Timothy Su

Parametrization of the ion–polar molecule collision rate constant by trajectory calculations

A recently reported calculation by chesnavich, Su and Bowers on classical trajectory study of thermal energy ion‐polar molecule capture collisions is further extended. (AIP)

Ion-Polar molecule collisions: the effect of ion size on ion-polar molecule rate constants; the parameterization of the average-dipole-orientation theory

Abstract Proton transfer rate constants from CH 5 + , C 2 H 5 + , C 3 H 7 + and C 4 H 9 + to NH 3 , CH 3 NH 2 , and (CH 3 ) 2 NH have been measured at thermal energies by ion cyclotron resonance techniques. The data are compared with the predictions of the average-dipole-orientation theory. In all cases, the CH 5 + , C 2 H 5 + and C 3 H 7 + ions transfer a proton with essentially unit efficiency. The C 4 H 9 + ions transfer protons with efficiencies between 0.6 and 0.9 depending on the substrate molecule. There is no evidence that the size of the ion has an appreciable effect on the magnitude of the proton transfer rate, at least within the accuracy of the ICR experiments. The average-dipole-orientation theory has been parameterized to permit determination of capture limit rate constants by reading a graph and making a simple calculation. A graph covering the range of polarizabilities and dipole moments of most molecules is included.

Theory of ion‐polar molecule collisions. Comparison with experimental charge transfer reactions of rare gas ions to geometric isomers of difluorobenzene and dichloroethylene

A classical theory for the ion‐permanent dipole interaction is developed that takes into consideration the thermal rotational energy of the polar molecule. The theory is formulated in terms of an r‐dependent average orientation angle θ (r) between the dipole and the line of centers of collision. The technique allows quantitative determination of the capture cross section as a function of ion‐polar molecule relative velocity. In addition, capture limit rate constants are readily calculated both at thermal energies and as a function of relative energy. Charge transfer rate constants from various rare gas ions to difluorobenzene and dichloroethylene isomers have been measured at thermal energies using ion cyclotron resonance spectroscopy. Rate constants are considerably larger than predicted by the capture theories for both polar and nonpolar isomers indicating long range electron jump is a prevalent mechanism for charge transfer in these systems. The average‐dipole‐orientation theory developed in this pape...

Collisions in a noncentral field: A variational and trajectory investigation of ion–dipole capture

Variational rate theory and trajectory methods are used to investigate the classical dynamics of capture on a noncentral long range potential. The ion–dipole surface is investigated in detail. An upper bound to the true thermal capture rate constant is derived using the variational approach. The upper bound is independent of the moment of inertia of the rotor and has only a multiplicative inverse square root dependence on the system reduced mass. For the ion–dipole surface the upper bound is in excellent agreement with trajectory calculations of the true capture rate constant and with experimental data on thermal heavy particle transfer rate constants between ions and polar neutrals. The trajectory calculations are done using an angular momentum conserved coordinate system which eliminates the need to solve two sets of Hamilton’s equations. A precise criterion for capture is derived and is used in the trajectory calculations. Reduced variables are introduced for the ion–dipole surface and it is shown that...

Multiple transition states in unimolecular reactions: A transition state switching model. Application to the C4H8 +⋅ system

A transition state switching model is developed for use in systems where more than one transition state occurs along the reaction coordinate. The model is cast in the perspective of both the unified statistical theory (UST) of Miller and of variational transition state theory. The basic assumptions are those common to transition state theory and RRKM–QET. A reaction branching analysis leads to reaction probabilities for a number of potential surfaces and appropriate expressions are delineated for both unimolecular and bimolecular reactions. The theory is developed from a microcanonical viewpoint and rigorously conserves both energy E and angular momentum J. Comparison is made with experimental data for the C4H8 +⋅ system where absolute unimolecular rate constants and branching ratios have been measured as a function of energy, bimolecular rate constants, and branching ratios measured at room temperature (the ethylene ion–molecule reaction), the lifetime of C4H8 +⋅ measured when formed by the ethylene ion–...

Kinetic energy, temperature, and derived rotational temperature dependences for the reactions of Kr+(2P3/2) and Ar+ with HCl

Rate constants for the reactions of Kr+(2P3/2) with HCl and DCl and of Ar+ with HCl have been measured as a function of reactant ion/reactant neutral average center‐of‐mass kinetic energy (〈KEc.m.〉 ) at several temperatures. The measurements were made using helium as the carrier gas. From these data we have derived the dependences of the rate constants on the rotational temperature of H(D)Cl. Rate constants for the reaction of Kr+(2P1/2) with HCl have also been measured as a function of temperature. The rate constants for all of the reactions were found to decrease with increasing temperature. The rate constants were also found to decrease with increasing 〈KEc.m.〉 at low 〈KEc.m.〉 but then to increase at higher 〈KEc.m.〉 . A significant rotational temperature dependence of the rate constant was derived for the reaction of Kr+(2P3/2) with H(D)Cl. The analogous derivation for Ar+ reacting with HCl showed the rate constant for this reaction to be independent of the rotational temperature of HCl within experime...

Ion-polar molecule collisions. A modification of the average dipole orientation theory: The cos θ model

Abstract The average dipole orientation (ADO) theory of ion-polar molecule capture collisions is reformulated in terms of the average cosine, cos θ . The earlier formulation used an average angle, θ , in the effective potential that is used to develop expressions for the cross section and rate constant. The cos θ and θ models give virtually identical results for thermal energy capture rate constants.

Ion-polar molecular collisions: the average quadrupole orientation theory

Abstract A classical theory for the ion—quadrupolar molecule interaction, termed the average quadrupole orientation (AQO) theory, is developed for molecules that belong to the D X point group. The theory utilizes a framework similar to the average dipole orientation (ADO) theory developed previously. The AQO theory indicates that there is a significant increase in the capture rate constant (0–40%) for molecules with large quadrupole moments and small polarizabilities. Empirical formulas have been obtained which permit determination of the thermal energy capture limit rate constants by making simple calculations. Thermal energy proton transfer reaction rate constant at 300 K from D 3 + to a number of quadrupolar molecules have been measured experimentally using ion cyclotron resonance spectroscopy. Experimental results are in good agreement with predictions of the AQO theory and are substantially less than predictions of the locked quadrupole theory.

Trajectory calculations of ion‐quadrupolar molecule collision rate constants

The trajectory method is used to calculate collision rate constants for ion‐quadrupolar molecule interactions for molecules belonging to the D∞h point group. The calculations utilize a framework similar to the trajectory technique developed previously for ion‐polar molecule interactions. The trajectory method gives a good upper bound to reaction rate constants. It predicts a larger effect of the quadrupole moment on collision rate constants than the AQO (average quadrupole orientation)theory. It is shown that the moment of inertia of the neutral is an important factor in determinng the collision rate constant.

Ion‐polar molecule collisions: Nonreactive collisions of Cl− with dichloroethylene and difluorobenzene

Momentum transfer rate constants for Cl− with dichloroethylene and difluorobenzene isomers have been studied by an ion cyclotron resonance line‐broadening technique. Rate constants for nonpolar isomers are compared to those predicted by the Langevin theory and the rate constants for polar molecules are compared to the average‐dipole‐orientation (ADO) theory. All experimental rate constants are consistently higher than theoretical predictions. The incremental effect of a permanent dipole on momentum transfer collision frequencies predicted by ADO theory is in satisfactory agreement with experimental results.