Abstract
The molecular mechanism of the decomposition reaction of nitroethyl benzoate (NEB) 1 yielding nitroethylene 2 and benzoic acid 3 has been studied within the Molecular Electron Density Theory (MEDT) using DFT methods at the B3LYP/6-31G(d) computational level. This decomposition reaction takes place through a one-step mechanism. Bonding Evolution Theory (BET) analysis of this reaction provides a complete characterisation of the electron density changes along the reaction. The reaction begins through the synchronous rupture of the O–C and C–H single bonds of NEB 1. Interestingly, while the rupture of the O–C single bond takes place heterolytically, that of the C5–H6 one takes place homolytically, yielding the formation of a pseudoradical hydrogen atom. These changes, which demand a high energy cost of 37.1 kcal mol−1, are responsible for the high activation energy associated with this decomposition reaction. Formation of the C–C double bond present in nitroethylene 2 takes place at the end of the reaction. The six differentiated phases in which the IRC associated with this reaction is divided clearly point out its non-concerted nature, thus ruling out the proposed pericyclic mechanism. This reaction, whose associated TS presents a more or less distorted six-membered cyclic structure in which all atoms may not necessarily be bound, is categorised as a pseudocyclic reaction.
Highlights
The thermal decomposition of alkyl esters to produce alkenes and carboxylic acids is a well-established process experimentally investigated a great number of times [1,2,3,4,5]
The present theoretical study has been divided into three sections: i) in Sect. 3.1, an electron localisation function (ELF) topological analysis and a natural population analysis (NPA) of the reagent nitroethyl benzoate (NEB) 1 are performed in order to characterise its electronic structure; ii) in Sect. 3.2, a Bonding Evolution Theory (BET) study of the decomposition reaction of NEB 1 is performed in order to characterise the molecular mechanism of this intramolecular process; and iii) in Sect. 3.3, an Molecular Electron Density Theory (MEDT) study of the decomposition reaction of NEB 1 based, on the one hand, the BET study, and, on the other hand, the analysis of the energies related to the different bonding changes taking place along to the reaction, is given with the aim of providing an explanation of its activation energy
One appealing procedure that provides a straightforward connection between the electron density distribution and the chemical structure is the quantum chemical analysis of Becke and Edgecombe’s ELF [22]
Summary
The thermal decomposition of alkyl esters to produce alkenes and carboxylic acids is a well-established process experimentally investigated a great number of times [1,2,3,4,5]. One of the fundamental goals for understanding a given chemical rearrangement is to analyse the changes of quantum mechanical observables such as the electron density along the reaction pathway. This approach enables a comprehensive picture of the chemical reactivity in terms of how and when chemical events, e.g. bond rupture and formation processes, will take place. As a continuation of the quantum chemical study about the decomposition reaction of nitroethyl carboxylates [18, 19], an MEDT study of the decomposition process of NEB 1 is performed in order to establish the molecular mechanism of this decomposition reaction and the nature of the electronic rearrangement along it
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