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Peroxides play a central role in many chemical and biological processes such as the Fenton reaction. The relevance of these compounds lies in the low stability of the O–O bond which upon dissociation results in radical species able to initiate various chemical or biological processes. In this work, a set of 64 DFT functional-basis set combinations has been validated in terms of their capability to describe bond dissociation energies (BDE) for the O–O bond in a database of 14 ROOH peroxides for which experimental values of BDE are available. Moreover, the electronic contributions to the BDE were obtained for four of the peroxides and the anion H 2 O 2 - at the CBS limit at CCSD(T) level with Dunning's basis sets up to triple- ζ quality providing a reference value for the hydrogen peroxide anion as a model. Almost all the functionals considered here yielded mean absolute deviations around 5.0 kcal mol - 1 . The smallest values were observed for the ω B97 family and the Minnesota M11 functional with a marked basis set dependence. Despite the mean deviation, order relations among BDE experimental values of peroxides were also considered. The ω B97 family was able to reproduce the relations correctly whereas other functionals presented a marked dependence on the chemical nature of the R group. Interestingly, M11 functional did not show a very good agreement with the established order despite its good performance in the mean error. The obtained results support the use of similar validation strategies for proper prediction of BDE or other molecular properties by DFT methods in subsequent related studies. [ABSTRACT FROM AUTHOR]

The adsorption of dibenzyl disulfide (DBDS) on a pentagonal bipyramid Cu7 cluster was investigated by using density functional calculations, from energetic and electronic viewpoints. The resulting complexes are mainly driven by Cu···S interaction, and an extra stabilization can be conferred by a secondary π···Cu weak interaction. They were classified as physi- or chemisorption according to their binding energy, and by applying a distortion/interaction decomposition model. Disulfide bond dissociation was observed in the most stable complexes, which includes higher distortion energy. From an electronic viewpoint, an electronic flow from copper to DBDS was observed. [ABSTRACT FROM AUTHOR]

The chemical reactivity of the first- and second-generation Grubbs catalysts has always been a significant issue in olefin metathesis. In the present work, we study the [2+2] cycloreversion/cycloaddition and the alkylidene rotation involved into the interconversion of the ruthenacyclobutane intermediate, through the reaction force and reaction force constant analysis. It has been found that the structural contribution controls the barrier energy in the interconversion of ruthenacyclobutane via [2+2] cycloreversion/cycloaddition, which is slightly lower in the second generation of Grubbs catalysts while its electronic contribution is slightly higher, which unveils a major rigidity and donor/acceptor properties of the NHC. This finding explains a greater structural contribution in the rate constant. Moreover, on the basis of the reaction force constant, the process can be classified as "two-stage"-concerted reactions, noting a more asynchronous process when the first generation is used as a catalyst. Finally, a similar analysis into the alkylidene rotation was performed. It was determined that [2+2] cycloreversion and alkylidene rotations take place in a sequential manner, the energy barrier is again controlled by structural reorganization, and the pathway is less asynchronous. [ABSTRACT FROM AUTHOR]

A synchronous, concerted chemical process is rigorously divided by the reaction force F( R), the negative gradient of V( R), into 'reactant' and 'product' regions which are dominated by structural changes and an intervening 'transition' region which is electronically intensive. The reaction force constant κ( R), the second derivative of V( R), is negative throughout the transition region, not just at the nominal transition state, at which κ( R) has a minimum. This is consistent with experimental evidence that there is a transition region, not simply a specific point. We show graphically that significant nonsynchronicity in the process is associated with the development of a maximum of κ( R) in the transition region, which increases as the process becomes more nonsynchronous. (We speculate that for a nonconcerted process this maximum is actually positive.) Thus, κ( R) can serve as an indicator of the level of nonsynchronicity. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

The reaction force F( ξ) is the negative gradient of the potential energy of a chemical process along the intrinsic reaction coordinate ξ. We extend the rigorous concept of F( ξ) to the 'activation strain model' of Bickelhaupt et al., to formulate the 'strain' force F( ξ) that retards a reaction and the 'interaction' force F( ξ) that drives it. These are investigated for a group of Diels-Alder cycloadditions. The results fully support the interpretation of the minimum of F( ξ) as defining the beginning of the transition from deformed reactants to eventual products. [ABSTRACT FROM AUTHOR]

We have analyzed the variation of the reaction force F( ξ) and the reaction force constant κ( ξ) along the intrinsic reaction coordinates ξ of the water-assisted proton transfer reactions of HX-N = Y (X,Y = O,S). The profile of the force constant of the vibration associated with the reactive mode, k( ξ), was also determined. We compare our results to the corresponding intramolecular proton transfers in the absence of a water molecule. The presence of water promotes the proton transfers, decreasing the energy barriers by about 12 - 15 kcal mol. This is due in part to much smaller bond angle changes being needed than when water is absent. The κ( ξ) profiles along the intrinsic reaction coordinates for the water-assisted processes show striking and intriguing differences in the transition regions. For the HS-N = S and HO-N = S systems, two κ( ξ) minima are obtained, whereas for HO-N = O only one minimum is found. The k( ξ) show similar behavior in the transition regions. We propose that this fine structure reflects the degree of synchronicity of the two proton migrations in each case. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

Hydroxyl radical reduction and peroxide bond breaking in hydrogen peroxide are reactions involved in various processes such as the Fenton reaction, which has applications as e.g. groundwater remediation. Here, we study these two reactions from a thermodynamical point of view through the bond dissociation energy (BDE) of the O-O bond in hydrogen peroxide and the electron affinity (EA) of the hydroxyl radical. High-level ab-initio calculations at the complete basis set (CBS) limit were carried out, and the performance of different DFT-based methods was addressed by following a specific classification on the basis of the Jacob’s ladder in combination with various Pople’s basis sets. The ab-initio calculations at the CBS limit are in agreement with experimental reference data and identify a significant contribution of the electron correlation energy to the BDE and EA. The studied DFT-based methods were able to reproduce the ab-initio reference values, although no functional was particularly detected as the best for both reactions. The inclusion of certain percentage of Hartree-Fock (HF) exchange in DFT functionals leads in most cases to smaller BDE and EA values, which might be related to the poor description of the two reactions by the HF method. Considering the computational cost, DFT methods provide better BDE and EA values than HF methods with an accuracy comparable to the MP2 or CCSD level of theory. Additionally, the quality of the hydrogen peroxide, hydroxyl radical and hydroxyl anion structures obtained from these functionals was compared to experimental reference data. In general, bond lengths were well reproduced and the errors in the angles were between one and two degrees with some systematic trend with respect to the basis set’s size. From our results we conclude that DFT methods present a computationally less expensive alternative to describe these two reactions that play a role in the Fenton reaction. The benchmark that is carried out in this study provides a systematic validation of various approximated Exc[ρ] functionals combined with different basis sets, which could serve as a stepping-stone for future research on the Fenton reaction. [ABSTRACT FROM AUTHOR]