Rate Constants For Product Formation Research Articles

Mechanism of imidazole inhibition of a GH1 β-glucosidase.

Imidazole is largely employed in recombinant protein purification, including GH1 β-glucosidases, but its effect on the enzyme activity is rarely taken into consideration. Computational docking suggested that imidazole interacts with residues forming the active site of the GH1 β-glucosidase from Spodoptera frugiperda (Sfβgly). We confirmed this interaction by showing that imidazole reduces the activity of Sfβgly, which does not result from enzyme covalent modification or promotion of transglycosylation reactions. Instead, this inhibition occurs through a partial competitive mechanism. Imidazole binds to the Sfβgly active site, reducing the substrate affinity by about threefold, whereas the rate constant of product formation remains unchanged. The binding of imidazole within the active site was further confirmed by enzyme kinetic experiments in which imidazole and cellobiose competed to inhibit the hydrolysis of p-nitrophenyl β-glucoside. Finally, imidazole interaction in the active site was also demonstrated by showing that it hinders access of carbodiimide to the Sfβgly catalytic residues, protecting them from chemical inactivation. In conclusion, imidazole binds in the Sfβgly active site, generating a partial competitive inhibition. Considering that GH1 β-glucosidases share conserved active sites, this inhibition phenomenon is probably widespread among these enzymes, and this should be taken into account when considering the characterization of their recombinant forms.

Monitoring helicase-catalyzed unwinding of multiple duplexes simultaneously.

Helicases catalyze the unwinding of duplex nucleic acids to aid a variety of cellular processes. Although helicases unwind duplex DNA in the same direction that they translocate on single-stranded DNA, forked duplexes provide opportunities to monitor unwinding by helicase monomers bound to each arm of the fork. The activity of the helicase bound to the displaced strand can be discerned alongside the helicase bound to the translocase strand using a forked substrate with accessible duplexes on both strands labeled with different fluorophores. In order to quantify the effect of protein-protein interactions on the activity of multiple monomers of the Bacteroides fragilis Pif1 helicase bound to separate strands of a forked DNA junction, an ensemble gel-based assay for monitoring simultaneous duplex unwinding was developed (Su et al., 2019). Here, the use of that assay is described for measuring the total product formation and rate constants of product formation of multiple duplexes on a single nucleic acid substrate. Use of this assay may aid characterization of protein-protein interactions between multiple helicase monomers at forked nucleic acid junctions and can assist with the characterization of helicase action on the displaced strand of forked duplexes.

Binding of pyridoxal, pyridoxal 5'-phosphate and derived hydrazones to bovine serum albumin in aqueous solution.

In the present paper, the kinetics of a reaction between bovine serum albumin (BSA) and pyridoxal, pyridoxal 5'-phosphate was studied, apparent rate constant of product formation and dissociation as well as binding constants were determined. Pyridoxal 5'-phosphate hydrazones of isonicotinic, picolinic, 2-furoic, thiophene-2-carboxylic, pyrazinoic acids binding to BSA was studied by spectrofluorimetry, stability constants of the associates were calculated from experimental data using maximal likelihood approach. The changes in the secondary structure of BSA induced by hydrazones addition were studied by IR spectroscopy. New freely available software for curve fitting was developed as a part of the software kit designed for the solution chemistry and used for a specific problem of this study, IR spectra processing.

Oxidative dehalogenation and denitration by a flavin-dependent monooxygenase is controlled by substrate deprotonation.

Enzymes that are capable of detoxifying halogenated phenols (HPs) and nitrophenols (NPs) are valuable for bioremediation and waste biorefining. HadA monooxygenase was found to perform dual functions of oxidative dehalogenation (hydroxylation plus halide elimination) and denitration (hydroxylation plus nitro elimination). Rate constants associated with individual steps of HadA reactions with phenol, halogenated phenols and nitrophenols were measured using combined transient kinetic approaches of stopped-flow absorbance/fluorescence and rapid-quench flow techniques. Density functional theory was used to calculate the thermodynamic and electronic parameters associated with hydroxylation and group elimination steps. These parameters were correlated with the rate constants of hydroxylation, group elimination, and overall product formation to identify factors controlling individual steps. The results indicated that the hydroxylation rate constant is higher when the pK a of the phenolic group is lower, i.e. it is more easily deprotonated, but not higher when the energy gap between the E LUMO of the C4a-hydroperoxy-FAD intermediate and the E HOMO of the phenolate substrate is lower. These data suggest that the substrate deprotonation has a higher energy barrier than the -OH transfer, and thus controls the hydroxylation step. For the group elimination, the process is controlled by the ability of the C-X bond to break. For the overall product formation (hydroxylation and group elimination combined), this analysis showed that the rate constant of product formation is dependent on the pK a value of the substrate, indicating that the overall reaction is controlled by substrate deprotonation. This step also likely has the highest energy barrier and thus controls the overall process of oxidative dehalogenation and denitration by HadA. This report is the first to identify a key mechanistic factor controlling the enzymatic processes of oxidative dehalogenation and denitration.

Electron-induced chemistry of methyl chloride caged within amorphous solid water

The interaction of low energy electrons (1.0-25 eV) with methyl-chloride (CD3Cl) molecules, caged within Amorphous Solid Water (ASW) films, 10-120 monolayer (ML) thick, has been studied on top of a Ru(0001) substrate under Ultra High Vacuum (UHV) conditions. While exposing the ASW film to 3 eV electrons a static electric field up to 8 × 10(8) V∕m is developed inside the ASW film due to the accumulation of trapped electrons that produce a plate capacitor voltage of exactly 3 V. At the same time while the electrons continuously strike the ASW surface, they are transmitted through the ASW film at currents of ca. 3 × 10(-7) A. These electrons transiently attach to the caged CD3Cl molecules leading to C-Cl bond scission via Dissociative Electron Attachment (DEA) process. The electron induced dissociation cross sections and product formation rate constants at 3.0 eV incident electrons at ASW film thicknesses of 10 ML and 40 ML were derived from model simulations supported by Thermal Programmed Desorption (TPD) experimental data. For 3.0 eV electrons the CD3Cl dissociation cross section is 3.5 × 10(-16) cm(2), regardless of ASW film thickness. TPD measurements reveal that the primary product is deuterated methane (D3CH) and the minor one is deuterated ethane (C2D6).

Kinetic modeling of cellulose hydrolysis with first order inactivation of adsorbed cellulase

Enzymatic hydrolysis involves complex interaction between enzyme, substrate, and the reaction environment, and the complete mechanism is still unknown. Further, glucose release slows significantly as the reaction proceeds. A model based on Langmuir binding kinetics that incorporates inactivation of adsorbed cellulase was developed that predicts product formation within 10% of experimental results for two substrates. A key premise of the model, with experimental validation, suggests that V max decreases as a function of time due to loss of total available enzyme as adsorbed cellulases become inactivated. Rate constants for product formation and enzyme inactivation were comparable to values reported elsewhere. A value of k 2/ K m that is several orders of magnitude lower than the rate constant for the diffusion-controlled encounter of enzyme and substrate, along with similar parameter values between substrates, implies a common but undefined rate-limiting step associated with loss of enzyme activity likely exists in the pathway of cellulose hydrolysis.

Atmospheric Heterogeneous Reactions of Benzo(a)pyrene

Abstract This experimental study deals with heterogeneous reactions of benzo(a)pyrene (BaP) with ozone, nitrogen dioxide and hydroxyl radicals. BaP was adsorbed on silica particles chosen here as a model of mineral atmospheric particles. Compound extractions were assisted by focused microwave and analyses were performed by gas chromatography coupled with mass spectroscopy in single ion monitoring mode. Pseudo-first order rate constants were obtained from the fit of experimental decays of particulate-BaP concentration versus reaction time. Second order rate constants were determined considering the different oxidant gaseous concentrations except for the case of hydroxyl radicals where only a pseudo-first order rate constant was proposed. Values obtained at room temperature are (2.1±0.5)×10−15 cm3 molecule−1 s−1 for (BaP + ozone), (5.8±1.4)×10−16 cm3 molecule−1 s−1 for (BaP + nitrogen dioxide) and (3.4±0.8)×10−2 s−1 for (BaP + OH) reactions. Products have only been investigated for the NO2 and the OH (in the presence of NOx) reactions. 1-, 3- and 6-nitrobenzo(a)pyrenes were detected as degradation products and quantified. Reaction rate constants for product formation are (3.7±0.9)×10−16 cm3 molecule−1 s−1 for 6-NBaP, (2.2±0.6)×10−17 cm3 molecule−1 s−1 for 1-NBaP and (5.3±1.3)×10−17 cm3 molecule−1 s−1 for 3-NBaP. 1-, 3- and 6-nitroBaP account respectively for approximately 5%, 12% and 83% of total nitrated species. If in the presence of only nitrogen dioxide, BaP was totally degraded within few minutes, only 20 to 25 % of the initial BaP led to nitrated compounds when reacting with OH (in the presence of NOx).

Cellulase deactivation based kinetic modeling of enzymatic hydrolysis of steam-exploded wheat straw

Applying mass action law and quasi-steady-state theory, two cellulase kinetic models namely Eqs. (5) and (8) were developed on the basis of the first and second order reactions of enzyme deactivation, respectively. The two models are compared according to analysis of experimental data from enzymatic hydrolysis steam-exploded wheat straw. Both simulation and prediction results show Eq. (8) has much higher accuracy than Eq. (5). Analysis of initial hydrolysis rate is also in accordance with Eq. (8) and against Eq. (5). Fitted values of k 2 (the rate constant of product formation), k de2 (the rate constant of enzyme deactivation) and K e (the equilibrium constant) determined from Eq. (8) are 0.4732 h −1, 0.4011 L/(h g), and 16.8597 g/L, respectively. The higher the enzyme concentration is, the larger the deactivation rate.

Kinetics of oxidation of hydroquinone to p-benzoquinone catalyzed by microporous metal-organic frameworks M3(BTC)2 [M = copper(II), cobalt(II), or nickel(II); BTC = benzene-1,3,5-tricarboxylate] using molecular oxygen

The kinetics of oxidation of hydroquinone (H2Q) to p-benzoquinone (Q) catalyzed by microporous metal-organic frameworks (MOFs) using molecular oxygen has been investigated. MOF catalysts with different metals and pore sizes, i.e. M3(BTC)2(H2O) x (M = CuII (1), CoII (2), or NiII (3); BTC = benzene-1,3,5-tricarboxylate; x = 3 for MOF 1 and 12 for 2 and 3), were used to catalyze the oxidation reaction. The apparent first-order reaction rate constant increased with increasing temperature, molar ratio of catalyst/H2Q, and pH of the medium. A kinetic model for the heterogeneous catalysis is proposed, and the apparent activation energy (E a), the association constants between the hydroquinone and the catalyst (K s), and the first-order rate constants for product formation in the MOF channels (k N) were calculated.

Butanolysis of 2‐methylbenzenediazonium ions: product distribution, rate constants of product formation, and activation parameters

AbstractWe have determined the product distributions, the rate constants of product formation and substrate loss, and the activation parameters for the butanolysis of 2‐methylbenzenediazonium, 2MBD, tetrafluoroborate in aqueous 1‐Butanol (BuOH) solutions by combining UV–VIS spectroscopy, high performance liquid chromatography (HPLC), and a derivatization protocol that traps unreacted 2MBD as a stable azo dye. BuOH/H2O solutions are miscible over a narrow composition range, but in reverse micelles composed of sodium dodecyl sulfate, SDS, BuOH, and water, are miscible between 45–80%. Two major and two minor dediazoniation products are observed, 2‐cresol, ArOH, 2‐butyl‐tolyl‐ether, ArOBu, and small amounts of 2‐chlorobenzene, ArCl (from HCl added to control solution acidity) and toluene, ArH (a reduction product). Product yields depend on experimental conditions, but quantitative conversion to products is achieved over the entire composition ranges investigated. The observed rate constants, kobs, obtained by monitoring 2MBD loss or by monitoring ArOH or ArOBu formation, are the same and they are only modestly affected by changes in the solution composition. The activation parameters obtained from the effect of temperature on kobs show that the enthalpy of activation is relatively high compared to those found in bimolecular reactions and the entropy of activation is small but positive. The results suggest that 2MBD is mainly sampling in the BuOH‐H2O rich interfacial region of the reverse micelle and are consistent with 2MBD decomposing through a DN + AN mechanism, i.e., a rate limiting formation of an aryl cation that reacts immediately with nucleophiles. Copyright © 2008 John Wiley & Sons, Ltd.

Hydrolysis of p-nitrophenyl picolinate catalyzed by metal complexes of N-alkyl-3,5-bis(hydroxymethyl)-1,2,4-triazole in CTAB micelles

Two lipophilic ligands containing triazole and hydroxyl groups, N-alkyl(C n H 2 n+1 )-3,5-bis(hydroxymethyl)-1,2,4-triazole ( n = 10 and 12), were synthesized. Effects of their Cu(II) and Ni(II) complexes on the hydrolysis of p-nitrophenyl picolinate (PNPP) in cetyltrimethylammonium bromide (CTAB) micelles have been investigated kinetically, and some kinetic parameters of the reactions were obtained by employing the ternary complex kinetic model for metallomicellar catalysis. It was found that Cu(II) complexes of these triazole-based ligands showed more effective catalytic activity on the hydrolysis of PNPP than Ni(II) complexes. Also, the apparent first-order rate constants for product formation in the metallomicellar phase ( k N ′ ), the association constants between the substrate and the binary complex ( K T ), and the association constants between the metal ion and the ligand ( K M ) increased with an increase in pH value, which may be attributed to an increase in the nucleophilicity of the hydroxyl groups in the ligand or the electrophilicity of the substrate at higher pH. In addition, at constant pH, k N ′ and K T increased with an increase in the hydrocarbon chain length of the ligand, while K M decreased.

Probing the role of tightly bound phosphoenolpyruvate in Escherichia coli 3-deoxy- d- manno-octulosonate 8-phosphate synthase catalysis using quantitative time-resolved electrospray ionization mass spectrometry in the millisecond time range

Escherichia coli 3-deoxy- d- manno-octulosonate 8-phosphate (KDO8P) synthase catalyzes the condensation of phosphoenolpyruvate (PEP) and d-arabinose 5-phosphate (A5P) to produce KDO8P and inorganic phosphate. The enzyme is often isolated with varying amounts of tightly bound PEP substrate. To better understand the role of tightly bound PEP in E. coli KDO8P synthase catalysis, a combination of transient kinetic methodologies including rapid chemical quench and mass spectrometry techniques such as time-resolved electrospray ionization mass spectrometry (ESI-TOF MS) were used to study the enzyme purified both in the PEP-bound state and in the unbound state. Pre-steady state burst and single-turnover experiments using radiolabeled [1- 14C] and [ 32P]A5P revealed significant kinetic differences between these enzyme preparations. The active sites concentrations for the bound and unbound states of the enzyme were almost the same (∼100%) and the product release for both states of the enzyme was rate limiting. However, the rate constant of product formation for the PEP-bound enzyme (125 s −1) was higher than that of the unbound enzyme (46 s −1). This was further confirmed by single-turnover experiments using radiolabeled [ 32P]A5P. Interestingly, when PEP was removed from the PEP-bound enzyme and external PEP was added before the kinetic experiments, both the pre-steady state burst and the single-turnover kinetic parameters were similar to those of the enzyme purified in the unbound state. The rate constants of product formation were determined as 44 s −1 (burst experiment) and 48 s −1 (single-turnover experiment). The reaction kinetics of the E. coli KDO8P synthase was also followed by time-resolved ESI mass spectrometry. To validate the suitability of this technique for conducting enzyme kinetics, the standard reaction of p-nitrophenyl acetate hydrolysis by chymotrypsin was analyzed by stopped-flow and time-resolved ESI-TOF MS. The rate constant of p-nitrophenol formation followed by stopped-flow spectrophotometry matched perfectly the rate constant of acetyl–chymotrypsin intermediate formation followed by time-resolved ESI-TOF MS (0.1 s −1). The catalytic properties of the PEP-bound and unbound states of the E. coli KDO8P synthase were then studied on a millisecond time scale. The changes in the intensity of E•PEP, E•KDO8P, and E•intermediate complexes as a function of time were quantified and the reaction kinetics were modeled using KinTekSim simulation software. An analysis of the reaction kinetics established the kinetic competence of the intermediate based upon the rate constants for substrate decay and product formation. The ability of time-resolved ESI-TOF MS to detect and monitor the kinetics for the reaction intermediate constitutes a significant advantage over the traditional rapid chemical quench technique. For all three states of the enzyme (PEP-bound, unbound, and PEP removed from the PEP-bound state) the rate constants obtained by time-resolved ESI-TOF MS matched the pre-steady state rates determined by rapid chemical quench. A comparison of reaction time courses for each state of the enzyme revealed that, in the case of PEP-bound enzyme, the enzymatic reaction reached completion faster than that for the unbound state. In summary, these studies led to the conclusion that bound PEP has an important role in catalysis, maintaining the enzyme in a conformational state optimal for catalytic activity, and established the kinetic competence of the reaction intermediate. This technique has broad applicability for the kinetic analysis of any enzyme system where the substrates, products, or intermediates are eluding the common detection techniques or as a method alternative to the widely used radioactivity assays.

Insights into the Mechanism of 3-Deoxy-D-arabino-heptulosonate 7-Phosphate Synthase (Phe) from Escherichia coli Using a Transient Kinetic Analysis

Escherichia coli phenylalanine-sensitive 3-deoxy-arabino-heptulosonate 7-phosphate synthase (DAHP synthase) catalyzes the net aldol condensation of phosphoenolpyruvate and erythrose 4-phosphate to form 3-deoxy-D-arabino-heptulosonate 7-phosphate and inorganic phosphate. For the first time, the presteady-state kinetic analysis of the Phe-sensitive DAHP synthase from E. coli is reported. The steady-state and presteady-state kinetic parameters of the DAHP synthase reconstituted with Mn(II), Cu(II), and Zn(II) were compared. These studies showed the following: 1) product release is rate-limiting for all of the three metal ions studied under physiologically relevant conditions; 2) concentration of the active sites of the metal-containing DAHP synthase is increasing from Mn- (30%) to Zn- (52%) and to Cu-DAHP synthase (88%); 3) rate constant for product formation is higher in Mn- (130-200 s(-1)) than Cu- (55 s(-1)) and Zn-DAHP synthase (6.8 s(-1)); and 4) steady-state rate (rate constant for product release) is higher for the Mn- (70 s(-1)) than for Cu- (5.6 s(-1)) and Zn-DAHP synthase (1.8 s(-1)). In addition, an examination of the reaction kinetics at lower pH reveals that for Cu-DAHP synthase, product release is no longer rate-limiting, whereas the Mn- and Zn-DAHP synthase show a slower rate of product formation, suggesting that the intermediate formation becomes rate-limiting in product formation. Also, a deuterium-isotope effect on the burst rate constant of product formation for Mn-DAHP synthase was observed at pH 6.0. This supports the hypothesis that the role of metal ion in E. coli DAHP synthase is to position the amino acids with the appropriate geometry required to coordinate and activate the water molecule.

Monitoring dediazoniation product formation by high-performance liquid chromatography after derivatization

A derivatization protocol that exploits the rapid reaction between arenediazonium ions and a suitable coupling agent followed by high-performance liquid chromatography analyses of the reaction mixture was employed to determine the product distribution, the rate constants for product formation and the association constant of 4-nitrobenzenediazonium, PNBD, ion with β-cyclodextrin, β-CD. The derivatization of PNBD with the coupling agent leads to the formation of a stable azo dye that prevents by-side reactions of PNBD with the solvents of the mobile phase, including water, or the metallic parts of the chromatographic system that would eventually lead to erroneous identification and quantification of dediazoniation products. The results show that in the presence of β-CD, nitrobenzene is formed at the expense of 4-nitrophenol, which is the major product in its absence. The observed rate constants for the interaction between PNBD and β-CD increase upon increasing [β-CD] showing a saturation profile indicative of the formation of an inclusion complex between PNBD and β-CD. By fitting the experimental data to a simplified Lineaweaver–Burk equation, the corresponding association constant and the maximum acceleration rate of β-CD towards PNBD were estimated. The protocol is applicable under a variety of experimental conditions provided that the rate of the coupling reaction is much faster than that of dediazoniation.

Isomerization of Indole. Quantum Chemical Calculations and Kinetic Modeling

Density functional theory calculations were carried out to investigate the pathways of the unimolecular isomerizations of indole. Equilibrium and transition-state structures were optimized by the Lee−Yang−Parr correlation functional approximation (B3LYP) using the Dunning correlation-consistent polarized double-ξ basis set. Energy values were calculated at the CCSD(T) level of theory. Rate parameters for all the steps on the surfaces of the indole isomerizations were evaluated using B3LYP frequencies and CCSD(T)//B3LYP energy values. 3H-Indole, which is a stable tautomer of indole, is a precursor in the formation of o-tolyl isocyanide that by −NC → −CN flip forms o-tolunitrile. The latter is one of the isomers of indole. A second isomer, phenylacetonitrile, is obtained directly from indole. A kinetics scheme containing all the elementary steps on the surfaces was constructed, apparent rate constants for product formation were calculated, and comparison with experimentally available formation rates was mad...

Determination of multiple steady states in an enzyme kinetics involving two substrates in a CSTR

The multiple steady states in an isothermal, constant-density CSTR involving two-substrates, enzyme- catalyzed reactions is determined by a zero eigenvalue analysis. The hysteresis and bistability occurs for a certain range of the rate constant of product formation from a ternary complex, kES1S2→P+E. A two-parameter (kES1S2→P+E, k0→S1) bifurcation diagram for several different values of flow rate kS1→0 is also presented. It shows that, to maintain the existence of the steady state multiplicity under a fixed flow rate, the larger the rate constant kES1S2→P+E is, the larger the feed concentration of a substrate is required and the wider the range of that exists. To maintain the existence of the steady state multiplicity for a lower flow rate, it is required to reduce the feed concentration of substrates.

Dediazoniations in Water: An Integrated Physical Organic Chemistry Experiment

Most organic chemistry textbooks include sections where arenediazonium salt chemistry is discussed but this material is not normally included in undergraduate laboratory experiments. We propose an integrated physical organic experiment that includes synthesis and isolation of an arenediazonium salt in the solid state and a kinetic study of its stability in water by employing both spectrophotometric and potentiometric measurements. Spectrophotometric kinetic data can be obtained by following the decrease in absorbance due to diazonium ion disappearance. Hydroxy-dediazoniations yield H3O+, so dediazoniation can be monitored by measuring pH changes with time, allowing determination of the rate constants for product formation. Comparison of the two rate constants allows simple mechanistic considerations: heterolytic dediazoniations proceed via a Dn + An mechanism, that is, rate-determining irreversible dissociation of the arenediazonium ion into N2 and an aryl cation that reacts immediately, at diffusion-cont...

Degradation of 3,5-dimethyl-tetrahydro-2H-1,3,5-thiadiazine-2-thione in aqueous aerobic media

Decomposition of 3,5-dimethyl-tetrahydro-2H-1,3,5-thiadiazine-2-thione (DTTT) has been studied by UV-visible spectroscopy (UV-VIS), high-performance liquid chromatography (HPLC), gas chromatography-mass spectrometry (GC-MS), and fast atom bombardment-mass spectrometry (FAB-MS). Hydrolysis rate constants for DTTT and rate constants for product formation were measured. The partially hydrolyzed and aged DTTT solutions have been separated and analyzed by GC-MS. Formaldehyde, 1,3-dimethyl-2-thiourea, N -methyl dithiocarbamic methyl ester, N -methyldithiocarbamic perthiomethyl ester, 2-methyl-5-dihydro-3H-4-thia-1,2-isooxazolidine-3-thione, 1,3,5-trimethyl-hexahydro-2H-1,3,5-triazine-2-thione, 4-methyl-5-methylimino-1,2,4-dithiazolidine-3-thi-one, and 2,4-dimethyl-1,2,4-thiadiazolidine-3,5-dithione were identified from the hydrolyzed DTTT solution. Methyl isothiocyanate, which is one of the intermediates formed during the decomposition of DTTT, was also monitored by UV-VIS and HPLC for the formation of common products and characterized by GC-MS. The decomposition pathways for DTTT are presented and are in accord with the limited literature data. Environmental impact assessment of the DTTT and its metabolites is summarized.

State-to-state rate constants for the collisional interaction of Xe(7p), Xe(6p′), and Kr(5p′) atoms with He and Ar

One-photon laser excitation of Xe(6s[3/2]2) and Kr(5s[3/2]2) atoms that were generated in a discharge-flow reactor was used to study the collisional relaxation of the Kr(5p′[3/2]1, [3/2]2, and [1/2]1), the Xe(7p[3/2]2, [3/2]1, [5/2]2, and [5/2]3), and the Xe(6p′[3/2]1, [3/2]2, and [1/2]1) states in He and Ar. Both cw and pulsed laser excitation techniques were utilized to obtain the total deactivation rate constants and product formation rate constants at 300 K. Collisions with He mainly produce Xe* and Kr* product states with small energy defects, but the rate constants can be as large as 20×10−10 cm3 atom−1 s−1, which correspond to thermally averaged cross sections of 150 Å2. Because of the rapid collisional coupling of populations in nearly isoenergetic levels, multicomponent exponential decay of the initially produced state is frequently observed. The deactivation rate constants for Ar are smaller than for He, but the product distributions tend to be more diverse than for He, and arguments based only on energy defects are not necessarily a good guide to the favored product state(s) from Ar collisions. The magnitude of the quenching cross sections for Ar is consistent with the crossing of an entrance channel with several diabatic exit channel potentials. However, the superlarge quenching cross sections for Xe*–He to just one or two product levels require special considerations. The Kr(5p′) and Xe(6p′ and 7p) rate constants with He are discussed with respect to collisional effects upon the use of cw optical pumping to convert populations in the metastable Xe(6s[3/2]2) and Kr(5s[3/2]2) levels to the Xe(6s′[1/2]0) and Kr([5s′[1/2]0) levels.

Aromatic nucleophilic substitution reaction of 2,4-dinitrohalogenobenzene and ethyl α-cyano-phenylacetate benzyl anion

The aromatic nucleophilic substitution reaction of 2,4-dinitrohalogenobenzene (DNHB; halogeno =F, Cl, Br) with ethyl α-cyanophenylacetate (ECPA) benzyl anion has been studied by means of UV and IR spectrophotometry as well as gas chromatography. The rates of intermediate formation and product formation were determined respectively. In contrast with common S N Ar reactions, the formation rate constant of the intermediates, substituted cyclohexadienates, was in the order of halogen =F>Br>Cl, whereas the rate constant of product formation was in the order F>Cl>Br, which revealed that the departure of the halogen atom was the rate-limiting step for the overall reaction in the presence of a sterically hindered nucleophile. These reactions could be catalyzed by the solvated electrons. The corresponding reaction intermediates, radical species, were detected by ESR and the reactions were proved to proceed mainly by a solvated-electron induced radical chain reaction mechanism.