On the aqueous solvation of AsO(OH)3vs. As(OH)3. Born-Oppenheimer molecular dynamics density functional theory cluster studies.

While arsenous acid, As(OH)3, has been the subject of a plethora of studies due to its worldwide ubiquity and its toxicity, pentavalent As in the form of arsenic acid, AsO(OH)3, has recently been found in rivers in central Mexico as the most abundant naturally occurring arsenic species. To better understand the solvation patterns of both toxic acids at the molecular level, we report the results of Born-Oppenheimer molecular dynamics simulations on the aqueous solvation of the AsO(OH)3 and As(OH)3 molecules at room temperature using the cluster microsolvation approach including 30 water molecules at the B3LYP/6-31G** level of theory. We found that the average per-molecule water binding energy is ca. 1 kcal mol-1 larger for the As(v) species as compared to the As(iii) one. To account for the asymmetry of both molecules, the hydration patterns were studied separately for a "lower" hemisphere, defined by the initially protonated oxygens, and for the opposite "upper" hemisphere. Similar lower hydration patterns were found for both As(iii) and As(v), with the same coordination number CN = 7. The upper pattern for As(iii) was found to be of a hydrophobic type, whereas that for As(v) showed the fourth oxygen to be hydrogen-bonded to the water network, yielding CN = 3.7; moreover, a proton "hopped" from the lower to the upper side, through the Grotthuss mechanism. Theoretical EXAFS spectra were obtained that showed good agreement with experimental data for As(iii) and As(v) in liquid water, albeit with somewhat longer As-O distances due to the level of theory employed. Proton transfer processes were also addressed; we found that the singly deprotonated H2AsO3- species largely dominated (99% of the simulation) for the As(iii) case, and that the deprotonated H2AsO4- and HAsO42- species were almost equally present (45% and 55%, respectively) for the As(v) case, which is in line with the experimental data pKa1 = 2.24 and pKa2 = 6.96. Through vibrational analysis the features of the Eigen and Zundel ions were found in the spectra of the microsolvated As(iii) and As(v) species, in good agreement with experimental data in aqueous solutions.

Aqueous solvation of Mg(ii) and Ca(ii): A Born-Oppenheimer molecular dynamics study of microhydrated gas phase clusters.

The hydration features of [Mg(H2O)n]2+ and [Ca(H2O)n]2+ clusters with n = 3-6, 8, 18, and 27 were studied by means of Born-Oppenheimer molecular dynamics simulations at the B3LYP/6-31+G** level of theory. For both ions, it is energetically more favorable to have all water molecules in the first hydration shell when n ≤ 6, but stable lower coordination average structures with one water molecule not directly interacting with the ion were found for Mg2+ at room temperature, showing signatures of proton transfer events for the smaller cation but not for the larger one. A more rigid octahedral-type structure for Mg2+ than for Ca2+ was observed in all simulations, with no exchange of water molecules to the second hydration shell. Significant thermal effects on the average structure of clusters were found: while static optimizations lead to compact, spherically symmetric hydration geometries, the effects introduced by finite-temperature dynamics yield more prolate configurations. The calculated vibrational spectra are in agreement with infrared spectroscopy results. Previous studies proposed an increase in the coordination number (CN) from six to eight water molecules for [Ca(H2O)n]2+ clusters when n ≥ 12; however, in agreement with recent measurements of binding energies, no transition to a larger CN was found when n > 8. Moreover, the excellent agreement found between the calculated extended X-ray absorption fine structure spectroscopy spectra for the larger cluster and the experimental data of the aqueous solution supports a CN of six for Ca2+.

Aqueous Solvation of SmI2: A Born-Oppenheimer Molecular Dynamics Density Functional Theory Cluster Approach.

We report the results of Born-Oppenheimer molecular dynamics (BOMD) simulations on the aqueous solvation of the SmI2 molecule at room temperature using the cluster microsolvation approach including 32 water molecules. The electronic structure calculations were done using the M062X hybrid exchange-correlation functional in conjunction with the 6-31G** basis sets for oxygen and hydrogen. For the iodine and samarium atoms the Stuttgart-Köln relativistic effective-core potentials were utilized with their associated valence basis sets. Starting from the optimized geometry of SmI2 embeded in the microsolvation environment, we find a swift substitution of the iodine ions by eight tightly bound water molecules around Sm(II). Through the Sm-O radial distribution function and the evolution of the Sm-O distances, the present study predicts a first rigid Sm(II) solvation shell from 2.6 to 3.4 Å, whose integration leads to a coordination number of 8.4 water molecules, and a second softer solvation sphere from 3.5 to ca. 6 Å. The Sm(II)-O radial distribution function is in excellent agreement with that reported for Sr2+ from EXAFS studies, a fact that can be explained because Sr2+ and Sm2+ have almost identical ionic radii (ca. 1.26 Å) and coordination numbers: 8 for Sr2+ and 8.4 for Sm2+. The theoretical EXAFS spectrum was obtained from the BOMD trajectory and is discussed in the light of the experimental spectra for Sm(III). Once microsolvation is achieved, no water exchange events were found to occur around Sm2+, in agreement with the experimental data for Eu2+ (which has a nearly identical charge-to-ionic radius relation as Sm2+), where the mean residence time of a water molecule in [Eu(H2O)8]2+ is known to be ca. 230 ps.

Born-Oppenheimer molecular dynamics studies of Pb(ii) micro hydrated gas phase clusters.

In this work, a theoretical investigation was made to assess the coordination properties of Pb(ii) in [Pb(H2O)n]2+ clusters, with n = 4, 6, 8, 12, and 29, as well as to study proton transfer events, by means of Born-Oppenheimer molecular dynamics simulations at the B3LYP/aug-cc-pVDZ-pp/6-311G level of theory, that were calibrated in comparison with B3LYP/aug-cc-pVDZ-PP/aug-cc-pVDZ calculations. Hemidirected configurations were found in all cases; the radial distribution functions (RDFs) produced well defined first hydration shells (FHSs) for n = 4,6,8, and 12, that resulted in a coordination number CN = 4, whereas a clear-cut FHS was not found for n = 29 because the RDF did not have a vacant region after the first maximum; however, three water molecules remained directly interacting with the Pb ion for the whole simulation, while six others stayed at average distances shorter than 4 Å but dynamically getting closer and farther, thus producing a CN ranging from 6 to 9, depending on the criterion used to define the first hydration shell. In agreement with experimental data and previous calculations, proton transfer events were observed for n≤8 but not for n≥12. For an event to occur, a water molecule in the second hydration shell had to make a single hydrogen bond with a water molecule in the first hydration shell.

Aqueous solvation of HgClOH. Stepwise DFT solvation and Born-Oppenheimer molecular dynamics studies of the HgClOH-(H2O)24 complex.

We address the aqueous solvation of HgClOH through a systematic study of stepwise hydration considering the HgClOH-(H2O)n structures with n = 1-24. After calibration of the DFT method, the electronic calculations have been carried out using the B3PW91 exchange-correlation functional. For n < 5 the main geometrical parameters and incremental binding energies are in agreement with counterpoise-corrected MP2/AVTZ static values and BO-MP2 dynamic averages. For n > 15 three direct water-Hg interactions appear during the hydration process and a pentacoordinated trigonal bipyramid apical pattern around Hg is found. 22 water molecules are needed to build the first solvation shell. Unlike microsolvated HgCl2, no stable equatorial trigonal bipyramid was found. Optimizations with the Polarizable Continuum Model lead to structures with extremely large Hg-O(water) distances because of a dominant solvation effect on the explicit water molecules; however, this overestimation diminishes for large values of n. A DFT Born-Oppenheimer molecular dynamics simulation at T = 700 K revealed the stability of the HgClOH-(H2O)24 complex with an average trigonal bipyramid Hg-coordination pattern, in accordance with the static cluster description. After thermalization is achieved, the exchange rate of the Hg-coordinated water molecules is estimated to be ca. 10(11) s(-1).

Aqueous solvation of Hg(OH)2: energetic and dynamical density functional theory studies of the Hg(OH)2-(H2O)n (n = 1-24) structures.

A systematic study of the hydration of Hg(OH)2 by the stepwise solvation approach is reported. The optimized structures, solvation energies, and incremental free energies of 1-24 water molecules interacting with the solute have been computed at the B3PW91 level using 6-31G(d,p) basis sets for the O and H atoms. The mercury atom was treated with the Stuttgart-Köln relativistic core potential in combination with an extended optimized valence basis set. One to three direct Hg-water interactions appear along the solvation process. The first solvation shell is fully formed with 24 water molecules. A stable pentacoordinated Hg trigonal bipyramid structure appears for n > 15. Density functional theory (DFT) Born-Oppenheimer molecular dynamics simulations showed the thermal stability of the Hg(OH)2-(H2O)24 structure at room temperature and the persistence of the trigonal bipyramid coordination around Hg. The Gibbs free energy for the first solvation shell is significantly larger for the fully solvated Hg(OH)2 than the one previously obtained for the HgCl2 case, due to σ-acceptor and π-donor properties involving the hydroxyl groups of the solute. This suggests that the transmembrane passage of Hg(OH)2 into the cell via simple diffusion is less favorable compared to the case when the metal is coordinated with two Cl groups.

On the stability of Be3: a benchmark complete active space self-consistent field + averaged quadratic coupled cluster study.

The optimized geometries and binding energies for the linear and triangular isomers of the beryllium trimer have been obtained through benchmark multireference averaged quadratic coupled cluster (AQCC) calculations using very large complete active space SCF (CASSCF) references (12 active electrons in 13 and 14 orbitals). Geometries were optimized with the cc-pV5Z basis, while the binding energies (including counterpoise correction) were obtained with the significantly larger aug-cc-pV5Z basis set. The binding energies (27.3 and 16.3 kcal/mol for the equilateral and linear isomers, respectively) are larger than the previous full CI benchmark values, while the corresponding Be-Be equilibrium distances of 4.101 and 4.088 a.u. are smaller. In view of the near-size consistency character of the CASSCF + AQCC method, the fact that all 12 electrons are fully correlated, the active reference space includes 14 orbitals, and the very large basis set used here, we propose to consider these results as reference data for Be(3). Using the electron pair localization function obtained at the CASSCF(12,15) level, it is clearly illustrated that the 2p orbitals lying in the molecular plane play a dominant role in the bonding pattern for the equilateral isomer.

Ab initio study of the spectroscopy of AgBr: a CASSCF + Averaged Coupled Pair Functional approach to the lowest excited states including spin-orbit couplings.

The nine lowest-lying singlet and triplet (X (1)Sigma(+), 2 (1)Sigma(+), 3 (1)Sigma(+), (3)Sigma(+), 1 (3,1)Pi, 2 (3)Pi, and (3,1)Delta) electronic states of AgBr were studied through state-specific Complete Active Space Self-Consistent Field with 16 active electrons in 12 orbitals followed by extensive Averaged Coupled Pair Functional and CIPT2 calculations with large optimized valence basis sets. The spin-orbit effects were included to obtain the Omega fine-structure states arising from the |Lambda S Sigma> parents. Even before the inclusion of the spin-orbit effects, the 2 (1)Sigma(+) and 3 (1)Sigma(+) states present shallow minima near the equilibrium geometry of the ground state. The 2 (1)Sigma(+) state has another minimum around 8.0 a.u. and is attractive up to 20 a.u. The lowest (3,1)Pi states were found to be totally repulsive while the (3,1)Delta states present deep minima around 4.8 a.u. Most of the calculated spectroscopic constants for the ground and B states are slightly improved with respect to the previous theoretical study using the much smaller CASSCF(16,10) reference wave functions [M. Guichemerre et al., Chem. Phys. 280, 71 (2002)]. The observed B<--X transition is confirmed as arising from the singlet-to-singlet 0(+)(2 (1)Sigma(+))<--0(+)(X (1)Sigma(+)) excitation around 31 900 cm(-1). However, at variance with the previous theoretical prediction, the C(Omega=0(+)) state is dominated around the equilibrium geometry of the ground state by the third (1)Sigma(+) state with a small contribution from the 2 (3)Pi state around 43,500 cm(-1); thus the X-C excitation is now explained as arising also from a singlet-to-singlet spin-allowed transition.