Dynamical effects of symmetry along a reaction path: mode specificity in the unimolecular dissociation of formaldehyde
If there is a geometrical symmetry (i.e., C/sub s/, C/sub 3v/, etc.) that is conserved along a reaction path (the steepest descent path in mass-weighted Cartesian coordinates from a transition state to reactants and to products), then it is shown that this leads to selection rules in the dynamical coupling between the reaction coordinate and the vibrational modes that are orthogonal to it: namely, states corresponding to different irreducible representations do not interact. Thus, even if one makes a statistical (i.e., transition-state theory) approximation to the dynamics within a given irreducible representation, i.e., a ''symmetry-adapted transition-state theory'', there can still be mode-specific effects between the different symmetries. The unimolecular decomposition of formaldehyde, H/sub 2/CO ..-->.. H/sub 2/ + CO, which has a planar reaction path, is taken as an example, and it is seen that the A' and A'' microcanonical rate constants differ by a factor of approx. 20 in the tunneling regime and still by a factor of 2 at approx. 5-6 kcal/mol above the classical threshold of the reaction.
- Research Organization:
- Lawrence Berkeley Lab., CA
- DOE Contract Number:
- AC03-76SF00098
- OSTI ID:
- 6164525
- Journal Information:
- J. Am. Chem. Soc.; (United States), Journal Name: J. Am. Chem. Soc.; (United States) Vol. 105:2; ISSN JACSA
- Country of Publication:
- United States
- Language:
- English
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