Branching ratios in activated systems

Journal of Physical Chemistry A

Volumn 101, Issue 1, 1997, Pages 19-24

  Craig, Stephen L ^a   Zhong, Meili ^a   Choo, Bryan ^a   Brauman, John I ^a  

^a STANFORD UNIVERSITY   (United States)

Author keywords

[No Author keywords available]

Indexed keywords

CALCULATIONS; ENTHALPY; ENTROPY; PHASE SPACE METHODS; QUANTUM THEORY; REACTION KINETICS; SURFACE TENSION;

ACTIVATED SYSTEMS; BRANCHING RATIOS; POTENTIAL ENERGY SURFACE;

ACTIVATION ENERGY;

EID: 0031546267     PISSN: 10895639     EISSN: None     Source Type: Journal    
DOI: 10.1021/jp961665j     Document Type: Article

References (31)

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20. The sum of states is also constrained by conservation of angular momentum, and a rigorous treatment would have to include that condition. In this work, we assume that the moments of inertia of the competing channels are equal, so that the only consequence of conserving angular momentum is to reduce the total energy available to the system (i.e., increase the apparent barriers to reaction by equal amounts). This is likely to be an appropriate assumption for cases in which the two transition states have comparable structure and, for this work, facilitates interpretation of the relationship between the BR and other characteristics of the potential energy surface.
21. While there may be two distinct complexes, interconversion between the complexes is expected to be extremely facile.
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24. We could, in principle, have selected two transition states with identical frequencies and therefore identical entropies of activation. We choose to examine the more general system in which there are competing effects due to entropy and enthalpy.
25. In the instance that the rate constant in the denominator is zero, the BR will be undefined. Switching the two rate constants so that the numerator is zero only temporarily avoids this problem, as the logarithm of the subsequent BR is then similarly undefined. The purpose of this paper, however, is to show how information about relative activation energies may be inferred from an observed BR. If either of the rate constants is zero, one cannot determine the relative activation energies of the two processes except to say that the total energy of the system lies between the critical energies of the reactions, and the subsequent analysis is irrelevant.
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31. 5,6 as a qualitative measure of the energy distribution in studies using the kinetic method. While Cooks' definition is more phenomenological, the RRK form in eq 8 may provide the basis for a more rigorous definition.