Experimental and theoretical studies of charge transfer and hydride transfer in the reactions of OD+ and C2H4

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 C2H4 at relative collision energies between 19.3 and 102 kJ mol−1 (0.20-1.06 eV). Charge transfer to form C2H4+ is a direct process occurring through large impact parameters. A comparison of the internal energy distributions of reaction products with the photoelectron spectrum of C2H4 is consistent with a Franck-Condon picture for long distance electron transfer. Charge transfer with H/D rearrangement to form C2H3D+ + OH does not occur, unlike the related system D2O+ + C2H4, in which comparable amounts of C2H4+ and C2H3D+ are observed. This difference is accounted for by the significantly smaller proton affinity of OH relative to H2O. Reactive processes are initiated by the formation of a protonated oxirane triplet diradical, which undergoes intersystem crossing to the singlet manifold. Formation of C2H3+ + 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 C2H3+ products may be formed above the dissociation threshold to C2H2+. Increasing collision energy results in enhanced yields of C2H2+ relative to C2H3+, consistent with unimolecular decay of the most highly excited C2H3+ 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.