Temperature dependence of bimolecular ion molecule reactions. The reaction C2H5++CH4= C3H7++H2

摘要

The reaction C2H5++CH4= C3H7++H2lsqb;Reaction (1)rsqb; was observed in pure methane in a pulsed electron beam high pressure ion source mass spectrometer. The rate constantsk1determined in the temperature range 250ndash;650thinsp;deg;K were found to increase with temperature. An Arrhenius plot ofk1gave a straight line which yields 6times;10minus;13cm3moleculeminus;1sdot;secminus;1for the preexponential factor and 2.5 kcal/mole for the activation energyE1. At temperatures below 250thinsp;deg;K, Reaction (1) was gradually replaced by (8): C2H5++2CH4= C3H9++CH4. It is concluded that both (1) and (8) proceed by the excited intermediate (C3H9+) ast; which may back react to C2H5++CH4, decompose to C3H7++H2, or be stabilized to C3H9+. The positive temperature dependence of (1) arises from an internal energy barrierBbetween the three center bonded structures: x x x x The lowest energy structure IV is formed by the addition of C2H5+to CH4. The H2elimination from C3H9+must proceed via structure III. The barrierB= Dgr;H8+Ea(1) = 6.6+2.5 kcal/mole is larger than the barrierAfor back dissociation of C3H9+to C2H5++CH4, whereA= Dgr;H8= 6.6 kcal/mole. Thus at low temperature back dissociation predominates, while at higher temperatures H2elimination becomes competitive.