Density Functional Theory Studies on the Structures and Water-Exchange Reactions of Aqueous Al(III)–Oxalate Complexes

The structures and water-exchange reactions of aqueous aluminum-oxalate complexes are investigated using density functional theory. The present work includes (1) The structures of Al(C2O4)(H2O)4+ and Al(C2O4)2(H2O)2– were optimized at the level of B3LYP/6-311+G(d,p). The geometries obtained suggest that the AlOH2 bond lengths trans to C2O42‑ ligand in Al(C2O4)(H2O)4+ are much longer than the AlOH2 bond lengths cis to C2O42‑. For Al(C2O4)2(H2O)2–, the close energies between cis and trans isomers imply the coexistence in aqueous solution. The 27Al NMR and 13C NMR chemical shifts computed with the consideration of sufficient solvent effect using HF GIAO method and 6-311+G(d,p) basis set are in agreement with the experimental values available, indicating the appropriateness of the applied models; (2) The water-exchange reactions of Al(III)–oxalate complexes were simulated at the same computational level. The results show that water exchange proceeds via dissociative pathway and the activation energy barriers are sensitive to the solvent effect. The energy barriers obtained indicate that the coordinated H2O cis to C2O42‑ in Al(C2O4)(H2O)4+ is more labile than trans H2O. The water-exchange rate constants (kex) of trans- and cis-Al(C2O4)2(H2O)2– were estimated by four methods and their respective characteristics were explored; (3) The significance of the study on the aqueous aluminum-oxalate complexes to environmental chemistry is discussed. The influences of ubiquitous organic ligands in environment on aluminum chemistry behavior can be elucidated by extending this study to a series of Al(III)–organic system

Theoretical Studies of the Formation Mechanisms, Thermodynamic Stabilities, and Water-Exchange Reactivities of Aluminum-Salicylate Complexes in Aqueous Solution

The formation mechanisms, thermodynamic stabilities, and water-exchange reactivities of 1:1 monomer aluminum–salicylate (Al–salicylate) complexes in acidic aqueous solution are investigated using the density functional theory-quantum chemical cluster model (DFT-CM) method. (1) The formation pathways for possible monodentate and bidentate Al–salicylate configurations are modeled with the gas phase-supermolecule-polarizable continuum model (GP-SM-PCM). It shows that the formation pathways for the Al–salicylate complexes follow the Eigen-Wilkins mechanism, where the dissociation of an inner-shell coordinated water of Al³⁺ is the rate-determining step. (2) The formation constants <i>K</i>_aq for different Al–salicylate configurations are estimated based on the total Gibbs free energy changes Δ<i>G</i>° for their overall formation pathways. It is indicated that in the acidic aqueous solution at pH ∼ 3, the main existence form of the 1:1 monomer Al–salicylate complex is the phenol-deprotonated bidentate Al­(Sal)­(H₂O)₄⁺ with six-membered ring. Its log <i>K</i>_aq is calculated as 13.8, in good agreement with the literature values of 12.9–14.5. (3) The water-exchange reactions are modeled for different Al–salicylate configurations. The water-exchange rate constant for Al­(Sal)­(H₂O)₄⁺ is estimated as log <i>k</i>_H2O = 3.9 s^–1, close to the experimental value of 3.7 s^–1. It proves again that this configuration is the dominant form under experimental conditions

Theoretical Investigation of Water Exchange on the Nanometer-Sized Polyoxocation AlO₄Al₁₂(OH)₂₄(H₂O)₁₂⁷⁺ (Keggin-Al₁₃) in Aqueous Solution

Theoretical Investigation of Water Exchange on the Nanometer-Sized Polyoxocation AlO4Al12(OH)24(H2O)127+ (Keggin-Al13) in Aqueous Solutio

Density Functional Theory Study on Aqueous Aluminum−Fluoride Complexes: Exploration of the Intrinsic Relationship between Water-Exchange Rate Constants and Structural Parameters for Monomer Aluminum Complexes

Density functional theory (DFT) calculation is carried out to investigate the structures, 19F and 27Al NMR chemical shifts of aqueous Al−F complexes and their water-exchange reactions. The following investigations are performed in this paper: (1) the microscopic properties of typical aqueous Al−F complexes are obtained at the level of B3LYP/6-311+G**. AlOH2 bond lengths increase with F− replacing inner-sphere H2O progressively, indicating labilizing effect of F− ligand. The Al−OH2 distance trans to fluoride is longer than other AlOH2 distance, accounting for trans effect of F− ligand. 19F and 27Al NMR chemical shifts are calculated using GIAO method at the HF/6-311+G** level relative to F(H2O)6− and Al(H2O)63+ references, respectively. The results are consistent with available experimental values; (2) the dissociative (D) activated mechanism is observed by modeling water-exchange reaction for [Al(H2O)6-iFi](3−i)+ (i = 1−4). The activation energy barriers are found to decrease with increasing F− substitution, which is in line with experimental rate constants (kex). The log kex of AlF3(H2O)30 and AlF4(H2O)2− are predicted by three ways. The results indicate that the correlation between log kex and AlO bond length as well as the given transmission coefficient allows experimental rate constants to be predicted, whereas the correlation between log kex and activation free energy is poor; (3) the environmental significance of this work is elucidated by the extension toward three fields, that is, polyaluminum system, monomer Al-organic system and other metal ions system with high charge-to-radius ratio

Electrochemical Studies of Guanosine in DMF and Detection of Its Radical Cation in a Scanning Electrochemical Microscopy Nanogap Experiment

This communication reports the findings of the investigation of the electrochemical (EC) oxidation of the important bimolecular guanosine (Gs) by scanning electrochemical microscopy (SECM) using carbon fiber ultramicroelectrodes (CF-UMEs) as the probe and substrate. The first attempt is to try to gain a steady-state voltammogram for EC oxidation of Gs at the CF-UME probe in aqueous buffer solutions with three different pH values. Experimental results indicate that due to serious adsorption of Gs on the CF-UME surface, an “S-shaped” steady-state voltammetric curve, which is required for SECM studies, cannot be obtained in aqueous solutions. To solve this adsorption problem, a series of experiments for studying the EC behavior of Gs in DMF are carried out. A well-defined “S-shaped” steady-state cyclic voltammogram (CV) could be achieved at the CF-UME in DMF containing 0.1M TBAPF6 as the supporting electrolyte. By combining several EC techniques, including cyclic voltammetry at glassy carbon (GC) macroelectrode and CF-UMEs, and chronoamperometry, the general chemical characteristics and EC behavior of Gs in DMF solution are studied. Furthermore, SECM detection of Gs•+, the radical cation of Gs electrogenerated in its first oxidation, is carried out by using feedback and tip generation/substrate collection modes in a nanogap configuration. Gs•+ has been electrochemically detected for the first time, with an estimated lifetime of ≤40 μs and E° = 1.55 V versus NHE for the Gs/Gs•+ couple

Density Functional Investigation of the Water Exchange Reaction on the Gibbsite Surface

The water exchange reactions on the gibbsite surface have been investigated by density functional calculations (B3LYP/6-31G(d) level) combining the supermolecular model and PCM model in this paper, and the water exchange rate constants on the gibbsite surface have also been predicted. In the proposed reaction pathways, the clusters Al6(OH)18(H2O)60 and Al6(OH)12(H2O)126+ are used as the models of gibbsite surface and protonated gibbsite surface respectively to examine the effect of protonation of gibbsite surface on the water exchange rate constants. The activation energy barriers ΔEs≠(aq) for Al6(OH)18(H2O)60 and Al6(OH)12(H2O)126+ are 28.6 and 27.2 kJ mol−1, respectively. The reaction energies ΔEs(aq) for Al6(OH)18(H2O)60 and Al6(OH)12(H2O)126+ are 2.9 and 14.4 kJ·mol−1, respectively, indicating that hexacoordinate aluminum in the gibbsite surface is more stable. The log kTST for Al6(OH)18(H2O)60 and Al6(OH)12(H2O)126+ are 6.5 and 7.5 respectively, and the log kex calculated by the given transmission coefficient for Al6(OH)18(H2O)60 and Al6(OH)12(H2O)126+ are 2.4 and 3.4 respectively, indicating that the protonation of gibbsite surface promotes the water exchange reaction of gibbsite surface and accelerates the dissolution rate of gibbsite. The relationship between the calculated free energy and experimental rate constants was explored, and according to this relationship, the log kex for Al6(OH)18(H2O)60 and Al6(OH)12(H2O)126+ are 2.5 and 3.1 respectively, close to the corresponding values calculated by the given transmission coefficient. The water exchange rate constant of gibbsite surface is close to those of K−MAl12(M = Al, Ga, and Ge) polyoxocations, but deviates from that of Al(H2O)63+, implying that the same reactions with similar structure have similar water exchange rate constants