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The agreement between the first three values of τ Γ * that are listed is scarcely surprising since semi-empirical potentials can be adjusted to give this coincidence (Porter and Karplus's value for * is low because they fitted their potential to an earlier estimate of the 'true' barrier height). Comparison of the other quantities in the table indicates how alike the potentials are in the region of the energy barrier. All the methods predict very similar internuclear distances in the transition state—and agree that it is linear.

5. Diagram illustrating the release of energy during exoergic chemical reactions [adapted from Polanyi, J. C , Accounts of Chemical Research, 5, 161 (1972)]. The bold lines represent the change in potential along the reaction path of minimum energy. On the attractive surface, most of the energy is released as the reactants approach and before the point is reached where r / r = r / r . 5 can also be used to illustrate a distinction between different types of potential surface that is important in discussions of reactive collision dynamics that will come l a t e r .

Van Nostrand, New York (1950) Only limited information about non-bound molecular states can be obtained from studying the continuous spectra that arise as a result of transitions to or from these states. However, molecular beam measurements on the elastic scattering of pairs of noble-gas atoms have now been m a d e ' with sufficient resolution, for the potential curves controlling the collisions to be determined accurately, by a numerical inversion technique which is the analogue of the R K R method that is used to treat spectroscopic data.