Abstract:An ab initio quantum-mechanical charge-field molecular-dynamics (QMCF-MD) simulation of the chromate ion in aqueous solution at ambient temperature was performed to study the structure and dynamics of this ion and its hydration shell. In contrast to conventional quantum-mechanical molecular-mechanics molecular-dynamics (QM/MM-MD) simulations, the QMCF-MD approach offers the possibility of investigating composite systems with the accuracy of a QM/MM method but without the time-consuming construction of solute-s… Show more
“…We believe that changes in electron density around the chromate ion act to weaken the interpolyhedral hydrogen-bond network (Figure 2), and thereby cause an additional inflation of the MgCrO 4 ·11H 2 O unit cell. Ab initio calculations reveal differences in the ability of various tetrahedral anions, including CrO 4 2− and SO 4 2− , to influence the structure of surrounding aqueous networks (Hinteregger et al , 2010), and the measurement of neutron single-crystal diffraction data from both MgCrO 4 heptahydrate and undecahydrate is desirable to characterize the details of any structural changes, particularly in the hydrogen-bond network. Figure 2.(Color online) The structure of MgCrO 4 ·11H 2 O, illustrating the interpolyhedral network of hydrogen-bonded water molecules.…”
A new hydrate of magnesium chromate is synthesized by quenching aqueous solutions of MgCrO4 in liquid nitrogen. MgCrO4·11H2O is isostructural with the rare mineral meridianiite (MgSO4·11H2O) being triclinic, $P{\bar 1}$, Z = 2, with unit-cell parameters a = 6.811 33(8) Å, b = 6.958 39(9) Å, c = 17.3850(2) Å, α = 87.920(1)°, β = 89.480(1)°, γ = 62.772(1)°, and V = 732.17(1) Å3 at −15 °C. The difference in unit-cell parameters between SO4- and CrO4-bearing species is only partially accounted for by the difference in S–O and Cr–O bond lengths; the remainder of the difference (over 90% in the cell volume) is attributed to weakening of the interpolyhedral hydrogen-bond network.
“…We believe that changes in electron density around the chromate ion act to weaken the interpolyhedral hydrogen-bond network (Figure 2), and thereby cause an additional inflation of the MgCrO 4 ·11H 2 O unit cell. Ab initio calculations reveal differences in the ability of various tetrahedral anions, including CrO 4 2− and SO 4 2− , to influence the structure of surrounding aqueous networks (Hinteregger et al , 2010), and the measurement of neutron single-crystal diffraction data from both MgCrO 4 heptahydrate and undecahydrate is desirable to characterize the details of any structural changes, particularly in the hydrogen-bond network. Figure 2.(Color online) The structure of MgCrO 4 ·11H 2 O, illustrating the interpolyhedral network of hydrogen-bonded water molecules.…”
A new hydrate of magnesium chromate is synthesized by quenching aqueous solutions of MgCrO4 in liquid nitrogen. MgCrO4·11H2O is isostructural with the rare mineral meridianiite (MgSO4·11H2O) being triclinic, $P{\bar 1}$, Z = 2, with unit-cell parameters a = 6.811 33(8) Å, b = 6.958 39(9) Å, c = 17.3850(2) Å, α = 87.920(1)°, β = 89.480(1)°, γ = 62.772(1)°, and V = 732.17(1) Å3 at −15 °C. The difference in unit-cell parameters between SO4- and CrO4-bearing species is only partially accounted for by the difference in S–O and Cr–O bond lengths; the remainder of the difference (over 90% in the cell volume) is attributed to weakening of the interpolyhedral hydrogen-bond network.
“…[4][5][6] On the other hand, the number of such studies of anions is scarce, and among the oxo anions studied with both simulations and experimental structure determination in aqueous solution includes only sulfate, selenate, sulfite, thiosulfate and chromate. [7][8][9][10][11][12] An experimental difficulty with hydrated anions is that the hydrogen bonds to the hydrating water molecules are relatively weak, and the exchange rates are normally faster than allowed to be studied with the experimental methods available today. Theoretical simulation is therefore an option to obtain information about water exchange mechanism and dynamics, as well as structure, of hydrated anions in aqueous solution.…”
Theoretical ab initio quantum mechanical charge field molecular dynamics (QMCF MD) has been applied in conjunction with experimental large angle X-ray scattering (LAXS) and EXAFS measurements to study structure and dynamics of the hydrated oxo chloro anions chlorite, ClO2−, chlorate, ClO3−, and perchlorate, ClO4−. In addition, the structures of the hydrated hypochlorite, ClO−, bromate, BrO3−, iodate, IO3− and metaperiodate, IO4−, ions have been determined in aqueous solution by means of LAXS. The structures of the bromate, metaperiodate, and orthoperiodate, H2IO63−, ions have been determined by EXAFS as solid sodium salts and in aqueous solution as well. The results show clearly that the only form of periodate present in aqueous solution is metaperiodate. The Cl-O bond distances in the hydrated oxo chloro anions as determined by LAXS and obtained in the QMCF MD simulations are in excellent agreement, being 0.01–0.02 Å longer than in solid anhydrous salts due to hydration through hydrogen bonding to water molecules. The oxo halo anions, all with unit negative charge, have low charge density making them typical structure breakers, thus the hydrogen bonds formed to the hydrating water molecules are weaker and more short-lived than those between water molecules in pure water. The water exchange mechanism of the oxo chloro anions resembles those of the oxo sulfur anions with a direct exchange at the oxygen atoms for perchlorate and sulfate. The water exchange rate for the perchlorate ion is significantly faster, τ0.5=1.4 ps, compared to the hydrated sulfate ion and pure water, τ0.5=2.6 and 1.7 ps, respectively. The angular radial distribution functions show that the chlorate and sulfite ions have a more complex water exchange mechanism. As the chlorite and chlorate ions are more weakly hydrated than the sulfite ion the spatial occupancy is less well-defined and it is not possible to follow any well-defined migration pattern as it is difficult to distinguish between hydrating water molecules and bulk water in the region close to the ions.
“…It is commonly assumed that chromate ions cannot penetrate an epoxy matrix due to size exclusion. For an epoxy system similar to the one used in this investigation, the free volume pore radius is reported to be 0.29 nm 17,18 , the radius of the anhydrous CrO 4 2− anion is 0.229 nm 30 , and the radius of the hydrated anion is calculated as 0.525 nm 31 . Based solely on size, penetration of hydrated chromate via polymer-free volume indeed does not seem possible, however, the migration of ions in polymers depends on the ionic interaction and is not fully understood 32 .…”
Transport of active species (i.e., ions) leaching from pigment particles incorporated in a polymer matrix is the main mechanism behind the anticorrosive performance of protective coatings. Understanding this mechanism is necessary for the effective design of the systems utilizing pigments less toxic than the most efficient chromate salts. It was demonstrated that anticorrosive pigment particles can themselves facilitate the transport of active species via the pathways formed after pigment leaching from a coating. It was also suggested that other paint components, e.g., certain additives, pigments, and fillers can be involved in the formation of transport pathways. Investigation of the possible influence of inert pigment (TiO2) on creating the pathways for chromate ion transport in polymer coatings was the primary objective of this work. In an experiment mimicking the transport of pigment species (i.e., chromate ions), a model epoxy coating containing particles of a single pigment (TiO2) was exposed to a chromate solution (aqueous, or with the addition of acetone as a polymer swelling agent). It was shown that the chromate ions can be transported in the epoxy film preferentially via the TiO2 particles/polymer matrix interface.
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