Rate Constant Data Research Articles

A kinetic and thermodynamic study on hydrolysis of sodium laurate in aqueous phase accompanied by transfer into oil phases containing different organic additives (I)

The kinetic and thermodynamic behavior at the interface between an aqueous solution of sodium laurate (NaLA) and various oil phases comprised primarily of benzene (Bz) and/or different organic compounds including amphiphiles has been investigated in regard to the hydrolysis of NaLA accelerated at the interface, transfer of lauric acid (LA) into oil phase and reverse transfer of Bz into aqueous phase in addition to interface tension. The contact of aqueous NaLA solution with the oil phase was found to accompany the mass transfer of LA and simultaneously promote the hydrolysis of NaLA in water phase. Analysis of the change of OH − ion concentration ([OH −]) over time allowed us to treat the events as a first order reaction. From the rate constant data the activation parameters such as the activation enthalpy and entropy, both of which control the transfer of LA molecules, were determined. The parameters were found to depend greatly on varied situations of the oil phase, being clearly able to explain the physicochemical behavior of the interface. Comparing the cases where the oil phase is one of the respective single systems such as Bz, dodecane (C 12) and dodecylbenzene (C 12Bz), C 12Bz resulted in the lowest rate constant. The transfer (or hydrolysis) rate was measured for the amphiphile-added oil systems as a function of amphiphile concentration. When 0.206 M C 16OH Bz came in contact with aqueous phase, emulsion formation at the interface layer was brought about with approximately zero activation enthalpy, leading to facile or spontaneous transfer of LA. In addition, UV absorbance representing the transfer of Bz from the oil phase to the aqueous phase also demonstrated the effects of added amphiphiles on the action of the interface.

Kinetics and mechanisms in carbocationic polymerization: The quest for true rate constants

AbstractThis article is a critical analysis of kinetic dataavailable on carbocationic polymerizations. A survey of published propagation rate constant (kp) data revealed several orders of magnitude differences. In this article, an explanation of this apparent discrepancy is offered with a case study involving the carbocationic polymerization of 2,4,6‐trimethylstyrene (TMS). With the polymerization mechanism originally proposed for this system, kp = 1.35 × 104 L mol−1 s−1 was extracted from experimental data with the Predici polyreaction package. The alternative mechanism yielded kp = 1.01 × 107 L mol−1 s−1, close to that predicted by Mayr's Linear Free Energy Relationship (LFER). We propose that true rate constants can only be obtained from direct competition experiments or from kinetic interpretation based on independently proven mechanisms. The second part of this review discusses critical analysis of the temperature and concentration dependence of various living IB systems. Comparison of the temperature dependence in systems initiated with 2‐ chloro‐2,4, 4‐ trimethylpentane (TMPCl)/TiCl4 from various laboratories yielded of ΔH ∼−25 and −34.5 kJ/mol for high and low TMPCl/TiCl4 ratios, respectively. Aromatic (cumyl‐type) initiators show ΔH ∼ −40 kJ/mol, whereas H2O/TiCl4 in the presence of the strong electron‐ pair donor dimethylacetamide gave ΔH = −12 kJ/mol. The significant differences indicate different underlying mechanisms with complex elementary reactions. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5394–5413, 2005

Methyl Transfer from CH3CoIIIPc to Thiophenoxides Revisited: Remote Substituent Effect on the Rates

A two-step mechanism of the reaction of CH(3)Co(III)Pc (Pc = dianion of phthalocyanine) with thiophenoxides in DMA has been confirmed, and the visible spectrum of the inactive transient, CH(3)Co(III)Pc(SAr)(-), has been determined. Rapid rates for ligation of CH(3)Co(III)Pc, yielding CH(3)Co(III)Pc(S-C(6)H(4)-X)(-), are virtually independent of X; this step proceeds probably by an I(d) mechanism. Kinetic data for the follow-up methyl-transfer step yield second-order rate constants and stability constants for CH(3)Co(III)Pc(S-C(6)H(4)-X)(-) consistent with those estimated from concentration dependence of the amplitude of the ligand-exchange step. Cyclic voltammetry provides first reduction potential for CH(3)Co(III)Pc(DMA) of -1.42 V vs Fc(+)/Fc, which makes an OSET mechanism unlikely. Homolytic decay of CH(3)Co(III)Pc(SAr)(-) has also been ruled out. All of the kinetic data, including Hammett's rho = -2.3 +/- 0.1, N-donor inhibition, and alkyl group effect, Me > Et, indicate that the reaction is a normal S(N)2 methyl transfer, only very fast. Methyl transfer to aliphatic thiolates is also rapid and follows the same S(N)2 mechanism. Exceptional methyl-transfer reactivity of the phthalocyanine model sharply contrasting with the inertness of methylcobaloxime is explained.

Theoretical Study of the Mechanism of NO2 Production from NO + ClO

The reaction of NO with ClO has been studied theoretically using density-functional and wave function methods (B3LYP and CCSD(T)). Although a barrier for cis and trans additions could be located at the RCCSD(T) and UCCSD(T) levels, no barrier exists at the B3LYP/6-311+G(d) level. Variational transition state theory on a CASPT2(12,12)/ANO-L//B3LYP/6-311+G(d) surface was used to calculate the rate constants for addition. The rate constant for cis addition was faster than that for trans addition (cis:trans 1:0.76 at 298 K). The rate constant data summed for cis and trans addition in the range 200-1000 K were fit to a temperature-dependent rate in the form kdi) = 3.30 x 10(-13)T(0.558) exp(305/T) cm3.molecule(-1).s(-1), which is in good agreement with experiment. When the data are fit to an Arrhenius plot in the range 200-400 K, an activation barrier of -0.35 kcal/mol is obtained. The formation of ClNO2 from ONOCl has a much higher activation enthalpy from the trans isomer compared to the cis isomer. In fact, the preferred decomposition pathway from trans-ONOCl to NO2 + Cl is predicted to go through the cis-ONOCl intermediate. The trans --> cis isomerization rate constant is kiso = 1.92 x 10(13) exp(-4730/T) s(-1) using transition state theory.

Model of the Inactivation of Bacterial Spores by Moist Heat and High Pressure

ABSTRACT: Formulae for the prediction of inactivation and accumulated lethality of bacterial spores under moist heat and high pressures were derived on the basis of classic thermodynamic and kinetics principles. The capability of the model to describe the inactivation of bacterial spores was verified using 2 independent data sets corresponding to Clostridium botulinum processed at 60°C to 75°C and Bacillus stearothermophilus processed at 92°C to 110°C. Both sets included pressures between 5 × 108 Pa and 7 × 108 Pa. The equation fit explained more than 86% of the variation of the rate constant data. The developed equations establish a strong foundation on which to compare high‐pressure processing treatments of different systems. This is especially useful because most systems have different transient temperature‐pressure conditions.

Studies on the formation of glutathionylcobalamin: any free intracellular aquacobalamin is likely to be rapidly and irreversibly converted to glutathionylcobalamin.

A decade ago Jacobsen and co-workers reported the first evidence for the presence of glutathionylcobalamin (GSCbl) in mammalian cells and suggested that it could in fact be a precursor to the formation of the two coenzyme forms of vitamin B(12), adenosylcobalamin and methylcobalamin (Pezacka et al. Biochem. Biophys. Res. Commun. 1990, 169, 443). It has also recently been proposed by McCaddon and co-workers that GSCbl may be useful for the treatment of Alzheimer's disease (McCaddon et al. Neurology 2002, 58, 1395). Aquacobalamin is one of the major forms of vitamin B(12) isolated from mammalian cells, and high concentrations of glutathione (1-10 mM) are also found in cells. We have now determined observed equilibrium constants, K(obs)(GSCbl), for the formation of GSCbl from aquacobalamin and glutathione in the pH range 4.50-6.00. K(obs)(GSCbl) increases with increasing pH, and this increase is attributed to increasing amounts of the thiolate forms (RS(-)) of glutathione. An estimate for the equilibrium constant for the formation of GSCbl from aquacobalamin and the thiolate forms of glutathione of approximately 5 x 10(9) M(-1) is obtained from the data. Hence, under biological conditions the formation of GSCbl from aquacobalamin and glutathione is essentially irreversible. The rate of the reaction between aquacobalamin/hydroxycobalamin and glutathione for 4.50 < pH < 11.0 has also been studied and the observed rate constant for the reaction was found to decrease with increasing pH. The data were fitted to a mechanism in which each of the 3 macroscopic forms of glutathione present in this pH region react with aquacobalamin, giving k(1) = 18.5 M(-1) s(-1), k(2) = 28 +/- 10 M(-1) s(-1), and k(3) = 163 +/- 8 M(-1) s(-1). The temperature dependence of the observed rate constant at pH 7.40 ( approximately k(1)) was also studied, and activation parameters were obtained typical of a dissociative process (DeltaH++ = 81.0 +/- 0.5 kJ mol(-1) and DeltaS++ = 48 +/- 2 J K(-1) mol(-1)). Formation of GSCbl from aquacobalamin is rapid; for example, at approximately 5 mM concentrations of glutathione and at 37 degrees C, the half-life for formation of GSCbl from aquacobalamin and glutathione is 2.8 s. On the basis of our equilibrium and rate-constant data we conclude that, upon entering cells, any free (protein-unbound) aquacobalamin could be rapidly and irreversibly converted to GSCbl. GSCbl may indeed play an important role in vitamin B(12)-dependent processes.

Kinetic Study of the Gas-Phase Reactions of Cl Radicals with 3-Pentanone and 3-Hexanone

The discharge flow mass spectrometric technique has been used to determine the absolute rate coefficients of the gas-phase reactions of Cl atoms with 3-pentanone (1) and 3-hexanone (2) in the temperature range 260−333 K and at a total pressure of 1 Torr. The rate constant data (k(1) and k(2) at different temperatures) were used to derive the Arrhenius expressions, k(1) = (1.5 ± 0.7) × 10-10 exp[(−281 ± 138)/T] cm3 molecule-1 s-1 and k(2) = (1.6 ± 0.4) × 10-9 exp[(−884 ± 69)/T] cm3 molecule-1 s-1. At 298 K and 1 Torr, the products of both reactions have been studied. Reactions 1 and 2 have been found to proceed through an abstraction mechanism, leading to HCl + C5H9(O) and HCl + C6H11(O), respectively. The branching ratio for the abstraction channel was determined at 298 K and 1 Torr, as 0.97 ± 0.10 for reaction 1 and 0.98 ± 0.08 for reaction 2. The results obtained are related to previous studies, and the mechanism and atmospheric implications for both reactions are discussed.

Kinetics of interaction of three carbamate pesticides with Indian soils: Aligarh district

The kinetics of interaction with soil of three carbamate pesticides (I, II, III) used as nematicides and herbicides was studied at four different temperatures from solutions of six soil samples of Aligarh district. The values obtained for rate constants for adsorption and desorption were in good agreement with those obtained from the Lindstrom model, which proved useful in the simultaneous evaluation of adsorption (k1) and desorption (k2) rate constants. The rate constants for pesticides were in the order III > I > II on all the six soil samples. The data for rate constants, activation energies, heats of activation, entropy of activation and thermodynamic parameters indicated a partly physical and partly chemical adsorption of pesticide on the soil surfaces. Adsorption occurred via coordination and/or protonation of the exchangeable cations with the amidic carbonyl group, and hydrogen bonding and dipole association at the crystal edge and basal surfaces. The adsorptivity of the soils may be attributed to the organic matter content and percentage clay content.

Class Projects in Physical Organic Chemistry: The Hydrolysis of Aspirin

A laboratory experiment is described that requires each student to determine the rate constant for the hydrolysis of aspirin at three different pH values. Individual results from each student are compiled to provide a class set of rate constant versus pH data. The students prepare a pH–rate profile for the hydrolysis of aspirin and analyze this profile to determine the mechanism and type of catalysis of the hydrolysis at different pH values.

A kinetic study on the conversion of methane to higher hydrocarbons in a radio-frequency discharge

This study investigated methane conversion with direct current discharge at low pressure in a radio frequency. The main gaseous products of the reaction were ethane, ethylene, acetylene and propane. This study was concentrated on the influence of discharge conditions on the conversion of methane to higher hydrocarbons. Reaction temperature, electron density and mean residence time were calculated from experimental data and mathematical relations. The maximum conversion of the methane was about 45% with the pure methane as a reactant. Ethane was the main product when the reaction occurred in the glow discharge. Ethane selectivity decreased with the increase of the gas temperature. The kinetics of reactions was also analyzed from possible reaction equations and various rate constant data. Consequently, the dissociation constant and the density of radicals could be obtained at any experimental conditions.

Patch size effect on diffusion and incorporation in dilute suspension of partially active spheres.

The normalized overall rate constant kp/kf for diffuse-and-incorporate processes in dilute suspension of spheres partially covered with randomly distributed nonoverlapping active patches is studied with a sped-up Brownian dynamic simulation scheme. The normalized overall rate constant is found to increase with decreasing characteristic size of the active patch under the condition of fixed fcover, the surface area fraction covered by the active patches. A scaling relation is proposed as (kp/kf)NDL/[(1 + P) - (1 + P/fcover)(kp/kf)NDL] approximately Np(1/2) with Np being the number of active patches on the sphere, P a parameter characterizing the relative dominance of surface incorporation over diffusion, and NDL signifying the nondiffusion-limited condition. This scaling relation is verified with rate constant data from the sped-up Brownian dynamic simulation. From this scaling relation, the maximum achievable kp/kf is derived to be (1 + P)/(1 + P/fcover). This result implies that kp/kf can approach unity under the diffusion-limited situation even for a partially active sphere by reducing the size of active patch, while it is not possible to achieve unit kp/kf for nondiffusion-limited systems.

The Mitochondrial Citrate Transport Protein: PROBING THE SECONDARY STRUCTURE OF TRANSMEMBRANE DOMAIN III, IDENTIFICATION OF RESIDUES THAT LIKELY COMPRISE A PORTION OF THE CITRATE TRANSPORT PATHWAY, AND DEVELOPMENT OF A MODEL FOR THE PUTATIVE TMDIII-TMDIII′ INTERFACE

The mitochondrial citrate transport protein (CTP) has been investigated by mutating 28 consecutive residues within transmembrane domain III (TMDIII), one at a time, to cysteine. A cysteine-less CTP that retains wild-type functional properties, served as the starting template. The single Cys CTP mutants were abundantly expressed in Escherichia coli, isolated, and functionally reconstituted in a liposomal system. The accessibility of each single Cys mutant to two methanethiosulfonate reagents was evaluated by determining the rate constants for inhibition of CTP function. These rate constants varied by over five orders of magnitude. With two independent data sets we observed peaks and troughs in the rate constant data at identical amino acid positions and a periodicity of 4 was observed from residues 123-137. Based on the pattern of accessibility we conclude that residues 123-137 exist as an alpha-helix. Although less certain, a combination of the rate constant data and the specific activity data with the single Cys mutants suggests that the alpha-helical secondary structure may extend to residue 113. Furthermore, the rate constant data define water-accessible and water-inaccessible faces of the helix. We infer that the water-accessible face comprises a portion of the substrate translocation pathway through the CTP, whereas the water-inaccessible surface faces the lipid bilayer. Finally, based on a combination of the CTP inhibition rate constant data and the existence of significant sequence identity with a transmembrane segment within glycophorin A that forms a portion of its dimer interface, a model for a putative CTP TMDIII-TMDIII' dimer interface has been developed.

Isodesmic Reactions for Transition States: Reactions of Cl Atoms with Methane and Halogenated Methanes

The performance of the technique of isodesmic reactions for transition states (IRTS) has been analyzed via application to 17 reactions of abstraction of hydrogen atoms from methane and halogenated methanes by Cl atom. A variety of quantum chemical methods and basis sets was used. The calculated energy barriers demonstrate linear correlations with those derived from modeling of the experimental rate constant data, in agreement with the prediction based on the IRTS formalism. The results of the study confirm the validity of the technique of isodesmic reactions for transition states for calculation of reaction rates and demonstrate the existence of method-specific systematic errors in calculations of reaction barriers. The technique of isodesmic reactions is directed at factoring out and eliminating these systematic errors. The predictive ability of the technique is directly related to the quality of the observed correlations. Average and maximum deviations from the best fit lines on the correlation plots de...

Modeling the Organic Nitrate Yields in the Reaction of Alkyl Peroxy Radicals with Nitric Oxide. 2. Reaction Simulations

Master equation calculations are used to model gas-phase literature experimental data for alkyl nitrate formation via the following reaction system of reversible reactions: (1) RO2 + NO ↔ ROONO, (2) ROONO ↔ RO + NO2, (3) ROONO ↔ RONO2, and (4) RONO2 ↔ RO + NO2 for R = CH3, i-C3H7, and 2-C5H11. The structures and thermochemistry of the stable species are based on electronic structure calculations described in the preceding companion paper in this issue (Lohr et al. J. Phys. Chem. A 2003, 107, xxx−xxx). Literature data for recombination rate constants are used to constrain the model calculations. Several transition state models and a range of energy transfer parameters are investigated. The results for R = CH3 show that a wide variety of plausible transition state models for k-4 gives good agreement with experiment for reaction (−4), because changes in assumed energy transfer parameters can compensate for differences between the transition state models. It is concluded that recombination reactions are good sources of absolute energy transfer parameters only when transition state properties are known with great accuracy. Although satisfactory models are obtained for the individual systems, the parameters cannot be transferred reliably from one system to another. Master equation models can be made to reproduce the experimental 2-pentyl nitrate yields from the title reaction as long as 〈ΔE〉down, the average energy transferred in deactivating collisions, is assumed to be surprisingly, and perhaps unphysically, small (∼25 cm-1), regardless of assumptions about the barrier to isomerization reaction 3. Several critical assumptions in the master equation models are examined, but none of them accounts for the small value of 〈ΔE〉down. It is concluded that new experiments should be carried out to verify or possibly revise the pressure-dependent alkyl nitrate yield data currently available in the literature.

Kinetics of the Unimolecular Decomposition of the C2Cl3 Radical

The C2Cl3 → C2Cl2 + Cl (1) reaction was studied using the laser photolysis/photoionization mass spectrometry technique. Rate constants were determined in time-resolved experiments as a function of temperature (569−683 K) and bath gas density ([He] = (3−24) × 1016 atoms cm-3, [N2] = 12 × 1016 molecules cm-3). C2Cl2 was observed as a primary product of reaction 1. Rate constants of reaction 1 are in the falloff, closer to the low-pressure limit, under the conditions of the experiments. The potential energy surface (PES) of reaction 1 was studied using a variety of quantum chemical methods. The results of the PES study indicate that the minimum energy path of the C2Cl3 dissociation proceeds through a shallow-well symmetric structure with the departing Cl atom equidistant from both carbon atoms; a PES saddle point exists between the nonsymmetric equilibrium C2Cl3 structure and the symmetric one. The results of quantum chemical calculations and the experimental rate constant data were used to create a model of reaction 1. The experimental dependence of the rate constant of C2Cl3 decomposition on temperature and pressure was reproduced in RRKM/master equation calculations. The reaction model provides expressions for the temperature dependences of the high-pressure-limit and the low-pressure-limit rate constants and the falloff broadening factors: k1∞ = (1.36 × 1013)T0.29 exp(−13625 K/T) s-1, k1°(He) = (4.89 × 1012)T-6.14 exp(−14497 K/T) cm3 molecule-1 s-1, k1°(N2) = (1.525 × 1012)T-5.95 exp(−14393 K/T) cm3 molecule-1 s-1, Fcent(He) = 0.33 exp(−T/1836 K) + 0.67 exp(−T/98 K) + exp(−3650 K/T), and Fcent(N2) = 0.35 exp(−T/1864 K) + 0.65 exp(−T/103 K) + exp(−3614 K/T). The experimental data are not sufficient to specify all the parameters of the model; consequently, some of the model parameters were obtained from quantum chemical calculations and from analogy with other reactions of radical decomposition. Thus, the parametrization is most reliable under conditions close to those used in the experiments.

A laboratory batch reactor test for assessing nonspeciated volatile organic compound biodegradation in activated sludge.

The relative rates of biodegradation and stripping and volatilization of nonspeciated volatile organic compounds (VOCs) in wastewater treated with aerobic activated-sludge processes can be quantified using a newly developed procedure. This method was adapted from the original aerated draft tube reactor test that was developed to measure biodegradation rate constants for specific volatile pollutants of interest. The original batch test has been modified to include solid-phase microextraction (SPME) fibers for sampling in the gas phase. The experimental procedure using SPME fibers does not require specific identification and quantitation of individual pollutants and can be used to evaluate wastewater with multiple VOCs. To illustrate use of this procedure, laboratory experiments were conducted using biomass and wastewater or effluent from three activated-sludge treatment systems. Each experiment consisted of two trials: a stripping-only trial without biomass and a stripping plus biodegradation trial using biomass from the activated-sludge unit of interest. Data from the two trials were used to quantify the rates of biodegradation by difference. The activated-sludge systems tested were a laboratory diffused-air reactor treating refinery wastewater, a full-scale surface aerated reactor treating a petrochemical wastewater, and a full-scale diffused-air reactor treating a variety of industrial effluents. The biodegradation rate constant data from each laboratory batch experiment were used in model calculations to quantify the fraction emitted (fe) and the fraction biodegraded (fbio) for each system. The fe values ranged from a maximum of 0.01 to a maximum of 0.32, whereas fbio values ranged from a minimum of 0.40 to a minimum 0.95. Two of these systems had been previously tested using a more complicated experimental approach, and the current results were in good agreement with previous results. These results indicate that biodegradation rate constant data from this laboratory method can be successfully used to predict the fate of VOCs in field-scale treatment units, and thus could potentially be used for demonstration of compliance with wastewater VOC emission regulations.

A New Model for Laser-induced Thermal Damage in the Retina

We describe a new model for laser-induced retinal damage. Our treatment is prompted by the failure of the traditional approach to accurately describe the image size dependence of laser-induced retinal injuries and by a recently reported study which demonstrated that laser injuries to the retina might not appear for up to 48 h post exposure. We propose that at threshold a short-duration, laser-induced, temperature rise melts the membrane of the melanosomes found in the pigmented retinal epithelial cells. This results in the generation of free radicals which initiate a slow chain reaction. If more than a critical number of radicals are generated then cell death may occur at a time much later than the return of the retina to body temperature. We show that the equations consequent upon this mechanism result in a good fit to the recent image size data although more detailed experimental data for rate constants of elementary reactions is still required. This paper contributes to the current understanding of damage mechanisms in the retina and may facilitate the development of new treatments to mitigate laser injuries to the eye. The work will also help minimize the need for further animal experimentation to set laser eye safety standards.

Heterogeneous electron-transfer kinetics for ruthenium and ferrocene redox moieties through alkanethiol monolayers on gold.

The standard heterogeneous electron-transfer rate constants between substrate gold electrodes and either ferrocene or pentaaminepyridine ruthenium redox couples attached to the electrode surface by various lengths of an alkanethiol bridge as a constituent of a mixed self-assembled monolayer were measured as a function of temperature. The ferrocene was either directly attached to the alkanethiol bridge or attached through an ester (CO(2)) linkage. For long bridge lengths (containing more than 11 methylene groups) the rate constants were measured using either chronoamperometry or cyclic voltammetry; for the shorter bridges, the indirect laser induced temperature jump technique was employed to measure the rate constants. Analysis of the distance (bridge length) dependence of the preexponential factors obtained from an Arrhenius analysis of the rate constant versus temperature data demonstrates a clear limiting behavior at a surprisingly small value of this preexponential factor (much lower than would be expected on the basis of aqueous solvent dynamics). This limit is independent of both the identity of the redox couple and the nature of the linkage of the couple to the bridge, and it is definitely different (smaller) from the limit derived from an equivalent analysis of the rate constant (versus temperature) data for the interfacial electron-transfer reaction through oligophenylenevinylene bridges between gold electrodes and ferrocene. There are a number of possible explanations for this behavior including, for example, the possible effects of bridge conformational flexibility upon the electron-transfer kinetics. Nevertheless, conventional ideas regarding electronic coupling through alkane bridges and solvent dynamics are insufficient to explain the results reported here.

Mechanisms for the ozonolysis of ethene and propene: Reliability of quantum chemical predictions

Reactions of ozone with ethene and propene leading to primary ozonide (concerted and stepwise ozonolysis) or epoxide and singlet molecular oxygen (partial ozonolysis) are studied theoretically. The mechanism of concerted ozonolysis proceeds via a single transition structure which is a partial diradical. The transition structures and intermediates in the stepwise ozonolysis and partial ozonolysis mechanisms are singlet diradicals. Spin-restricted and unrestricted density functional methods are employed to calculate the structures of the closed-shell and diradical species. Although the partial diradicals exhibit moderate to pronounced instability in their RDFT and RHF solutions, RDFT is required to locate the transition structure for concerted ozonolysis. Spin projected fourth-order Møller–Plesset theory (PMP4) was used to correct the DFT energies. The calculated pre-exponential factors and activation energies for the concerted ozonolysis of ethene and propene are in good agreement with experimental values. However, the PMP4//DFT procedure incorrectly predicts the stepwise mechanism as the favored channel. UCCSD(T) predicts the concerted mechanism as the favored channel but significantly overestimates the activation energies. RCCSD(T) is found to be more accurate than UCCSD(T) for the calculation of the concerted mechanism but is not applicable to the diradical intermediates. The major difficulty in accurate prediction of the rate constant data for these reactions is the wide range of spin contamination for the reference UHF wave functions and UDFT solutions across the potential energy surface. The possibility of the partial ozonolysis mechanism being the source of epoxide observed in some experiments is discussed.

Non-Markovian effects in the radiationless decay of rhodamine 3B+ in water : ethanol mixtures

The intramolecular nonradiative decay of the first excited singlet of Rhodamine 3B cation, is studied in the full range of composition of water : ethanol mixtures with different polarities and viscosities. The correlation of experimental data of absorption, emission and rate constants, knr, with the dielectric and shear viscosity parameters of these mixtures shows that friction effects determine the reaction rates. These were modelled using the phenomenological dependence of radiationless decay rates which is expressed in terms of Kramers–Grote–Hynes theory. The reactant well wavenumber (ω0/2πc=30 cm−1) obtained previously for the activated radiationless decay of Rhodamine 3B in a variety of solvents provides a means to estimate the reaction transmission coefficients (κ) in the mixtures. The trend of variation with the composition reflects the dependence on the viscoelastic response of water : ethanol mixtures. The clustering of ethanol molecules at low ethanol content disrupts the water structure and induces enhanced hydrophobicity around the chromophore which manifests in non-Markovian effects on the reaction rates, knr.