In this work, we have calculated bond dissociation enthalpies of selected C-H and O-H bonds in sterols using two semi-empirical quantum chemistry methods AM1 and PM3.

The conformational analysis of the para-menthane (PM) and 1,2,4-trihydroxy-para-menthane (TPM) is performed using the quantum chemical density functional theory (DFT) and ab initio Møller-Plesset perturbation theory up to the second order (MP2). In TPM, three hydroxyl groups generate eight stereoisomers comparing to the four para-menthane stereoisomers. From the thermodynamics point of view, the most preferred conformations show the chair-shaped configuration of the cyclohexane ring. The obtained energy barriers for the isopropyl group rotation in the chair-shaped stereoisomers are between 35 and 45 kJ mol−1. The crystal structure as well as the solvated TPM stereoisomer isolated from the Tea tree oil, Melaleuca alternifolia (Maiden & Betche) Cheel, were investigated experimentally. Isolated stereoisomer corresponds to the most energetically preferred conformation and the calculated structural data agree very well with the results from the X-ray and nuclear magnetic resonance measurements. Finally, the influence of the conformation and the presence of the intramolecular hydrogen bonds on the homolytic OH bond dissociation enthalpies and proton affinities were also discussed with respect to the simple alcohols (methanol, iso-propanol, iso-pentanol, tert-butanol, cyclohexanol) and phenol.

Theoretical study of 2-phenylpyrrole molecule using various quantum-chemical approaches A systematic theoretical study of 2-phenylpyrrole (PhPy) is presented for its neutral and monocharged states. The calculations were performed using the semiempirical Austin Model 1 (AM1) method, ab initio Møller-Plesset perturbation theory up to the second-order (MP2), density functional theory (DFT) and its tight-binding approximation (DFTB+). The comparison of the obtained equilibrium geometries showed that the C—C bond lengths in the phenylene ring are practically identical for the neutral state. Electric charging leads to significant changes in the geometry with respect to the neutral state. The C—N bonds in PhPy are elongated and the negative charging produces the out-of-plane distortion of N—H bond from the aromatic ring plane. The anionic state of the investigated molecule is connected with a higher perturbation of bond length alternation in both rings in comparison to the cationic state. The vibrationaly broadened absorption spectra, based on the on-the-fly molecular dynamics (MD) simulations, are also presented and compared with experimental spectra. Although the DFTB+ method has the tendency to planarize the investigated molecular structure, the agreement of simulated absorption spectra based on the MD DFTB+ geometries with TD-DFT calculations is acceptable.

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 BSLYP/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 = f(sigma(p)) 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 arc. It has been also observed that the larger the atom, the smaller corresponding BDEs.

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.

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.

The applicability of the semiempirical Self-Consistent Charge Density-Functional Based Tight Binding approach (SCC-DFTB) is tested and compared with the reference ab initio Møller–Plesset perturbation theory up to the second-order (MP2) and density functional theory (DFT). In this context, the calculations of 2-phenylthiophene and 2,2′-bithiophene molecules are presented with respect to the optimal geometries and torsional potentials. The vibrational broadenings of absorption spectra based on the combined on-the-fly molecular dynamics (MD) simulations were analyzed for the studied model molecules as well as for selected representative oligomeres or star-shaped molecules built from the thiophene and phenyl units. Our calculations showed that the SCC-DFTB method has the tendency to planarize the mutual distortion between the thiophene–thiophene or thiophene–phenyl units and to overestimate the energy barriers for perpendicular structures with respect to the benchmark MP2 and DFT results. The semiempirical TD-B3LYP//MD(SCC-DFTB) or ZINDO//MD(SCC-DFTB) simulations at a temperature of 300K produce narrower absorption spectroscopic bands comparing to the reference TD-B3LYP//MD(B3LYP) results. The temperature increase to 600K ensures the higher population of distorted MD(SCC-DFTB) geometries and consequently a more realistic shape of the absorption bands with respect to the internal vibrational and rotational modes.

In this article, we have studied para- and meta-substituted thiophenols in order to study the effect of various electron-donating and electron-withdrawing groups in the gas-phase and in the four solvents on the enthalpies of homolytic and heterolytic S–H bond cleavage. Reaction enthalpies related to hydrogen atom transfer (HAT), single electron transfer–proton transfer (SET–PT) mechanism and sequential proton loss electron transfer (SPLET) mechanisms were studied using B3LYP/6-311++G** method. Solvent contribution to the enthalpies was computed employing integral equation formalism IEF-PCM method. Obtained results were confronted with available experimental data. Besides, reaction enthalpies and substituent effects were compared with previously published data for phenols with identical group ofsubstituents. In studied environments, thermodynamically favored reaction pathway was determined.

For 15 para -substituted sterically hindered phenols, i.e. phen ols with large tert -butyl groups in the two ortho positions, the reaction enthalpies related to thre e mechanisms of phenolic antioxidants action: ( i) hydrogen atom transfer (HAT), ( ii ) single-electron transfer ‐ proton transfer (SET-PT), and ( iii ) sequential proton loss electron transfer (SPLET) in gas-phase, were calculated using DFT/B3LYP/6-311++G** method. Computed enthalpies were compared with available experimental values and with data obtained for para-substituted phenols. Obtained reaction enthalpies were also cor related with Hammett constants, σp. Electron-donating groups lower BDE, IP and ETE and induce an increase in PA and PDE. Electron-withdrawing groups cause a decrease in PA and PDE and a rise in BDE, IP and ETE. Dependences of studied reaction enthalpies on Hammett constants can be considered linear. In the case of HAT and SPLET mechanisms, we have found linear dependences between corresponding enthalpies (BDE, PA, ETE) and length of phenolic C‐O bond. Linear dependence between this bond length and Hammett constant, σp, has been obtained, too.

In this work, O–H bond dissociation enthalpies (BDE) of non-substituted phenol and 26 metaand para-substituted phenols were calculated using semiempirical DFTB+ method. In comparison with available experimental BDE values, calculated BDEs are overestimated by ca 40 kJ mol –1 . DFTB+ method predicts lower substituent induced changes in BDE in

In this work, we have calculated bond dissociation enthalpies of O-H bonds in selected poly substituted phenols using density functional theory (B3LYP) with 6-311++G** basis set.

Subject Laboratory practice in physical chemistry covers all major fields in physical chemistry; thermodynamics, electrochemistry, chemical kinetics, colloid chemistry, and properties of molecules. In 2009, we introduced new equipment based on Coach Lab laboratory measurement system. The system has also been successfully employed in the further education of secondary-school teachers of physics, chemistry and biology for four years.