Electrooxidation of Chlorpromazine in Aqueous and Micellar Media and Spectroscopic Studies of the Derived Cationic Free Radical and Dication Species

Summary.  The electrochemical behaviour of chlorpromazine has been examined in phosphate buffers in aqueous as well as micellar media at a pyrolytic graphite electrode surface. Two oxidation peaks were obtained in linear sweep voltammetry of chlorpromazine. The first peak corresponds to the formation of the cationic free radical, which on further 1e⁻-oxidation gives a dication. The spectroscopic changes and kinetics of the cationic free radical and dication species generated during electrooxidation of chlorpromazine were investigated in both media. The decay of the dication was studied chronoamperometrically and was found to follow first-order kinetics with a half-life of ∼25 ms. Surfactants affect both E _p and i _p values. The anionic surfactant SDS has been found to catalyze the reaction of the free radical cation and the dication.     

Influence of Glycerol on Nitrogenase Reactions

The influence of glycerol on the ATPase reaction of nitrogenase and reduction of the substrate (acetylene) is studied. Glycerol inhibits the ATPase nitrogenase reaction dependent on an electron donor. The reaction rate is halved at a glycerol concentration of 11% in the medium when the solution viscosity increases only 1.31 times. The electron donor–independent (decoupled) ATPase reaction of nitrogenase is inhibited to a lesser extent. The activation energies (E _a) of reactions studied in the presence of glycerol are determined. Despite the inhibition effect, glycerol in a concentration of 7.5% does not affect the E _a of acetylene reduction. The introduction of glycerol significantly decreases the E _a of the electron donor-dependent ATPase reaction. In the absence of glycerol, this reaction limits the nitrogenase reaction: E _a = 14 ± 1.4 kcal/mol at temperatures higher than 21°C and E _a = 50 ± 10 kcal/mol at temperatures below 21°C, which are close to the E _a of acetylene reduction. In the presence of 7.5% glycerol, the E _a = 0.7 ± 0.6 kcal/mol at temperatures above 21°C and the E _a = 2.4 ± 0.6 kcal/mol at temperatures below 21°C. This indicates that the reactions of substrate-binding and ATPase sites are decoupled in the presence of glycerol, and the step of substrate reduction becomes the limiting step of the nitrogenase reaction. Glycerol also has a noticeable effect on the E _a of the electron donor-independent ATPase reaction and the shape of the plot of logw vs. 1/T for this reaction. The data obtained indicate the specific interaction of glycerol with nitrogenase in the region of the ATPase site perhaps due to the distortion of the structure of hydrogen bonds, and this interaction changes the limiting step of the nitrogenase reaction.     

Kinetics of iodination of hydrogen sulfide by iodine and the heat of formation of the SH radical

The kinetics of the gas-phase thermal iodination of hydrogen sulfide by I₂ to yield HSI and HI has been investigated in the temperature range 555–595 K. The reaction was found to proceed through an I atom and radical chain mechanism. Analysis of the kinetic data yields log k (l/mol·sec) = (11.1 ± 0.18) – (20.5 ± 0.44)/θ, where θ = 2.303 RT, in kcal/mol. Combining this result with the assumption E_?1 = 1 ± 1 kcal/mol and known values for the heat of formation of H₂S, I₂, and HI, ΔH_f,298⁰(SH) = 33.6 ± 1.1 kcal/mol is obtained. Then one can calculate the dissociation energy of the HS? H bond as 90.5 ± 1.1 kcal/mol with the well-known values for ΔH_f,298⁰ of H and H₂S.     

Free radical decomposition of bromoacetonitrile in liquid cyclohexane

The kinetics of the gamma radiation induced free radical chain decomposition of BrCH₂CN in liquid cyclohexane (RH) was investigated over the temperature range of 60–170°C. In addition competitive experiments in the presence of CCl₄ were carried out between 80 and 180°C. For the reactions the following Arrhenius expressions were derived: where θ = 2.303RT in kcal/mol. The effect of CN substitution on the activation energies of reactions (2) and (3) was evaluated based on the present and previously published results. The CN group effect on halogen atom abstraction [reaction (2)] is discussed in terms of inductive and enthalpic factors. The differences E₃ ? E(CH₃ + RH) and E(CCl₂CN + RH) ? E(CCl₃ + RH), which yield a value of about 5.5 kcal/mol, are considered to reflect the cyano stabilization effect at the radical center confirming D(CH₂(CN)–H) ～ 93 kcal/mol.     

Ab initio quantum-mechanical calculations of the propagation enthalpies for the radical and anionic polymerizations of formaldehyde and methanimine

The gas phase enthalpies of formation for oligomeric radicals and anions H(CH₂NH)_n^* and H(CH₂O)_n^* were theoretically determined by ab initio quantum-mechanical calculations with n in the range 1 to 6. From these results, the reaction enthalpies for each of the first five propagation steps of the polymerization were estimated for methanimine (H₂C = NH) and formaldehyde (H₂C = O). At the same step of oligomerization, enthalpies associated with anionic polymerizations are always more negative than enthalpies corresponding to radical polymerizations, but the difference between them decreases with increasing n. Both Delta;H (propagation) vs. n curves tend rapidly, particularly for radical polymerizations, towards an asymptotic value independent of the mode of polymerization and equal to - 12 kcal/mol for formaldehyde and - 14 kcal/mol for methanimine. Experimental data for the gas phase polymerization of formaldehyde are in good agreement with our theoretical value. These results demonstrate that heats of polymerization can be reasonably estimated by intensive calculation methods if a careful choice of the reaction mimicking the propagation step is done.     

Theoretical study of the CO catalytic oxidation on Au/SAPO‐11 zeolite

Quantum chemistry calculations were carried out, using ONIOM2 methodology, to investigate the CO adsorption and oxidation on gold supported on Silicoaluminophospates (SAPO) molecular sieves Au/SAPO‐11 catalysts. Two models were studied, one containing one Au atom per site (Au? SAPO‐11), and the other with two Au atoms per site (Au₂? SAPO‐11). The results reveal that the CO adsorption and oxidation are exothermic on Au/SAPO11 with an ΔE of ?41.0 kcal/mol and ΔE = ?52.0 kcal/mol, for the adsorption and oxidation, respectively. On the Au₂? SAPO‐11 model, the CO adsorption and oxidation reaction occur, with a ΔE of ?29.7 kcal/mol and ?52 kcal/mol, respectively. According to our results, the oxidation reaction exhibits an Eley‐Rideal type mechanism with adsorbed CO. The theoretical calculations reveal that this type of material could be interesting to disperse Au and consequently to strengthen its catalytic use for different reactions. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem 110:2573–2582, 2010     

Mechanism of the chain process forming H2 in the photooxidation of formaldehyde

A mechanism for the formation in a chain of H₂, CO, and HCOOH in the photooxidation of formaldehyde is proposed. This mechanism is initiated by the addition of HO₂ to formaldehyde. Hydrogen atoms are produced by the thermal dissociation of the HOCH₂O radical: HOCH₂O → H + HCOOH; ΔH = + 3.2 kcal/mol [5]. Photolysis of CH₂O? O₂? NO mixtures and product analysis were carried out in conjunction with kinetic simulation yielding an estimate for the activation energy of the dissociation reaction : E₅ = 14.9 ± 1.0 kcal/mol. Previous observations of this chain process are considered in view of this mechanism.     

Kinetics of the Copolymerization of Methyl Vinyl Ketone and Acrylamide Initiated by the Adduct of Methyl Vinyl Ketone and Imidazole

N-(Butyl-3-one)imidazole acts as an initiating adduct which is formed in the anionic polymerization of methyl vinyl ketone (MVK) induced by imidazole (Im) and is directly formed from Im and the MVK monomer. The kinetics of the anionic homopolymerization of MVK and acrylamide (AAm) under argon in the presence of the adduct were investigated in tetrahydrofuran (THF). The rate of polymerization for the MVK system is expressed as R_p = k[Adduct] [MVK], where k = 3.1 × 10^?6 L/(mol·s)in THF at 30°C. The overall activation energy, E_a , was found to be 5.34 kcal/mol. The R_p for the AAm system is expressed as R_p = k[Adduct] [AAm], where k = 6.8 × 10^?6 L/(mol·s) in THF at 30°C, with E_a 7.78 kcal/mol. The mechanism of the polymerization induced by the initiator adduct is discussed on the basis of these results.     

High-temperature degradation of polyacrylic acid in aqueous solution

Poly(acrylic acid) (PAA) was decarboxylated in aqueous solution as a function of pH and ionic strength in the temperature range 100–350°C. Degradation kinetics were first order with respect to acid functionality. The rate of decarboxylation at high pH (E_α = 51.1 kcal/mol) was much slower than that at low pH (E_α = 20.3 kcal/mol). At intermediate pH, the reactivity was found to depend on the degree of dissociation of PAA as a functions of ionic strength, pH, and temperature. No monomer was observed in the reaction product. © 1996 John Wiley & Sons, Inc.     

A computational investigation on the photochemistry of the popular spin-trap agent N-tert-butyl-α-phenylnitrone (PBN) and thermal isomerization pathways of its photoproduct oxaziridine

Computational investigation on the low-lying photo-excited states of N-tert-butyl-α-phenylnitrone (PBN), a well-known spin-trap agent, has revealed its photo-product (oxaziridine) formation channel. The S₀-S₂ vertical excitation in PBN is subsequently followed by a non-radiative decay pathway through S₂/S₁ and S₀/S₁ conical intersections (CIs) with CNO-kinked structures, situated around 23 kcal/mol and 45 kcal/mol below the vertically excited S₂ state, respectively. The reverse photo-process of PBN formation involves photo-excitation of oxaziridine to its S₂ and S₃ photo-excited states. The forward photo-isomerization leads to the trans-oxaziridine with a backside CNO kink (trans-OX_B) while the reverse path studied by us, connects its front-side CNO-kinked analogue (trans-OX_F) with the PBN. Our search for the reverse thermal reaction paths from these two oxazirdines has led to their corresponding transition states, one at 35 kcal/mol and the other at 27 kcal/mol above trans-OX_F and trans-OX_B geometries, respectively. They lead to two different isomers (E and Z) of PBN which supports the reported nature of products from the trans-oxaziridine in this thermal reaction. The inversion path of the chiral nitrogen atom of this N-tert-butyl-oxaziridine (barrier 21 kcal/mol) has also been tracked. This reaction path has been compared with that of the N-methyl (barrier 30 kcal/mol) and N-acyl (barrier 10.5 kcal/mol) oxaziridine analogues.     

Kinetics and equilibria in the system Br + t-BuO2H ⇆ HBr + t-BuO2. OH bond dissociation energy in t-BuO2-H

The kinetics and equilibria in the system Br + t-BuO₂H ? HBr + t-BuO₂· have been measured in the range of 300–350 K using the very low pressure reactor (VLPR) technique. Using an estimated entropy change in reaction (1) ΔS₁ = 3.0 ± 0.4 cal/mol·K together with the measured ΔG₁, we find ΔH₁ = 1.9 ± 0.2 kcal/mol and DHº (t-BuO₂-H) = 89.4 ± 0.2 kcal/mol ΔH_f·(tBuO₂·) = 20.7 kcal/mol and DH^º (t-Bu-O₂) = 29.1 kcal/mol. The latter values make use of recent values of ΔH_f·(t-Bu) = 8.4 ± 0.5 kcal/mol and the known thermochemistry of the other species. The activation energy E₁ is found to be 3.3 ± 0.6 kcal/mol, about 1 kcal lower than the value found for Br attack on H₂O₂. It suggests a bond 1 kcal stronger in H₂O₂ than in tBuO₂H.     

The kinetics and equilibrium of the gas-phase reaction CH3CF2Br + I2 = CH3CF2I + IBr the C-Br bond dissociation energy in 1,1-difluorobromoethane

The kinetics and equilibrium of the gas-phase reaction of CH₃CF₂Br with I₂ were studied spectrophotometrically from 581 to 662°K and determined to be consistent with the following mechanism: A least squares analysis of the kinetic data taken in the initial stages of reaction resulted in log k₁ (M^?1 · sec^?1) = (11.0 ± 0.3) - (27.7 ± 0.8)/θ where θ = 2.303 RT kcal/mol. The error represents one standard deviation. The equilibrium data were subjected to a “third-law” analysis using entropies and heat capacities estimated from group additivity to derive ΔH_r° (623°K) = 10.3 ± 0.2 kcal/mol and ΔH_rr (298°K) = 10.2 ± 0.2 kcal/mol. The enthalpy change at 298°K was combined with relevant bond dissociation energies to yield DH°(CH₃CF₂ - Br) = 68.6 ± 1 kcal/mol which is in excellent agreement with the kinetic data assuming that E₂ = 0 ± 1 kcal/mol, namely; DH°(CH₃CF₂ - Br) = 68.6 ± 1.3 kcal/mol. These data also lead to ΔH_f°(CH₃CF₂Br, g, 298°K) = -119.7 ± 1.5 kcal/mol.     

Pulse radiolysis studies of poly(methyl methacrylate) containing pyrene

A pulse radiolysis study of poly(methyl methacrylate) in the presence of pyrene has been carried out in the temperature range 100–295 K. The concentration of pyrene was changed from 10⁻³ to 10⁻¹ mol dm⁻³. The absorption/emission spectra and kinetics of solute excited states and solute radical ions were investigated. It was found that pyrene excited states were formed as a result of their radical ion recombination in a time scale up to seconds. The decay of solute radical ions was influenced by photobleaching and can be described by a time-dependent rate constant. The activation energy of Py ions decay was temperature dependent and was equal to 35.7 and 1.2 kJ/mol for temperatures >T_γ and <T_γ, respectively, where T_γ ∼ 175 K represented the transition temperature responsible for γ-relaxation. The reaction mechanism was proposed. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1209–1215, 1998     

Ionic species in irradiated poly(methyl methacrylate). A pulse radiolysis and ESR study

Poly(methyl methacrylate) free of initiators was synthesized by γ-irradiation and cast into transparent films. The samples were investigated by ns pulse radiolysis at various temperatures, and by ESR spectroscopy after γ irradiation at 77°K. Short-lived transients with optical absorptions at 440 and 725 nm were observed. The 440 nm absorption has been ascribed to the cation and the 725 nm absorption (εG = 3000 M^?1 cm^?1 (100 eV)^?1, τ_1/2 = 190 ns at ?13°C, E_a = 6.5 kcal/mol) to the anion. These assignments are based on ESR data of samples of poly(methyl methacrylate) and pivalic acid methyl ester deuterated at the ester deuterated at the ester and α-methyl groups, respectively, and subjected to thermal annealing and photobleaching. The anion decomposes on photobleaching by loss of the ester ·CH₃ radical, and the cation is proposed to decay by loss of the ·CH₃ radical from the α-methyl group. The thermal decay of the anion is discussed.     

Evidence for production of the hydroxycarbonyl radical in the decomposition of formic acid on platinum

Formic acid molecules highly diluted in argon were passed through a clean platinum screen at 420–730 K and condensed onto an 8-K CsI window. The well-known decomposition products, CO₂, CO, and H₂O, were observed in the infrared spectra of the resulting matrices. In addition, new absorptions which are attributed to the OCOH free radical were also observed. Experiments with partially deuterated formic acids confirmed that the carbon-hydrogen bond of the formic acid was lost in the formation of the new intermediate species. The activation energy for CO₂ production, E_a = 3.5 ± 0.2 kcal/mol, was determined by monitoring its appearance rate at several different catalyst temperatures.     

Kinetics and thermodynamics of anionic polymerization of 2-methoxy-2-oxo-1,3,2-dioxaphosphorinane

Kinetic activation parameters and thermodynamic functions describing the reversible anionic polymerization of 2-methoxy-2-oxo-1,3,2-dioxaphosphorinane (1,3-propylene methyl phosphate) were determined. Enthalpy and entropy of the anionic propagation ? depropagation equilibrium were found to be close to those found previously by the present authors for the cationic polymerization, while the activation parameters of propagation and depropagation differ substantially for both processes and reflect the differences in the involved mechanisms. Thus, data for anionic polymerization (and cationic polymerization in parentheses) are: ΔH_1s° = ?0.7 kcal/mole (?1.1); ΔS_1s° = ?2.8 cal/mole-deg (?5.4); ΔH_p^? = 26.7 kcal/mole, and ΔS_p^? = ?6.1 cal/mole-deg. The polymers obtained have low degrees of polymerization (DP _n ≤ 10) because of the extensive chain transfer, leaving cyclic end groups in macromolecules. The presence, structure and concentration of the end groups have been determined by ¹H-, ³¹P-, and ¹³C-NMR spectra.     

Reaction of H + ketene to formyl methyl and acetyl radicals and reverse dissociations

Thermochemical properties for reactants, intermediates, products, and transition states important in the ketene (CH₂?C?O) + H reaction system and unimolecular reactions of the stabilized formyl methyl (C·H₂CHO) and the acetyl radicals (CH₃C·O) were analyzed with density functional and ab initio calculations. Enthalpies of formation (ΔH_f°₂₉₈) were determined using isodesmic reaction analysis at the CBS‐QCI/APNO and the CBSQ levels. Entropies (S°₂₉₈) and heat capacities (C_p°(T)) were determined using geometric parameters and vibrational frequencies obtained at the HF/6‐311G(d,p) level of theory. Internal rotor contributions were included in the S and C_p(T) values. A hydrogen atom can add to the CH₂‐group of the ketene to form the acetyl radical, CH₃C·O (E_a = 2.49 in CBS‐QCI/APNO, units: kcal/mol). The acetyl radical can undergo β‐scission back to reactants, CH₂?C?O + H (E_a = 45.97), isomerize via hydrogen shift (E_a = 46.35) to form the slight higher energy, formyl methyl radical, C·H₂CHO, or decompose to CH₃ + CO (E_a = 17.33). The hydrogen atom also can add to the carbonyl group to form C·H₂CHO (E_a = 6.72). This formyl methyl radical can undergo β scission back to reactants, CH₂?C?O + H (E_a = 43.85), or isomerize via hydrogen shift (E_a = 40.00) to form the acetyl radical isomer, CH₃C·O, which can decompose to CH₃ + CO. Rate constants are estimated as function of pressure and temperature, using quantum Rice–Ramsperger–Kassel analysis for k(E) and the master equation for falloff. Important reaction products are CH₃ + CO via decomposition at both high and low temperatures. A transition state for direct abstraction of hydrogen atom on CH₂?C?O by H to form, ketenyl radical plus H₂ is identified with a barrier of 12.27, at the CBS‐QCI/APNO level. ΔH_f°₂₉₈ values are estimated for the following compounds at the CBS‐QCI/APNO level: CH₃C·O (?3.27), C·H₂CHO (3.08), CH₂?C?O (?11.89), HC·CO (41.98) (kcal/mol). © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 35: 20–44, 2003     

Free radical copolymerization of sulfur dioxide with 1-alkynes

Free radical copolymerization of SO₂ with 1-alkynes (AY) was studied by evaluation of the copolymerization rate under controlled conditions of copolymerization temperature and monomer concentration product ([AY][SO₂]). The poly(alkyne sulfone)s always contained equimolar units of SO₂ and alkyne, regardless of the copolymerization conditions. Using 1-hexyne (HY) and 1-octyne (OY) as comonomers of SO₂, the values of ceiling temperature (T_c) were determined: when [HY][SO₂] = [OY][SO₂] = 0.25 mol²/L², the values of T_c were 90.5 and 84.5°C, respectively. T_c increases with increasing monomer concentration product. The activation energies for propagation (E_p) and depropagation (E_d) of the SO₂-alkynes copolymerization system were investigated, using the SO₂? OY copolymerization system, and estimated to be 12.2 and 26.7 kcal/mol, respectively. The value of E_d is high compared with that of the copolymerization of SO₂ and 1-butene (20.3 kcal/mol), demonstrating that the free radical endings (～ OY? SO₂ and ～ SO₂? OY) are difficult to depropagate, compared with those formed from the copolymerizaton of SO₂ and 1-butene. ΔS and ΔH_p, calculated from experiments, were found to be ?37.7 cal/mol K and ?14.3 kcal/mol, respectively     

The very low-pressure pyrolysis of phenyl methyl sulfide and benzyl methyl sulfide. The enthalpy of formation of the methylthio and phenylthio radicals

The kinetics and mechanisms of the unimolecular decompositions of phenyl methyl sulfide (PhSCH₃) and benzyl methyl sulfide (PhCH₂SCH₃) have been studied at very low pressures (VLPP). Both reactions essentially proceed by simple carbon-sulfur bond fission into the stabilized phenylthio (PhS·) and benzyl (PhCH₂·) radicals, respectively. The bond dissociation energies BDE(PhS-CH₃) = 67.5 ± 2.0 kcal/mol and BDE(PhCH₂-SCH₃) = 59.4 ± 2 kcal/mol, and the enthalpies of formation of the phenylthio and methylthio radicals ΔH° _,298K(PhS·, g) = 56.8 ± 2.0 kcal/mol and ΔH°_{f, 298K}(CH₃S·, g) = 34.2 ± 2.0 kcal/mol have been derived from the kinetic data, and the results are compared with earlier work on the same systems. The present values reveal that the stabilization energy of the phenylthio radical (9.6 kcal/mol) is considerably smaller than that observed for the related benzyl (13.2 kcal/mol) and phenoxy (17.5 kcal/mol) radicals.     

Chemistry of trifluorostyrenes and their dimmers: 3. Solvent effect on the kinetic parameters of the thermal cyclodimerization of α, β, β-trifluorostyrene1

Rate constants for the thermal cyclodimerization of α, β, β-trifluorostyrene (TFS) were determined in six solvents at 393°K. The products of this reaction were mixtures of roughly equal amounts of cis-trans isomers. The rate constants in 3 solvents, were calculated according to Arrhenius equation. In n-hexane, log A = 6.02±0.18, E_a= 19.5±0.3 kcal.mol^?1; in glyme, logA = 5.31 ± 0.19, E_a= 18.0±0.3 kcal.mol^?1; in methanol, IogA=4.93±0.13, E_a=17.1±0.3 kcal mol^?1. All data are consistent with a stepwise radical mechanism, and our reaction in this solvent series obeys an isokinetic relationship, with β = 478°K.