To understand the mechanism of aqueous acid dissociation, which plays a fundamental role in aqueous chemistry, the ionic dissociation of HX acids (X=F, Cl, Br, and I) in water clusters up to hexamer is examined using density functional theory and Møller-Plesset second-order perturbation methods (MP2). Further accurate analysis based on the coupled clusters theory with singles and doubles excitations agrees with the MP2 results. The equilibrium structures, binding energies, electronic properties, stretching frequencies, and rotational constants of HX(H(2)O)(n) and X(-)(H(3)O)(+)(H(2)O)(n-1) are calculated. The dissociated structures of HF and HCl can be formed for n>/=4, while those of HBr and HI can be formed for n>/=3. Among these, the dissociated structures of HX (X=Cl, Br, and I) are more stable than the undissociated ones for n>/=4, while such cases for HF would require much more than six water molecules, in agreement with previous reports. The IR spectra of stable clusters including anharmonic frequencies are predicted to facilitate IR experimental studies. Undissociated systems have X-H stretching modes which are highly redshifted by hydration. Dissociated hydrogen halides show three characteristic OH stretching modes of hydronium moiety, which are redshifted from the OH stretching modes of water molecules.

Interaction optimized basis sets for correlated ab initio calculations on the water dimer J. Chem. Phys. 111, 3812 (1999); 10.1063/1.479684 Ab initio calculation of the interaction potential for the krypton dimer: The use of bond function basis sets Efficient elimination of basis set superposition errors by the local correlation method: Accurate ab initio studies of the water dimer Ab initio calculations with various large basis sets have been performed on the water dimer in order to study the structure, energetics, spectra, and electrical properties. As a reference system, the calculations of the water monomer were also performed. The second order M0ller-Plesset perturbation theory (MP2) using a large basis set (O:13s,8p,4d,2j/H:8s,4p,2d) well reproduces various water monomer experimental data except for the somewhat underestimated absolute energy and hyperpolarizability. The monomer energy calculated with the fourth-order M0ller-Plesset perturbation theory (MP4) with the above basis set is -76.407 hartrees, which is only 0.073 hartree above the experimental energy. To compare the theoretical and experimental dimer structures and thermal energies accurately, we summarized the quantum statistical thermodynamic quantities with corrections for anharmonic vibration, rotation, rotation-vibration coupling, and internal rotation. With the correction for the anharmonic binding potential and rotation, the predicted interoxygen distance of the dimer is 2.958 A, which is so far the closest to the experimental value -2.976 A. The predicted dimer dipole moment is 2.612 D, which is the first agreement with experiment (2.60--2.64 D). The predicted frequency shift of the dimer with respect to the monomer is in good agreement with experiment. With the MP2 calculation using the large basis set, the basis set superposition error correction (BSSEC) of the dimer is only 0.33 kcal/mol, which is by far the smallest among the MP2 results reported. Without BSSEC, the predicted binding energy, enthalpy, free energy, and entropy are all in good agreement with experiment within the error bounds, whereas with BSSEC, some of them seem to be slightly off the experimental error bounds. Nevertheless, the results with BSSEC can be more reliable than those without BSSEC. w is the vibrational angular frequency corresponding to the harmonic component of the potential. We also use the following additional notations:v=liw/kT. A. A weak anharmonic potentialLet us consider a potential of

The global minimum energy structures of the water hexamer predicted by widely used analytic water potentials are very different from each other, while the cyclic hexamer does not appear to be a low-lying energy structure. However, high levels of ab initio calculation predict that a number of low-lying energy conformers including the cyclic conformer are almost all isoenergetic due to the balance of two-body and nonadditive interactions. For modeling of water potentials, we suggest that the binding energy of the dimer be between −5.0 and −4.7 kcal (mol dimer)−1, while the three-body corrections be taken into account to a large extent.

Using extensive ab initio calculations including electron correlation, we have studied structures, thermodynamic quantities, and spectra of hydrated sodium ions [Na(H2O)+n (n=1–6)]. Various configurations were investigated to find the stable structures of the clusters. The vibrational frequency shifts depending on the number of water molecules were investigated along with the frequency characteristics depending on the presence/absence of outer-shell water molecules. The thermodynamic quantities of the stable structures were compared with experimental data available. Entropy-driven structures for n=5 and particularly for n=6 are noted in the calculations, which can explain the peculiar experimental thermal energies. On the other hand, the enthalpy effect to maximize the number of hydrogen bonds of the clusters with the surrounding water molecules seems to be the dominant factor to determine the primary hydration number of Na+ in aqueous solution. The nonadditive interactions in the clusters are found to be large. To have reliable binding energies, up to four-body interactions need to be included. Nevertheless, as a good approximation to calculate the total electron correlation energy, only water–water two-body interactions can be taken into account because other many-body interactions are partially offset.

Polychlorinated dibenzo-p-dioxins (PCDDs) are one of the most intensively studied chemical pollutants. However, the absence of reliable thermodynamic data on PCDDs seriously limits quantitative understanding of their formation and distribution at combustion sources. By carrying out a DFT calculation (B3LYP/6-31G**) for thermodynamic properties of 75 PCDD congeners, we find that previous thermodynamic estimations on the gas-phase enthalpies of formation (∆H f ) and Gibbs free energy of PCDDs were seriously misestimated, particularly for highly chlorinated congeners, largely as a result of underestimating the intramolecular Cl-Cl repulsion energy. The disagreement between the DFT calculation and other methods becomes progressively larger with increasing chlorine substitution. Octachlorodibenzo-p-dioxin (OCDD) that has been considered as the most thermodynamically stable congener in previous calculations turns out to be much less stable. The differences in calculated ∆H f values between OCDD and the most toxic congener, 2,3,7,8-tetrachlorodibenzop-dioxin, or other laterally chlorinated (2,3,7,8-substituted) toxic congeners are within 1 kcal/mol. Most ∆H f values for congeners with five to eight chlorines differ by only 1-2 kcal/mol, since the decreasing electronic energies with increasing the number of chlorines in PCDDs are counterbalanced by increasing Cl-Cl repulsion energy. The intramolecular chlorine repulsion effects in PCDDs are systematically analyzed by using isodesmic reactions.