Rate Constants for Strong Collisions

To our thinking, the usual RRKM procedure is often not transparent enough to show directly the influence of the various contributing and often uncolain molecular parameters. For this reason, a simplified representation of the strong-collision rate constants has been suggested, Mn the form of equation (20). 

Here the harmonic density of vibrational states in many practical cases is adequate represented by the simple semi-classical density of states  

For higher precision, Whitten and Rabinovitrii s or HaaihofTs apf oxima-tions are quickly evaluated. The vibrational partititm fimetion is given by equation (22). The anharmonicity correction factor can be estimated  

More precise formulae appear to be necessary only if the anharmonicity effects of the molecule at high excitations are better known. Also, the exact interfragment potential and the corresponding centrifugal maxima have to be available to determine better rotational factors equations (24) and (25) have been based on an interfragment potential at the distances important for centrifugal maxima of the form V(g) oIl - 2( ,/ ) ], which corresponds to only the attractive part of a Lennard-Jones potential). 

Finally, hindered-rotor potentials have to be taken into account when there are internal rotations. As long as no precise information on these often uncertain molecular parameters is available, the use of the simplified expressions (20)—(26) is recommended. One generally obtains agreement of better than a factor of 2 with the corresponding complete RRKM calculations (without sqxiration of vibrations and rotations). 

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Table 5.1. Apparent internal relaxation rate constants for strong collision reactions...

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