Proton transfer dynamics on highly attractive potential energy surfaces: Induced repulsive energy release in O<sup>−</sup> + HF at high collision energies

1997

J. Phys. Chem. A

We present a crossed beam study of the title reaction over the collision energy range from 0.47 to 1.20 eV, over which the dynamics of particle transfer are direct. The data include vibrational state populations and vibrational-state dependent angular distributions. Over the entire collision energy range, the product vibrational state distributions are inverted, and the extent of that inversion as measured by the vibrational surprisal increases with increasing collision energy. The energy dependence of the product vibrational state distributions is partially consistent with mixed energy release in the Heavy + Light−Light mass combination, but it more closely resembles the behavior of proton transfer in the O- + HF system at low energies, where the attractive well induces corner cutting trajectories, leading to partitioning of increased reagent translation into product vibration. The most probable rotational energy is approximately constant as a function of vibrational quantum number and is effectively independent of collision energy. This saturation effect is consistent with the softening of the low energy repulsive wall as the collision geometry changes from collinear to bent. Several features of the data can be rationalized with extant ab initio calculations, but other features, especially the vibrational state populations, require additional theoretical insight for complete understanding.

Section: Results and Analysismentioning

confidence: 99%

Section: Results and Analysismentioning

confidence: 99%

Vibrational State-Resolved Study of the O^- + D₂ Reaction: Direct Dynamics from 0.47 to 1.20 eV

Carpenter

1997

J. Phys. Chem. A

“…(u,). As described in detail in previous publications from our laboratory, 19,20 the energyindependent differential cross section was recovered from the lab data by inverting the transformation relation ͑3͒ with concomitant removal of the dispersion of the beam velocities,…”

Section: Results and Analysismentioning

confidence: 99%

Vibrational state-resolved study of the O−+H2 reaction: Isotope effects on the product energy partitioning

Lee

The Journal of Chemical Physics

1999

The deuterium isotope effect on product energy partitioning in the O Ϫ ϩH 2 particle transfer reaction is investigated in a crossed molecular beam experiment. Vibrational-state-resolved angular distributions are measured at six collision energies between 0.20 and 0.77 eV for the O Ϫ ϩH 2 reaction and at seven collision energies between 0.22 and 1.20 eV for the O Ϫ ϩD 2 reaction. The fraction of the total available energy deposited into product vibration is significantly larger in the deuterium system than in the hydrogen system. This effect is greatest at the lowest collision energies where OD Ϫ products are formed with more than twice as much vibrational energy as OH Ϫ products. The isotopic systems display similar trends in the product angular distributions, which extend over the full range of scattering angles at low energies and shift towards the forward direction as the collision energy is increased. These observations are discussed in terms of a competition between reaction mechanisms. An insertion-migration mechanism, yielding products with moderate vibrational excitation, is especially important at the lower energies. The insertion process leads to the isotope effect in the product energy partitioning which is explained in terms of Franck-Condon factors. As the energy increases, larger impact parameter collisions are able to proceed through a direct mechanism, yielding more tightly forward-scattered, vibrationally excited products. Since direct mechanisms show isotopically independent energy partitioning, the overall isotope effect diminishes with increasing energy as more collisions become purely direct. Bimodal rotational state distributions help strengthen the claim that two distinct reaction mechanisms produce the particle transfer product.

“…The deuterium ion transfer process to form NH 3 D + exhibits many of the characteristics of direct proton transfer that we have been observed in previous systems. [48][49][50] In strongly exothermic proton transfer systems, the dynamics appear to be direct, partitioning the majority of the total energy into product vibration. As examples of the heavy + light-heavy mass combination in which the light particle is transferred in reaction, such systems exhibit the energy transfer motif of "mixed energy release", [35][36][37][38] in which the reaction occurs with the cleaving and incipient bonds in an extended configuration.…”

Section: Discussionmentioning

confidence: 99%

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

2004

J. Phys. Chem. A

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.