Arsenate and Arsenite Reaction Kinetics with Ferric Hydroxides Using Quantum Chemical Calculations

Received: 01.10.2018. Accepted: 25.10.2018. Published: 31.10.2018.The knowledge of the mechanism involved in the process of adsorption and desorption of arsenate and arsenite with ferric hydroxides is important to address the water toxicity problems and to tackle the adverse effect of these substances in nature. An essential outcome of previous studies on the kinetics of the arsenate adsorption on aluminum and iron oxide was that the adsorption is a two-phase (bi-phase) process. Quantum mechanical calculations using density functional theory were used to determine the thermodynamic variables governing the adsorption process to get an insight into the stability of the complexes formed. The previous investigation showed that the positively charged ferric hydroxide cluster had better stability at neutral pH. The chemisorbed charged monodentate complexes had Gibbs free energy of reaction –55.97 kcal/mol where the bidentate complex formation had Gibbs free energy of reaction –62.55 kcal/mol. The bidentate complex having a negative charge had more Gibbs free energy of reaction compared to uncharged one. The results of the study indicate that Gibbs free energy for the reaction has a significant role in controlling the kinetics of the adsorption and sorption process of arsenate on ferric hydroxide clusters.K. Giri acknowledges financial support from UGC, Govt. of India for Start-up Project Funding. S. Santra and G. V. Zyryanov thank Russian Science Foundation (Ref # 18‑73‑00301) for financial help

Effect of Alkylation on the Kinetic Stability of Arsenodiester and Organoarsenicals against Hydrolysis: A Theoretical Study

Received: 14.06.2018. Accepted: 27.07.2018. Published: 30.07.2018.Arsenic diesters have same structural and chemical properties as Pi (phosphate) diester. Beside this structural similarity, arsenate is not considered by cellular processes to replace phosphate. Quantum calculation reveals that this happens due to very high hydrolysis rate of Asi diester (As–O-bond-based compounds) as compared to Pi, but how organoarsenicals (As–C-bond-based compounds) that are produced by alkylation of Asi survive in highly aqueous tissues of marine organisms? We found that this alkylation results in lower hydrolysis rate of Asi diester. Our work concluded that alkylating effort by our body on Asi is to avoid structural ambiguity with phosphate and excrete out arsenic in the form of organoarsenicals from body

Quantum dynamics of (H<SUP>−</SUP>, HD) collisions at low energies

Time-independent quantum mechanical results are reported for H&#8722;+HD (v=1, j=0) rearrangement reaction at low energies, for a range of total angular momentum values. Initial state selected reaction probability and integral reaction cross section values, when plotted as a function of relative translational energy reveal a number of prominent resonances arising from the quasibound states of the H&#8722;...HD van der Waals complex. It is shown that the nonreactive vibrational quenching dominates over both the exchange reactions and that H2 formation is more favourable than HD

irc_movies

Movie files showing IRC calculations for all the computed compounds

Supplementary Geometries

Optimized Geometries of all reactants and TS structures

Data from: Effect of arsenate substitution on phosphate repository of cell: a computational study

The structural analogy with phosphate derives arsenate into various metabolic processes associated with phosphate inside the organisms. But it is difficult to evaluate the effect of arsenate substitution on the stability of individual biological phosphate species, which span from a simpler monoester form like pyrophosphate to a more complex phosphodiester variant like DNA. In this study, we have classified the physiological phosphate esters into three different classes on the basis of their structural differences.This classification has helped us to present a concise theoretical study on the kinetic stability of phosphate analogue species of arsenate against hydrolysis. All the calculations have been carried out using QM/MM methods of our Own N-layer Integrated molecular Orbital molecular Mechanics. For quantum mechanical region we have used M06-2X density functional with 6-31+G(2d,2p) basis set and for molecular mechanics region AMBER force field. The calculated rate constants for hydrolysis show that none of the phosphate analogue species of arsenate has a reasonable stability against hydrolysis

Influence of reagent rotation on (H<SUP>&#8722;</SUP>, D<SUB>2</SUB>) and (D<SUP>&#8722;</SUP>, H<SUB>2</SUB>) collisions: a quantum mechanical study

Time-independent quantum mechanical (TIQM) approach (helicity basis truncated at k=2) has been used for computing differential and integral cross sections for the exchange reaction H&#8722;+D2 (v=0, j=0-4)&#8594;HD+D&#8722; and D&#8722;+H2 (v=0, j=0-3)&#8594;HD+H&#8722; in three dimensions on an accurate ab initio potential energy surface. It is shown that the j-weighted differential reaction cross section values are in good agreement with the experimental results reported by Zimmer and Linder at four different relative translational energies (Etrans=0.55, 0.93, 1.16 and 1.48 eV) for (H&#8722;, D2) and at one relative translational energy (Etrans=0.6 eV) by Haufler et al. for both (H&#8722;, D2) and (D&#8722;, H2) collisions. The j-weighted integral reaction cross section values are in good agreement with the crossed beam measurements by Zimmer and Linder in the Etrans range 0.5-1.5 eV and close to the guided ion beam results by Haufler et al. for (H&#8722;, D2) in the range 0.8-1.2 eV. Time-dependent quantum mechanical (TDQM) results obtained using centrifugal sudden approximation are reported in the form of integral reaction cross section values as a function of Etrans in the range 0.3-3.0 eV for both reactions in three dimensions on the same potential energy surface. The TDQM reaction cross section values decline more sharply than the TIQM results with increase in the initial rotational quantum number (j) for the D2 molecules in their ground vibrational state (v=0) for (H&#8722;, D2) collisions. The computed j-weighted reaction cross section values are in good agreement with the experimental results reported by Zimmer and Linder for (H&#8722;, D2) collisions and guided ion beam results by Haufler et al. for both (H&#8722;, D2) and (D&#8722;, H2) collisions for energies below the threshold for electron detachment channel

Ground and excited states of the monomer and dimer of certain carboxylic acids

The ground-state properties of the monomer and the dimer of formic acid, acetic acid, and benzoic acid have been investigated using Hartree-Fock (HF) and density functional theory (DFT) methods using the 6-311++G(d,p) basis set. Some of the low-lying excited states have been studied using the time-dependent density functional theory (TDDFT) with LDA and B3LYP functionals and also employing complete-active-space-self-consistent-field (CASSCF) and multireference configuration interaction (MRCI) methodologies. DFT calculations predict the ground-state geometries in quantitative agreement with the available experimental results. The computed binding energies for the three carboxylic acid dimers are also in accord with the known thermodynamic data. The TDDFT predicted wavelengths corresponding to the lowest energy n-π∗ transition in formic acid (214 nm) and acetic acid (214 nm) and the π-π∗ transition in benzoic acid (255 nm) are comparable to the experimentally observed absorption maxima. In addition, TDDFT calculations predict qualitatively correctly the blue shift (4-5 nm) in the excitation energy for the π-π∗ transition in going from the monomer to the dimer of formic acid and acetic acid and the red shift (~19 nm) in π-π∗ transition in going from benzoic acid monomer to dimer. This also indicates that the electronic interaction arising from the hydrogen bonds between the monomers is marginal in all three carboxylic acids investigated