Rate kernel theory for pseudo-first-order kinetics of diffusion-influenced reactions and application to fluorescence quenching kinetics

Theoretical foundation of rate kernel equation approaches for diffusion-influenced chemical reactions is presented and applied to explain the kinetics of fluorescence quenching reactions. A many-body master equation is constructed by introducing stochastic terms, which characterize the rates of chemical reactions, into the many-body Smoluchowski equation. A Langevin-type of memory equation for the density fields of reactants evolving under the influence of time-independent perturbation is derived. This equation should be useful in predicting the time evolution of reactant concentrations approaching the steady state attained by the perturbation as well as the steady-state concentrations. The dynamics of fluctuation occurring in equilibrium state can be predicted by the memory equation by turning the perturbation off and consequently may be useful in obtaining the linear response to a time-dependent perturbation. It is found that unimolecular decay processes including the time-independent perturbation can be incorporated into bimolecular reaction kinetics as a Laplace transform variable. As a result, a theory for bimolecular reactions along with the unimolecular process turned off is sufficient to predict overall reaction kinetics including the effects of unimolecular reactions and perturbation. As the present formulation is applied to steady-state kinetics of fluorescence quenching reactions, the exact relation between fluorophore concentrations and the intensity of excitation light is derived.     

The dependence of quinine fluorescence quenching on ionic strength

The Stern-Volmer constant for the quenching of quinine fluorescence by chloride ions has been found to be markedly dependent on acid concentration. Steady-state and time-resolved fluorescence measurements under different acid and salt concentrations have further shown that the decrease in quenching arises from the influence of increasing ionic strength on the diffusion-controlled rate constant for the bimolecular quenching process. Two possible mechanisms for this dependence are discussed: a decrease in the intrinsic rate constant for the reaction due to the kinetic salt effect, and a decrease in the effective encounter distance due to screening of the charges on the reactants. © 1996 John Wiley & Sons, Inc.     

The temperature dependence and the mechanism of the SO2 (3B1) quenching reactions

The Arrhenius parameters have been determined for the SO₂(³B₁) quenching reaction (9), SO₂(³B₁) + M → (SO₂ ? M), for 21 different molecules as quenching partner M. The rate constants were calculated from phosphorescence lifetime measurements made over a range of reactant pressures and temperatures. Excitation of the SO₂ (³B₁) molecules was accomplished by two very different methods: (1) a 3829 Å laser pulse generated the triplet directly through absorption within the “forbidden” SO₂ (³B₁) → SO₂ (¹A₁) band; (2) a broadband Xe-flash system generated SO₂(³B₁) molecules and triplets were formed subsequently by intersystem crossing, SO₂(¹B₁) + M → SO₂(³B₁) + M. The measured rate constants were independent of the method of triplet formation employed. For the atmospheric gases, the activation energies (kcal/mole) were identical within the experimental error: N₂, 2.9 ± 0.4; 0₂, 3.2 ± 0.5; Ar, 2.8 ± 0.6; CO₂, 2.8 ± 0.4; CO, 2.7 ± 0.4; CH₄, 2.5 ± 0.6. This energy corresponds to the first region of the SO₂(³B₁) → SO₂(¹A₁) absorption spectra in which Brand and coworkers observe strong perturbations. It is suggested that the quenching in these cases results largely from the physical process involving potential energy surface crossing to another electronic state. Activation energies for SO₂(³B₁) quenching by the paraffinic hydrocarbons show a regular decrease in the series ethane, neopentane, propane, n-butane, cyclohexane, and isobutane, which parallels closely the decrease in C? H bond energies in these compounds. These and other data are most consistent with the dominance of chemical quenching in these cases. The rate constants for the olefinic and aromatic hydrocarbons and nitric oxide show only very small variations with temperature change, and they are near the kinetic collision number. These data support the hypothesis that quenching in these cases is associated with the formation of a charge-transfer complex and subsequent chemical interactions between the SO₂(³B₁) molecule and the π-system of these compounds.     

The anomalous temperature dependence of enzyme-catatlyzed reactions

Breaks or curvatures in the plot of log v_br vs. 1/T have been described in the literature for a number of reactions. If sufficiently small temperature intervals are used in the investigation of the temperature dependence (STI method) a trend line is obtained instead. It is then possible to distinguish regions of linear behavior and regions of anomalous behavior. The effects of e. g. the pH, the ionic strength, and added salts as well as of the size of the enzyme and substrate molecules on the anomalies were examined. The anomalous temperature dependence of the reaction rate was found to be a particularly sensitive indicator of the reaction event on the enzyme. The course of the reaction is evidently affected even by relatively small changes in conformation, particularly in the region of the active center.     

Kinetics of partly diffusion controlled reactions. I. Transient and apparent transient effect in fluorescence quenching

Analysis of decay curves of electronically excited molecules A* versus time in presence of quencher B leads to the determination of kinetic data for the reaction (i.e. for diffusion limited reactions, the experimental collisional distance σ′ greater than the true collisional distance σ) and the sum D of diffusion coefficients of both reactants). Experimental fluorescence steady state measurements (Stern-Volmer representation) are inconsistent with calculated curves using classical Smoluchowski's model with σ′and D. The difference between this has been interpreted by complex formation in the ground state between A and B. But, this assumption is unnecessary if we take into account a “static” quenching arising from B molecules located at distances between σ and σ′ from A*. A theoretical model based upon this principle is described; good agreement between the model and experimental results was obtained.     

The temperature dependence of the fluorescence lifetime of pinacyanol iodide

The fluorescence decay time of pinacyanol (1,1′-diethyl-2,2′-carbocyanine) iodide has been measured in the temperature range 95–225 K. The results indicate that two separate internal-conversion processes are operative. One of these involves torsional relaxation and is viscosity dependent.     

Kinetics of partly diffusion controlled reactions. X. Use of a high pressure cell in fluorescence quenching experiments

The increase of the pressure over a liquid results in an increase of its viscosity, which makes possible the study of the mechanism of diffusion-controlled, or not, fluorescence quenching processes. This technique has been applied to the quenching of the fluorescence of two substituted anthraquinones by N,N-dimethyl-p-toluidine in acetonitrile solution.

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The temperature dependence of the micellisation of chlorpromazine hydrochloride in aqueous solution

The association characteristics of the weakly associating drug chlorpromazine hydrochloride have been examined over the temperature range 10–35 °C by means of conductimetric measurements. Critical micelle concentrations (cmc) have been determined by the application of a recently developed numerical method [Pérez-Rodríguez et al. (1998) Langmuir 14:4422] especially designed for the analysis of the association pattern in highly polydisperse systems of low aggregation number. The cmcs determined in this manner are used in combination with the mass-action model to obtain the thermodynamic parameters of the micellisation process, in particular the surface and hydrophobic contributions to the free energy. The use of exact forms of equations for the thermodynamics of micellisation applicable to systems of low aggregation number leads to values of the enthalpy of micellisation in reasonable agreement with experimentally determined values. Received: 6 January 2000/Accepted: 9 February 2000     

Water-induced fluorescence quenching of aniline and its derivatives in aqueous solution

Nonradiative deactivation processes of excited aniline and its derivatives in aqueous solution were investigated by steady-state and time-resolved fluorescence measurements to reveal characteristic solvent effects of water on the relaxation processes of excited organic molecules. The magnitude of nonradiative rate (k_nr) of excited aniline derivatives increased significantly in water compared to that in organic solvents (cyclohexane, ethanol, and acetonitrile). The fluorescence lifetime measurements in organic solvent/H₂O mixed solvents suggested that the fluorescence quenching in water was not due to exciplex formation but due to interactions with a water cluster. From temperature effect experiments on the fluorescence lifetime and quantum yield of aniline, N-methylaniline, and N,N-dimethylaniline, the apparent activation energies for the nonradiative deactivation rate in water were determined as 21, 30, and 41 kJ mol^-1, respectively. Upon substitution of hydrogen atoms in the aromatic ring of aniline derivatives for deuterium atoms resulted in normal deuterium isotope effect in cyclohexane, i.e. k_nr decreased by deuterium substitution, while in water the same deuterium substitution led to an increase in k_nr (the inverse isotope effect). The inverse isotope effects implied that a direct internal conversion to vibrationally higher excited states in the electronically ground state is not a dominant mechanism but the transition to a close-lying energy level, e.g. the relaxation to charge transfer to solvent (ctts) state, would be associated with the quenching mechanism in water.     

New combinatorial approach for the investigation of kinetics and temperature dependence of surface reactions in thin organic films

In this paper we present a new, simple, and reproducible method for the rapid determination of the temperature dependence of solution phase surface reactions of organic thin films on solid supports. Instead of estimating the extent of reaction for many separate samples for many different temperatures sequentially, we exploit in our new high throughput combinatorial approach surface reactions carried out under a thermal gradient followed by position-resolved contact angle (CA) measurements. The reaction kinetics, activation energies, and entropies are, thus, accessible on the basis of measurements on a very limited set of samples that differ in reaction times. The kinetics and temperature dependence of surface reactions of the previously studied alkaline hydrolysis of 11,11'-dithiobis(N-hydroxysuccinimidylundecanoate) (NHS-C10) self-assembled monolayers (SAMs) on gold, as well as the ester hydrolysis in SAMs of the novel disulfide 11,11-dithiobis(tert-butylundecanoate) (t-Bu-C10), were investigated in detail using the conventional sequential and the new combinatorial approach. The reaction kinetics, corresponding apparent rate constants k, and activation energies Ea, as well as activation entropies deltaS double dagger, determined according to both approaches agree well with each other to within the experimental error. Hence, these parameters can be quantitatively determined using the described combinatorial approach. A comparison of the reactions of the two model systems indicated that the transition state is tighter for the acid-catalyzed ester hydrolysis in SAMs of the novel disulfide t-Bu-C10 compared to the hydrolysis of the ester groups in SAMs of NHS-C10 on gold.     

Chemical kinetics of reactions in the unfrozen solution of ice

Some reactions are accelerated in ice compared to aqueous solution at higher temperatures. Accelerated reactions in ice take place mainly due to the freeze-concentration effect of solutes in an unfrozen solution at temperatures higher than the eutectic point of the solution. Pincock was the first to report an acceleration model for reactions in ice,1 which successfully simulated experimental results. We propose here a modified version of the model for reactions in ice. The new model includes the total molar change involved in reactions in ice. Furthermore, we explain why many reactions are not accelerated in ice. The acceleration of reactions can be observed in the cases of (i) second- or higher-order reactions, (ii) low concentrations, and (iii) reactions with a small activation energy. Reactions with a buffer solution or additives in order to adjust ion strength, zero- or first-order reactions, or reactions containing high reactant concentrations are not accelerated by freezing. We conclude that the acceleration of reactions in the unfrozen solution of ice is not an abnormal phenomenon.     

Influence of temperature on quenching of fluorescence in compounds of the coumarin series

A study has been made of the influence of temperature on the quantum yield of fluorescence of 3,4-benzocoumarin in ethanol or n-hexane, in the temperature interval 77–300 K. A mechanism of fluorescence quenching is proposed, consisting of thermal activation of intercombination conversion between neighboring energy levels that differ in orbital nature.Poltava Branch, Academy of Sciences of Engineering Cybernetics of the Ukraine, 3 Koval' Street, Poltava 314601, Ukraine. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 32, No. 4, pp. 247–250, July–August, 1996. Original article submitted September 26, 1995.     

Low temperature kinetics of unstable radical reactions

Recent advances in Earth and satellite based observations of molecules in interstellar environments and in planetary atmospheres have highlighted the lack of information regarding many important gas-phase formation mechanisms involving neutral species at low temperatures. Whilst significant progress has been made towards a better understanding of radical-molecule reactions in these regions, the inherent difficulties involved in the investigation of reactions between two unstable radical species have hindered progress in this area. This perspective article provides a brief review of the most common techniques applied to study radical-radical reactions below room temperature, before outlining the developments in our laboratory that have allowed us to extend such measurements to temperatures relevant to astrochemical environments. These developments will be discussed with particular emphasis on our recent investigations of the reactions of atomic nitrogen with diatomic radicals.     

The quenching of chlorine fluorescence in the gas phase

Laser induced fluorescence has been observed from the B³Π(O⁺_u) state of chlorine. Lifetimes have been measured from 19 to 208 Pa, but the radiative lifetime is too long to be extracted from the data. A rate coefficient of 3.2 × 10[su13] cm³ mol?1 s?1 was measured for quenching by Cl₂ or Ar, the removal process being collision induced predissociation of the ³Π(O⁺_u) state.     

Magnetic field dependence of delayed fluorescence of pyrene in solution

The magnetic field effect on the intensities of monomer (I_M) and excimer (I_D) delayed fluorescence of pyrene has been found to be identical. This result suggests a common spin-selective step in the processes giving rise to I_M and I_D, and is discussed in the light of previously proposed mechanisms. An unusual solvent effect is found on the I_M (I_D) versus field strength curves. This is attributed to photochemical doublet-radical formation. Doublet-radical formation affects the field dependence of I_M and I_D through spin selectivity in the triplet quenching reaction.     

Influence of the temperature on the fluorescence quenching of pheophytin α by toluquinone

The influence of the temperature on the constant for the fluorescence quenching of pheophytin a by toluquinone in various solvents has been investigated. It has been shown that the quenching occurs in alcohols according to both static and dynamic mechanisms. The static quenching is caused by the formation of complexes of two types: charge-transfer complexes of ordinary structure and more stable complexes, in which the donor and acceptor are bound to one another by the alcohol molecules.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 1, pp. 115–118, January–February, 1985.     

Oxidation kinetics and fluorescence quenching of coumarin-1 laser dye with peroxodisulfate

Summary The fluorescence of coumarin-1 laser dye (C1; 7-diethylamino-4-methyl-coumarin) proved to be sensitive to the presence of peroxodisulfate (S₂O ₈ ^2– ). The emission intensity decreases with increasing in peroxodisulfate concentration. The decrease is assigned to a quenching process connected with a ground state peroxodisulfate oxidation of the dye. The kinetics of the reaction have been investigated spectrophotometrically at 382 nm. The reaction follows second order kinetics, being first order for each reactant. The reaction rate ispH dependent; higher rates are observed in alkaline media. Addition of sodium dodecyl sulfate (SDS) retards the oxidation process remarkably.

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Kinetik der Oxidation und Fluoreszenzlöschung des Laserpigments Coumarin-1 mit Peroxodisulfat

Zusammenfassung Die Fluoreszenz des Laserpigments Coumarin-1 (C1; 7-Diethylamino-4-methyl-coumarin) reagiert auf die Anwesenheit von Peroxodisulfat (S₂O ₈ ^2– ). Die Emissionsintensität nimmt mit steigender Peroxodisulfatkonzentration ab. Die Abnahme wird einem Löschvorgang zugeschrieben, der mit einer Oxidation des Grundzustands durch Peroxodisulfat verbunden ist. Die Kinetik der Reaktion wurde bei 382 nm spektrophotometrisch untersucht. Sie verläuft nach zweiter Ordnung (nach erster Ordnung bezüglich jedes Reaktanden). Die Reaktionsgeschwindigkeit istpH-abhängig; in alkalischen Medien werden höhere Geschwindigkeitskonstanten gefunden. Zusatz von Natriumdodecylsulfat (SDS) hemmt die Oxidation beträchtlich.