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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 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]

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]