Rotational spectra and structures of the Ar2–H35Cl/37Cl trimers

Abstract

Microwave rotational spectra have been observed for both Cl isotopes of the Ar2–HCl trimer with the pulsed nozzle Fourier transform method using the Flygare Mark II spectrometer. The Cl nuclear quadrupole hyperfine structure was analyzed for each of the transitions and the coupling constants and line centers determined. Sixteen transitions were observed in the 2 to 15 GHz region for the 35Cl species and 11 for 37Cl. The line centers were fitted to obtain ground state rotational and quartic centrifugal distortion constants A″, B″, C″, τ1, τ2, τaaaa, τbbbb, and τcccc. For Ar2–H35Cl, the values are 1733.857, 1667.932, 844.491, −0.1170, −0.0292, −0.1199, −0.0802, and −0.0079 MHz, respectively, and for Ar2–H37Cl: 1733.824, 1606.877, 828.497, −0.1121, −0.0279, −0.1205, −0.0737, and −0.0075 MHz. The equilibrium geometry is determined to be T shaped with C2v symmetry and the H end of the HCl closest to the Ar2. Large amplitude slightly anisotropic torsional motion of the HCl is evident from the hyperfine constants. The chlorine isotopic substitution enables the torsional displacement of the Ar2 dimer to be estimated. It is found to be an average of 7.3° with respect to the b axis. With allowance for the torsional effects, a value of 3.861 Å is obtained for the Ar–Ar distance and 4.005 Å for the Ar to Cl distance. An approximate, harmonic force field analysis is based on the centrifugal distortion constants. A comparison is given of the force constants and Ar–Ar distances for the Ar2–HCl and Ar2–H/DF trimers.