Rotational spectra, structures, and dynamics of small Arm–(H2O)n clusters: The Ar2–H2O trimer

Abstract

Rotational spectra have been observed for the Ar2–H2O trimer and several of its isotopomers with the Balle/Flygare pulsed nozzle, Fourier transform microwave spectrometer. Analysis gives a planar T-shaped structure with C2v symmetry and the bidentate protons pointed at the argons. Two sets of asymmetric top transitions were found for the species with H2O, D2O, or H2 18O. Several lines of evidence support assigning them to internal rotor states of the water, the upper set to Σ(000) and the lower to Σ(101) as in the Ar–H2O dimer. Support includes: Hyperfine interaction constants which differ for the two states of water; Systematic aspects of the rotational constants such as B’s that are little affected by isotopic substitution; and MMC calculations which indicate the importance of rovibrational coupling. The Ar–Ar distance for the Σ(000) state of the trimer is estimated to be 3.822 Å and the center of mass (Ar2) to center of mass (H2O) distance to be 3.173 Å. An intriguing result is finding the hyperfine interaction constants of the water in the trimer to be very nearly the same as those reported earlier for the dimer. This shows the two dynamic states of the water are the same in the dimer and trimer, as is the average projection of the water C2 axis onto the inertial frame.