Limited ionicity in poor protic ionic liquids: Association Gibbs energies.

Protic ionic liquids (PILs), made from anhydrous mixtures of Bronsted acids HA and bases B (HA + B → BH+ + A-), occasionally suffer from limited ionicity. In cases of "poor" PILs (<10% ionicity, e.g., using carboxylic acids), past simulations have hinted that ion-pair association, more than incomplete proton transfer, is at fault. To improve upon the Fuoss equation for predicting the degree of ion pairing, new electrostatic equations (including induced dipoles) are presented, for ion-pair and other associations that occur in anhydrous amine/carboxylic acid mixtures. The equations present the association Gibbs energies ΔGA (and thus the association constants KA) as functions of three fundamental properties: the acid/base mixing ratio (n = xA/xB), the HA-to-B proton-transfer strength (ΔpKa,ε=78), and the dielectric constant (relative permittivity) of the mixture (ε). Parameter values were obtained from fits to constant-dielectric quantum chemistry data (obtained and presented here). These ΔGA functions were then used to predict ΔGioniz values for the net ion-generating (autoionization) equilibrium in carboxylic acid/amine mixtures: 2B(HA)n⇄B(HA)n-dHB++A(HA)n+d-1 -, where n = xA/xB and d = degree of disproportionation. The agreement with experiment was excellent, demonstrating that these equations could have useful predictive power.

Kinetics and Mechanism of the Antioxidant Activities of C. olitorius and V. amygdalina by Spectrophotometric and DFT Methods.

The kinetics and mechanism of the antioxidant activities of the methanolic extract of the leaves of two vegetables [Corchorus olitorius (C. olitorius) and Vernonia amygdalina (V. amygdalina)] have been studied using experimental and theoretical approaches. The kinetics (second order and pseudo-first order) of the 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activities of the leaf extracts at varying times (30-90 min) were determined using the UV-visible spectrophotometry method at λmax = 517 nm, whereas the mechanism was studied by density functional theory at two levels of functionals (B3LYP and LC-ωPBE) using bond dissociation enthalpy and adiabatic ionization potential values. Molecular properties such as the highest occupied molecular orbital, lowest unoccupied molecular orbital, electronegativity (χ), electrophilicity (ω), hardness (η), and softness (S) of the predominant phenolic antioxidants were also compared. The second-order kinetics is favored by both plants rather than pseudo-first order; however, V. amygdalina with a second-order rate constant k 2 of 0.0152 (mM)-1 min-1 is faster in scavenging DPPH radicals than C. olitorius with a k 2 value of 0.0093 (mM)-1min-1. Chlorogenic acid and luteolin-7-O-ß-glucuronide, which are the most abundant phenolic acid antioxidant in C. olitorius and V. amygdalina, both preferably scavenge the DPPH radical via a hydrogen atom transfer mechanism. This is evident from their lower bond dissociation enthalpy values than the adiabatic ionization potential values. Successful molecular docking of these phenolic compounds indicates that both compounds form favorable interactions with the therapeutic target, xanthine oxidase.