A crossed molecular beam study has been made of reactive cross section as a function of collision energy Sr(ET) for all isotopic variants of the abstraction reaction H′+H″Br→H′H″+Br. The apparatus incorporates, for reagent preparation, a supersonic source of variable-energy H or D atoms, and, for product detection, a tunable vacuum ultraviolet laser to obtain laser-induced fluorescence of Br. The cross-section functions indicate that the threshold energy for reaction is <1 kcal/mol. At enhanced collision energy of ET = 7 kcal/mol, the observed order of reactivity in the isotopic series designated (H′,H′′) was (D,H)≳(D,D)≳(H,H)≳(H,D). As noted in a previous report from this laboratory [Int. J. Chem. Kinet., Laidler Festschrift (in press)] the favorable kinematics for (D,H) as compared with (H,D) can be understood in terms of lengthened interaction time for D atom reaction (compared with H) and diminution in the time required for HBr (compared with DBr) to rotate into the preferred alignment for reaction. The effect is illustrated here in terms of a simple model of reaction. The experimental data obtained in this work at low collision energy, in conjunction with 300 K rate constants obtained by others, suggest that close to threshold, kinematic effects are supplanted by threshold effects, yielding Sr(H,D)≳Sr(D,H), the inverse of the principal isotope effect at enhanced collision energy.

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A crossed molecular beam study has been performed on the nonadiabatic reactions F(2P3/2) [F(2P1/2)]+HBr(DBr)→HF(DF)+Br(2P3/2) [Br(2P1/2)]. Atomic F came from a seeded supersonic jet, so that the cross sections Sr(Br) and Sr(Br) could be measured as a function of collision energy ET = 1–11 kcal/mol. The reagent ratio [F]/[F] was varied by means of a variable temperature F atom source. Products Br and Br were detected by tunable vacuum ultraviolet laser-induced fluorescence (VUV LIF). The reaction cross sections showed no threshold, but a steep decline with increasing ET; the barrier to reaction on the FHBr surface is <1 kcal/mol. The ratio [Br]/[Br] showed no correlation with [F]/[F] in the reagents, indicating that the source of Br was not the adiabatic process F+HBr→HF+Br, but a nonadiabatic process F+HBr→HF+Br, Br. The results at high collision energy indicate that there is a substantial barrier to the reaction F+HBr→HF+Br. The cross section ratio for the two branches of F+HBr, i.e., Sr(Br)/Sr(Br), is 0.056±0.004 at room temperature. This ratio declines slightly with increasing collision energy, and is sensitive to isotopic substitution: for F+DBr at 300 K, Sr(Br)/Sr(Br) = 0.0101±0.0016. The velocity dependence and isotope effect of the branching ratio are discussed in terms of an ’’energy exchange’’ occurring well along the exit valley of the ground electronic state potential-energy surface, in which V–E transfer between nascent HF and Br results in a hop onto the upper potential-energy surface.

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'.

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