Bridging Theory with Experiment: A Benchmark Study of Thermally Averaged Structural and Effective Spectroscopic Parameters of the Water Molecule

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Abstract
Extending our previous study on the equilibrium structures of the major isotopologues of the water molecule (Császár et al. J. Chem. Phys. 2005, 122, 214305), temperature-dependent averaged structural parameters (for example, rg- and ra-type distances, their related root-mean-square amplitudes, and moments corresponding to the probability distribution functions of interatomic distances), effective rotational constants, and low-order vibration−rotation interaction constants have been determined for two major symmetric isotopologues of water, H216O and D216O. The nuclear motion treatments employed full quantum mechanical variational procedures which utilized the accurate adiabatic semiglobal PESs of water isotopologues named CVRQD (Barletta et al. J. Chem. Phys. 2006, 125, 204307). The temperature-dependent molecular structural parameters are based on expectation value computations and Boltzmann averaging in the temperature range 0−1500 K. The precise computed average internuclear, inverse internuclear, rms amplitude, and anharmonicity parameters could support a future gas electron diffraction (GED) investigation, though water isotopologues are far from being ideal species for GED analyses. Using a clearly defined and general formalism applicable to molecules of any size, we have evaluated vibrationally averaged effective rotational constants as expectation values using inertia tensor formulas in the Eckart frame for vibrational states of H216O and D216O. While such variationally determined rotational constants do not correspond strictly to constants resulting from fits performed by spectroscopists, the expected good agreement is found for the A and B rotational constants for both isotopologues. Low-order vibration−rotation interaction constants, the so-called α- and γ-constants, have also been determined from the computed rotational constants; the latter were derived for the first time.

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