The rotational spectra of five isotopic species of the Ar-SiO complex have been observed at high-spectral resolution between 8 and 18 GHz using chirped Fourier transform microwave spectroscopy and a discharge nozzle source; follow-up cavity measurements have extended these measurements to as high as 35 GHz. The spectrum of the normal species is dominated by an intense progression of a-type rotational transitions arising from increasing quanta in the Si-O stretch, in which lines up to v = 12 (~14 500 cm-1) were identified. A structural determination by isotopic substitution and a hyperfine analysis of the Ar-Si17O spectrum both suggest that the complex is a highly fluxional prolate symmetric rotor with a vibrationally averaged structure between T-shaped and collinear in which the oxygen atom lies closer to argon than the silicon atom, much like Ar-CO. To complement the experimental studies, a full dimensional potential and a series of effective vibrationally averaged, two-dimensional potential energy surfaces of Ar + SiO have been computed at the CCSD(T)-F12b/CBS level of theory. The equilibrium structure of Ar-SiO is predicted to be T-shaped with a well depth of 152 cm-1, but the linear geometry is also a minimum, and the potential energy surface has a long, flat channel between 140 and 180°. Because the barrier between the two wells is calculated to be small (of order 5 cm-1) and well below the zero-point energy, the vibrationally averaged wavefunction is delocalized over nearly 100° of angular freedom. For this reason, Ar-SiO should exhibit large amplitude zero-point motion, in which the vibrationally excited states can be viewed as resonances with long lifetimes. Calculations of the rovibrational level pattern agree to within 2% with the transition frequencies of normal and isotopic ground state Ar-SiO, and the putative Ka = ±1 levels for Ar-28SiO, suggesting that the present theoretical treatment well reproduces the salient properties of the intramolecular potential.




The work in Cambridge is supported by NSF Grant Nos. CHE-1058063 and CHE-1566266. R.D. is supported by the US National Science Foundation (No. CHE-1566246).

Keywords and Phrases

Crystal Atomic Structure; Geometry; Ground State; Isotopes; Microwave Spectroscopy; Molecular Physics; Potential Energy; Potential Energy Surfaces; Quantum Chemistry; Rate Constants; Silicon Oxides; Spectral Resolution, Equilibrium Structures; Fourier Transform Microwave Spectroscopy; High Spectral Resolution; Intramolecular Potential; Structural Determination; Theoretical Treatments; Transition Frequencies; Vibrationally Excited State, Silicon Compounds

International Standard Serial Number (ISSN)

0021-9606; 1089-7690

Document Type

Article - Journal

Document Version

Final Version

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