STRONG COLLISION BROADENING FACTORS FROM THEORIES OF UNIMOLECULAR RATE COEFFICIENTS

The broadening factor representation of falloff behavior of unimolecul ar rate coefficients as a function of pressure is essentially an expre ssion of deviations from a referential strict Lindemann falloff curve. There are advantages in recasting the broadening factor as a function of a new variable u, a bounded function of pressure. It is shown anal ytically that, as a function of u, F has a relative minimum and is alw ays concave upward. The important features of F(u) are therefore the l ocation and the value of its minimum. Broadening factors for a wide ra nge of temperatures were calculated, in the strong collision limit, fr om standard RRKM theory for aziridine inversion and from a variational version of RRKM theory for methane dissociation. An extension of the Kassel model yields analytic results for F which are compared to the R RKM results for F and to the standard Kassel model results for F. The extended Kassel model is found to be better than its standard version in describing the temperature dependence of the RRKM-based broadening factors but cannot fully reproduce the diverse behavior which the latt er display. These differences are interpretable in terms of energy dep endence of transition-state sums of states and reactant molecule densi ties of states given by the theories.