Pre-reactive complexes

The vibrationally excited states of H2-OH have enough energy to decay either to H2 and OH or to cross the barrier to reaction. Time-dependent experiments have been carried out to monitor the non-reactive decay (to H2 + OH), which occurs on a timescale of microseconds for H2-OH but nanoseconds for D2-OH [52, 58]. Analogous experiments have also been carried out for complexes in which the H2 vibration is excited [59]. The reactive decay products have not yet been detected, but it is probably only a matter of time. Even if it proves impossible for H2-OH, there are plenty of other pre-reactive complexes that can be produced. There is little doubt that the spectroscopy of such species will be a rich source of infonnation on reactive potential energy surfaces in the fairly near future. 

In many cases the XB adduct is a pre-reactive complex (or intermediate) formed prior to chemical reaction or significant charge transfer [18] (see also the chapter by Legon in this volume). The stronger interactions easily evolve into different molecular species if concentration, temperature, solvent polarity, or other parameters are changed [60]. The 1 1 complex that dihalogen... 

Fig. 4 Halogen-bonded adducts are pre-reactive complexes, which, under convenient conditions, can lead to covalent bonds breaking and forming. Perfluoroalkylation occurs when the complexes between iodoperfluoroalkanes and anilines are heated or irradiated in certain solvents...

Rode J, Klos J, Rajchel L, Szczesniak MM, Chalasinski G, Bouchachenko A (2005) Interactions of open-shell clusters ab initio study of pre-reactive complex 0(3P)+HC1. J Phys Chem A 109 11484... 

The potential energy surface of the BrCH2CH2CH3 -h Cl is depicted in Figures 11.7A, 11.7B, and 11.7C. The a hydrogen abstraction is characterized by two different branches as shown in Figure 11.7A. The enthalpy of reaction is predicted to be -3.3 kcalmol-i at the CCSD(T)/6-311+-hG(2df,2p)//MP2(full)/6-31G(d) level of theory. The pre-reactive complexes that were foimd, trans and gauche, have... 

Results from this study confirms the experimental findings that the reaction of n-bromopropane with OH radicals should be slower than the reaction with Cl atoms. The present results show that pre- and post-reaction complexes are important in the hydrogen abstraction reactions. A detailed study of these rate constants that incorporates the contribution of pre-reactive complexes, the multi-chaimel nature of these reactions, and temperature dependence is necessary. The results also find that there are subtle reaction preferences for the abstraction of site specific hydrogen on n-bromopropane. While knowledge of the dominant products of the... 

FIGURE 14.1 MP2/6-311G optimized OH-toluene pre-reactive complex. The indicated... 

The OH interaction (either via an addition or abstraction route) with toluene has been studied using computational chemistry [26-29]. Uc et al. [28] specifically showed that the mechanism for OH interaction with toluene involves the formation of a stable pre-reactive complex (depicted in Figure 14.1) when the radical approaches the aromatic ring at a van der Waals distance. 

Many radical-molecule reaction mechanisms have been shown to be complex and to involve a fast equilibrium between the reactants and the pre-reactive complex, followed by the irreversible formation of products. An example of this mechanism for toluene is shown in Reactions Ibi and Ibii, respectively,... 

The results of the rate constant calculations by d Anna et al,156 seem to confirm this reaction mechanism. In Fig. 25 is shown the temperature dependence of the observed and calculated rate constants. The rate constant k describes the rate of formation of the post-reaction adduct under the assumption that the pre-reactive adducts are not stabilized by collisions, whereas kadd describes the kinetics of formation of the stable pre-reactive complexes at a total pressure of 1 bar. Thus the overall rate constant for the decay of reactants (denoted in the figure by a solid line) is given by the sum k + k. The values of k predicted by d Anna et al.156 distinctly underestimate the reaction rate at low temperatures, but they approach the results of measurements at temperatures above 700 K. The limiting rate constants kadd, and kadd,0 for the addition channels were analyzed in terms of statistical unimolecular rate theory. Results of the calculations show a fall-off behavior of the reaction kinetics under typical atmospheric conditions corresponding to a total pressure of 1 bar. Therefore, all kadd values were derived from the... 

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