Singlet Delta State Research Articles

Singlet oxygen generation by the reaction of acrolein with peroxynitrite via a 2-hydroxyvinyl radical intermediate

Acrolein (2-propenal) is an environmental pollutant, food contaminant, and endogenous toxic by-product formed in the thermal decomposition and peroxidation of lipids, proteins, and carbohydrates. Like other α,β-unsaturated aldehydes, acrolein undergoes Michael addition of nucleophiles such as basic amino acids residues of proteins and nucleobases, triggering aging associated disorders. Here, we show that acrolein is also a potential target of the potent biological oxidant, nitrosating and nitrating agent peroxynitrite. In vitro studies revealed the occurrence of 1,4-addition of peroxynitrite (k2 = 6 × 103 M−1 s−1, pH 7.2, 25 °C) to acrolein in air-equilibrated phosphate buffer. This is attested by acrolein concentration-dependent oxygen uptake, peroxynitrite consumption, and generation of formaldehyde and glyoxal as final products. These products are predicted to be originated from the Russell termination of •OOCH=CH(OH) radical which also includes molecular oxygen at the singlet delta state (O21Δg). Accordingly, EPR spin trapping studies with the 2,6-nitrosobenzene-4-sulfonate ion (DBNBS) revealed a 6-line spectrum attributable to the 2-hydroxyvinyl radical adduct. Singlet oxygen was identified by its characteristic monomolecular IR emission at 1,270 nm in deuterated buffer, which was expectedly quenched upon addition of water and sodium azide. These data represent the first report on singlet oxygen creation from a vinylperoxyl radical, previously reported for alkyl- and formylperoxyl radicals, and may contribute to better understand the adverse acrolein behavior in vivo.

Reaction of Aniline with Singlet Oxygen (O21Δg).

Dissolved organic matter (DOM) acts as an effective photochemical sensitizer that produces the singlet delta state of molecular oxygen (O21Δg), a powerful oxidizer that removes aniline from aqueous solutions. However, the exact mode of this reaction, the p- to o-iminobenzoquinone ratio, and the selectivity of one over the other remain largely speculative. This contribution resolves these uncertainties. We report, for the first time, a comprehensive mechanistic and kinetic account of the oxidation of aniline with the singlet delta oxygen using B3LYP and M06 functionals in both gas and aqueous phases. Reaction mechanisms have been mapped out at E, H, and G scales. The 1,4-cycloaddition of O21Δg to aniline forms a 1,4-peroxide intermediate (M1), which isomerizes via a closed-shell mechanism to generate a p-iminobenzoquinone molecule. On the other hand, the O21Δg ene-type reaction forms an o-iminobenzoquinone product when the hydroperoxyl bond breaks, splitting hydroxyl from the 1,2-hydroperoxide (M3) moiety. The gas-phase model predicts the formation of both p- and o-iminobenzoquinones. In the latter model, the M1 adduct displays the selectivity of up to 96%. A water-solvation model predicts that M1 decomposes further, forming only p-iminobenzoquinone with a rate constant of k = 1.85 × 109 (L/(mol s)) at T = 313 K. These results corroborate the recent experimental findings of product concentration profile in which p-iminobenzoquinonine represents the only detected product.

Enhancement of hydrogen sulfide oxidation via excitation of oxygen molecules to the singlet delta state

Comprehensive analysis of the effect of the presence of excited singlet oxygen molecules O2(a1Δg) on the enhancement of ignition and combustion in the H2S–air mixture is conducted on the basis of developed reaction mechanism. This mechanism incorporates the results of ab initio calculations of potential energy surfaces for the H2S+O2(X3Σg−) and H2S+O2(a1Δg) systems. The existing experimental data on the ignition delay and laminar flame speed in the H2S–air mixture are used for the validation of the constructed reaction mechanism. It has been demonstrated that the abundance of O2(a1Δg) molecules even in small amounts (only 1% in total oxygen) could substantially intensify the chain-branching in the H2S–air mixture. This allows one to reduce notably the ignition delay, especially at low initial temperatures (T0 = 600–800K), and to increase the flame speed. The major effect is observed for the fuel-lean and stoichiometric mixtures. So, at ϕ = 0.6, the presence of 5% O2(a1Δg) molecules in total oxygen increases the flame speed by 25%. Kinetic mechanisms, responsible for shortening the ignition delay and for the increase of flame speed, are discussed in a detailed manner.

P23 - Singlet molecular oxygen generated in dark biological process

Ultraweak chemiluminescence arising from biomolecules oxidation has been attributed to the radiative deactivation of singlet molecular oxygen [1O2] and electronically excited triplet carbonyl products involving dioxetane intermediates. As examples, we will discuss the generation of 1O2 from lipid hydroperoxides, which involves a cyclic mechanism from a linear tetraoxide intermediate. The generation of 1O2 in aqueous solution via energy transfer from the excited triplet acetone arising from the thermodecomposition of dioxetane a chemical source, and horseradish peroxidase-catalyzed oxidation of 2-methylpropanal, as an enzymatic source, will also be discussed. The approach used to unequivocally demonstrate the generation of 1O2 in these reactions is the use of 18O-labeled hydroperoxide / triplet dioxygen (18[3O2]), the detection of labeled compounds by HPLC coupled to tandem mass spectrometry (HPLC-MS/MS) and the direct spectroscopic detection and characterization of 1O2 light emission. Characteristic light emission at 1,270nm, corresponding to the singlet delta state monomolecular decay was observed. Using18[3O2], we observed the formation of 18O-labeled 1O2 (18[1O2]) by the chemical trapping of 18[1O2]with the anthracene-9,10-diyldiethane-2,1-diyl disulfate disodium salt (EAS) and detected the corresponding 18O-labeled EAS endoperoxide usingHPLC-MS/MS. The combined use of the thermolysis of a water-soluble naphthalene endoperoxide as a generator of 18O labeled 1O2 and the sensitivity of HPLC-MS/MS allowed the study of 1O2reactivity toward biomolecules. Photoemission properties and chemical trapping clearly demonstrate that the production of hydroperoxide and excited carbonyls generates 18[1O2], and points to the involvement of 1O2 in physiological and pathophysiological mechanism.Supported by FAPESP (2012/12663-1), CAPES, INCT Redoxoma (FAPESP/CNPq/CAPES; 573530/2008-4), NAP Redoxoma (PRPUSP; 2011.1.9352.1.8), CEPID Redoxoma (FAPESP; 2013/07937-8).

Theoretical Study of the Reaction of Ethane with Oxygen Molecules in the Ground Triplet and Singlet Delta States

Quantum chemical calculations are carried out to study the reaction of ethane with molecular oxygen in the ground triplet and singlet delta states. Transition states, intermediates, and possible products of the reaction on the triplet and singlet potential energy surfaces are identified on the basis of the coupled-cluster method. The basis set dependence of coupled-cluster energy values is estimated by the second-order perturbation theory. The values of energy barriers are also refined by using the compound CBS-Q and G3 techniques. It was found that the C(2)H(6) + O(2)(X(3)Σ(g)(-)) reaction leads to the formation of C(2)H(5) and HO(2) products, whereas the C(2)H(6) + O(2)(a(1)Δ(g)) process produces C(2)H(4) and H(2)O(2) molecules. The appropriate rate constants of these reaction paths are estimated on the basis of variational and nonvariational transition-state theories assuming tunneling and possible nonadiabatic transitions in the temperature range 500-4000 K. The calculations showed that the rate constant of the C(2)H(6) + O(2)(a(1)Δ(g)) reaction path is much greater than that of the C(2)H(6) + O(2)(X(3)Σ(g)(-)) one. At the same time, the singlet and triplet potential surface intersection is detected that leads to the appearance of the nonadiabatic quenching channel O(2)(a(1)Δ(g)) + C(2)H(6) → O(2)(X (3)Σ(g)(-)) + C(2)H(6). The rate constant of this process is estimated with the use of the Landau-Zener model. It is demonstrated that, in the case of the existence of thermal equilibrium in the distribution of molecules over the electronic states, at low temperatures (T < 1200 K) the main products of the reaction of C(2)H(6) with O(2) are C(2)H(4) and H(2)O(2), rather than C(2)H(5) and HO(2). At higher temperature (T > 1200 K) the situation is inverted.

Comprehensive analysis of combustion enhancement mechanisms in a supersonic flow of CH4–O2 mixture with electric-discharge-activated oxygen molecules

Mechanisms of ignition and combustion enhancement in a supersonic CH4–O2 reactive flow behind an inclined shock wave front are analyzed numerically when chemically active species are produced by electric discharges with different values of reduced electric field. The analysis is carried out based on thermally nonequilibrium kinetic models both for the discharge and for the shock-induced combustion, taking into account the disruption of thermodynamic equilibrium between vibrational and translational degrees of freedom of the reacting molecules and treating the vibrational-electronic-chemistry coupling. It is demonstrated that the presence of electronically excited O2 molecules O2(a 1Δg) and as well as O atoms in oxygen plasma allows one to intensify the chain mechanism of CH4–O2 combustion even if the energy put to O2 molecules in the discharge is sufficiently low. The excitation of O2 molecules to the singlet states O2(a 1Δg) and is more preferable in shortening the induction zone length than the production of O atoms in the course of dissociation of O2 molecules due to electron impact. However, both excitation of O2 molecules to the singlet delta and singlet sigma states and production of O atoms in an electric discharge are much more effective in combustion enhancement than merely heating the mixture.

Kinetic mechanism of CO–H 2 system oxidation promoted by excited singlet oxygen molecules

The features of oxidation kinetics in CO–H 2O and CO–H 2 mixtures comprising singlet oxygen O 2( a 1Δ g ) and O 2( b 1 Σ g + ) molecules are considered. The proposed kinetic model is based on the syngas oxidation mechanism previously developed by authors, which was thoroughly tested against a vast set of experimental data, and incorporates recent ab initio studies of elementary processes with singlet oxygen molecules. An additional comparison with measurements of ignition delay in pure CO–O 2 mixture behind reflected shock wave was performed. It was shown that, in thermally non-equilibrium case, when treating the experiments on the ignition in the CO–O 2 mixture in the shock tube one must consider the process of delayed excitation of electronic degrees of freedom of oxygen molecules in the post-shock region. In thermally equilibrium case, for example, investigating the ignition in a flow reactor, it is necessary to take into account the contribution of CO + O 2( a 1Δ g ) channel to the oxidation rate. Modelling studies showed that the abundance of singlet delta oxygen in the CO–O 2, CO–H 2O–O 2 and CO–H 2–O 2 mixtures results in the acceleration of oxidation process and, as a consequence, allows to shorten significantly the induction time and decrease the ignition temperature even at a small content of O 2( a 1Δ g ) molecules in normal oxygen. It was demonstrated that the excitation of O 2 molecules to the singlet delta state is many time more effective as regards to accelerating the oxidation process and decreasing the ignition temperature than mere heating the mixture.

Theoretical Study of the Reaction of Carbon Monoxide with Oxygen Molecules in the Ground Triplet and Singlet Delta States

Quantum chemical calculations were carried out to study the reaction of carbon monoxide with molecular oxygen in the ground triplet and singlet delta states. Transition states and intermediates that connect the reactants with products of the reaction on the triplet and singlet potential energy surfaces were identified on the base of coupled-cluster method. The values of energy barriers were refined by using compound techniques such as CBS-Q, CBS-QB3, and G3. The calculations showed that there exists an intersection of triplet and singlet potential energy surfaces. This fact leads to the appearance of two channels for the triplet CO+O(2)(X(3)Σ(g)(-)) reaction, which produces atomic oxygen in the ground O((3)P) and excited O((1)D) states. The appropriate rate constants of all reaction paths were estimated on the base of nonvariational transition-state theory. It was found that the singlet reaction rate constant is much greater than the triplet one and that the reaction channel CO+O(2)(a(1)Δ(g)) should be taken into consideration to interpret the experimental data on the oxidation of CO by molecular oxygen.

Pulsed diode laser-based monitor for singlet molecular oxygen

Photodynamic therapy (PDT) is a promising cancer treatment. PDT uses the affinity of photosensitizers to be selectively retained in malignant tumors. When tumors, pretreated with the photosensitizer, are irradiated with visible light, a photochemical reaction occurs and tumor cells are destroyed. Oxygen molecules in the metastable singlet delta state O2(1Delta) are believed to be the species that destroys cancerous cells during PDT. Monitoring singlet oxygen produced by PDT may lead to more precise and effective PDT treatments. Our approach uses a pulsed diode laser-based monitor with optical fibers and a fast data acquisition system to monitor singlet oxygen during PDT. We present results of in vitro singlet oxygen detection in solutions and in a rat prostate cancer cell line as well as PDT mechanism modeling.

Observation of O2 1.27 μm dayglow by SPICAM IR: Seasonal distribution for the first Martian year of Mars Express

The O2(a1Δg) molecule is a result of the photodissociation of ozone in the Martian atmosphere and may be used as a tracer for atmospheric ozone mostly above ∼20 km since the singlet delta state is quenched by carbon dioxide at lower altitudes. The SPICAM IR acousto‐optic tunable filter (AOTF) spectrometer is a part of the SPICAM experiment onboard Mars Express. It is able to measure the O2 singlet delta emission band at 1.27 μm with a resolving power of ∼2200. We present the first seasonal map of the O2 emission covering the entire Martian year. Maximal values of O2 emission are observed during late winter to early spring at high latitudes in both hemispheres. We report the highest dayglow intensity of 30 MR in the southern hemisphere (70°–80°S latitudes) at Ls = 185°–195° and of 26 MR in the north polar regions (latitudes 70°–80°N) at Ls = 10°–20°. The lowest emissions are measured during the southern hemisphere summer near perihelion (Ls = 270°–330°), with an upper limit of 1–2 MR. At low latitudes (30°S–30°N) the seasonal evolution of the O2 emission shows a distinct maximum (5–7 MR) near aphelion. This is consistent with the maximum in O3 occurring at this time of the year, which has been reported from Earth, from satellite, or by modeling studies. The comparison of our results with previous measurements carried out from Earth shows quantitative differences that can be attributed to differences in local time of the observation or interannual variability of the Martian ozone layer.

Activation of molecular oxygen by infrared laser radiation in pigment-free aerobic systems.

With the goal of mimicking the mechanisms of the biological effects of low energy laser irradiation, we have shown that infrared low intensity laser radiation causes oxygenation of the chemical traps of singlet oxygen dissolved in organic media and water saturated by air at normal atmospheric pressure. The photooxygenation rate was directly proportional to the oxygen concentration and strongly inhibited by the singlet oxygen quenchers. The maximum of the photooxygenation action spectrum coincided with the maximum of the oxygen absorption band at 1270 nm. The data provide unambiguous evidence that photooxygenation is determined by the reactive singlet (1)Delta(g )state formed as a result of direct laser excitation of molecular oxygen. Hence, activation of oxygen caused by its direct photoexcitation may occur in natural systems.

Reaction Chemistry in the Afterglow of an Oxygen−Helium, Atmospheric-Pressure Plasma

The reaction chemistry in the afterglow of a non-equilibrium, capacitive discharge, operated at 600 Torr total pressure with (0.5 to 5.0) × 1017 cm-3 of oxygen in helium, has been examined by ultraviolet absorption spectroscopy, optical emission spectroscopy, and numerical modeling. The densities of the active species, O(3P), O2(1Δg), O2(1Σg+), and O3, have been determined as a function of the operating conditions. At RF power densities between 6.1 and 30.5 W/cm3 and a neutral temperature of 100 ± 40 °C, the plasma generated (0.2 to 1.0) × 1016 cm-3 of O(3P) and O2(1Δg), (0.2 to 2.0) × 1015 cm-3 of O2(1Σg+), and (0.1 to 4.0) × 1015 cm-3 of O3. After the power was turned off, the singlet-sigma and singlet-delta states decayed within 0.1 and 30.0 ms, respectively. The concentration of oxygen atoms remained constant for about 0.5 ms, then fell rapidly due to recombination with O2 to form O3. It was found that the etching rate of polyimide correlated with the concentration of oxygen atoms in the afterglow, indicating that the O atoms were the active species involved in this process.

Singlet Sigma: The “Other” Singlet Oxygen in Solution

AbstractThe lowest excited electronic state of molecular oxygen, O2(a1‐DLg), is often called simply singlet oxygen. This singlet delta state is an acknowledged and well‐studied intermediate in many solution‐phase photosystems. However, the second excited electronic state of oxygen, O2(b1δg+), is also a singlet. It has recently become possible to monitor this singlet sigma state in solution, which, in combination with studies of the singlet delta state, contributes to a better understanding of a variety of general problems in chemistry.

Configuration interaction studies of the HeH+ molecular ion. III. Singlet pi and delta states

The method of superposition of configurations is applied to the singlet pi and delta states of HeH+ which correlate to the separated atom states of principal quantum number less than or equal to 3. The calculations are carried out for internuclear separations R in the range 0(0.1)34.5, 35.0(0.5)50.0 a.u. The delta state calculations are also extended to R=65. The work is a continuation of a study of HeH+ for which the singlet sigma states have already been reported. In addition to the energy eigenvalues, matrix elements for radial and angular coupling and the dipole and gradient matrix elements are calculated for all combinations of the singlet sigma, pi, and delta states. Plots and tables of the calculated eigenvalues and matrix elements are presented. Where possible the calculated data are compared with the results of previous work. The results of this work are useful for theoretical collision studies of H++He and H+He+.

Polydiene Oxidations with Singlet Oxygen

Abstract Excited molecular oxygen in its singlet delta (1Δg) state can be made chemically in homogeneous solution and in the gas-phase by the electrodeless discharge of ground state oxygen. Both techniques have been used to perform oxidations of polydiene systems. Solutions of high cis, trans, and vinyl polybutadienes have been treated with singlet oxygen produced in situ. Only the high cis and high trans were oxidized, apparently by different mechanisms. Squalene, a model for polyisoprene, has been oxidized in solution and the initially formed hydroperoxides reduced and analyzed and found identical to the product from a photosensitized oxidation. Cis-polybutadiene films were treated with gas-phase singlet oxygen and the extent of surface oxidation was monitored spectroscopically and chemically.