ROTATIONAL SPECTRUM, INTERNAL ROTATION, AND STRUCTURE OF $Ar-CH_{3}Cl$

$^{1}$ J. M. Steed, L. S. Berstein, T. A. Dixon, K. C. Janda, and W. Klemperer, J. Chem. Phys. 71, 4189 (1979). $^{2}$ R. L. DeLeon and J. S. Muenter, J. Chem. Phys., 75, 1113 (1981). Address of Fraser, Suenram, and Lovas: Molecular Spectroscopy Divsion, National Bureau of Standards, Gaithersburg, MD 20899.Author Institution:Rotational spectra of $Ar-CH_{3}Cl$, for both Cl isotopes, have been observed using a pulsed nozzle Fourier transform microwave spectrometer. Lower resolution (- 10MHz) molecular beam electric resonance spectra of these complexes have been previously reported by Steed $et al.^{1}$ and DeLeon and $Muenter^{2}$. Steed concluded from the observed spectral complexity that $Ar-CH_{3}Cl$ is non-symmetrical with a low barrier to internal rotation of the $CH_{3}Cl$ subunit. DeLeon and Muenter tentatively assigned the spectrum to a rigid rotor. The high resolution spectra that we have obtained allow a definitive assignment of the rotational transitions because of the characteristics Cl nuclear quadrupole hyperfine splittings. The results that we obtain are consistent with the interpretation of Steed. Our assignment for the ground internal rotor state differs from that of DeLeon and Muenter. For the ground internal rotor state the following spectroscopic constants (in MHz) have been obtained: [FIGURE] The large shift in frequency of the $1_{01}-1_{10}$ transition with internal rotor state $(v_{A}=12040.973(4) MHz$ and $\nu_{E}=14977.107(8) MHz$ implies nearly free internal rotation of the $CH_{3}Cl$ subunit. The symmetry axis of the $CH_{3}Cl$ subunit is nearly perpendicular $(-81^{\circ})$ with the line joining the center of mass of the $CH_{3}Cl$ subunit to the Ar. The center of mass seperation is 3.77 A. Centrifugal distortion analysis yields the weak bond stretching force constant and stretching frequency: 0.016(4) mdyn/A and $34(4) cm^{-1}$ respectively