![]() When the energy transfer from a polyatomic molecule is modeled in classical mechanics, an ambiguity arises, namely, what the reference of measurement of the energy content of the molecule should be. Accordingly, in RRKM calculations the energy is always referred to the zero-point energy of the reactant and the transition state, respectively. In dissociation and in reactive systems in general, however, the zero-point energy of the spectator modes is supposed to be inactive in the sense that only the energy exceeding it can be converted from vibrational modes to the motion along the reaction path (not considering the change of the frequencies and of the zero-point energy of “spectator” modes when the molecule moves from its equilibrium geometry to that in the transition state). ![]() of 20 such modes (not even considering the other modes in such a molecule), if not “frozen,” is equivalent to the energy needed for CC or CH bond dissociation. For example, the zero-point energy corresponding to a C–H stretch mode of about 3500 cm − 1 is about 5 kcal mol − 1 or 21 kJ mol − 1, i.e., the z.p.e. György Lendvay, in Comprehensive Chemical Kinetics, 2019 2.11 Setting of the zero of energy measurement in the excited moleculeįor large molecules, the vibrational zero-point energy is very large, comparable to the height of the potential barrier to reaction.
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