On simple gas reactions

Abstract

We set ourselves the task to investigate London's reaction rate theory of adiabatic processes for the simplest case of the three atom reaction Y+XZ?YX+Z in linear configuration. It appeared that our understanding of this reaction is crucially affected by a term that has not been taken into consideration until now, the exchange energy between the two outer atoms (Y and Z), which acts to enhance the saturationand activation mechanism of the valence forces. The investigation was carried out as follows: 1. In a first step the entire binding energy (potential energy) obtained from the band spectra was considered as resonance energy, such that the potential energy of the system of atoms could be calculated as a function interatomic distances and the result be plotted in a contour diagram ("resonance energy surface"). In this way the significance of the interaction of the outer atoms for the energy content and the configuration of the transition state becomes apparent; it raises the energy of the latter (a-contribution to the heat of activation) and causes an extension of the affected molecule. This term essentially is responsible for the instability of the transition state (H3). 2. Further, we attempted to take into account the Coulomb contribution to the binding energy. Starting from Heitler-London theory and the calculations by Sugiura we presented this function for the atom pair H2 and then calculated the Coulomb term of the binding energy for the reaction of three H-atoms as a function interatomic distances ("Coulomb well)." Subsequently, we reduced the "resonance energy surface" according to the calculated contributions of exchange and Coulomb terms to the binding energy, and then superimposed the corrected "resonance energy surface" and the Coulomb well. 3. In order to extract from the resulting potential energy surface the heat of activation, one has to consider the zero point energy of initial and transition state. It follows from the shape of the surface that the quasi-elastic binding force and, hence, the zero point energy of the transition state amounts to just a fraction of that of the initial state, such that almost the entire zero point energy of the initial state may contribute to the heat of activation. The magnitude of the latter obtained in this way (13 kcal), within the uncertainties of the approximations, agrees with the value of 4 to 11 kcal measured by A. Farkas for the reaction H+H2 para ?H2 ortho +H. In addition, the discussion of other reactions, though not as detailed, supports the adiabatic picture that can explain the high reactivity of free atoms. 4. The dynamics of the reaction may be represented (to the extent that the equations of motion are applicable) as the motion of an image point in the potential field plotted as a function of distances between the atoms. It appears that it is justified to identify the height of the saddle of the energy landscape with the heat of activation. In addition, one may deduce what kind of energy (vibrational or translational) must be provided as activation energy. © by Oldenbourg Wissenschaftsverlag, München.