Spectra of water dimer from a new ab initio potential with flexible monomers

Abstract

Communication: Determination of the bond dissociation energy (D0) of the water dimer, (H2O)2, by velocity map imaging The Journal of Chemical Physics 134, 211101211101 (2011) We report the definition and testing of a new ab initio 12-dimensional potential for the water dimer with flexible monomers. Using our recent accurate CCpol-8s rigid water pair potential [W. Cencek, K. Szalewicz, C. Leforestier, R. van Harrevelt, and A. van der Avoird, Phys. Chem. Chem. Phys. 10, 4716 (2008)] as a reference for the undistorted monomers' geometries, a distortion correction has been added, which was taken from a former flexible-monomer ab initio potential. This correction allows us to retrieve the correct binding energy D e = 21.0 kJ mol −1 , and leads to an equilibrium geometry in close agreement with the one obtained from benchmark calculations. The kinetic energy operator describing the flexible-monomer water dimer has been expressed in terms of Radau coordinates for each monomer and a recent general cluster polyspherical formulation describing their relative motions. Within this formulation, an adiabatic scheme has been invoked in order to decouple fast (intramolecular) modes and slow (intermolecular) ones. Different levels of approximation were tested, which differ in the way in which the residual potential coupling between the intramolecular modes located on different monomers and the dependence of the monomer rotational constants on the dimer geometry are handled. Accurate calculations of the vibration-rotation-tunneling levels of (H 2 O) 2 and (D 2 O) 2 were performed, which show the best agreement with experiments achieved so far for any water potential. Intramolecular excitations of the two monomers were calculated within two limiting cases, to account for the lack of non-adiabatic coupling between intramolecular modes due to the intermolecular motion. In the first model, the excitation was assumed to stay either on the donor or the acceptor molecule, and to hop between the two moieties upon donor-acceptor interchange. In the second model, the excitation remains on the same molecule whatever is the dimer geometry. Marginal frequency differences, less than 2 cm −1 , were obtained for all modes, and the resulting infrared shifts are in good agreement with experiments.

Low Temperature Kinetics of the First Steps of Water Cluster Formation

“…The dissociation energy, D 0 , is a key parameter in this analysis. For the (1,1) case, we employ the experimental D 0 value of 1105 cm −1 [22], within a few cm −1 of high-level theoretical values [23,24]. For the (1,2) case, we employ the theoretical D 0 value of 2726 cm −1 [25], which is in good agreement with the experimental value of 2650±50 cm −1 [26].…”

Rotationally-Resolved Spectroscopy of the Donor Bending Mode of (D2O)2

“…High-resolution spectra of the intramolecular modes provide additional data to support theoretical work on the water dimer, including vibrational frequency shifts and tunneling barriers in the excited vibrational states. Recent potentials have been developed that attempt to include the effect of intramolecular vibrations, 29 but only some of the intramolecular vibrations have been experimentally measured at high resolution. Huang and Miller 21 measured rotationally resolved spectra of the O− H stretching bands of (H 2 O) 2 , and the equivalent O−D stretching bands of (D 2 O) 2 were later measured by Paul et al 22 Low-resolution spectra of the bending modes of (H 2 O) 2 have also been obtained, 30 but up to this point in time, no rotationally resolved spectra of the bending modes of water dimer have been recorded.…”

The water dimer I: Experimental characterization

“…Leforestier et al presented new results using a rigid water pair potential calculated at the coupled cluster level of theory, then adapted into a flexible pair potential. A decoupling of the fast and slow monomeric modes, similar to that used to develop the VRT(MCY-5f) model, was again employed to simplify the calculation [102]. This new model, CCpol-8sf, displays extreme accuracy with respect to VRT energy levels and precisely predicted the binding energy of the water dimer.…”