Ligand sequential replacement on chromium(III)-aqua complexes by L-alanine and other biological amino acids: a kinetic perspective

The ligand sequential replacement on chromium(III)-aqua complexes by L-alanine in slightly acidic aqueous solutions (pH range: 3.55-5.61) has been kinetically followed by means of UV-Vis spectrophotometry. A two rate constant model has been applied to fit the absorbance-time data, corresponding to the formation (k1) and decay (k2) of an intermediate not reactive enough to be in steady state (long-lived intermediate). The kinetic orders of the amino acid were fractional (0.40 ± 0.03 for k1 and 0.40 ± 0.02 for k2). The two steps showed base catalysis, and the activation energies were 60 ± 3 (for k1) and 83 ± 6 (for k2) kJ mol-1. The rate constants for the coordination of the first L-alanine ligand followed the sequence CrOH2+ < Cr(OH)2+ < Cr(OH)3, Cr3+ being almost inactive. This suggests that the increase in the reaction rate with increasing pH was caused by the enhancement of the lability of the Cr(III)-aqua bonds induced by the presence of hydroxo ligands. The activation parameters for a series of ligand substitution on Cr(III)-aqua complexes by organic molecules yielded a statistically significant enthalpy-entropy linear plot with an isokinetic temperature of 296 ± 21 K

Complexació de crom(III) per L-alanina: un estudi cinètic

Treballs Finals de Grau de Química, Facultat de Química, Universitat de Barcelona, Any: 2018, Tutor: Joaquín F. Pérez de BenitoThe kinetics of the reaction of substitution of aqua ligands in chromium(III) complexes by Lalanine has been studied with the aid of a spectrophotometric technique in aqueous media under slightly acidic conditions (pH 3.55 – 5.61). The process did not follow the usual pseudofirst order pattern, even under a large excess of amino acid with respect to the metal ion. On the contrary, the rate decreased much faster than a pseudo-first order reaction would. A two consecutive reaction model has been applied involving the formation (rate constant k1) and decay (rate constant k2) of a long-lived intermediate. Both rate constants decreased with the initial concentration of Cr(III) (due to the pH decrease) and increased with the initial concentration of organic ligand (fractional orders), whereas an increase of the medium ionic strength resulted in an increase of k1 and a decrease of k2, both steps presenting base catalysis and the corresponding activation energies being 60.2 ± 3.3 and 83.3 ± 5.9 kJ mol-1. The rate constants for the replacement of an aqua ligand by L-alanine at 25.0 ºC followed the sequence 2+ CrOH (1.77 M-1 s-1) < + 2 Cr(OH) (128 M-1 s-1) < 3 Cr(OH) (3.07 104 M-1 s-1). The UV-Vis spectrum of the long-lived intermediate was intermediary between those of the inorganic reactant and the reaction product. The spectra of the final reacting mixtures revealed the coexistence of at least four different complexes in equilibrium. A mechanism in agreement with the available experimental data has been proposed, involving an elementary reaction sequence for each experimental rate constant, and starting both with the breakage of a Cr(III)-aqua chemical bond as a previous (slow) step to the coordination of the organic ligan

Estudi cinètic de la complexació de crom(III) per l’àcid L-glutàmic

Treballs Finals de Grau de Química, Facultat de Química, Universitat de Barcelona, Any: 2017, Tutor: Joaquín F. Pérez de BenitoThe complexation of chromium(III) by L-glutamic acid / hydrogen L-glutamate ion in aqueous media under slightly acidic conditions (pH 2.46 - 5.87) has been studied by the use of UV-Vis spectroscopy. Although the reaction in the presence of a large excess of organic ligand is considered in the chemical literature to be a pseudo-first order kinetic process, the rate-time plots corresponding to the formation of the reaction product show a distinct bell-shaped profile, caused by the accumulation of a long-lived intermediate. Given that this intermediate is not reactive enough for the steady state approximation to apply, an integrated rate law involving a double-exponential function has been used to obtain two rate constants for each kinetic experiment, corresponding to the formation of the long-lived intermediate from the reactants (k1) and to its decay into the reaction products (k2). Both rate constants increased with the initial concentration of hydrogen L-glutamate ion in a double-reciprocal linear relationship, k1 decreased with the ionic strength whereas k2 increased, both evidenced the existence of base catalysis and the corresponding activation energies were 83 ± 10 and 95 ± 5 kJ mol-1, respectively. The spectroscopic data indicated the formation of several complexes, differing in the number of organic ligands per chromium atom and in their acid-base properties. A mechanism in agreement with the available experimental data has been proposed, involving the breakage of a Cr(III)-aqua chemical bond as a previous step to the coordination of the organic ligan

Ab initio molecular dynamics simulations of lanthanide coordination structures in water and in faujasite

Atomic and molecular resolution can provide unique insights into the ambiguous mechanisms by which lanthanum increases the hydrothermal stability of faujasite in cracking catalysts, as well as in cation exchange in faujasite. The structures of the lanthanide aqua ions were resolved with density functional theory calculations and ab initio molecular dynamics (AIMD) simulations within ~0.05 Å of experimental results. Reaction energies were quantified by predicting the first hydrolysis constant of lanthanide aqua ions within ~1.1 pKa units using AIMD with rare event simulation techniques and electronic structure calculations. The capture of structural and reaction trends in the lanthanide aqua ion served as a benchmark for implementing similar methods in lanthanum-exchanged faujasite. In faujasite, AIMD simulation identified the preferred binding site of lanthanum. AIMD with a rare event simulation technique was used to quantity the free energy of faujasite aluminum tetrahedra deprotonation, with and without lanthanum exchanged in faujasite. The presence of lanthanum makes faujasite deprotonation energetically more favorable, thus making faujasite less hydrophilic. The local structure of water confined in faujasite was simulated with AIMD. The model faujasite structure was modified to produce a series of systems to study the influence of confinement, hydrophilicity, and cation exchanged on the local structure of water as quantified from radial distribution functions. Increases in hydrophilicity in hydrogen-exchanged faujasite disrupts the confined water structure. While lanthanum ions compensate for a larger magnitude of charge, equivalent number of sodium ions have a higher probability of occupying sites interacting with the supercage and thus disrupt the local structure of water more significantly than lanthanum