Electronic structures, vibrational analyses, stabilization energies and amorphicity were analyzed for the most stable configurations of the neutral gas phase clusters of calcium carbonate (CaCO 3 ) n (n = 2-7). Minimum-energy structures were generated through simulated annealing using a mix of molecular dynamics/semiempirical optimization and then full optimization at the ab initio level (RHF/6-31G*). HF-level results are calibrated versus MP2/6-31G* results. Though many clusters are symmetric, the structural arrangements are not crystalline and resemble neither calcite nor aragonite. In general, greater symmetry means a lower stabilization energy. The BSSE-and zero point energy-corrected stabilization energies appear to plateau at about -113 kcal/mol when n = 7. Amorphous clusters start appearing when n = 4 and the lowest-energy minima for n C 5 are of C 1 symmetry. No solvent effect is necessary to induce amorphism.

We studied hydrated calcium oxalate and its ions at the restricted Hartree-Fock RHF/6-31G* level of theory. Performing a configurational search seems to improve the fit of the HF/6-31G* level to experimental data. The first solvation shell of calcium oxalate contains 13 water molecules, while the first solvation shell of oxalate ion is formed by 14 water molecules. The first solvation shell of Ca(II) is formed by six water molecules, while the second shell contains five. At 298.15 K, we estimate the asymptotic limits (infinite dilution) of the total standard enthalpies of hydration for Ca(II), oxalate ion and calcium oxalate as -480.78, -302.78 and -312.73 kcal mol(-1), resp. The dissociation of hydrated calcium oxalate is an endothermic process with an asymptotic limit of +470.84 kcal mol(-1).

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We studied microhydrated calcium thiosulfate and its ions at the restricted Hartree-Fock RHF/6-31G* level of theory. A semiempirical molecular dynamics search of progressively more hydrated species provided lowest-energy configurations that were then fully optimized and characterized as energy minima at the RHF/6-31G* level of theory. The first solvation shell of calcium thiosulfate contains 18 water molecules, while the first solvation shell of thiosulfate ion consists of 15 water molecules. QTAIM calculations show hydrogen bonding to sulfur. At 298.15 K, we estimate the total standard enthalpies of hydration for thiosulfate ion and calcium thiosulfate at infinite dilution as -301 kcal mol(-1) and -335 kcal mol(-1), respectively. The dissociation of hydrated calcium thiosulfate at infinite dilution is predicted to be an endothermic process with an enthalpy of 262 kcal mol(-1). Based on some experimental data, the predominant form of calcium thiosulfate in solution is predicted to be the contact ion pair.

We studied microhydrated calcium fluoride, calcium carbonate and their ions at the MP2/6-311++G** level of theory and found that water–water non-covalent interactions destabilize the solvation shell, and are compensated by cooperative hydrogen bonds.

<p>On-line sample pretreatment (clean-up and analyte preconcentration) was coupled to sequential injection chromatography for the determination of 2, 4-D (2,4-dichlorophenoxyacetic acid) and atrazine in corn samples. Prior to their analysis, microwave-assisted approach was evaluated and optimized for the extraction of 2,4-D and atrazine in corn samples. Extraction was carried out using MeOH:water (30:70 v/v) solvent at 50°C for 15 min. The on-line SPE-HPLC/UV approach combined reversed solid-phase extraction using strata X sorbent with MeOH:H₂O (80:20 v/v) at 1 mL min^-1 as eluent for the enrichment of the analytes. C18 monolithic column with acetonitrile:10 mM acetate buffer pH 4 (20:80, v/v) was employed as mobile phase at 2 mL min^-1 flow rate for separation of the compounds before UV detection. Enrichment factors up to 13.4 were achieved with a 10 mL sample volume. The developed procedure showed linear response ranges from 0.07–0.70 mg kg⁻¹for 2,4-D and atrazine with correlation coefficients &gt;0.993. The LODs were 0.03 and 0.02 mg kg^-1for 2,4-D and atrazine, respectively with RSD ranged from 4.0 to 7.2 % at 0.07 and 0.30 mg kg^-1. The recoveries of 2,4-D and atrazine in corn samples were from 82.6 and 98.2%. The proposed method showed good recoveries and reasonable precision for herbicide analysis in corn samples avoiding the time-consuming batch sample pretreatment step, thus minimizing risks of sample contamination and analyte losses.</p>

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