Lewis M. Bass

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–...

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.

Kinetics of ion–molecule collision complexes in the gas phase. Experiment and theory

Essential elements of a transition switching model formulation of statistical rate theory are presented. The model is developed for use on rather complex polyatomic potential surface. Data on the energy dependence of the absolute unimolecular rate constants and branching ratios, thermal biomolecular rate constants and branching ratios including isotopic-substitution studies, and kinetic-energy distributions for the lowest-energy pathways are presented for the C4H+˙8, C4H+˙6 and C6H+˙6 reaction systems. Detailed comparisons are made between experiment and the transition-state switching model. Comments are directed toward tests of the fundamental assumptions of statistical rate theory and toward detailed mechanistic interpretation of the specific systems studied.

Unimolecular and bimolecular reactions in the C4H6+⋅ system: Experiment and theory

New experimental data on the metastable reactions of C4H6+⋅ are presented. The bimolecular reactions between C2H2+⋅ and C2H4 and between C2H4+⋅ and C2H2 have been reinvestigated using a tandem ICR. Total rate constants and branching ratios were measured and isotopic scrambling investigated using deuterium labeling. The experimental data reported here and the PIPECO data of Werner and Baer [J. Chem. Phys. 62, 2900 (1975)] are compared with the predictions of the transition state switching model. Good agreement was found between the predictions of the model and the experimental data. The branching ratios were found to be strong functions of both energy and angular momentum. In the bimolecular reaction between C2H2+ and C2H4 charge transfer appears to occur predominently via a loosely bound intermediate which only samples a small volume of the available C4H+⋅6 phase space.

Theory of ion—molecule collisions: The effect of the anisotropy in the polarizability on the collision rate constant

A classical treatment is used to estimate the increase in ion—molecule collision rates attributable to the anisotropy of the molecular polarizability, for linear and symmetric top molecules. The treatment used is directly analogous to the average dipole orientation (ADO) theory and the average quadrupole orientation (AQO) theory. Rate constants calculated for a number of thermal energy proton transfer reactions from D3+ to neutral species at 300 K show that the anisotropy will have a relatively insignificant effect on these rates (0–2%). An expression is obtained for the maximum effect (in the limit of zero neutral rotational energy) of the anisotropy on reaction rates. It is shown that the anisotropy effect will usually not be important, except at very low temperatures, and will generally be smaller than the quadrupole effect.

Ion-Molecule Association Reactions

The overall reaction for the general ion-molecule association process is

Ion-molecule association reactions: reaction sequences initiated by protonated methanol (MeOH2+) in methanol; experiment and theory

{A^ + } + B\xrightarrow{{{k_2}}}A{B^ + }

Experimental and theoretical investigations of the unimolecular dissociation of nascent ion-molecule clusters: diaquahydrogen(1+) ion (H2O.H3O+), diamminehydrogen(1+) ion (NH3.NH4+), and carbon oxide(1+) ion (CO2.CO2+)

(1) where k2 is the second-order rate constant for formation of the association product AB. In this paper we will deal with association reactions which proceed through the formation of a long-lived excited intermediate complex according to the mechanism Open image in new window where kf, kb, ks, kr and kd represent the rate constants for formation of the excited complex, back-dissociation of the complex, stabilization of the complex via collisions with bath gas M, stabilization of the complex via radiative emission of a photon, and dissociation of the complex into some fragment channel other than the reactant channel. Note that the incorporation of more than one such channel into mechanism 2 will not be discussed here, but is straight-forward. For each of the specific systems which will be discussed here either kr or kd or both will be zero.