06-2X calculations gave a slightly better result for the angular orientation of the CH₃I moiety. MP2 calculations with a 6-311G(d,p) basis set on all atoms were also performed, but were generally not as effective at reproducing the experimentally derived geometry. None of the calculations proved satisfactory for predicting the nuclear quadrupole coupling constants of the iodine. The computed barriers to internal rotation of the methyl group ranged from 8 to 94 cm⁻¹, depending on the method and basis set used.