Ab initio study of the oxidation of CH3SH to CH3SSCH3

The thermodynamic and kinetic aspects of the oxidation of methanethiol to dimethyl disulfide by oxygen were studied using ab-initio molecular techniques. The reaction was considered as a multi-step process. Geometrical parameters and molecular energies for critical points were determined and characterized at the MP2(Full)/6-31G* level. Afterwards, a study was carried out employing different methods of theoretical model chemistry (G2, G2(MP2), G3 and CBS-4 MB), for which the equilibrium geometry at MP2(Full)/6-31G* was assumed as starting point. The steps governing the rate of the whole reaction were determined and prove to be the same for any level of theory employed. Although the MP2(Full)/6-31G* level is able to afford a good qualitative description of the reaction pathway, the quantitative aspect is less satisfactory. Results from the methods of theoretical model chemistry, on the other hand, accord well with experimental data, if available, and are very useful for studying the thermodynamic and kinetic aspects of chemical reactions

BOND-DISSOCIATION IN HYDROGEN-PEROXIDE AND IN THE HYDROGEN-PEROXIDE RADICAL-ANION - A-MO ABINITIO MCSCF APPROACH

The problem of bond dissociation in hydrogen peroxide and in its monoanion was tackled with the ab initio MCSCF method by employing a triple-zeta (TZP) contracted basis set, augmented by polarization functions. For hydrogen peroxide the calculated energies for homolytic O-O and O-H bond cleavages are underestimated (by about 12 kcal mol-1) with respect to bond dissociation enthalpies obtained from heats of formation. This result seems to arise from not taking dynamical correlation effects properly into account, as emerges from the comparison with the results of MP2/TZP calculations. For the monoanion of hydrogen peroxide the equilibrium geometry turns out to be almost all planar with one hydrogen atom shifted between the oxygen atoms and the unpaired electron mostly localized on the outer oxygen atom. The two processes: H2O2- (X 2A)-->OH(X 2PI) + -OH(X 1SIGMA+) and H2O2-(X 2A)-->O-(X 2p(u)) + H2O(X 1A1) require 36.15 and 23.82 kcal mol-1, respectively (41.06 and 30.14 kcal mol-1 at MP2/TZP level), and the latter is likely to be the more favoured path in the chemical reactivity of hydrogen peroxide in the presence of electron donating molecules (i.e. Fenton's reagent)

THERMOCHEMICAL PROPERTIES AND HOMOLYTIC BOND-CLEAVAGE OF ORGANIC PEROXYACIDS AND PEROXYESTERS - AN EMPIRICAL-APPROACH BASED ON AB-INITIO MO CALCULATIONS

Heats of formation DELTAH(f)-degrees of a number of peroxyacids RC(O)OOH, peroxyesters RC(O)OOR' and of the radicals RC(O), RC(O)O, RC(O)OO and C(O)OOR', related to possible routes of their thermal decomposition, were estimated by employing empirical approaches. R and R' are a wide set of alkyl and aryl groups. The approaches were based on isodesmic reactions, atom equivalent, group additivity and molecular mechanics (MM2) calculations. Isodesmic reactions and atom equivalent schemes were employed with HF/6-31G* ab initio total molecular energies. The MM2 calculations were applied to peroxyacids and peroxyesters and the values of DELTAH(f)-degrees obtained were corrected by a constant quantity because they appeared systematically in excess with respect to those obtained by the other approaches. In atom equivalent and group additivity methods, new parameters were evaluated in order to be able to calculate the heats of formation of these molecules. Values of DELTAH(f)-degrees obtained from the different approaches turn out to be consistent within +/-2 kcal mol-1. For the RC(O) radicals with R = H, CH3, C6H5 and CF3, a comparison with values reported in the literature is possible and agreement can be considered satisfactory. For the RC(O)O radicals with R = H, CH3, C6H5, the DELTAH(f)-degrees values estimated in this work from isodesmic reactions and atom equivalent approaches are 7-10 kcal mol-1 higher than those reported in the literature. This difference appears to be due to improper evaluation of the total molecular energy with HF/6-31G* ab initio calculations for this radical, owing to symmetry breaking in SCF methods. With the heats of formation calculated for peroxyacids, peroxyesters and for the radicals structurally related to these molecules, homolytic dissociation energies for the various bonds were derived and discussed as a function of the structural features of the R and R' groups. Possible routes for the thermal decomposition of the peroxyesters were examined

Complete basis set model chemistry applied to molecules of increasing molecular complexity: Thermochemical properties of organic sulfur derivatives

To estimate the thermochemical properties, bond dissociation energies and atomization energies of sulfur organic derivatives, the complete basis set (CBS) method was employed at the lower computational level (CBS-4) owing to the large molecular size of a number of the molecules chosen. By comparison with experimental values, calculated values of thermochemical properties are subject to error, which increases in line with the increase in molecular complexity. The main source of error affecting the calculated enthalpy of formation stems from the difference between the energy of the molecule and that of the single atoms: the greater the size of the molecule, the greater the accumulation of error. By acting on the empirical correction to the CBS energy and minimizing the error due to the contribution of the single atoms to the dissociation energy a parameter di for each atom i is obtained. Application of these corrections does not greatly affect the heats of formation of the small molecules included in test sets employed for previous comparisons of calculated and experimental values, while there is a great improvement in the case of large molecules, for example, diphenyl disulfide. The mean absolute deviation turns out to be 2.52, which is greater than that obtained in recent reexaminations of model chemistry methods Including the G3 and G3(MP3) approaches. The improvement in the results calculated for large molecules, whose heats of formation are calculated with large errors at the CBS-4 level, in comparison also with the CBS-4M version, justify our approach

Ground-state molecular stabilization of substituted ethylenes. A theoretical mo ab-initio thermochemical study

The origin of the rotational barrier around the partial C=C double bond in substituted ethylenes is discussed with reference to the stabilization of the conformational minimum (GS) and of the rotational transition state (TS). Molecules with different polar character of the double bond were chosen, ranging from ethylene to olefins with strong push-pull character. The enthalpies of hydrogenation, DeltaH(hydr.), and of formation, DeltaH(f)(0), of these molecules were employed to obtain the stability of GS; these thermochemical properties were calculated with MO ab-initio theory at HF/6-311G**//HF/6-311G** and MP2/6-311G**//HF/6-311G** levels and with CBS-4M model chemistry. The stabilization of TS was derived from the torsional potential for rotation around the C=C bond. The lowering of the energy content of GS of substituted ethylenes, referring to ethylene, is accompanied by an even greater stabilization of TS, thus a lowering of the rotational barrier with respect to ethylene is generally found in these molecules. (C) 2001 Elsevier Science B.V. All rights reserved

A theoretical approach to the factorization of the effects governing the barrier for internal rotation around the C(sp(2))-C(sp(3)) bond into alpha-substituted toluenes

The effects governing the barrier for internal rotation in a number of alpha-substituted toluenes C6H5CH2X (X = Cl, F, CH3, C(CH3)(3), CF3, CCl3) and alpha,alpha-disubstituted toluenes C6H5CHX2 (X = Cl, Fl were interpreted using a model which factorizes the energy barrier into three components, namely, a) hyperconjugation, b) electrostatic effects and c) van der Waals interactions. The potential energy profiles for internal rotation of the CH2X and CHX2 rotors were calculated at molecular orbital MO ab initio level with a 6-31G* basis set and analysed by means of a truncated Fourier series in the V-2 and V-4 terms. The hyperconjugative contributions were estimated employing natural bond orbitals (NBO) derived from the 6-31G* wave functions in a scheme of acceptor-donor intramolecular interactions. The donor and acceptor hyperconjugative contributions, with respect to the pi system of the benzene ring, of each bond constituting the CH2X and CHX2 rotors were found to contribute additively to the hyperconjugative effect of the whole rotating group. Electrostatic effects and van der Waals interactions were tentatively estimated with empirical formulas. The separate contributions of these effects were compared, albeit at a qualitative level, with the total molecular energy and their relative weight discussed. The rotational barriers of benzylchloride and benzalchloride are mainly controlled by hyperconjugative effects. In benzylfluoride and benzalfluoride, the hyperconjugative effects are active to the same extent as in chlorine derivatives but the barrier is mainly controlled by electrostatic effects. In the compounds with bulky X groups (X = C(CH3)(3), CF3 and CCl3), hyperconjugation plays a less important role than van der Waals interactions and electrostatic effects, and the relative weight of these effects differs for the substituents examined

CONFORMATIONAL PROPERTIES OF PEROXYACIDS, PEROXYESTERS AND OF STRUCTURALLY RELATED RADICALS - A THEORETICAL AB-INITIO MO APPROACH

The geometrical features relative to the lowest conformational minimum, and absolute and relative energy contents of a number of peroxyacids RC(O)OOH (R = H, CH3, CF3, C6H5) and methyl esters (R = H, CH3, CF3) have been calculated at the ab initio MO level with the 6-31G* basis set. For the peroxyacids with R = H and CH3, the most stable conformation obtained from this calculation is of cis type (referring to internal rotation around the O-O bond) and agrees with that found experimentally. This conformation is the most stable one also for the other peroxyacids examined and for the peroxyesters. Poor agreement is, however, found between calculated and experimental geometrical parameters relative to the O-O and O-H bond lengths, and to the OOC and OOH bond angles. Comparison of calculated structural features of peroxyacids with those of the corresponding acids shows that stabilization of the planar cis conformation of the former compounds is due to intramolecular hydrogen bonding. This property influences the energy difference between cis and trans forms, DELTAE, which, for the same R, is larger in the peroxyacid than in the peroxyester. The DELTAE values increase approximately with the bulk of R. The energy content, conformational features and geometrical parameters of the more stable conformation have been analysed within the same theoretical approach for the radicals RC(O)O, RC(O)OO and C(O)OOR'(R' = H, CH3), formally derived from homolytic bond dissociation of the peroxyacids and peroxyesters examined. The sigma(RC(O)O and C(O)OOR') and pi (RC(O)OO) characters, atom spin densities and relative stabilities of the conformers of these radicals are discussed