Water Isotopomers Research Articles

First principles computation of thermochemical properties beyond the harmonic approximation. I. Method and application to the water molecule and its isotopomers

The anharmonic potential energy surface of water has been computed ab initio using an augmented coupled cluster method and various basis sets. Whereas the Pople 6–311 G family is manifestly unsatisfactory, Huzinaga–Dunning basis sets perform quite well. The [5s4p2d1f,3s2p] surface reproduces harmonic frequencies and anharmonicity constants to better than about 2 and 1 cm−1, respectively. For quantitative agreement with experiment, both f functions on oxygen and inclusion of core correlation seem to be prerequisite. Comparison with various experimentally derived force fields reveals that the ab initio force field is of comparable quality. From the best computed force field, a set of spectroscopic constants has been derived for all important isotopomers of water. Using a hybrid analytic/direct summation method recently developed by the present authors, the thermodynamic functions gef(T), hcf(T), S0, and Cp are computed including exact account of anharmonicity and rovibrational coupling, and very good analytical approximations to centrifugal distortion and quantum rotation effects. The computed functions substantially revise previous literature results at high temperatures. Differences between thermodynamic functions from various computed force fields are an order of magnitude smaller than these errors. Thermodynamic tables in JANAF style from 100 to 3000 K, as well as a full set of rovibrational spectroscopic constants, are presented. It is concluded that obtaining force fields of near-spectroscopic accuracy, and thermodynamic tables of very high accuracy, is presently feasible for small polyatomic molecules.

Decoupled isotopomer vibrational frequencies in cubic ice: A simple unified view of the Fermi diads of decoupled H2O, HOD, and D2O

The existing infrared spectroscopic data for the isotopomers of water [H2O, D2O, (HOD)2, and HOD] decoupled in cubic ice at 90 K are reviewed and combined with new results to complete the infrared data for the internal vibrational modes. An assignment of the observed absorption bands, including the perturbed Fermi diads for νs in resonance with 2ν2, that largely follows established views is offered. This assignment is shown to be internally self-consistent by the analysis of the Fermi diads within a single framework based on the simplest representation of the effects of Fermi resonance and using the Burneau–Corset value for the Fermi-interaction parameter appropriate to cubic ice (60 cm−1 for H2O). It is shown that the inclusion of the decoupled-HOD diads significantly lowers the estimated value of νs for H2O and, consequently, allows the downshift of this mode to the observed frequency value (3225 cm−1), through resonance with 2ν2 (where ν2=1735 cm−1), to be closely modeled using the relatively small Fermi-interaction parameter of Burneau and Corset.