H + H2 reaction

D Mello M, Duneczky C and Wyatt R E 1988 Recursive generation of individual S-matrix elements application to the collinear H + H2 reaction Chem. Phys. Lett. 148 169... 

Park T J and Light J C 1989 Accurate quantum thermal rate constants for the three-dimensional H + H2 reaction J. Chem. Phys. 91 974... 

Evidently, the dissociation energies of the H—H and Cl—H bonds are very close and the triplet repulsion in the transition states of these reactions is, therefore, almost identical. Nevertheless, the quantities Eeo and re in these two reactions differ very considerably. The reason for this is that the H—H bond is nonpolar, while the Cl—H bond is polarized its AEA 92.3 kJ mol 1 (Equation [6.29]). As in the HC1 molecule, in the transition state there is evidently a strong attraction between Cl and H, which in fact induces a decrease in re and Ee0. If the Cl + H2 reaction was characterized by the same parameter re = 3.69 x 10-11m as the H + H2 reaction, an activation energy of Ee0 = 56.5 kJ mol 1 would be obtained for that reaction. The difference between the observed and expected activation energies (A ,ea = 36.7—56.5 = —19.8 kJ mol 1) must be attributed to the influence of the unequal electronegativities of the hydrogen and the chlorine atoms on Ec(, in the Cl + H2 reaction. 

T. P. Softley At 10 meV collision energy for the H + H2 reaction the collision energy resolution is approximately 3 meV. This is due to the translational temperature of the beam and angular divergence primarily. 

T. P. Softley At the present time, we do not say that there are no rotational effects on the cross section of the H + H2 reaction but that our signal-to-noise levels at the lowest collision energies (where these effects might occur) are not sufficiently good to draw any conclusions (see the current chapter). 

Figure 2. Dotted lines are the eigen-reaction-probabilities p (E) for the collinear H + H2 reaction. The solid line is their sum, the cumulative reaction probability N(E).

Figure 3 shows the CRP N(E) for the collinear and three-dimensional versions of the H + H2 reaction. The nonmonotonic energy dependence seen in N(E) for the collinear case (Fig. 3a) is a result of transition state-violating dynamics. In the 3d case (Fig. 3b), however, N(E) is monotonically increasing that is, the additional averagings involved in 3d causes TST to be a better approximation (a well-known phenomenon). 

Trajectory surface-hopping (TSH) calculations have been accomplished for H3+ on DIM hypersurfaces.2,498 A surface-hopping mechanism has also been suggested as applicable to several other systems, including various He2+ charge-transfer reactions,495 the H -H2 reaction,499 and the C+-H2 radiative association process.500... 

The search for reactive resonances in the H+H2 reaction (and its isotopomers) has a long history [1-5, 111, 112]. The existence of resonances was first... 

I. dose-coupling method for collinear H+H2 reaction, Mol. Phys. 22 (1971) 881. 

R.T. Skodje, R. Sadeghi, H. Koppel, J.L. Krause, Spectral quantization of transition state dynamics for the three-dimensional H+H2 reaction, J. Chem. Phys. 101 (1994) 1725. 

The VB simplified model of ground-state potential energy surface H3 system considered as transition state and stabilization valleys of the H + H2 reaction is also an early problem, belonging to the history of physical chemistry under the name London-Eyring-Polanyi-Sato (LEPS) model that continues to serve as basis of further related developments [17,18], The actual analysis is a new a focus on the JT point of this potential energy surface able to absorb results of further renewed CASCCF type calculations on this important system. 

TABLE VI Properties of Linear Symmetric (Dx)1) Saddle Point for H + H2 Reaction ... 

Examples are the oxygen electrode, Equation (1), or the hydrogen electrode (i.e. the H+/H2 reaction), Equation (2). Electron transfer proceeds regularly with the reacting species adsorbed at an electrode surface. In many cases, electrocatalysis by the electrode metal plays an important role. Associated chemical reactions, such as protonation and dissociation, render the reaction mechanisms complex. This is true in particular for the oxygen electrode. 

As indicated previously, it is desirable to consider the individual electrode reactions independently. One might suppose that this could be achieved by characterizing the individual electrodes as described in Section 3.1.3. However, for reasons of sound thermodynamics, another method has been established. It was decided to relate all electrode reactions to one common reference electrode. Electrochemists have chosen the H+/H2 reaction under standard conditions (ct 1+ = 1M p 12 = 1 bar) as such a general reference electrode. It is termed the normal hydrogen electrode or the standard hydrogen electrode (SHE). Thus, whenever E and E° values are presented for individual electrode reactions (half cells), it is understood that these values pertain to a complete cell in which the SHE constitutes the second electrode. 

The idea of the existence of a boundary between reactants and products can be traced to the scientific memoirs of Marcelin published in 1915 [26], It was not until 1931 that this idea began to percolate into the thinking of the chemistry community. In that year Eyring and Polanyi published their seminal article on the calculation of the absolute reaction rate for the collinear H + H2 reaction [27], It was in this article, which must be viewed as the origin of the modern theory of chemical reactions, that the concept of a TS separating reactants from products is first quantified. They defined it in terms of the morphology of the potential energy surface. 

The development of theory for reliable calculations of chemical dynamics has two components the construction of accurate, ab initio, multidimensional potential energy surfaces (PESs) and the performance of reactive scattering calculations, either by time-independent (TI) or time-dependent (TD) methods, on these surfaces. Accurate TI quantum methods for describing atom-diatom reactions, in particular for the benchmark H + H2 reaction, have been achieved since 1975.[1,2,3] Many exact and approximate theories have been tested with the H + H2 reaction.[4,5]... 

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