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Martin Michalík, Adam Vagánek and Peter Poliak

Abstract

A chemical microsolvation model for solution phase bond dissociation enthalpy (BDE) evaluation by means of molecular dynamics is presented. In this simple model, the primary solvent effect on the BDE values was estimated by placing of five water molecules nearby the studied functional groups evenly. Furthermore, the secondary solvent effect was reflected using the conductor like screening model (COSMO). From the quantum-chemical point of view, the molecular dynamics simulations based on the B3LYP functional in rather small basis set were performed. Despite of the constitutional limitations of the proposed model, the obtained O-H and N-H BDE values in phenol (363 kJ mol-1) and aniline (369 kJ mol-1) are in good agreement with the experimental solution phase data.

Open access

Peter Poliak and Adam Vagánek

Abstract

In this work, C-H bond dissociation enthalpies (BDE) and vertical ionization potentials (IP) for various hydrocarbons and ketones were calculated using four density functional approaches. Calculated BDEs and IPs were correlated with experimental data. The linearity of the corresponding dependences can be considered very good. Comparing two used functionals, B3LYP C-H BDE values are closer to experimental results than PBE0 values for both used basis sets. The 6-31G* basis set employed with both functionals, gives the C-H BDEs closer to the experimental values than the 6-311++G** basis set. Using the obtained linear dependences BDEexp = f (BDEcalc), the experimental values of C-H BDEs for some structurally related compounds can be estimated solely from calculations. As a descriptor of the C-H BDE, the IPs and 13C NMR chemical shifts have been investigated using data obtained from the B3LYP/6-31G* calculations. There is a slight indication of linear correlation between IPs and C-H BDEs in the sets of simple alkanes and alkenes/ cycloalkenes. However, for cycloalkanes and aliphatic carbonyl compounds, no linear correlation was found. In the case of the 13C NMR chemical shifts, the correlation with C-H BDEs can be found for the sets of alkanes and cycloalkanes, but for the other studied molecules, no trends were detected.

Open access

Adam Vagánek, Lenka Rottmannová and Michal Ilčin

Abstract

In this work, the substituent effect on the Se-H bond dissociation enthalpy (BDE) for benzeneselenol and ten para-substituted benzeneselenols was investigated. The set of various electron-donating and electron-withdrawing substituents was used. The gas-phase bond dissociation enthalpies were calculated using B3LYP/6-311++G** method. Obtained trends were compared with those found for para-substituted phenols and thiophenols for the same set of substituents. While the BDE = ƒ(σp) dependences for phenols and thiophenols exhibit very good linearity, for benzeneselenols, the linearity is rather insufficient. It was found for oxygen, sulphur and selenium that the larger the atom is, the weaker the substituent induced changes in corresponding BDE values are. It has been also observed that the larger the atom, the smaller corresponding BDEs.

Open access

Lenka Rottmannová, Adam Vagánek, Ján Rimarčík, Vladimír Lukeš and Erik Klein

S—H Bond Dissociation Enthalpies in para- and meta-Substituted Thiophenols: Correlation with Thiophenolic C—S Bond Length

For mono-substituted anilines, phenols, and thiophenols it has been found that N—H, O—H and S—H bond dissociation enthalpies (BDE) depend on Hammett constants approximately linearly. For substituents placed in meta position, linearity of found dependences is usually considerably worse in comparison to para-substituted molecules. Therefore, their applicability for prediction of changes in BDE using substituent Hammett constant may be limited. In this work, we have found that the length of thiophenolic C—S bond, R(C—S), or its shortening after hydrogen atom abstraction, ΔR(C—S), represent suitable descriptors of substituent induced changes in S—H BDE. For fifteen studied meta-substituted thiophenols, these geometry descriptors correlate with S—H BDEs considerably better than Hammett constants.

Open access

Peter Poliak and Adam Vagánek

Abstract

In this work, the influence of the inter-ring dihedral angles and their deformation on the energetics of diphenylamine molecule and its radical is investigated by the B3LYP/6-311++G** approach. Our approximated bond dissociation enthalpy of diphenylamine is 370.0 kJ mol-1 and it is in good agreement with the recently published experimental data. The potential functions of both the molecule and the radical with respect to the mutual aromatic ring orientations are presented. The potential function for the molecule is of a double-barrier type, whereas the radical possesses a single-barrier function. The calculated total electronic energies are used to approximate the change of the bond dissociation enthalpy with the twisting of the dihedral angle. The dependence of the bond dissociation enthalpy on the dihedral angle is represented by a single-barrier type function. The dependence of the nitrogen atom spin density on the studied dihedral angle is also discussed.

Open access

Peter Škorňa, Adam Vagánek, Peter Poliak and Erik Klein

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.