ACCSA sudden approximation

At low temperature the classical approximation fails, but a quantum generalization of the long-range-force-law collision theories has been provided by Clary (1984,1985,1990). His capture-rate approximation (called adiabatic capture centrifugal sudden approximation or ACCSA) is closely related to the statistical adiabatic channel model of Quack and Troe (1975). Both theories calculate the capture rate from vibrationally and rotationally adiabatic potentials, but these are obtained by interpolation in the earlier work (Quack and Troe 1975) and by quantum mechanical sudden approximations in the later work (Clary 1984, 1985). 

Figure 6. Temperature dependence of the reaction efficiency per collision for the reactions of OD + CH3Cl (open circles), 0D D20 + CH3Cl (filled circles and 0D (D20)2 + CH3Cl (half-filled circles). The reaction efficiency per collision is the experimental rate constant divided by the calculated collision rate constant, calculated by Clary using the adiabatic capture centrifugal sudden approximation (ACCSA) (28). For experimental reasons (29), the measurements were made with completely deuterated...

In the present study, we employ the ACCSA method which involves a combined rotationally adiabatic capture and centrifugal sudden approximation. The method enables the computation of cross-sections and rate constants which are state-selective in the initial rotational states of the reactant molecule. The ACCSA approach has been successful for ion-dipole reactions which involve proton transfer. Predicted rate constants for the reactions -f HCN H2CN + H2 and HCO + HCN - - CO are in... 

When the interaction depends on the orientation of the neutral molecule, as is the case, for example, for ion-dipole reactions, the simple treatment outlined above is no longer appropriate. The adiabatic channel method is often used in this context [32]. The rotational energy levels in the separated reactants are coupled with the orbital energy levels to define a set of channels for the collision complex. The number of open states, N(EJ), is the number of channels with an energy maximum below the energy E. Examples of this approach include the adiabatic channel centrifugal sudden approximation (ACCSA) of Clary [33] and the statistical adiabatic channel model (SACM) of Troe and co-workers [34]. 

---