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

Abstract

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.

Mg(II) and Ca(II) Microsolvation by Ammonia: Born–Oppenheimer Molecular Dynamics Studies

“…From the BOMD trajectories, we obtained the vibrational spectra of the clusters employing the same approach as in refs and . The vibrational frequencies are helpful in identifying isomers and the arrangement of solvated ion clusters.…”

“…In fact, at finite temperature several previous studies using the BOMD scheme have shown that the main structural features of the microsolvated species obtained after thermalization have been achieved and are found to be in very good agreement with the experimental data. 15,52,54,55,58 The average Ca−N distance is 2.54 Å for the NH 3 molecules in the first shell, while those in the second shell stayed between 4.1 and 5.3 Å. The ADF (Figure 8) shows that the coordination pattern remains close to octahedral geometry, again being more flexible than the Mg 2+ one.…”

“…Here we present a detailed study of the structural and dynamic properties of [Mg­(NH 3 ) n ] 2+ and [Ca­(NH 3 ) n ] 2+ clusters with n = 2–6, 8, 10, 20, and 27 through Born–Oppenheimer molecular dynamics (BOMD) simulations. We have employed this powerful methodology with great success in the study of the aqueous solvation of Mg 2+ , Ca 2+ , and Pb 2+ cations ,, as well as in the study of the solvation of several types of metallic molecules. − We stress that in these cases nanodroplet cluster models (with around 30 solvent molecules) have quite successfully reproduced the solvation structure observed in the liquid phase.…”

Study of the Elusive Hydration of Pb2+from the Gas Phase to the Liquid Aqueous Solution: Modeling the Hemidirected Solvation with a Polarizable MCDHO Force-Field

“…The theoretical EXAFS (L 3 -edge) spectra were calculated following the work of Merkling et al as previously applied in the study of aqueous solvation of the cations Mg 2+ and Ca 2+ , of the acids As­(OH) 3 and AsO­(OH) 3 , and of the molecules SmI 2 and SmBr 2 . The spectra were obtained as an average of the spectra of 500 equally separated configurations occurring in the simulations.…”

Dynamics of Water in the Solvation Shell of an Iodate Ion: A Born–Oppenheimer Molecular Dynamics Study

“…However, we note that, to the best of our knowledge, it is the only existing study which has investigated some of the dynamical aspects of the solvation shell of an iodate ion in water. There have also been studies on microsolvation and bulk solvation of various other ions and salts − using Born–Oppenheimer molecular dynamics simulations of clusters. These studies have correctly reproduced many structural, energetic and dynamic properties of ions and of neutral molecules dissociating into several ions in solution using about 30–40 water molecules.…”