Rate Constants For Addition Research Articles

Covalent Catalysis by Pyridoxal: Evaluation of the Effect of the Cofactor on the Carbon Acidity of Glycine

First-order rate constants for deprotonation of the alpha-imino carbon of the adduct between 5'-deoxypyridoxal (1) and glycine were determined as the rate constants for Claisen-type addition of glycine to 1 where deprotonation is rate determining for product formation. There is no significant deprotonation at pH 7.1 of the form of the 1-glycine iminium ion with the pyridine nitrogen in the basic form. The value of kHO for hydroxide ion-catalyzed deprotonation of the alpha-imino carbon increases from 7.5 x 10(2) to 3.8 x 10(5) to 3.0 x 10(7) M(-1) s(-1), respectively, with protonation of the pyridine nitrogen, the phenoxide oxyanion, and the carboxylate anion of the 1-glycine iminium ion. There is a corresponding decrease in the pKas for deprotonation of the alpha-imino carbon from 17 to 11 to 6. It is proposed that enzymes selectively bind and catalyze the reaction of the iminium ion with pKa = 17. A comparison of kB = 1.7 x 10(-3) s(-1) for deprotonation of the alpha-imino carbon of this cofactor-glycine adduct (pKa = 17 by HPO4(2-) with k(cat)/K(m) = 4 x 10(5) M(-1) s(-1) for catalysis of amino-acid racemization by alanine racemase shows that the enzyme causes a ca 2 x 10(8)-fold acceleration of the rate of deprotonation the alpha-imino carbon. This corresponds to about one-half of the burden borne by alanine racemase in catalysis of deprotonation of alanine.

The reactivity of carotenoid radicals with oxygen

The possibility that carotenoid radicals react with oxygen to form chain-carrying peroxyl radicals has been postulated to account for the reduction in antioxidant effetiveness displayed by some carotenoids at high oxygen concentrations. The primary objective of the work described in this paper was to measure the rate constants for oxygen addition to a series of carotenoid radicals and to examine any influence of carotenoid structural features on these rate constants. Laser flash photolysis has been used to generate long-lived carotenoid radicals (PhS–CAR√) derived from radical addition reactions with phenylthiyl radicals (PhS√) in benzene. The PhS–CAR√ radicals are scavenged by oxygen at rates that display a moderate dependence on the number of conjugated double bonds (ndb) in the carotenoid. The rate constants range from ∼103 to ∼104 M− 1 s− 1 for ndb = 7–11. The data also suggest that the presence of terminal cyclic groups may cause an increase in the rate constant for oxygen addition.

Kinetic analysis of the cross reaction between dithioester and alkoxyamine by a Monte Carlo simulation

AbstractA model reaction of dithioester and alkoxyamine is proposed to probe the reversible addition–fragmentation chain transfer (RAFT) process. The kinetics of the model reaction is analyzed and compared with that of pure alkoxyamine homolysis with a Monte Carlo simulation. Although the pure alkoxyamine obeys the law of persistent radical effect, the model reaction results in higher concentration of the persistent radical during the main period of the reaction. However, for a very fast RAFT process or a very low addition rate constant, the time dependence of the persistent radical concentration is quite close to that of pure alkoxyamine. Furthermore, the cross termination between the intermediate and alkyl radicals causes a retardation effect for the model reaction when the intermediate is relatively long‐lived. The Monte Carlo simulation indicates that it is feasible to measure the individual rate constants of the RAFT process, such as the rate constant of addition, with a large excess of alkoxyamine. In addition, the special feature of the system with different leaving groups in the alkoxyamine and dithioester is also discussed. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 374–387, 2007

Search for High Reactivity and Low Selectivity of Radicals toward Double Bonds: The Case of a Tetrazole-Derived Thiyl Radical

The search for new radical structures having both low selectivity and high reactivity toward the addition reaction onto alkenes can be of interest in organic synthesis or polymer chemistry and has led us to propose a new tetrazole-derived thiyl radical. The reactivity of this sulfur-centered structure is compared to that of an aminoalkyl radical also efficient for alkene addition Worthwhile results are obtained; the new structure is more reactive on the complete range of alkenes with addition rate constants higher than 107 M(-1) s(-1) for both electron-deficient (acrylonitrile, ...) or electron-rich (vinylether, ...) double bonds. Quantum mechanical calculations have allowed a better understanding of this unique feature.

Radical−Molecule Reactions for Aromatic Growth: A Case Study for Cyclopentadienyl and Acenaphthylene

Polycyclic aromatic hydrocarbon growth from acenaphthylene and cyclopentadienyl was investigated by using the B3LYP/6-31G(d,p) and BH&HLYP/6-31G(d,p) levels of theory as well as transition state theory. The reaction pathways of cyclopentadienyl bearing hydrocarbons are different from those without these moieties and cannot be adequately accounted for by the existing acetylene addition and aryl-aryl addition mechanisms. The reaction mechanisms identified in this paper lead to the formation of fluoranthene, aceanthrylene, and acephenanthrylene. Rate constants of the radical-molecule addition and subsequent intramolecular addition steps predict that the 1,2 double bond in acenaphthylene is much more reactive than the 3,4 and 4,5 double bonds. Fluoranthene is the most abundant product produced at high temperatures and the yield of acephenanthrylene is bigger than that of aceanthrylene. The computational results are discussed in light of pyrolysis experiments on CPD-indene and CPD-acenaphthylene mixtures conducted by Prof. Mulholland's research group reported in a previous work.

An ESR Approach to the Estimation of the Rate Constants of the Addition and Fragmentation Processes Involved in the RAFT Polymerization of Styrene

AbstractThe reversible‐addition‐fragmentation chain transfer (RAFT) controlled radical polymerization of such vinylic monomers as styrene (= ethenylbenzene) has gained increasing popularity in current years. While there is a general agreement on the mechanism of RAFT polymerization, there is an ongoing debate about the values of the rate constants of its key steps, i.e., the addition of the propagating radicals to the mediator and the fragmentation of the resulting spin adducts. By carrying out an ESR spectroscopic investigation of the AIBN‐initiated polymerization of styrene (AIBN = 2,2′‐azobis[2‐methylpropanenitrile]), mediated by benzyl (diethoxyphosphoryl)dithioformate (5) as RAFT agent, we were able to detect and characterize four different radical species involved in the process. By reproducing their concentration–time profiles through a kinetic model, the addition and fragmentation rate constants at 90° of the propagating radicals to and from the mediator were estimated to be ca.107 M−1 s−1 and ca. 103 s−1, respectively. The validity of the kinetic model was supported by hybrid meta DFT calculations with the BB1K functional that predicted addition‐ and fragmentation‐rate‐constant values in good agreement with those estimated from the ESR experiments.

Respective Contributions of Polar vs Enthalpy Effects in the Addition/Fragmentation of Mercaptobenzoxazole-Derived Thiyl Radicals and Analogues to Double Bonds

The formation and the reactivity of three selected sulfur-centered radicals formed from mercaptobenzoxazole, mercaptobenzimidazole, and mercaptobenzothiazole toward four double bonds (methyl acrylate, acrylonitrile, vinyl ether, and vinyl acetate) are investigated. The reversibility of the addition/fragmentation reaction in these widely used photoinitiating systems of radical polymerization was studied, for the first time, through the measurement of the corresponding rate constants by time-resolved laser spectroscopy. The combination of these results with quantum mechanical calculations clearly evidences that, contrary to previous studies on other aryl thiyl radicals, the addition rate constants (ka) are governed here by the polar effects associated with the very high electrophilic character of these radicals. However, interestingly, the back-fragmentation reaction (k-a) is mainly influenced by the enthalpy effects as supported by the relationship between the rate constants and the addition reaction enthalpy DeltaHR. The addition and fragmentation rate constants calculated from the transition state theory (TST) are in satisfactory agreement with the experimental ones. Therefore, molecular orbital (MO) calculations offered new opportunities for a better understanding of the sulfur-centered radical reactivity.

N2 Hydrogenation Promoted by a Side-On Bound Hafnocene Dinitrogen Complex

Reduction of the hafnocene diiodide (η5-C5Me4H)2HfI2 with excess sodium amalgam under 1 atm of dinitrogen yielded the hafnocene dinitrogen complex [(η5-C5Me4H)2Hf]2(μ2,η2:η2-N2). X-ray diffraction has established a strongly activated, side-on bound, bridging N2 ligand with an N−N bond length of 1.423(11) Å. Exposure of the dinitrogen compound to 1 atm of dihydrogen at 23 °C resulted in rapid hydrogenation of the N2 ligand, affording the hydrido hafnocene diazenido complex [(η5-C5Me4H)2HfH]2(μ2,η2:η2-N2H2). Confirmation of N2 hydrogenation has been provided by NMR and IR spectroscopic studies and X-ray diffraction. Pseudo-first-order observed rate constants for H2 addition have been measured by monitoring the disappearance of a principally LMCT band centered at 886 nm [(η5-C5Me4H)2Hf]2(μ2,η2:η2-N2) as a function of time. The hafnocene complex hydrogenates dinitrogen approximately 4 times faster than the corresponding zirconocene compound. Divergent chemistry from zirconium is observed upon thermolysis of [(η5-C5Me4H)2HfH]2(μ2,η2:η2-N2H2), which results in cyclopentadienyl methyl group cyclometalation rather than N−N bond scission.

Addition of Benzoyl Radicals to Butyl Acrylate: Absolute Rate Constants by Time-Resolved EPR

The addition rate constants of five differently substituted benzoyl radicals to butyl acrylate were determined by continuous-wave time-resolved EPR (TR-EPR at X-band (9 GHz) and at W-band (95 GHz)) spectroscopy. Remarkably, the reactivity of the benzoyl radicals, is divided into two domains: One is established at low (<1.25 M) concentrations of butyl acrylate with rate constants in the range of 106 mol-1 s-1. At alkene concentrations above ca. 1.25 M, the addition reaction slows down by ∼1 order of magnitude. This decrease of the reactivity coincides with a dramatic increase of the macroscopic viscosity in the reaction medium.

Thiyl Radical Interaction with Pyrimidine C5−C6 Double Bond

Addition and elimination interaction of thiyl radicals with the C5-C6 double bond in pyrimidines was studied by the pulse radiolysis technique in aqueous solution with the use of different monitoring systems. For this purpose, p-thiocresol, cysteamine hydrochloride, and mercaptoethanol were used. The rate constants of addition and elimination of thiyl radicals were determined by applying the modified version of ACUCHEM (computer program for modeling complex reaction systems). Aliphatic thiyl radicals add to the pyrimidine C5-C6 double bond with k = 1.0-3.0 x 10(7) dm3 mol(-1) s(-1), whereas elimination takes place with k = 0.7-2.0 x 10(5) s(-1). Quantum chemical calculations at the B3LYP/6-31G(d)/PCM level show that the addition should occur at the C6 position of the pyrimidine ring and that the energy of interaction between thiyl radicals and the pyrimidine double bond C5-C6 is low.

Distribution Kinetics Model for the Sol−Gel Transition Critical Exponent

The pregelation weight-average molecular weights of cross-linked polymers and silica-based sol−gels diverge at the sol−gel transition, where time t approaches the gel time tc. The divergence follows a power law, (1 − t/tc)-γ, with a critical exponent larger than the γ = 1 prediction of classic Flory−Stockmayer theory and of our previous cross-linking model. In this paper we modify the cross-linking mechanism by allowing a fractional exponent, 0 < ω ≤ 1, for the mass-dependent rate kernel to match the critical exponents of real systems. Moment equations solved by closure approximations show that γ is determined solely by ω, in agreement with an existing analytical solution. The rate constants for cross-linking, addition, and condensation polymerization have essentially no effect on γ. We also obtained for the first time an analytical expression for gel time, in agreement with moment closure solutions for values of ω other than 0 or 1. When ω is either 3/4 or 5/6, good approximations to experimental values of γ for silica sol−gel or polymer, respectively, are found.

Theoretical study of OH addition to α-pinene and β-pinene

Quantum chemical calculations have been employed to determine the structures and energetics of the radicals arising from the OH addition to α-pinene and β-pinene. The rate constants of OH addition to α-pinene and β-pinene are calculated using the canonical variational transition state theory (CVTST). At the B3LYP/6-31G(d,p) level of theory, the high-pressure limit rate constants at 300 K are 5.1 × 10 −11 cm 3 molecule −1 s −1 for α-pinene and 6.1 × 10 −11 cm 3 molecule −1 s −1 for β-pinene, consistent with experimental studies. These results provide the isomer-specific rate constants and branching ratios for OH addition to α-pinene and β-pinene.

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.

Kinetic studies on the addition of dipropylamine to starch acrylate

The kinetics of nucleophilic addition of dipropylamine to vinyl groups of starch acrylate has been investigated. The reactions were carried out under pseudo-first order conditions in tetrahydrofuran with excess amine within the temperature range from 25 to 45oC. Pseudo-first-order rate constants for addition and the activation energy have been determined.

Prediction of Initiation Efficiency in Radical Photopolymerization: The Case of Benzoyl and Isopropylketyl Radicals

The addition of two carbon centered radicals R (benzoyl and isopropylketyl) as initiating radicals to acrylonitrile, methyl acrylate, methyl vinyl ether, and vinyl acetate as monomers M was examined in detail by using molecular orbital calculations. The influence of the radical and monomer structures on the reactivity of the various R/M systems were studied. It was found that both polar and enthalpy effects account for the activation energies for this reaction. The quantitative evaluation of these effects allows to predict quite well the change of the experimental addition rate constants.

The Kinetics of Z/E Isomerization of Methyl Oleate Catalyzed by Photogenerated Thiyl Radicals

The reaction of beta-mercaptoethanol with methyl oleate in tertbutyl alcohol was investigated under photochemical conditions. The time-dependent Z/E isomerization of the unsaturated moiety as well as the thiol-adduct formation were analyzed on the basis of radical generation rates and established rate constants. This provides precise room-temperature rate constants for the reversible thiyl radical addition to both Z and E isomers. The rate constants for HOCH2CH2S* addition to the Z and E forms were found to be 1.6 x 10(5) and 2.9 x 10(5) M(-1) S(-1), respectively. For the beta-elimination of the thiyl radical from the alkyl radical adduct, the rate constants were 1.7 x 10(7) and 1.6 x 10(8) S(-1), the faster associated with the formation of the E isomer. DFT-BB1K calculations on 2-butene assign this large preference of fragmentation for the E isomer to a lower activation barrier for the formation of the E transition state from the equilibrium radical adduct structure.

Absolute determinations of the addition rate constants for the reactions of O(3P) atoms with halogenated propenes and butenes. A structure–activity relationship for the estimation of rate constants

AbstractThe kinetics of the reactions of O(3P) atoms with CF3CHCH2, CF3CClCCl2, CF3CFCClCF3 and CF3CClCClCF3 were studied at 298 K using a discharge flow tube system. This is the first absolute kinetic study of these reactions. The overall rate constants based on the measured afterglow reactions were (3.4±0.4) × 10−14, (3.3±0.6) × 10−14, (1.3±0.3) × 10−14 and (1.9±0.4) × 10−14 cm3 molecule−1 s−1, respectively. The experiments were carried out under pseudo‐first‐order conditions with [O(3P)]0 ≪ [alkene]0. The effect of substituent atoms or groups on the reactivity was analyzed. A simple method, using a structure–activity relationship based on the structure of the alkene, was applied for the first time to the reactions of O(3P) with haloalkenes. It is shown that this approach is useful in obtaining an initial estimate of unknown rate constants for reactions of O(3P) with alkenes. Copyright © 2004 John Wiley &amp; Sons, Ltd.

Direct DFT dynamics study of the addition reaction CF 2[dbnd6]CHF+H→product

The addition reactions CF 2 CHF+H→product have been studied using direct density functional theory dynamics method. All of the information along the minimum energy path (MEP) was calculated at the BHandHLYP/aug-cc-pVDZ level of theory. Energetic data along the MEP were further refined at the BHandHLYP/aug-cc-pVQZ level with the BHandHLYP/aug-cc-pVDZ optimized geometries. The barrier heights for forward reactions of C1 and C2 addition were obtained as 4.66 and 3.21 kcal mol −1, respectively. Reaction rate constants and activation energies were calculated for the temperature range 250–2500 K by the canonical variation transition state theory incorporating zero-curvature tunneling and small-curvature tunneling corrections. The calculated results show that the rate constants at the room temperature are in satisfactory agreement with the experimental values, and the C2 addition reaction is about 1 order of magnitude faster than that of the C1. The tunneling corrections are important for the rate constants of the hydrogen atom addition to CF 2 CHF.

Gas-phase reactions of divalent Ni complex ions with acetonitrile: chelate ring size, inductive, and steric effects

Ni(II) complexes of a series of pentadentate polyamine ligands have been reacted with CH 3CN in the gas phase using a modified quadrupole ion trap mass spectrometer. The ligands have structural features such that upon complexation, chelate ring size, sterics, and inductive effects can be evaluated in the gas phase. Rate and equilibrium constants for CH 3CN addition to the metal complexes show that there is a general decrease in the gas-phase reactivity as the chelate ring size is increased. Density functional theory calculations at the B3LYP/LANL2DZ level of theory have been used to obtain minimum energy structures and Mulliken charges for the complexes. The decreased reactivity observed as the chelate ring size is increased correlates with a decrease in the atomic charge on the metal. A larger chelate ring size enhances ligand flexibility and improves the overlap of the ligand's donor atoms with the metal center. Adding methyl groups adjacent to or on the nitrogen donor groups of a ligand also decreases the rate and equilibrium constants for the reactions of a given complex with CH 3CN. Analysis of Mulliken charges for these complexes indicates that both inductive and steric effects are responsible for lower complex reactivity. These results suggest that while the gas-phase reactivity of a metal complex with CH 3CN is very dependent on the functional groups directly bound to the metal, in some cases steric effects can conceal the correlation between reactivity and coordination structure.

Mechanisms of Heterogeneous Processes in the System SiO2+ CH4: III. Products of &gt;Si&lt;O2Group Methylation

The methods of optical, ESR, and IR spectroscopy were used to obtain data on the structure and mechanism for the formation of the products in the reaction of dioxasilirane groups (≡Si–O)2Si Si(O⋅)(OCH3) + CH4 → >Si(OH)(OCH3) + ⋅CH3 and >Si(O–⋅CH2)(OH) + ⋅CH3 → >Si(OH)(OC2H5). The ratio of the rate constants of methyl radical addition to (≡Si–O)2Si: and (≡Si–O)2Si <O2 at room temperature was determined experimentally (4.6 ± 1.0).