Experimental and theoretical studies of charge transfer and hydride transfer in the reactions of OD + and C 2H 4

Abstract

We present a crossed beam and density functional theory (DFT) study of the dynamics of charge transfer and hydride transfer in collisions of OD + with C 2H 4 at relative collision energies between 19.3 and 102 kJ mol −1 (0.20–1.06 eV). Charge transfer to form C 2H 4 + is a direct process occurring through large impact parameters. A comparison of the internal energy distributions of reaction products with the photoelectron spectrum of C 2H 4 is consistent with a Franck–Condon picture for long distance electron transfer. Charge transfer with H/D rearrangement to form C 2H 3D + + OH does not occur, unlike the related system D 2O + + C 2H 4, in which comparable amounts of C 2H 4 + and C 2H 3D + are observed. This difference is accounted for by the significantly smaller proton affinity of OH relative to H 2O. Reactive processes are initiated by the formation of a protonated oxirane triplet diradical, which undergoes intersystem crossing to the singlet manifold. Formation of C 2H 3 + + HOD, nominally a hydride transfer reaction, is shown to occur at the lowest collision energy through transient singlet intermediates in which the timescale for rate-limiting hydrogen atom migration corresponds to a significant fraction of a rotational period. Formation of hydride transfer products is sufficiently exothermic (ΔH = −489 kJ mol −1) that a fraction of the C 2H 3 + products may be formed above the dissociation threshold to C 2H 2 +. Increasing collision energy results in enhanced yields of C 2H 2 + relative to C 2H 3 +, consistent with unimolecular decay of the most highly excited C 2H 3 + products. However, the very small translational energy releases at all collision energies also require significant vibrational excitation in the HOD products; the most probable internal excitation in HOD is approximately 60 kJ mol −1 at all collision energies.