The Absolute Rates of Reaction of Hydrogen with the Halogens

Abstract

In this paper we apply the statistical-mechanical method which has only recently been developed for reaction rates to three new groups of reactions. These are the reactions of I2, Cl and Br with the various kinds of hydrogen molecules. In the first type (I2+H2→2HI) symmetry properties enable one to divide the six internal modes of vibrations for the four coplanar atoms into three sets with one, two and three modes of vibration in each so that in obtaining the frequency nothing greater than a cubic is solved. This treatment of the frequencies will apply to a very broad class of reactions. The potential surfaces probably yield too high a zero-point energy for the activated state. In spite of this result they give the right relative rates for the various isotopic reactions because mass enters very strongly into what in the past has been thought of as the collision factor. The study of isotopic reactions in conjunction with the new theory of rates gives a detailed knowledge of the potential in the activated state and can be carried much farther than has been done in this preliminary treatment. The treatment has been carried far enough however to show its general applicability to these types of processes.

Semi‐empirical potential energy surfaces for hydrogen‐atom transfer reactions

“…The stretching and bending frequencies in the activated complex are low [l 1, 231, whereas the present values, which are similar to Eyring's results [24], are considerably higher, therefore requiring a higher tunneling correction. I n addition to the possible sources of error in the present surfaces discussed above, Klein [25] has suggested that the overlap integral between the end atoms in 1H-2H-Cl might differ considerably from the integral for the two adjacent atoms (2H-C1 value for the diatomic halides based on the fact that they have similar molecular orbitals may not be strictly valid since the 5pn orbitals of H I may have appreciable bonding power [26].…”

Valence bond theory for chemical dynamics

“…Valence bond theory has been used as a framework for modeling chemical reaction dynamics since the early days of quantum mechanics 2, 3, 20–24, 28, 29, 40, 41, 45, 48, 50, 51, 67, 72, 79, 81, 86, 87, 89, 90, 92, 95–99, 101–104, 106, 112, 210–286. In part this may be explained by the fact that valence bond theory often provides a concise description of multireference character, and transition states are more likely than stable molecules to exhibit multireference character.…”

“…The method has proved to be a very efficient way to fit potential energy surfaces,312, 313 and it can be made even more efficient by using different amounts of data to interpolate different elements of the Hessian of the resonance integral314 and by adjusting the diagonal valence‐bond matrix elements as well as the off‐diagonal ones 320. Thus, the lessons learned from valence bond theory1–3, 13, 20–112, 210–286 can be imported into MCMM to provide improved representations of reactive potential energy surfaces.…”

Molecular thermodynamics for chemical reaction design

“…This was first applied by Eckert and Boudart (1963) to the bimolecular pyrolysis of hydrogen iodide. The model of Wheeler et al (1936) in equilibrium could be anticipated. Such effects are not unexpected, as dense gases can often have excellent solvent properties, interacting strongly with dilute species.…”