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Abstract
Potential energy surfaces for three unimolecular elimination reactions: , , and were calculated using a variety of quantum chemical methods. It was shown that, in all the three cases, the transition state in the first step of the reaction leads to the production of the complex intermediates based on van der Waals interactions. In addition to the fact that the three complexes appear as intermediates on the potential energy surfaces, which means that they are not free entities, the entropy values of the two elimination products are far above those of the complexes due to their additional Sackur-Tetrode entropy. Moreover, the three vibrational frequencies of the H2O group in the (CH3)3COH complex and the H–Cl and H–F stretch frequencies in CH3CF3 and CH3CH2CH2Cl are quite different (see the various tables). The energy levels of the complexes were found to be below those of the corresponding decomposition products. Rate constants for the elimination processes were calculated from the potential energy surfaces using transition-state theory and were compared to available experimental data.
Highlights
There is a large number of thermal unimolecular elimination reactions where part of a molecule is eliminated from a large organic one
The potential energy surfaces describing the unimolecular eliminations were believed for quite a while to be rather simple processes where the reactant generates the products via one transition state with no intermediates
We have shown in the past [11] that the potential energy surfaces of hydrogen bond, and van der Waals complex existed, for example, prior to the H2 elimination from 2,3-dihydrobenzofuran to produce benzofuran and hydrogen
Summary
There is a large number of thermal unimolecular elimination reactions where part of a molecule is eliminated from a large organic one. The potential energy surfaces describing the unimolecular eliminations were believed for quite a while to be rather simple processes where the reactant generates the products via one transition state with no intermediates. The structure and the energetics of complexes, such as van der Waals complexes, hydrogen bonds, and others have recently been discussed in the literature [7,8,9,10]. We have shown in the past [11] that the potential energy surfaces of hydrogen bond, and van der Waals complex existed, for example, prior to the H2 elimination from 2,3-dihydrobenzofuran to produce benzofuran and hydrogen. A recent work by El-Nahas et al [12] describes a very reliable structures and energetics of several unimolecular eliminations on the basis of quantum chemical calculations, but it hardly provides a discussion of whether these proceed via one or two step mechanisms. The future eliminated molecule is remote from the remaining part of the complex relative to the reacting molecule, but it is still a part of the original structure
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