Isotope effects in abstraction and exchange reactions H + H′Br

Abstract

A preliminary report is given of relative reactive cross sections for four abstraction reactions H + H'Br -HH' + Br with attacking atom (A) H or D, and atom under attack (B) H or D.The pattern of reactive cross sections, as obtained in a crossed molecular beam experiment at a collision energy ET = 7 kcal/mol, indicates S,Tile atoms in parentheses are A and B. We describe a three-dimensional classical trajectory (CT) study on a potential-energy surface proposed in 1969 by Parr and Kuppermann (PK); the CT results are in fair accord with experiment. It is suggested that (D,H) has the largest cross section because it exhibits the most favorable relative timing of A approach to BC rotation. On the basis of CT it appears that the same sequence of cross sections and the same rationale may be applied to the exchange reactions H t BrH' -HBr + H'.

The dynamics of the reaction of O(1D) with HBr studied by crossed molecular beams and time-resolved Fourier transform spectroscopy

“…The presence of translationally hot o(~P), however, may not be important to the results reported here, for several reasons: first, it is a minor constituent (~1 0 % ) of the photodissociation products. Second, the reactive cross sections of processes such as this decrease at the extremely high relative translational energy of the o(~P) atom (33,34) and finally, increased reagent translational energy causes reduced product vibrational excitation in reactions such as this (35). This latter effect, in the case of the reaction reported here, causes the OH from the o(~P) reaction to be created predominantly in the lowest vibrational levels.…”

“…We note parenthetically that the observation of high rotational excitation in the products of the gas phase reaction, to which we have referred earlier, does not contradict the insertion mechanism because Monte Carlo simulations of the o('D) + HCl reaction using a statistical model suggest (60) that the high OH rotational excitation arises from angular momentum constraints and has no unique dynamical origin. It has been noted that the energy partitioning of both the ground state and excited state reactions resembles that of the direct abstraction reaction of F with HBr (34,61). The observation, however, of a marked propensity for production of the II(A') A component in the OH product of the o('D) + HCI reaction (13) implies that the half-filled IT orbital of OH lies preferentially oriented in its rotational plane and remains in the plane of the dissociating triatomic complex.…”

Spin-orbit effects in gas-phase chemical reactions

“…Since the spin-orbit splitting in atomic fluorine is comparable to kT, variation of the temperature of a F atom source was utilized to vary the relative spin-orbit populations. 131 In order to change the temperature without substantially altering the laboratory velocity, an effusive source of fluorine, which was prepared by thermal dissociation of F2, was employed. In principle, a variable-temperature supersonic source could be used if the seed gas mixture were varied to keep the velocity constant as the temperature was changed.…”

Search for Br∗ production in the D+DBr reaction

“…To date, the only previous experimental studies [10][11][12] of H + HBr were unable to answer whether or not Br ‫ء‬ is formed. They photolyze Br 2 to create hot Br and Br ‫ء‬ atoms, but these become thermalized by collisions with excess argon.…”