Reduced parabolic model for radical addition reactions |
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Authors: | E. T. Denisov |
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Affiliation: | (1) Institute of Problems of Chemical Physics, Russian Academy of Sciences, 1 prosp. Akad. Semenova, 142432 Chernogolovka, Moscow Region, Russian Federation |
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Abstract: | The transition state of addition of free radicals and atoms to multiple bonds is considered as a result of intersecting of two parabolic potential curves. One of them characterizes the stretching vibration of the attacked multiple bond, and another curve characterizes the stretching vibration of the bond formed in the transition state. The force constant of the latter is calculated by an empirical equation that correlates the force constant with the bond dissociation energy. In the framework of this model, the thermally neutral activation energy (Ee0) and the elongation of the attacked and formed bonds (re) in the transition state were calculated from the experimental data (activation energy (Ee) and enthalpy of reaction (He)) for the addition of an H atom and methyl, alkoxyl, aminyl, triethylsilyl, and peroxyl radicals to the C=C bond and the addition of H and CH3 to the C=O and CC bonds. Analysis of the data obtained showed that Ee0 depends linearly on the |He| + Ee sum, i.e., Ee0/kJ mol–1 = 14.2 + 0.61 · (Ee – He), and the bond elongation in the transition state for addition of the most part of radicals to ethylene and acetylene vary within (0.65–0.87)·10–10 m. The factors affecting the activation energy of the radical addition reactions are discussed.Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1542–August, 2004. |
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Keywords: | atom double bond reduced intersecting parabolas model transition state radical addition force constant of chemical bond free radical triplet repulsion triple bond bond elongation activation energy bond dissociation energy electro negativity of atoms enthalpy of reaction |
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