Abstract  Rate coefficients and branching ratios for unimolecular reactions of alkoxyl radicals in the temperature range 200 to 1500 K and in the pressure range 10(-2) to 10(4) bar are presented. Special emphasis is given to the change of the relative importance of different reaction channels. This is evident for longer chain alkoxyl radicals which at lower temperatures undergo isomerisation via an energetically favourable six-member ring transition state. At temperatures above >600 K, however, the rates of the alternative decomposition pathways become faster than that of isomerisation.

The theoretical approach to derive unimolecular rate coefficients makes use of extensive (G2/G3) ab initio calculations. These are used as input data for RRKM calculations to which a tunnelling probability of the generalised Eckart type has been included on the microcanonical level. A subsequent multi-channel master equation (ME) treatment yields thermal rate coefficients and complete fall-off curves for arbitrary pressure and temperature conditions.

The kinetics of two linear chain alkoxyl radicals, 1-butoxyl and 2-pentoxyl, have been analysed to show the expected complex temperature and pressure behaviour resulting from the competition of multiple accessible reaction channels, each with its specific p, T-dependence. Additional results are reported for the minimum temperatures where channel switching can occur, the trajectory of these characteristic points on the p, T-surface, and the temperature dependence of the fractional ratios of the unimolecular reaction channels and the bimolecular reaction with oxygen at ambient atmospheric pressure.

Abstract  The study disclosed herein provides for the first time a detailed experimental support for the general mechanism of the cigarette-smoke-derived chemiluminescence, as an example par excellence of the excited-state generation in a chemically complex aerosol medium. The mechanism involves chemiexcitation in a unimolecular transformation of the smoke-borne free radical species. However, the concentration of these radicals, [r∙], obeys a bimolecular (second-order) kinetics and depends on a particulate-phase content (total particulate matter, TPM) of the cigarette smoke. The decrease in [r∙] with increasing the TPM amount manifests radical-scavenging propensity of the smoke particulate phase. Astonishingly, no energy transfer takes place from the primary excited light-emitting species to luminophoric molecules abundant in the smoke. The reported results build up fundamentals of a facile chemiluminescence assay for free radical properties of the smoke. The experimental approaches developed for this study are of general scope and may be used for mechanistic elucidation of the excited-state generation in chemical systems and environments of an arbitrary complexity.

Abstract  Solvent effects on the kinetics for hydrogen abstraction from a lactone antioxidant were determined for alkoxyl and nitroxyl radicals; their reactivity differ by about 7 orders of magnitude. A decrease by approximately 12 and approximately 35 were determined for H-abstraction by tert-butoxyl and nitroxyl radicals, respectively, upon changing the solvent from hexane to acetonitrile. Results of solvent and isotope studies indicate that the antioxidant properties of lactone antioxidants should be attributed to the enol, not the lactone. [reaction: see text]

Abstract  The kinetics of the title reactions have been studied by relative-rate methods as a function of temperature. Relative-rate coefficients for the two decomposition channels of 2-methyl-2-butoxyl have been measured at five different temperatures between 283 and 345 K and the observed temperature dependence is consistent with the results of some previous experimental studies. The kinetics of the two decomposition channels of 2-methyl-2-pentoxyl have also been investigated, as a function of temperature, relative to the estimated rate of isomerisation of this radical. Room-temperature rate coefficient data for the two decomposition channels of both 2-methyl-2-pentoxyl and 2-methyl-2-butxoyl (after combining the relative rate coefficient for this latter with a value for the rate coefficient of the major channel, extrapolated from the data presented by Batt et al., Int. J. Chem. Kinet., 1978, 10, 931) are shown to be consistent with a non-linear kinetic correlation, for alkoxyl radical decomposition rate data, previously presented by this laboratory (Johnson et al., Atmos. Environ., 2004, 38, 1755-1765).

Abstract  Photoacoustic Calorimetry is used to determine the a-C-H bond dissociation Energy (BDE) in 1,2,3,4-tetrahydronaphthalene (tetralin) in the neat liquid as well as in competition with hydrogen abstraction from tetrahydrofuran (THF). Also, the a-C-H BDE in THF was determined in the neat liquid. The experiments found to be 82.9 +/- 1.2 and BDE(gas) (THF) to be 92.1 +/- 1.6 kcal mol(-1). The revised global rate constant of hydrogen abstraction from THF is found to be 4.6 x 10(6) M(-1) s(-1) at 297 K.

Abstract  The consumption rates of three monolignols (p-coumaryl, coniferyl, and sinapyl alcohols) and eight analogues using horseradish peroxidase (HRP)-H2O2 as an oxidant were measured and compared with the anodic peak potentials thereof measured with cyclic voltammetry. 3-Monosubstituted p-coumaryl alcohols, i.e., 3-methoxy-, 3-ethoxy-, 3-n-propoxy-, and 3-n-butoxy-p-coumaryl alcohols, had faster reaction rates than p-coumaryl alcohol. This is most probably due to the electron-donating effect of alkoxyl groups. However, the reaction rates gradually decreased with an increase in the molecular weight of the alkoxyl groups. Furthermore, t-butoxyl group, which is a very bulky substituent, caused an extreme reduction in the reaction rate, even though its electron-donating effect was almost the same as that of other alkoxyl groups. The reaction rates of 3,5-disubstituted p-coumaryl alcohols, especially 3,5-dimethyl-p-coumaryl alcohol, were very low compared with 3-monosubstituted p-coumaryl alcohols. These results suggest that there are three main factors of hindrance during the approach of monolignols to the active site of HRP. First, from the results of 3-monoalkoxy-p-coumaryl alcohols, it was suggested that the volume of substituents could decrease their oxidation rates. Second, from the results of 3,5-disubstituted p-coumaryl alcohols, it was suggested that local steric hindrance by the amino residues quite near the heme decreased the oxidation rates. Third, from the results of the substrates with hydrophobic substituents at their 3,5-positions, we suggested that hydrophilicity near heme would decrease their oxidation rates.

Abstract  3-(3-t-Butoxyl)succinimidyl acetyl chloride was obtained by a series of reactions starting from malic acid. 3-(3-t-Butoxyl)succinimidyl acetyl chloride reacted with Schiff bases 1a similar to 1h to yield 8 new 3-(3-t-butoxyl)succinimidyl monocyclic P-lactam derivatives 2a similar to 2h. The structures of the new compounds were confirmed by IR, H-1 NMR, MS spectra and elemental analysis.

Abstract  A simple method for the preparation of 5-epi-nojirymycin (5-epi-DNJ) by a reductive amination of sugar derived alkoxyamines is presented. The latter were prepared in situ from the respective alkylated sugar oximes, which were obtained from the readily available methyl alpha-D-glucoside in a few well defined steps. The stereoselectivity of the reductive amination/cyclisation step depended on the size of the alkyl group in the oxime moiety and was best for the derivative decorated with a bulky tert-butoxyl group (5-epi-DNJ:DNJ = 82:18). The stereochemical outcome of this reaction was rather surprising since the analogous process performed for sugar alkylamines usually provides predominantly derivatives of DNJ. (C) 2016 Elsevier Ltd. All rights reserved.

Abstract  A kinetic study on the effect of acetic (AcOH) and trifluoroacetic acid (TFA) on hydrogen abstractions from the C-H bonds of basic substrates by the cumyloxyl radical (CumO(center dot)) was carried out in acetonitrile. With tetrahydropyran no significant effect on k(H) was observed after acid addition. With the more basic tertiary amines acid addition led to greater than 4-orders of magnitude decreases in k(H). Protonation at nitrogen decreases the degree of overlap between the alpha-C-H sigma* orbital and the lone-pair leading to an increase in the strength of the C-H bond and a destabilization of the radical formed after abstraction. Evidence that C-H deactivation extends up to the gamma-C-H bonds and for the reversibility of this approach was also provided. With TFA no reaction was observed up to [amine] = [TFA], pointing towards stoichiometric protonation. At [amine] > [TFA], k(H) values that are very similar to those measured in the absence of added acid were obtained. With the weaker acid, AcOH, no reaction was observed up to [AcOH]/[amine] similar to 4, and a curved plot was observed with increasing [amine], as a result of the acid-base equilibrium between AcOH and the amine. With 1,4-dimethylpiperazine, a quantitative evaluation of the C-H deactivation determined by sequential protonation of one or both nitrogen centers was obtained. These observations show that protonation provides an extremely efficient, precise and tunable method for the deactivation of the C-H bonds of basic substrates allowing moreover for careful control over the hydrogen abstraction selectivity. The implications of this approach are discussed.

Abstract  Surface binding between the carboxylic group and the TiO2 nanoparticle is important in the potentially applicable dye-sensitized mesoporous TiO2 crystalline solar cell. However, the nature of the surface binding is still not clear enough. We propose a new method to study the surface binding between the carboxylic group and the TiO2 surface atoms by using an interface-sensitive molecular probe. The principle underlying this method is based on the triplet-triplet absorption of the probe molecule all-trans-retinoic acid (ATRA) formed by the photoinduced charge recombination between the TiO2 nanoparticle and the probe molecule. By characterizing the individual triplet absorption spectra of ATRA in different forms, including the synthesized ester-like linkage model compound tri-butoxyl titanium (IV) all-trans-retinoate (Ti(IV)(OBu)(3)RCOO; RCOO = retinoate), the composition of different surface binding forms of carboxylic groups on TiO2 nanoparticle surfaces in hexanol has been determined. The result shows that the simple adsorption form (anion, hydrogen bonding) comprises about 3% of the total adsorbed form, the ester-like form accounts for 63%, and chelating together with bridging form accounts for 34%.

Abstract  Non-conventional techniques, such as power ultrasound (US) has been used to promote one-pot synthesis of second generation ionic liquids (ILs), reducing reaction times and improving yields. Because of the emerging importance of the ILs as green materials with wide ranging applications and our general interests in green processes such as sonication, 1-butoxyl-3-butyl imidazolium bromide (alkoxyl-functionalized) and their derivatives were synthesized using a facile and green US assisted procedure. Their structures were characterized by FT-IR, H-1-NMR, C-13-NMR and mass spectroscopy.

Abstract  The primary and secondary free radical intermediates in the gamma radiolysis of poly(D,L-lactide-co-glycolide) (PLGA) and clonazepam loaded PLGA microspheres were investigated by matrix EPR spectroscopy in the temperature range 77-298 K. Drug-polymer interactions were found to be important leading to significant deviations of the G(radicals) from the additivity law. In particular, in the mixed system a stabilization of the polymer matrix with respect to the radiation damage was detected, witnessed by a decrease of the overall polymer radicals yield which is accompanied by an increase of the drug radicals yield. These effects have been attributed to the scavenging properties of the nitro group with respect to electrons and polymer radicals. It is conceivable that such conclusions be of general application for all pharmaceutical formulations containing drugs bearing nitro groups in their chemical structure. (C) 2003 Elsevier Science Ltd. All rights reserved.

Abstract  5-[4-(2-Benzimidazolethiol)butoxyl]phenyl-10,15,20triphenylporphyrinatoiron(III) chloride, BzImPT(Ph) (PFeCl)-Cl-III, and its derivatives BzImPT(p-XC(6)H(4))-(PFeCl)-Cl-III(X = Me, OMe, Cl) were prepared and characterized by elemental analysis, H-1-n.m.r., i.r., u.v.-vis., mass spectra, e.s.r. and m.c.d. spectra. The hydroxylation of cyclohexane catalyzed by these complexes with molecular oxygen in the absence of added axial ligand was examined. The results indicated that these complexes had higher catalytic activity, due to the intermolecular coordination of the terminal groups to the iron centre which was indicated by their m.c.d. spectra. The effect of added reducing agent on the turnover numbers (TN) is discussed.

Abstract  Coordination properties between two new tailed porphyrin iron (III) complexes, 5-[o(-)(4-(1-imidazolyl)butoxyl)phenyl]-10, 15, 20-triphenylporphyrinatoiron (III) chloride {[Fe-III(ImTPP)]Cl} and 5-[p-(4-(3-pyridyloxyl)butoxyl]phenyl-10, 15, 20-triphenylporphyrinatoiron (III) chloride {[Fe-III(PyTPP)]Cl}, with nitrogenous ligands and the equilibrium constants for the addition of these ligands to [Fe-III P]Cl in chloroform were studied by spectral technique, Experimental results demonstrated that [Fe-III(PyTPP)]Cl, just similar to [Fe-III(TPP)]Cl, form 1:2 adducts with imidazole or pyridines, respectively, but [Fe-III(ImTPP)]Cl forms only 1:1 adducts with pyridines and form 1: 2 adduct with imidazole. The equilibrium constants for the addition of nitrogenous ligands to [Fe-III(ImTPP)]Cl are 10 similar to 10(3) times as big as those for [Fe-III(TPP)]Cl system. We believe that this result is from the fact that the axial coordination of these ligands to [ Fe-III(ImTPP)]Cl induces the coordination of terminal imidazole group with ferric iron in [Fe-III(ImTPP)]Cl and this kind of coordination mode strengthens the bonding between ferric ion in [Fe-III(ImTPP)]Cl and nitrogen atom in ligand. The novel coordination mode and its structure were identified by the experimental data from UV-vis, H-1 NMR and low-temperature EPR.

Abstract  Kinetic solvent effects on hydrogen abstractions involving C-H donors (dienes, ethers, alkylbenzenes) have been corroborated by experiment and theory (UB3LYP/6-311++G**, polarized continuum model). To single out the effect of solvent polarity, rate constants for scavenging of the cumyloxyl radical and fluorescence quenching of 2,3-diazabicyclo[2.2.2]oct-2-ene were obtained in binary aprotic mixtures of ethylacetate and acetonitrile. Polar solvents result in a selective stabilization of the reactants (see TOC graphic), which results in slower rate constants.

Abstract  Primary, secondary, and tertiary alkoxyl radicals add exo-selectively to the olefinic pi-bond in bicyclo[2.2.1]heptene to afford exo-2-alkoxybicyclo[2.2.1]kept-3-yl radicals, which are trapped with BrCCl3 preferentially from the endo face to furnish beta-bromohydrine ethers in 23-67% yield. (c) 2007 Elsevier Ltd. All rights reserved.

Abstract  An amphiphilic derivative of carboxymethylchitosan (CMCS), (2-hydroxyl-3-butoxyl)propyl-CMCS (HBP-CMCS), was used as an organic additive in the precipitation process of calcium carbonate (CaCO3). HBP-CMCS molecules can interact with calcium ions, the functional groups of which act as active sites for the nucleation and crystallization of CaCO3. Simultaneously, HBP-CMCS molecule also functionalizes as a colloidal stabilizer to prohibit the sedimentation of the grown CaCO3 crystals, depending upon the molar ratio of the initial Ca2+ ions to the repeat units of HBP-CMCS molecules. The combination investigations of scanning electron microscopy, X-ray diffraction, and Fourier transform infrared spectroscopy on the precipitated CaCO3 crystals proved that concentrations of HBP-CMCS and Ca2+ exert great influence on the crystallization habit of CaCO3, such as the nucleation, growth, morphology, crystal form, etc. The formation of the peanut-shaped CaCO3 particles suggests the template effect of HBP-CMCS molecules on the aggregation behavior of CaCO3 nanocrystals.

Abstract  Using competitive methods, the rates of hydrogen-atom abstraction by a typical perfluoro-n-alkyl radical, n-C7F25; have been determined for a series of silane and stannane reductants including (n)Bu(3)Sn-H (2.03 X 10(8) M(-1) s(-1)), Et(3)Si-H (7.5 X 10(5)), (TMS)(2)SiMe-H (1.63 X 10(7)) and (TMS)(3)Si-H (5.1 X 10(7)), all of which exhibited substantial rate enhancements relative to their analogous reductions of hydrocarbon radicals. The reduction by PhSH is, in contrast, similar to 400-times slower than for hydrocarbon radicals. Transition state polar effects are invoked to rationalize the relative reactivity of perfluoro- versus hydrocarbon radicals in these hydrogen-transfer reactions. A Hammett study for H-atom transfer from arene thiols (rho+ = -0.56) provided further substantiation for this conclusion. A discussion of the relative reactivity of t-butoxyl and perfluoro-n-alkyl radicals is presented.

Abstract  Using a highly stable and excellently performed three-component photorefractive (PR) polymer composite, poly(N-vinylcarbazole):1-n-butoxyl-2,5-dimethyl-4-(4'-nitrophenylazo)benzene:2,4,7-trinitro-9-fluorenone optical image storage was demonstrated. The dark storage time was 5-7h. The PR response time was measured to be about 200 ms at an intensity of 1 W/cm(2) with an applied electric field of 84 V/mu m. A proof-of-principle experiment on real-time correction of distorted images based on phase conjugattion of four-wave mixing geometry was also carried out.

Abstract  Rate constants have been measured at 25 degrees C in 13 solvents (S) for abstraction of the phenolic hydrogen atom from alpha-tocopherol (TOH) by tert-butoxyl (BO.), k(TOH/BO)(S), and by 2,2-diphenyl-1-picrylhydrazyl (DPPH.), k(TOH/DO)(S), and in eight solvents for abstraction of the phenolic hydrogen atom from phenol by cumyloxyl, k(PhOH/CumO)(S), and DPPH., k(PhOH/DPPH)(S). Over the range of solvents examined k(TOH/BO)(S) and k(TOH/DPPH)(S) vary by a factor of Ca. 65, and k(PhOH/CumO)(S) and k(PhOH/DPPH)(S) vary by a factor of ca. 120. in accordance with a prediction(5) the kinetic solvent effect is essentially identical for the same substrate and is independent of the attacking radical. That is, for almost any pair of solvents, A and B (k(TOH/BO)(A)/k(TOH/BO)(B))/(k(TOH/DPPH)(A)/k(TOH/DPPH)(B)) approximate to 1.0. The same applies with phenol as the substrate. Exceptions to this 1:1 relationship occur when one of the reactions becomes partly diffusion-controlled and in the solvent tert-butyl alcohol in which DPPH. shows a larger reactivity than would be expected. The absolute magnitudes of the alkoxyl and DPPH rate constants in the same solvent differ by a factor of over 1 000 000 (10(6)) for alpha-tocopherol and by 10 000 000 000 (10(10)) for phenol! We have therefore confirmed, under extreme conditions, a new, unifying principle for free radical chemistry in solution.

Abstract  By using Fourier-Transform Infrared (FTIR) absorption spectroscopy, rate coefficients in the range of 10(-16) to 10(-14) cm(3) molecule(-1) s(-1) have been determined for the hydrogen atom abstraction reactions of several substrates including halogenated organic compounds and amines by t-butoxy radical generated from the uv photolysis of t-butyl nitrite in the gas phase. Arrhenius parameters for selected reactions have been measured in the temperature range 299-318 K. Transition states and activation barriers for such reactions have been computed with the help of GAUSSIAN 03 software and found to match very well with the experimental values. (c) 2006 Elsevier B.V. All rights reserved.

Abstract  Di(tert-butyl) trioxide in a solution of CFCl3 (Freon-11) at -23 degreesC exists in equilibrium with the tert-butoxyl and tert-butylperoxyl radicals virtually without irreversible decomposition. The above radicals decompose ozone to dioxygen with a high effective rate constant, which is proprotional to the square root of the (BuOOOBut)-O-t concentration. The kinetic scheme describing the found relationships was proposed.