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The hydrogen-exchange reaction

Perhaps the most famous reaction surface is that for H+H2 → H2+H. The H3 surface has been calculated to an accuracy of ¼ kcal/mol at 267 points using Cl wave functions [B.Liu., J.Chem.Phys., 58, 1925 (1973); P. Siegbahn and B. Liu, J. Chem. Phys., 68, 2457 (1978)], and an analytical potential function has been fitted to these points [D.G. Truhlar and C.J. Horowitz, J. Chem. Phys., 68, 2466 (1978)].  From this surface, rate constants in good agreement with experiment have been calculated [M.C. Colton and G.C. Schatz, Int. J. Chem. Kinet., 18, 961 (1986); G.C. Schatz, Chem. Phys. Lett., 108, 532 (1984)]. See also J.V. Michael et al., Science, 249, 269 (1990). More-accurate PESs for H3 are H. Partridge et al., J. Chem. Phys., 99, 5951 (1993) and A.I. Boothroyd et al., J. Chem. Phys., 104, 7139 (1996).

Chemistry professor and theoretician David C. Clary of University College, London, comments that the hydrogen-exchange reaction is "the simplest chemical reaction, and to really understand the fundamentals of chemical reactions, this is the benchmark that has to be understood first."

But for now, he and his coworkers are focusing on the hydrogen-exchange reaction's dual mechanism. "What seems to be happening in the forward-scattered process," Althorpe says, "is that the H and D2 become trapped together in the form of a prereactive H-D2 complex, which rotates around in space. The H and D2 are being trapped on the reagent side of the reaction barrier, before the D2 bond breaks."

When Wrede attempts to model his behavior using Newtonian (classical) mechanics, "then there is some trapping, but this occurs on the product side of the bar rier," Althorpe says. "Hence, classical mechanics says the trapping occurs after the D2 bond has broken, but quantum mechanics, the true picture, tells us that the trapping occurs before the D2 bond breaks."

The group is currently studying why the classical and quantum simulations differ. "These are challenging calculations," Althorpe says, "because by being trapped on the H + D2 side of the barrier, the complex is obscured by a large cloud of H atoms that are bouncing straight off the D2 molecules without reacting. We therefore need to find a way of disentangling the HD2 complex from this large cloud of departing H atoms, and that is what we are working on right now. It's a bit like trying to find your relatives who are waiting at the airport while a crowd of people is streaming out of the exit."

ZARE  NOTES  THAT  he is "waiting for Stuart to complete further calculations to learn more about the nature of the delayed forward-scattered HD product. Fernandez-Alonso and I have written a review article on just this problem in the press." [Ann. Rev. Phys. Chem., 53 (2002)].

Zare adds that the hydrogen-exchange reaction reveals its secrets very reluctantly. the new study shows, he says, "that we still have so much to learn, even about the simplest of chemical reactions."