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

The reaction C2H5++CH4 = C3H7++H2 [Reaction (1)] was observed in pure methane in a pulsed electron beam high pressure ion source mass spectrometer. The rate constants k1 determined in the temperature range 250–650 °K were found to increase with temperature. An Arrhenius plot of k1 gave a straight line which yields 6×10−13 cm3 molecule−1⋅sec−1 for the preexponential factor and 2.5 kcal/mole for the activation energy E1. At temperatures below 250 °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+) * 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 barrier B between the three center bonded structures: x x x x The lowest energy structure IV is formed by the addition of C2H5+ to CH4. The H2 elimination from C3H9+ must proceed via structure III. The barrier B = ΔH8+Ea(1) = 6.6+2.5 kcal/mole is larger than the barrier A for back dissociation of C3H9+ to C2H5++CH4, where A = ΔH8 = 6.6 kcal/mole. Thus at low temperature back dissociation predominates, while at higher temperatures H2 elimination becomes competitive.