Peter Škorňa

Solvation enthalpies of the proton in polar and non-polar solvents: Theoretical study

Abstract In spite of the importance of proton transfer in solution-phase processes, there is still no systematic theoretical study of proton solvation enthalpies. We have investigated the solvation enthalpies of the proton in seven solvents of various polarities (benzene, chloroform, acetone, methanol, ethanol, DMSO, water) using the Integral Equation Formalism Polarized Continuum Model (IEF-PCM). All computations were performed at the B3LYP and BHLYP levels of theory with aug-cc-pVDZ, aug-cc-pVTZ and aug-cc-pVQZ basis sets. Our calculations have shown that the B3LYP and BHLYP functionals provide similar solvation enthalpies. Finally, differences in the solvation enthalpy of the proton values stemming from the various basis sets do not exceed 6 kJ mol-1, with exception of DMSO and chloroform. Distance between H+ and the acceptor atom of the solvent molecule is the shortest in the case of water. It has been also found that the B3LYP distances are slightly longer.

Acidic and alkaline bimolecular hydrolysis of substituted formanilides. Computational analysis and modelling of substitution effects

Abstract In this article, the study of 67 compounds representing various para-, meta- and ortho- substituted formanilides is presented. These molecules and the products of their acidic and alkaline hydrolysis were studied using DFT quantum chemical methods in order to calculate the reaction enthalpies. These enthalpies are correlated with the hydrolysis rate constants, kH, published for the acid-catalysed acyl cleavage bimolecular (AAC2) mechanism and the modified base-catalysed acyl cleavage bimolecular (BAC2) mechanism. The found linear dependences can be used for the prediction of rate constants of non-synthesised formanilide derivatives.

Gallic acid: thermodynamics of the homolytic and heterolytic phenolic O—H bonds splitting-off

Abstract The DFT study of primary antioxidant action of gallic acid and its carboxylic anion is presented in the gas-phase, benzene and water. Corresponding reaction enthalpies for three possible mechanisms was calculated using B3LYP/6-311++G** method. Bond dissociation enthalpy (BDE) and proton dissociation enthalpy (PDE) of 4-OH group was found to be the lowest in gas-phase as well as in both solvents approximated by IEF-PCM model. Ionization potentials (IPs) were higher than BDEs in all cases. Deprotonation of carboxylic group result in increased antioxidant potency as drop in BDE, proton affinities (PAs) and IPs was indicated in all environments.

Theoretical study of the first step of SPLET mechanism: O–H bond cleavage in the mono-substituted benzoic acids

Abstract The first step of SPLET mechanism in solution phase for 15 benzoic acid derivatives was studied from the thermodynamic point of view. For this purpose, proton affinities (PAs) of corresponding carboxylate or phenoxide anions were computed by means of DFT method employing B3LYP and M062X functionals with 6-311++G** basis set and SMD or IEF-PCM solvent model. The substituent effect on PAs was analyzed in terms of Hammett constants, σp. Found dependences exhibit satisfactory linearity and enable quick estimation of solution phase PAs from the Hammett constants.

Theoretical study of the energetics of carboxylic O–H bond cleavage in the para- and meta-substituted benzoic acid derivatives

Abstract Gas phase bond dissociation enthalpies (BDE) of para- and meta-substituted benzoic acids and proton affinities (PA) of their carboxylate anions were investigated using the B3LYP/6-311++G** method for 15 substituents with various electron-donating and electron-withdrawing effects. The employed computational method provided BDE and PA values in fair agreement with experimental data. The substituent effect on BDEs and PAs was analyzed in terms of Hammett constants. Found dependences exhibit satisfactory linearity and enable quick estimation of BDEs and PAs from the Hammett constants.

Theoretical 1H(Se—H) NMR shifts in meta-substituted Ph—XH (X = O, S, Se)

Abstract A systematic comparative theoretical study has been performed on a series of fourteen metasubstituted selenophenols. The optimal geometries were calculated using the density functional theory (DFT) and the Nuclear Magnetic Resonance parameters were computed by applying the Gauge Including Atomic Orbital (GIAO) method. The calculated NMR shifts were correlated with the Hammett constants. The obtained results were also compared with the theoretical data obtained for thiophenols and phenols. Our results indicate the linear dependence between the gas-phase NMR shifts and Hammett constants. However, the presence of large selenium atoms is able to suppress significantly the substituent effect in meta position. Therefore six substituents (Me, OH, MeCO, COOMe, COOEt and CF3 groups) were excluded from the data evaluation. Correlations with the fundamental stretching vibration frequencies of the mode with the dominant Se-H vibration have not been found.