Effects of peroxide bonds and their degradation products on the dielectric relaxations of polystyrene

Polystyrene radically polymerized in atmosphere of air is composed of bisegment (C-A) or trisegment (C-A-C) block copolymers consisting of styrene segment (A) and styrene peroxide segment (C). Dielectric measurements of a system of copolymers of styrene and oxygen were obtained above the glass temperature. Three primary relaxations, a, b, and c, in order of descending temperature, were found corresponding to three microphases: styrene phase (phase a), styrene peroxide phase (phase c), and an intermediate phase (phase b) which contains a low concentration of peroxide bonds. An alternating copolymer of styrene and oxygen exhibits the relaxation c alone. With heat treatment above the glass temperature, relaxation c and subsequently relaxation b vanish with thermal degradation of peroxide bonds. The sum of relaxation strengths is linearly related to the content of peroxide bonds which was evaluated by the elementary analysis and DTA. Below the glass temperature, the temperature dependence of dielectric loss of carefully purified polystyrene without peroxide bonds shows very weak peaks which correspond to γ (200°K at 10 kHz) and δ (50°K at 10 kHz) peaks, respectively, in the activation plot. When low molecular degradation products of peroxide bonds are occluded or impurities such as benzaldehyde are added into the specimen, the height of the γ peak is appreciably enhanced, indicating that the reorientation of small polar molecules in polystyrene accompanies the vibration of the phenyl group about the C? C₆H₅ bond which gives rise to the γ relaxation.     

The energies of dissociation of the N—O bonds in pyridine 1-oxide derivatives

The enthalpies of combustion of some pyridine derivatives in the solid state have been measured by precision bomb calorimetry, and their enthalpies of formation have been calculated. The enthalpies of sublimation of these compounds have been determined from the experimental temperature dependences of saturated vapor pressure using the Clausius-Clapeyron equation. The enthalpies of combustion, formation, and sublimation are the following (kJ mol^–1): -3360.9±2.1, -0.5±2.1, and 79.1±1.3, respectively, for 4-methylpyridine 1-oxide; -2551.0±1.7, 11.7±1.7, and 89.1±2.5, respectively, for 4-nitropyridine 1-oxide;-2355.6±1.3, 102.1±1.3, and 106.3±2.9 for 2,4,6-trinitropyridine 1-oxide; and -2287.6±1.3, 34.3±1.3, and 101.7±2.9 for 2,4,6-trinitropyridine. The enthalpies of formation in the solid state and the enthalpies of sublimation of pyridine derivatives obtained together with the literature data allowed the energies of dissociation of the donor-acceptor N—O bonds in pyridine 1-oxides to be calculated.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 660–662, April, 1995.     

Thermochemical properties of tert-butyl and cumyl derivatives of peroxide compounds

Standard enthalpies of combustion, formation, and vaporization (sublimation) of five organic peroxides have been determined according to calorimetry. Influence of the nature of substituents on the energy of peroxide bond has been established.     

Evaluation of the bond energy terms for the various types of boron-nitrogen bonds

Summary We have determined a series of bond energy terms in compounds containing dative, single, double and/or triple boron-nitrogen bonds. We describe various interesting applications based on these bond energy terms namely the determination of enthalpies of atomization and stabilization energies. More particularly, the conventional ring strain energies of three- and four-membered small ring containing boron and nitrogen atoms could be determined and the aromaticity of borazine, reexamined.Dedicated to Professor Alberte Pullman     

Energetics of intra- and intermolecular bonds in ω-alkanediols

The enthalpies of combustion and vaporization at 298.15 K of the-alkanediols were determined with a CRMT rocking calorimeter equipped with a micro-bomb and a Tian-Calvet calorimeter equipped with a Knudsen effusion cell, respectively. The enthalpies of formation in the condensed and gaseous phases and the enthalpies of atomization are calculated. They depend linearly on the number of carbon atoms. Furthermore, the results permit derivation of the enthalpy of an intermolecular hydrogen bond in liquid-alkanediols and calculation of mean enthalpies of the C-C and C-OH bonds. All these thermochemical quantities are discussed in relation to the structural particulars of the molecules.     

The enthalpies of formation and reorganization of aromatic radicals

The enthalpies of formation of some biphenyl derivatives were determined. A "double difference" method for calculating the enthalpies of formation of aromatic radicals and the bond dissociation energies was proposed. The enthalpies of formation of the radicals biphenyl, diphenyl oxide, and phenyl oxide were determined. The energies of reorganization of these radicals as well as phenyl and 4-, 3-, and 2-pyridyls were calculated. The sums of the energies of the chemical bonds in the molecular moieties transformed into radicals upon the decomposition of chemical compounds were found to be constant for different compounds. The energies of the chemical bonds in arenes were determined.     

Metal fluorides form strong hydrogen bonds and halogen bonds: measuring interaction enthalpies and entropies in solution

The organometallic compound trans-(tetrafluoropyrid-2-yl)bis(triethylphosphine)-fluoronickel(II) (NiF) is shown to serve as a strong hydrogen bond and halogen bond acceptor in solution via intermolecular interactions with the fluoride ligand. The nature of the interactions has been confirmed by multinuclear NMR spectroscopy. Experimental binding constants, enthalpies, and entropies of interaction with hydrogen-bond-donor indole and halogen-bond-donor iodopentafluorobenzene have been determined by 19F NMR titration. In toluene-d8 solution indole forms a 1:1 and 2:1 complex with NiF (K1 = 57.9(3), K2 = 0.58(4)). Interaction enthalpies and entropies are -23.4(2) kJ mol-1 and -44.5(8) J mol-1 K-1, respectively, for the 1:1 complex; -14.8(8) kJ mol-1 and -53(3) J mol-1 K-1, respectively, for the 2:1 complex. In toluene-d8 solution iodopentafluorobenzene forms only a 1:1 complex (K1 = 3.41(9)) with enthalpy and entropy of interaction of -16(1) kJ mol-1 and -42(4) J mol-1 K-1, respectively. A marked solvent effect was observed for the halogen bond interaction. NMR titrations in heptane solution indicated formation of both 1:1 and 2:1 complexes of iodopentafluorobenzene with NiF (K1 = 21.8(2), K2 = 0.22(4)). Interaction enthalpies and entropies are -26(1) kJ mol-1 and -63(4) J mol-1 K-1, respectively, for the 1:1 complex; -21(1) kJ mol-1 and -83(5) J mol-1 K-1, respectively, for the 2:1 complex. There is a paucity of such experimental energetic data particularly for halogen bonds despite substantial structural data. These measurements demonstrate that halogen bonds are competitive with hydrogen bonds as intermolecular interactions and provide a suitable benchmark for theoretical calculations and quantitative input into design efforts in supramolecular chemistry and crystal engineering.     

Theoretical study on the oxidation mechanism of thiourea by hydrogen peroxide with water and hydroxyl assistance

This article presents a theoretical study on the oxidation reaction of thiourea by hydrogen peroxide in water or alkaline solutions using density functional and ab initio theories. This work also focuses on the analysis of the thermodynamic and kinetic properties of the predicted oxidation mechanism of thiourea using density functional and ab initio theories. The calculated results show that the activation energies, activation enthalpies, and activation Gibbs free energies of the reaction decreased and the releasable reaction energies, enthalpies and Gibbs free energies increased with the cooperation of water or hydroxyl anion. We conclude that the oxidation reaction of thiourea by hydrogen peroxide in water or alkaline solutions was easier and more completed than that in the gas state. The calculated results are consistent with the experiments.     

The standard enthalpies of formation and thermal stability of diacyldiperoxides

Bomb calorimetry was used to determine the standard enthalpies of combustion and formation of eight crystalline aliphatic diacyldiperoxides with high molecular weights and low decomposition temperatures. A comparison of the calculated peroxide bond energy with the enthalpies of sublimation of these substances shows that the latter cannot in principle be determined experimentally.     

Uptake of methacrolein into aqueous solutions of sulfuric acid and hydrogen peroxide

Multiphase acid-catalyzed oxidation by hydrogen peroxide has been suggested to be a potential route to secondary organic aerosol (SOA) formation from isoprene and its gas-phase oxidation products, but the kinetics and chemical mechanism remain largely uncertain. Here we report the first measurement of uptake of methacrolein into aqueous solutions of sulfuric acid and hydrogen peroxide in the temperature range of 253-293 K. The steady-state uptake coefficients were acquired and increased quickly with increasing sulfuric acid concentration and decreasing temperature. Propyne, acetone, and 2,3-dihydroxymethacrylic acid were suggested as the products. The chemical mechanism is proposed to be the oxidation of carbonyl group and C═C double bonds by peroxide hydrogen in acidic environment, which could explain the large content of polyhydroxyl compounds in atmospheric fine particles. These results indicate that multiphase acid-catalyzed oxidation of methacrolein by hydrogen peroxide can contribute to SOA mass in the atmosphere, especially in the upper troposphere.     

Estimates of the energy of intramolecular hydrogen bonds

A method for the estimation of the energy of intramolecular hydrogen bonds in conjugated systems existing in a variety of conformations is presented. The method is applied to determine the intramolecular hydrogen bond energy in 3-aminopropenal and 3-aminopropenthial. According to the proposed estimation scheme, the intramolecular H-bond energies are found to be of the order of 5-7 kcal/mol. These results are compared with those obtained by using other estimation schemes as well as with the recent results by other authors. Also, the H-bond energies in dimers and trimers of the two molecules are calculated and compared with the corresponding data for internally hydrogen-bonded monomers. This comparison shows that the bond equalization effect is primarily due to proton donor-proton acceptor proximity. In comparison with intermolecular hydrogen bonds, the rigidity of the chelate skeleton enhances this proximity effect. The same effect can be seen in systems with intermolecular hydrogen bonds, although its magnitude is diminished because of the absence of additional forces which pull the proton donor and proton acceptor groups toward each other. No specific resonance-assisted origin of the intramolecular hydrogen bond energy seems to be needed to elucidate the energetics of these bonds.     

The formation of hydrogen peroxide in metallic reductors

The formation of hydrogen peroxide in various types of metallic reductors both in the presence and absence of oxygen has been studied. Only when oxygen is rigorously excluded is peroxide undetectable. The oxidimetric determination of iron is seriously affected by this peroxide when the test solution and reductor are open to atmospheric oxygen. Systems which are completely oxygen-free give satisfactory results.     

Convenient peripheral aroyloxylation reactions of porphyrins and chlorophyll-a-based chlorins with benzoyl peroxide

A practical and efficient methodology for the formation of C–O bonds on the porphyrin/chlorin periphery was developed. The aroyloxy-substituted porphyrins and chlorins related to chlorophyll-a at the β- and meso-positions, respectively, were conveniently synthesized by the free radical substitution reaction with benzoyl peroxide and its homologs.     

The atmospheric chemistry of hydrogen peroxide: a review

This paper reviews the atmospheric chemistry of hydrogen peroxide, taking into account the formation processes of both gas-phase and aqueous H2O2, and the reactions involving hydrogen peroxide in the gas phase and in atmospheric hydrometeors. Gas-phase hydrogen peroxide mainly forms upon dismutation of the hydroperoxyl radical, a product of the reactions between atmospheric hydrocarbons, hydroxyl radicals, nitric oxide, and oxygen. Aqueous hydrogen peroxide originates from the dissolution of the gaseous one, the reduction of molecular oxygen, a series of reactions involving dissolved ozone, and the irradiation of anthraquinones, aromatic carbonyls, and semiconductor oxides. The reactions involving aqueous H2O2 are very important in the context of the chemistry of the atmosphere. They include oxidation of S(IV) to S(VI), photolysis, the Fenton reaction in the presence of Fe(II), and possibly the formation of peroxynitrous acid. Within this framework, the correlation of hydrogen peroxide with other atmospheric components and the time trends of hydrogen peroxide in the atmosphere are easily accounted for.     

Theoretical analysis of the alternative additions of radicals to multiple bonds

The alternative additions of the hydrogen atom and methyl, aminyl, and methoxyl radicals to the double bond of CH₂=Y (Y = CHR, CR₂, CHCH=CH₂, CHPh, NH, O) compounds are theoretically analyzed using the intersecting parabolas method and DFT. The enthalpies, activation energies, and geometric parameters of the transition state in the reactions R^· + CH₂=Y → RCH₂Y^· and R^· + CH₂=Y → RYC^·H₂ are calculated. The results obtained by the two methods are compared with experimental data. The competing alternative radical additions to the multiple bonds are governed by the enthalpies of the reactions.     

Adsorption of hydrogen peroxide on functionalized mesoporous silica surfaces

MCM-41 and MSU-H mesoporous silicas were successfully functionalized with hydrogen bonds forming organic moieties, which have been proven by elemental analysis. Both moieties, based on oxygen and nitrogen containing groups, were introduced with high efficiency—the amount of carbon in all cases exceeded 10 % and the elemental ratios suggest binding to the surface through two or three Si–O–Si bonds. Hydrogen peroxide adsorption was conducted in its aqueous solutions and the amount adsorbed was determined using the ferric thiocyanate method. Results are presented as a function of hydrogen peroxide concentration in aqueous solution from 5 to 30 %. Both functionalized silicas show increased adsorption capacity when compared with that of their unfunctionalized analogues. The surface modified with nitrogen-based organic moiety revealed better adsorption properties as well as higher resistance against oxidation. MSU-H silica, due to its larger pore diameter, provides more space to bind hydrogen peroxide molecules and thus was found to have higher adsorption capacity: it adsorbed up to four times more hydrogen peroxide than MCM-41.     

The shape of the potential energy curves for NHN(+) hydrogen bonds and the influence of non-linearity

The potential energy curves for proton motion in NHN(+) hydrogen bonds have been calculated to investigate whether different methods of evaluation give different results: for linear H bonds most curves calculated along the NH direction are, as expected, identical with those along NN; for intramolecular H bonds it is very important to take into account the non-linearity and the potential energy curve calculated along the NH direction can be very far from the curve correctly describing the proton transfer. Other factors which influence the proton-transfer process are steric hindrance and presence of anions which modify the proton motion. In the analysis of the proton transfer process it is very important to take changes in the structure of the rest of the molecule into account, which is connected with exchange of energy with the surroundings. Comparison of adiabatic and non-adiabatic curves shows that they are significantly different for very bent hydrogen bonds and for hydrogen bonds with steric constraints for which the proton transfer process must be accompanied with relaxation of the whole molecule. Comparison of the potential-energy curves for compounds with very short H bonds emphasizes that the term 'strong H bond' needs to be qualified. For intermolecular H bonds shortening of the bond is connected with linearization. But for intramolecular H bonds the NN distance cannot be used as the only measure of H bond strength.     

Effects of indole and imidazole derivatives on the radiation- and peroxide-induced transformations of ethanol

The interaction of indole, imidazole, and their derivatives with α-hydroxyethyl radicals has been studied by the radiation and peroxide initiation of free-radical processes. The enthalpies of H-atom addition to the multiple bonds of the test compounds, which characterize their oxidation properties, have been calculated within the framework of the density functional theory. The set of experimental and theoretically calculated data indicate that serotonin or β-carboline alkaloids (harmine, harman, and harmaline) inhibit the formation of 2,3-butanediol—the main radiolysis product of deaerated ethanol—mainly due to reduction and addition or the oxidation of α-hydroxyethyl radicals, respectively. Enhancement of oxidation properties in the above order of β-carboline alkaloids has been observed. Pyrrole, indole, melatonin, imidazole, 1-methylimidazole, and 2-mercapto-1-methylimidazole exhibit low reactivity toward α-hydroxyethyl radicals.     

Cooperative effect of hydrogen bonds in the complexes of aliphatic alcohols with proton acceptors in chloroform

Effect of solvation on the frequency of stretching vibrations of O-H groups of aliphatic alcohols in complexes with acetonitrile, dimethyl sulfoxide, diethyl ether, pyridine, tetrahydrofuran, and triethylamine in various media was studied by IR spectroscopy. We found that the strength of hydrogen bonds in the studied complexes increased considerably at the interaction with chloroform as a solvent. The contribution of cooperative effect was estimated. The enthalpies of cooperative effect in the complexes consisting of three different molecules are estimated for the first time.     

Thermochemical Study of Ethylene Glycol Mixtures with N,N-Disubstituted Amides of Carboxylic Acids

The enthalpies of mixing of N'N-disubstituted amides of formic, acetic acids with ethyleneglycol were considered in view of the data on the enthalpies of mixing of the same compounds with form-amide and water. The exothermicity of amide-ethylene glycol interactions results from the formation of heterocomponent H bonds stronger than the H bonds in ethylene glycol. Solvophilic solvation and solvophobic effects are much weaker in nonaqueous solutions with low concentrations of amides than in water.