Collisional Deactivation of Intermediate C4H8+ Ion in the Photolysis of Ethylene at 1048–1067 Å

Abstract

The vacuum ultraviolet of photolysis of ethylene with argon resonance radiation leads to ion formation with a quantum yield (ionization efficiency) of 0.218 ± 0.015. Ethylene ion immediately forms an adduct with ethylene with an internal energy of 58 kcal/mole above the most stable 2-butene or methylpropene ion configurations. Neutralization of the C4H8+ ions by charge exchange with toluene, dimethylamine, or trimethylamine yields 2-butene and methylpropene as neutral counterparts to ions assumed to have a similar structure. The relative abundance of the isomers is dependent on the concentrations of charge acceptor and of inert gases. Results are interpreted in terms of an equilibrium between the ionic precursors of 2-butene and methylpropene. A transition state corresponding to a loose methylcylopropane ion structure, with a barrier of about 30 kcal/mole, is postulated for the isomerization. The effects of inert additives and ethylene, which does not react rapidly with C4H8+ ion, are ascribed to successive energy loss by collision and a resultant change in the equilibrium constant. This model is developed quantitatively and it is shown that, within the framework of some limiting assumptions, energy transfer in nonreactive ion–molecule collisions is considerably less than anticipated on the basis of an intimate complex. This is ascribed to inefficient energy loss for ion–molecule collisions at large impact parameters where redissociation occurs rapidly as a result of small increases in over-all angular momentum.