The Mechanism of Processes Initiated by Excited Atoms III. Photo-Sensitized Hydrocarbon Reactions

Abstract

The photo‐sensitized reactions of the hydrocarbons are discussed in terms of the following reaction scheme, in which A^* indicates the photo‐sensitizing atom and MH₂ the hydrocarbon: $$\begin{array}{c}{\mathrm{A}}^{*}+{\mathrm{MH}}_2\to \mathrm{A}+\mathrm{MH}+\mathrm{H},\\ {\mathrm{A}}^{*}+{\mathrm{MH}}_2\to \mathrm{A}+{\mathrm{MH}}_2^{*},\\ {\mathrm{MH}}_2^{*}+{\mathrm{MH}}_2\text{→2}{\mathrm{MH}}_2,\\ {\mathrm{MH}}_2^{*}\to \mathrm{M}+{\mathrm{H}}_2,\\ {\mathrm{MH}}_2^{*}\to \mathrm{MH}+\mathrm{H}.\end{array}$$ The first and fifth reactions are followed by polymerizations. Expressions for the quenching rate, the rate of formation of M, and the rate of formation of polymer, are obtained in terms of the original rate constants, and a number of special cases of practical interest are treated. The relative rates of the reactions are discussed with reference to the experimental data, particular regard being paid to the energy relationships, the general kinetic behavior, and the influence of temperature. Evidence is adduced in favor of the following: (1) the strength of the C–H bond in ethylene is about 100 kcal., (2) there is a triplet‐excited state of acetylene with an excitational energy of 10–30 kcal., (3) conversion of a large amount of electronic energy into vibrational energy proceeds with difficulty, the process taking place much more readily when some of the energy can be taken up as electronic energy of the colliding molecule.