Theoretical study of the OH−(H2O)2 system: Nature and importance of three-body interactions

Abstract

The nature and importance of nonadditive three-body interactions in the ionic OH−(H2O)2 cluster have been studied by supermolecule Mo/ller–Plesset (MP) perturbation theory and coupled-cluster method, and by symmetry-adapted perturbation theory (SAPT). The convergence of the SAPT expansion was tested by comparison with the results obtained from the supermolecule Mo/ller–Plesset perturbation theory calculations through the fourth order (MP2, MP3, MP4SDQ, MP4), and the coupled-cluster calculations including single, double, and approximate triple excitations [CCSD(T)]. It is shown that the SAPT results reproduce the converged CCSD(T) results within 10%. The SAPT method has been used to analyze the three-body interactions in the clusters OH−(H2O)n, n=2,3,4,10, with water molecules located either in the first or the second solvation shell. It is shown that at the Hartree–Fock level the induction nonadditivity is dominant, but it is partly quenched by the Heitler–London and exchange-induction/deformation terms. This implies that the induction energy alone is not a reliable approximation to the Hartree–Fock nonadditive energy. At the correlated level, the most important contributions come from the induction-dispersion and the MP2 exchange energies. The exchange-dispersion and dispersion nonadditivities are much smaller, and for some geometries even negligible. This suggests that it will be difficult to approximate the three-body potential for OH−(H2O)2 by a simple analytical expression. The three-body energy represents only 4%–7% of the pair CCSD(T) intermolecular energy for the OH−(H2O)2 cluster, but can reach as much as 18% for OH−(H2O)4. Particular attention has been paid to the effect of the relaxation of the geometry of the subsystems.

Hydration shell structure of the OH−(H2O)n=1–15 clusters from a model potential energy function

“…Those of Wei et al 36 correspond to the DFT/PLAP3/Sadlej-basis computational scheme, those of Turki et al 35 correspond to MP2 calculations with constrained intramolecular geometries, those of Tuñón et al 33 to the HF/6-31ϩG* level, and finally those by Grimm et al 37 correspond to the MP2/DZP͑s,p͒ scheme.…”

“…Namely, a significant elongation of the proton-donor to the ion molecular bond has been calculated. 35 Low energy structures are displayed in Fig. 7 for the n ϭ2 and 3 clusters and in Fig.…”

“…In the following, low energy structures for the nϭ2 -4 clusters generated through quenching of various molecular dynamics trajectories at periodic time intervals are compared to ab initio conformations [32][33][34][35][36][37] as they appear in the literature. One thing we should keep in mind is that it is not possible to make a direct comparison for the following reasons.…”

“…There are several theoretical works, [30][31][32][33][34][35][36][37] at different levels of approximation, which have investigated the energetics and structures of low energy conformations of the OH Ϫ ͑H 2 O͒ n clusters. Most of the work is devoted to the examination of structural and thermodynamic properties, like binding energies and enthalpies of the first three nϭ1 -3 clusters.…”

Hydrated copper and gold monovalent cations: Ab initio study

“…1-3 Hydration chemistry of monovalent systems including alkali metal ions has been intensively studied experimentally [4][5][6][7][8][9][10][11][12] and theoretically. [13][14][15][16][17][18][19][20][21][22] Their characteristic coordination reflects the intrinsic properties of ions.…”