Unimolecular Dissociation Rate Research Articles

Kinetics of interaction of N epsilon-fluorescein isothiocyanate-lysine-23-cobra alpha-toxin with the acetylcholine receptor

We have studied the kinetics of N epsilon-fluorescein isothiocyanate-lysine-23 cobra alpha-toxin (FITC-toxin) binding to the membrane-associated acetylcholine receptor from the Torpedo californica electric organ. The fluorescent toxin not only enabled us to monitor the binding reaction continuously but also to examine simultaneously the enhancement of ligand fluorescence and the increase in steady state polarization of fluorescence associated with binding of the alpha-toxin. Over the range of concentrations employed, both parameters yielded identical kinetic constants, suggesting that the enhancement of fluorescence of fluorescein and its immobilization are occurring in the same time frame. Both an initial rate analysis and the integrated rate expression showed association to be a simple, reversible bimolecular process. The apparent second-order association rate constant derived from the integrated rate analysis was constant within a factor of 2 over a 40-fold concentration range (6.7 +/- 1.7 X 10(3) M-1 S-1). The unimolecular dissociation rate constant was found to be 3.3 +/- 0.5 X 10(-5) S-1.

Unimolecular fragmentations of internal energy selected 1,3-butadiene cations

Photoelectron—photoion coincidence spectroscopy was used to study the dissociations of electronically excited 1,3-butadiene cations. Appearance energies and breakdown curves have been measured for the dominant fragment ions. The breakdown diagram was used to discuss the competitive fragmentation pathways. The dependence of the unimolecular dissociation rate constant on the internal energy of the parent cations was determined in the low energy range of the first excited electronic state (A˜2Au).

Unimolecular reactions of N2O and CO2 at high pressure

An apriori calculation in the framework of the theory of radiationless transitions is presented for the non-adiabatic spin-forbidden reactions of N2O and CO2. The calculation is two-dimensional, incorporating the two stretching degrees of freedom, and the microscopic reaction process is formulated using the relative coordinate system of Rosen as modified by Beswick and Jortner. All microscopic parameters required for the calculation are derived from spectroscopic data; no adjustable parameter is included. For each reaction, a purely theoretical reaction probability function k(E) is synthesised and used to derive a theoretical Arrhenius expression for the infinite-pressure reaction rate. The calculated high-pressure unimolecular dissociation rates for both reactions are in reasonable agreement with experiment. High-pressure recombination rate coefficients are also presented for CO2; they exhibit positive activation energies which increase from 0.5 kcal/mol at room temperature to 8 kcal/mol at 5000 K.

Thermal gas‐phase decomposition of chloroethylenes. II. Vinyl chloride

AbstractThe thermal gas‐phase decomposition of vinyl chloride has been studied behind shock waves over the temperature range of 1350‐1900°K and the density range of 7 × 10−7‐1.5 × 10−3 mol/cm3 (at 1600°K) in mixtures of C2H3Cl highly diluted with argon. The ultraviolet absorption of C2H3C was recorded at 230 nm as a function of time. The decomposition proceeds via molecular elimination of HCl. The unimolecular dissociation rate is pressure dependent at all but the highest pressures applied. Application of modified HKRR theory results in the rate expression for the limiting high pressure rate constant, and in a collision efficiency of derived from the limiting low‐pressure rate constant.

Nonlinear resonance and stochasticity in intramolecular energy exchange

The dynamics of intramolecular vibrational energy transfer is studied, with particular reference to its effect on the unimolecular dissociation rates of isolated molecules. The molecule is represented as a set of classical, coupled, nonlinear (anharmonic) oscillators which transfer energy through resonant interactions. We suggest that isolated nonlinear resonances (which predominate at low energies) lead to trapping of the vibrational energy of the system, hence to slow vibrational relaxation, while resonance overlap at higher energies leads to rapid energy exchange and the random lifetime distribution assumed by RRKM theory. Results are presented for some simple model systems, and the approach is compared with other recent theoretical descriptions of vibrational relaxation in isolated molecules.

Calculated R.R.K.M. unimolecular dissociation rate constants for hydrazine

Unimolecular rate constants for hydrazine dissociation by thermal and chemical activation have been calculated according to the R.R.K.M. theory. The two activated complex models used in the calculations represent plausible upper and lower bounds to the rate constants. The calculations are mainly directed toward establishing expected decomposition to stablilization ratios of N2H4 produced by combination of NH2 radicals; however, a general comparison to available experimental data for hydrazine dissociation is made.

Cubic Potential Surfaces in the Transition-State Theory of Unimolecular Reactions

The frequency factor of a unimolecular dissociation rate is enhanced if the molecular vibrations are loosened during the approach to the activated state; this loosening may be regarded as affecting the partition function or entropy in the nonreactive degrees of freedom. These ideas are illustrated here in terms of cubic potential surfaces, with examples of linear molecules and of the effect of freeing an internal rotation. As a more general but related point, the uniqueness of the reaction coordinate of transition-state theory is discussed with some reference to isotope effects.

New Formulation of Gaseous Unimolecular Dissociation Rates

First Page

XII.—On the Theory of Unimolecular Gas Reactions: A Quantum Harmonic Oscillator Model

SynopsisThe writer's theory of unimolecular dissociation rates, based on the treatment of the molecule as a harmonically vibrating system, is put in a form which covers quantum as well as classical mechanics. The classical rate formulæ are as before, and are also the high-temperature limits of the new quantum formulæ. The high-pressure first-order rate k∞ is found first from the Gaussian distribution of co-ordinates and momenta of harmonic systems, and is justified for the quantum-mechanical case by Bartlett and Moyal's phase-space distributions. This leads to a re-formulation of k∞ as a molecular dissociation probability averaged over a continuum of states, and to a general rate for any pressure of the gas.The high-pressure rate k∞ is of the form ve-F/kT, where v and F depend, in the quantum case, on the temperature T; but v is always between the highest and lowest fundamental vibration frequencies of the molecule. Concerning the decline of the general rate k with pressure at fixed temperature, k/k∞ is to a certain approximation the same function of as was tabulated earlier for the classical case, apart from a constant factor changing the pressure scale in the quantum case.