Reaction Dynamics of H₂O⁺ (D₂O⁺) + NH₃ Studied with Crossed Molecular Beams and Density Functional Theory Calculations

Abstract

The charge transfer and proton transfer reactions between H 2 O + (D 2 O + ) and NH 3 were studied at collision energies below 1 eV using the crossed molecular beam technique and density functional theory (DFT) calculations. The reaction products include NH 3 D + formed by deuterium ion transfer, NH 3 + produced by charge transfer, and NH 2 D + formed by charge transfer with H/D exchange. These three products are formed in the ratio of 1.0:2.0:1.2, respectively. The center of mass flux distributions of the product ions for all three reaction channels exhibit asymmetry, with maxima close to the velocity and direction of the precursor ammonia beam, characteristic of direct reactions. The internal energy distributions of the products of charge transfer are independent of collision energy, are very narrow (∼0.55 eV) and are peaked at the reaction exothermicity of 2.55 eV. The NH 2 D + products have very similar distributions, peaking at the reaction exothermicity and having widths of ∼0.70 eV. The mechanisms for these reaction channels are discussed on the basis of the DFT calculated potential energy surfaces, which identify three electrostatically bound complexes of the form [H 2 O‚NH 3 ]+ that mediate reaction. Theory suggests that H/D exchange to form NH 2 D + may occur through the complex NH 3 D + ‚‚‚OD, in which an internal rotation of the NH 3 D + unit exchanges hydrogen and deuterium atoms. The large fraction of NH 2 D + reaction products observed indicates that this exchange takes place at a rate nearly 3 orders of magnitude faster than the predictions of statistical theory.

Reaction dynamics of OH+(Σ−3)+C2H2 studied with crossed beams and density functional theory calculations

“…The proton transfer and charge transfer products are clearly formed in large impact parameter collisions, suggesting that the centrifugal barrier associated with the long-range attractive potential of the approaching reactants prevents the reactants from reaching the short range part of the potential where C-O bond formation occurs. In previous works, 4,5,[23][24][25] we have shown that the electrostatic complexes that characterize reactions proceeding through large impact parameters decay with rates near 10 14 s −1 , further suggesting that processes that may occur via slow isomerization pathways will result in product levels below the detection limit.…”

“…The proton transfer process forming C 2 H 3 + exhibits many characteristics of direct proton transfer that we have studied recently in this laboratory. [3][4][5][23][24][25]31 This proton transfer reaction is another example of a heavy+ light-heavy ͑H+LH͒ system in which a light particle, H, is transferred between heavier molecular fragments. The potential energy surface for this transfer, expressed in scaled and skewed coordinates, 32 is characterized by a very acute angle, 18°, between the entrance and exit channels.…”

Hydride transfer reaction dynamics of OD++C3H6

“…Similar observations were also reported in the D 2 O + +NH 3 system previously studied in our laboratory. 25 The explanation for this small but unusual decrease in the incremental translation of the products may arise from the possibility that the bent configurations in the current system become increasingly important at higher energies, leading to additional rotational excitation in the fragments. The corner cutting trajectories that produce vibrationally excited products at lower collision energies are replaced by trajectories at higher translational energy that are more effective at reaching the compressed configurations that facilitate translation in the separating products.…”

Imaging the dynamics of ion–molecule reactions

“…In many of the systems investigated by Farrar and co-workers, charge transfer is dominant over proton transfer. Through a variation of the charge state (OH +/À + C 2 H 2 ) 69,70 or the proton donor (H 2 O + /H 3 O + + NH 3 ), 71,72 they were able to investigate the proton transfer dynamics. A direct stripping mechanism was found to be the only reaction pathway.…”