R. F. Vasil’ev

Antioxidant Activity of Chalcones: The Chemiluminescence Determination of the Reactivity and the Quantum Chemical Calculation of the Energies and Structures of Reagents and Intermediates

Six antioxidants from the class of chalcones (ArOH), compounds from which flavonoids are obtained in nature, were studied. The antiradical activity of chalcones and a number of related compounds was determined by a chemiluminescence method using the scavenging of peroxide radicals ROO· + ArOH → ROOH + OAr· (with the rate constant k7) in a model reaction of diphenylmethane (RH) oxidation. The structures and energies of the reagents and intermediates were determined by semiempirical quantum chemical (PM3, PM6) calculations. 3,4-Dihydroxychalcone and caffeic acid, which have a catechol structure, that is, two neighboring OH groups in phenyl ring B, exhibited high antioxidant activity (k7 ≈ 107 l mol−1 s−1); this is consistent with the lowest bond strengths D(ArO-H) of 79.2 and 76.6 kcal/mol, respectively. The abstraction of a hydrogen atom by the ROO· radical is the main reaction path of these compounds; however, the low stoichiometric coefficients of inhibition (f = 0.3–0.7) suggest a contribution of secondary and/or side reactions of ArOH and OAr·. In the other chalcones, the ArO-H bond is stronger (D(ArO-H) = 83–88 kcal/mol) and the antioxidant activity is lower (k7 = 104–105 l mol−1 s−1).

Chemiluminescence quantum yield in permanganate reduction reactions in acid medium

Chemiluminescence quantum yields for the reactions of permanganate with oxalic, tartaric, and citric acids; hydrazine; KBr; and FeSO4 in aqueous solutions of sulfuric acid have been measured. The maximum quantum yield reaches 1.2 × 10−5 einstein/mol with the chemiexcitation yield being 2%. Hence, the relatively low chemiluminescence quantum yield is due to a low yield of light emission by chemiexcited particles, rather than the low chemiexcitation yield.

Dioxetane formation in the phenol-inhibited oxidation of hydrocarbons

The structures and energies of possible products of the reaction between the phenoxy radical OAr• and dioxygen are calculated using the PM6 semiempirical method. The possible adducts are the peroxyl radical OArOO• and dioxetane radicals. The latter are the cyclization products resulting from the addition of the terminal atom of OArOO• to a carbon atom of the Ar ring. This reaction is nearly thermoneutral and is, therefore, likely.

Evolution of bond orders in the reversible reactions of alkylene oxides with R-β-hydroxyalkyl sulfides and the role of the intermolecular hydrogen bond in initiation and at intermediate reaction stages

Semiempirical (PM3) calculations of the changes in the structures and energies of the reactants, intermediates, transition states, and final products have been carried out for the reversible reaction of β-hydroxyethyl methyl sulfide with propylene oxide. The evolution of the electron density distribution during the reaction is analyzed. It is demonstrated that the transformation proceeds via two intermediate products and the O⋯H⋯O bridge persists throughout the reaction pathway.

Chemiluminescence in reactions of manganese(III) reduction by lactic acid: Two-electron mechanism of chemiexcitation

Chemiluminescence is generated during reduction of manganese(III) ions with lactic acid to have a quantum yield as high as 0.1, a record-breaking chemi-excitation value for inorganic emitters. Kinetic features of the chemiluminescence and a nonradical mechanism of lactic acid oxidation lead to the conclusion that chemiexcitation results from two-electron reduction of manganese(IV) ions in the complex with lactic acid.

Phenomenological features of chemiluminescence in manganese(III) reduction reactions

Chemiluminescence is generated in the reduction reactions of Mn(III) in sulfuric acid solutions; however, it is absent during the reduction of Mn(III) with iron(II) sulfate or potassium bromide. The dependences of the light yield and the kinetic parameters of chemiluminescence upon the concentration of reagents have been determined for the reactions of Mn(III) reduction with malonic acid. The rate of Mn(III) consumption during the light pathway of the reaction (i.e., reaction that results in chemiexcitation) depends on the squared Mn(III) concentration. The light pathway loses in competition with the dark pathway. The chemiexcitation of Mn2+ follows either one- or two-electron mechanism, but the complex character of the reaction does not allow making selection between these two routes.

Role of chemiexcited particles in permanganate reduction by citric acid: Investigation with spectrophotometric and chemiluminescence methods

Potassium permanganate reduction by citric acid in the presence of sulfuric acid includes stages of chemigeneration of electronically excited Mn(II) and emission of photons by it. The electronic absorption, reactant concentration, and chemiluminescence kinetics have been investigated. It has been shown that the chemiexcited species Mn(II) (chemiluminescence emitter) acts as a permanganate reduction catalyst.

Extremely Efficient Antioxidants Evolved into Environment from Aging Polymeric Materials: Chemiluminescent Monitoring

The paper describes a novel phenomenon: the evolution of very efficient acceptors of peroxyl radicals from aging polymeric materials into the gaseous phase. A chemiluminescence method for detection and determination of these “technogenic” antioxidants and for evaluation of their kinetic parameters is outlined and discussed. The method is based on the competition between the self-reaction of peroxyl radicals giving rise to light and scavenging the peroxyls by the antioxidants resulting in quenching. The rate constant for the reaction of peroxyls with the technogenic antioxidants is very high, approximately 109 l/(mol·s), which is close to the diffusion-limited rate and may mean that these species are free radicals (probably with bulky substituents at a partly delocalized electron center) as the reactions of peroxyls with molecules are known to be much slower. Acceptors have been detected in the ambient air: the probable sources are polymeric fabrics, parts, waste, etc. in the environment. The concentration of technogenic antioxidants may reach 10−8 – 10−7 mol/l in the air. It decreases on breathing: the antioxidants penetrate from the air into tissues and are able to suppress the free-radical oxidation of biological membrane lipids, thereby correcting the excitability level of the central nervous system and modifying behavior of experimental animals.

Quenchers and inhibitors of chemiluminescence in the reduction reactions of Mn3

The effect of NaF and CH3CN on the chemiluminescent reactions of Mn(III) reduction in solutions of sulfuric acid is studied. It is established that NaF is an inhibitor of these reactions, and the quenching of chemiluminescence under the effect of NaF is nominal. Acetonitrile does not affect the kinetics of reactions under similar conditions and acts as a specific quencher of the chemiexcited emitter. The Stern-Volmer quenching constant is 52 ± 11 M−1.

Kinetics of ethylbenzene oxy-chemiluminescence in the presence of antioxidants from tissues of the marine invertebrate Eupentacta fraudatrix: Estimating the concentration and reactivity of the natural antioxidants

Extracts from tissues of the holothuria Eupentacta fraudatrix suppress the oxy-chemiluminescence of ethylbenzene in chlorobenzene initiated by azobisisobutyronitrile thermolysis and enhanced by energy transfer to 9,10-dibromoanthracene. The decrease of the chemiexcitation rate in the reaction between peroxyl radicals is caused by radical interception by the antioxidants contained in the extracts. The concentrations of the antioxidants in the extracts and the rate constants of their reactions with peroxyl radicals have been estimated from chemiluminescence kinetics.