The homogeneous gas phase H₂D₂ metathesis at room temperature: Reaction induced by specific vibrational excitation

Abstract

We have demonstrated that reactions for which substantial activation energies are needed can be induced to occur at room temperature via specific vibrational excitation.Indeed, the indications are that the atom-switching reactions for which E, > 25 kcal take place with high probability only when the activation energy is localized in the vibrational mode. In this preliminary report on the utilization of the stimulated Raman effect to generate substantial populations in the critical vibrational states required for the homogeneous atom exchange between H Z and Dz, we first summarized the historical development of the concept. The experimental arrangement is then described and the analytical results tabulated; the observed dependence on relative concentrations is semiquantitatively rationalized on the basis of a model proposed in 1964. Independent shock tube and molecular beam investigations were similarly accounted for. Attention is called to the discrepancy between the generally concordant experimental results and the a6 inztio quantum mechanical calculations of the potential energy surface for 4H atoms.The bimolecular, four-center metathesis is a reaction type that is used with great ireluctance by chemical kineticists in formulating reaction mechanisms, even though the presumed prototype(1) 1-1 + H-H $ 2 H-Iis treated as such in all textbooks on kinetics published as recently as 1968. The fact that the rate of production of HI from the gaseous elements is directly proportional, over the temperature range 600" i 100"K, to the product [H2][12] was accepted for many decades as an argument in favor of a bimolecular exchange which passed through a four-center transition state, presumed to have a trapezoidal structure [ 11. Whereas the uniqueness of this conclusion was questioned as early as 1926, the simplicity of the model proved too enticing to relinquish. During more than half a century, few if any authentic analogous reactions were un-93

Crossed molecular beam studies of proton transfer, unimolecular decay, and isotope scrambling in the reactions H2+ + H2O, H2+ + D2O, and D2+ + H2O

“…However, in the studies at hand, the product HOD+ is observed at collision energies below 1 eV, with H2+ reagents which have up to 2.6 eV of internal excitation. The role of reagent internal excitation in promoting the four-center H2 + D2 reaction has been emphasized in the laser excitation experiments of Bauer et al,31 the interpretation of which is consistent with the present work in which H2+ vibrational excitation may be efficacious in surmounting a symmetry-imposed barrier.…”

Reaction of cyanogen and hydrogen behind reflected shock waves

“…For the simpler H2 + D2 reaction, these equations have been solved with the result that the value of z changes from 1.2 to 3 without an induction period. 22 It has been suggested that the reaction of C2N2 with D2 to form DCN would be faster than C2N2 with H2 if VEX were the dominant mechanism.23 This prediction is related to the observation reported in the single-pulse shock tube study of the H2 + D2 exchange that the rate depended more strongly on the concentration of D2 than on H2.21. 24 In the temperature range 1050-1200°K, the rate of D2 with C2N2 was found to be faster than that of H2 with C2N2 although the rate decreased with increasing temperature.23…”

Reaction of hydrogen and carbon dioxide behind reflected shock waves

“…The importance of the vibrational energy content of the reactants has been emphasized and the observed nonlinear time dependence for product formation supports the contention that there exists a nonequilibrium step(s) in the exchange mechanism. 21 The determination of non-zero-order dependencies for the inert gas diluent argues also for a multistep process involving excitation of the reactants prior to exchange.…”