Jan Turulski

How much do the ion-dipole capture rate constants obtained by transition state theory differ from those given by trajectory calculations?

The ion-dipole capture rate constant is calculated by using the macrovariational version of transition state theory for the conservation of resultant orbital-rotational momentum. The results are obtained in the nearly analytical form and show practically no differences with the results obtained by much more time consuming trajectory calculations.

Double Stark effect in the theory of ion-dipole capture processes

Equations have been derived to describe the energy of a linear dipole in the weak external rotating electric field. This formalism is used to estimate the effect of the principle of resultant momentum conservation on the rate constant for the ion–dipole capture calculated according to the statistical adiabatic channel model.

Effect of quantization of rotational energy on the ion-molecule capture

Abstract Failure to account for the quantized character of the energy of the molecule is one of the main reasons for the differences between measured and classically calculated rate constants for the ion-molecule charge transfer reactions. The quantization is especially important at low temperatures even for molecules that feature small dipole momenta.

Quantum effects in the ion-dipole capture

Abstract A microcanonical version of the transition state theory, TST, combined with the statistical adiabatic channel model, SACM, show the importance of quantum effects induced by the changes in angular momentum in the ion-dipole captures that occur at low energies. At extremely low energies the coupling of orbital and rotational motion becomes the rate determining factor. Hence, the theories based on the Born-Oppenheimer approximation are completely inadequate at such energies. The correct rate parameters that account both for the quantization of angular momentum and the coupling of orbital-rotational motions are reported for some model systems.

The rate of ion-linear dipole capture at extremely low temperatures estimated with the use of the statistical adiabatic channel model

Abstract The rate constant for ion—linear dipole capture is estimated over a range of very small energies using the microcanonical version of the statistical adiabatic channel model (SACM). The energy dependence is found to be very diverse and not following any simple pattern. The dimensionless parameters y and ξ affect the issue considerably. Depending on these parameters, the function rate constant as a function of energy may increase either monotonously or stepwise, it may attain a plateau value at some energy and next, having surpassed some threshold point, may further increase monotonously at a rate that is affected by the magnitude of ξ and y .

Classical transition state theory for the ion—linear quadrupole capture

Abstract The classical treatment using a microcanonical version of the transition state theory shows that the ion-linear-quadrupole capture rate constant depends on two dimensionless parameters, B3 and ζ which are defined as B3=Qq/(2Eα3q6) 1 4 and ζ=Mα2q2/IQ2. Not only is the capture rate affected by the absolute value of B3 but also by its sign. This sign is, in turn, determined by the sign of the product of the charge on the ion and the quadrupole moment of the molecule. The sign effect undergoes a change to the opposite on going from one asymptotical region, ζ→∞, to another, ζ→0. Based on such a result the contradictions in the earlier theories can be reconciled.

The classical Langevin rate constant for ion/molecule capture: when, if at all, is it constant?

Abstract Using the statistical adiabatic channel model (SACM) to estimate the rate of capture of an ion by a spherically symmetrical molecule, and including quantization of the orbital momentum, leads to the reduced capture rate constant that depends on two dimensionless parameters characterizing the system. The form of such a dependence indicates, however, that for all of the typical ions as well as for all of the typical spherical molecules the capture rate constant approximates to the classical Langevin rate constant. The rate can only be appreciably faster if the system features a very small reduced mass, for instance, a thermal electron plus a molecule. Also the importance of two quantum effects, the overbarrier reflection from the potential barrier and the tunnelling through this barrier, was examined with the use of three different barriers to approximate the potential barrier for the polarization complex. The functions that describe this barrier realistically, the symmetrical Eckart function and the Dirac comb, indicate that any contribution to the capture rate constant for all of the ion-molecule systems considered from the former effect can be ignored over the whole range of temperatures. The latter effect, tunnelling through the barrier, is important but only from the systems that feature reduced masses so small as to become physically unrealistic. The physically realistic systems do not undergo any tunnelling unless the temperature is extremely low, decreasing below the characteristic rotational temperature of the molecule.

Simple theory of the charge transfer rate in ion/molecule reactions. Part 1. Derivation of the theory

Abstract Simple equations are derived for calculation of the charge transfer rates for unichannel ion-polar molecule reactions occurring in the gas phase at moderate temperatures. Also, equations are given yielding the magnitudes of some kinetic and molecular parameters characterizing a transition state involved in these reactions.

Implementation and limitations of the transition-state theory for ion–molecule systems with non-spherical dividing surfaces

Some inconsistencies in the use of transition–state theory (TST) to calculate rate constants for ion-polar molecule reactions are indicated. For strongly polar molecule involving small orbital angular momenta there is, for a certain range of orbital angular momenta and orientation angles, no way to define the critical dividing surface, S*, and the notion of capture becomes meaningless. For collisions involving large orbital angular momenta S* can be defined for all orientation angles but will be spherically unsymmetrical. The surface should be chosen so that the flux normal to it is a minimum. While conventional TST versions are shown to fail, the equations are derived that ensure the minimum flux for a given S*. The iterative version of TST is formulated that makes it possible to find the optimum S*.

Simple theory of the charge transfer rate in ion/molecule reactions. Part 2. Experimental verification

Abstract A comparison of theoretical calculations with experiment is presented. For simple proton and hydride anion transfers, the agreement is satisfactory, especially for exothermal processes. Some sources of error are discussed: the multiple reflection from the potential barrier, the effect of increasing the height of the barrier of the activated complex, and the effect of additional channels.