Average dipole orientation, ADO

For highly exothermic SN2 reactions, which have a central barrier significantly lower in energy than that of the reactants, association of the reactants may be the rate controlling step in TST.1 The SN2 rate constant can then be modeled by a capture theory9 such as VTST,10 average dipole orientation (ADO) theory,11 the statistical adiabatic channel model (SACM),12 or the trajectory capture model.13... 

The reactions of PH3 with ions in the gas phase generally were investigated by mass spec-trometric methods. The experimental rate constants were occasionally compared to maximum rate constants which were calculated by using the average dipole orientation (ADO) theory [1 to 3]. 

A large number of approximate theories have been proposed for ion-dipole reactions. Some of these include the average dipole orientation (ADO) approximation and its extension to include conservation of angular momentum (the AADO method ), various transition-state theories involving variational and statistical modifications, the semiclassi-cal perturbed rotational state (PRS) approximation, classical trajectory studies, the adiabatic invariance method, and the statistical adiabatic channel model (SACM). 

The products form by charge exchange, proton abstraction, or elimination. Reaction mechanisms were frequently discussed see for example [4, 8, 9]. Multiple H-D exchange, as in the formation of OH from D2O, was occasionally observed, but its minor importance indicates a short lifetime of the reactive intermediate complex [19]. The average dipole orientation (ADO) theory usually predicts the rate constants satisfactorily an exception is the value calculated for the reaction with N2O which is much too small [8]. Calculated structural parameters of the adduct of NHJ with NH3 are given on p. 269. 

Addressing this limitation, Su and Bowers developed the average dipole orientation (ADO) theory in which the orientation angle of the dipole with respect to the line of centres of collision is averaged as a function of ion-molecule separation distance. This concept leads to a locking constant , c, which is multiplied by the cos 6 term this constant has been parametrized as a function of for the range of... 

A more realistic description was provided by Su and Bowers and is known as the average dipole orientation (ADO) theory [22,23]. The ADO rate coefficient is given by... 

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