A quantum chemical study of photochemical processes in the reaction Se + O<Subscript>2</Subscript> → SeO<Subscript>2</Subscript> with allowance for the spin orbit interaction

DFT, SA-MCSCF, and MRMP/MCQDPT2 methods in the 6-311++G(2d) basis set are employed to consider the features of the formation reaction of key intermediates (SeOO, Se(O₂)) and photochemical dissociation of selenium dioxide with the formation of singlet oxygen. The cross-sections of potential energy surfaces of SeO₂, Se(O₂), and SeOO are constructed and the terms of their ground and excited states are analyzed at the SeOO dissociation limit with regard to spin-orbital interaction. Possible formation channels of ¹O₂ (¹Δ _g , ¹Σ _g ⁺) reactive oxygen species during the decay of the excited states of selenium oxocomplexes are revealed. The effect of the spin-orbit interaction on the character of electronic spectrum transitions and zero field splitting in oxygen is estimated.     

Singlet oxygen in the environmental sciences

This review of the part played by the singlet states of molecular oxygen in the environment deals with atmospheric aspects. There are five bound excited states of molecular oxygen that correlate with two ground state, ³P, oxygen atoms. Of these, three are singlets, although the other two states (triplets) are closely associated with singlet oxygen processes, especially in the mesosphere. A weakly bound quintet state has been invoked, as well, in explaining some aspects of the physical chemistry of the singlet species. Of the three singlet states, the a¹Δ_g is the most familiar. It has a low excitation energy, a long radiative lifetime, and is rather resistant to collisional deactivation in the gas phase. As a consequence, its chemistry has been susceptible to detailed study in the laboratory. These investigations, coupled with estimates of production rates, suggest that O₂(a¹Δ_g) is probably not important in initiating much chemical change in the lower atmosphere, at least in the gas phase; excited molecules dissolved in water droplets may promote chemical change under special circumstances. In the stratosphere and mesosphere, each of the bound excited states gives rise to characteristic emission features of the airglow, both by day and by night. The observational data, obtained from the ground, and from balloons, high-flying aircraft, rockets and satellites is surveyed as a background to examining the chemical and photochemical mechanisms by which the different states become excited. These mechanisms clearly differ by day and by night, and they also depend on the altitude from which the emission comes. The most intense feature of the oxygen dayglow, the Infrared Atmospheric Band, comes from O₂(a¹Δ_g) that is produced in the photolysis of ozone. Because dayglow measurements are sometimes used to derive ozone concentrations and altitude profiles in the atmosphere, the efficiency of production of the species in the photolysis of ozone is examined critically, and some unexpected laboratory findings are reported. The b¹Σ⁺ _g state of oxygen is excited during the day largely by resonance scattering, although some is also populated by energy transfer from O(¹D) to O₂. At night, recombination of O(³P) atoms is the most likely source of excitation of all the states of oxygen. Laboratory experiments that bear on these processes are reviewed, and theoretical estimates of the partitioning of recombination events between the different states are presented. Direct recombination into the a¹Δ_g and b¹Σ⁺ _g states is unlikely to be efficient enough to produce the observed concentrations of these species, and some indirect process is thus implicated. Laser excitation experiments show that quenching of the three higher excited (ungerade) states of oxygen by O₂ and, especially, N₂, can generate O₂(b¹Σ⁺ _g) with high efficiency; similar experiments demonstrate explicitly that the quenching of O₂(b¹Σ⁺ _g) by the atmospheric gases yields O₂(a¹Δ_g). A consistent excitation scheme for the nightglow emissions is presented; this scheme also pays attention to the “auroral green” line produced by the ¹S state of atomic oxygen, the intensities of which in the atmospheres of Earth and Venus provide some clues about the excitation of the molecular states. Finally, the laboratory studies are shown to indicate that the formation of excited molecular oxygen from vibrationally rich hydroxyl (OH) radicals is unlikely to be of major importance in the atmosphere.     

Photoactivation mechanism of orthovanadate-like (V=O)O3 species

The photoluminescence spectrum and action spectrum for the photooxidation of orthovanadate-like (V=O)O₃ species exhibiting photoluminescence at 520 nm indicate that the triplet excited state T₁ of the orthovanadate-like species, which is formed from the singlet excited states S₁ and S₂ by intersystem crossing, is directly involved in the photooxidation of cyclohexane into cyclohexanone in the presence of molecular oxygen.     

Photophysical Behavior of Angelicins and Thioangelicins: Semiempirical Calculations and Experimental Study

The decay processes of the lowest excited singlet and triplet states of five methylated angelicins (4,6,4′-trimethyl-angelicin, MA, and four methylated thioangelicins, MTA; see Scheme 1) were investigated in live solvents by stationary and pulsed fluorometric and flash photolytic techniques. In particular, the solvent effects on absorption, fluorescence, quantum yields of fluorescence (φ_F) and triplet formation (φ_T), lifetimes of fluorescence (τ_F) and the triplet state (τ_T) and the quantum yields of singlet oxygen production (φ_Δ) were investigated. Semiempirical (ZINDO/S-CI) calculations were carried out to obtain information (transition probabilities and nature) on the lowest excited singlet and triplet states. The quantum mechanical calculations and the solvent effect on the photophysical properties showed that the lowest excited singlet state (S¹) is a partially allowed π,π* state, while the close-lying S₂ state is n,π* in nature. The efficiencies of fluorescence, S₁→T₁ intersystem crossing (ISC) and S₁→ S₀ internal conversion (IC) strongly depend on the energy gap between S₁, and S₂ and are explained in terms of the so-called proximity effect. In fact, for MA in cyclohexane, only the S₁→ S₀ internal conversion is operative, while in acetonitrile and ethanol, where the n.π* state is shifted to higher energy, the efficiencies of fluorescence and ISC increase significantly. The energy gap between S₁ and S₂ increases in MTA, where the furanic oxygen is replaced by a sulfur atom. Consequently, the solvent effect on the photophysical parameters of MTA is less marked than for MA; e.g. fluorescence and triplet-triplet absorption are also detectable in the nonpolar cyclohexane. The lowest excited singlet state of molecular oxygen O₂(¹D_g) was produced efficiently in polar solvents by energy transfer from the T₁ state of MA and MTA.     

Origin of the size dependence of Au nanoparticles toward molecular oxygen dissociation

Density functional theory calculations using a plane-wave basis set and a generalized gradient approach exchange-correlation potential have been carried out to study the dissociation of molecular oxygen by Au nanoparticles and its dependence with particle size. The analysis of the energy related data shows that the reactivity is dominated by the energy barrier height from adsorbed O₂ to the dissociated state and by the stability of molecular oxygen on the nanoparticle. The energy barrier is found to be only slightly dependent on the particles size where large variations are found for the adsorption energy of the O₂ molecule on the different nanoparticles. A careful analysis of the electronic structure shows that the driving force for O₂ adsorption by these nanoparticles is the existence of a clear gap between occupied and unoccupied states for the naked particle. This allows accommodating bonding states with O₂ below the Fermi level resulting in a strong interaction. On the contrary, the Au nanoparticles with a more metallic electronic structure have necessarily to accommodate bonding and antibonding states below the Fermi level with a concomitant weaker interaction with O₂.     

Photosensitized Generation of Singlet Oxygen from (Substituted Bipyridine)ruthenium(II) Complexes

Photophysical properties in dilute MeCN solution are reported for seven Ru^II complexes containing two 2,2′‐bipyridine (bpy) ligands and different third ligands, six of which contain a variety of 4,4′‐carboxamide‐disubstituted 2,2′‐bipyridines, for one complex containing no 2,2′‐bipyridine, but 2 of these different ligands, for three multinuclear Ru^II complexes containing 2 or 4 [Ru(bpy)₂] moieties and also coordinated via 4,4′‐carboxamide‐disubstituted 2,2′‐bipyridine ligands, and for the complex [(Ru(bpy)₂(L)]²⁺ where L is N,N′‐([2,2′‐bipyridine]‐4,4′‐diyl)bis[3‐methoxypropanamide]. Absorption maxima are red‐shifted with respect to [Ru(bpy)₃]²⁺, as are phosphorescence maxima which vary from 622 to 656 nm. The lifetimes of the lowest excited triplet metal‐to‐ligand charge transfer states ³MLCT in de‐aerated MeCN are equal to or longer than for [Ru(bpy)₃]²⁺ and vary considerably, i.e., from 0.86 to 1.71 μs. Rate constants k_q for quenching by O₂ of the ³MLCT states were measured and found to be well below diffusion‐controlled, ranging from 1.2 to 2.0⋅10⁹ dm³ mol⁻¹ s⁻¹. The efficiencies f of singlet‐oxygen formation during oxygen quenching of these ³MLCT states are relatively high, namely 0.53 – 0.89. The product of k_q and f gives the net rate constant k for quenching due to energy transfer to produce singlet oxygen, and k_q−k equals k, the net rate constant for quenching due to energy dissipation of the excited ³MLCT states without energy transfer. The quenching rate constants were both found to correlate with ΔG^CT, the free‐energy change for charge transfer from the excited Ru complex to oxygen, and the relative and absolute values of these rate constants are discussed.     

A CI pseudopotential-based description of the low-lying states of Ag O2

Extensive SCF -LCAO -MO variational and perturbative configuration interaction (CI ) calculations framed within an effective core potential approximation have been performed to determine the two experimentally observed geometrical isomers of A_g O₂ and the interconversion route between them. These structural forms, associated to the ground-state local minima, yield virtually the same energy, and their spontaneous interconversion is strongly indicated, which agrees fairly well with the experimental measurements. The reaction A_g + O₂ → A_g O₂ was theoretically analyzed along a CI fully optimized energy pathway for the ground and various excited states, within C_2v and C_s symmetry. Although a tight-ion pair (A O) character is predicted for the ground state at the equilibrium geometries, its dissociation leads to neutral rather than to ionic fragments. The study of the reaction path within C_s symmetry shows an avoided crossing between the ground state and another ²A″ potential curve where the former correlates adiabatically with the reactants A_g(²S) + O₂(¹Δ_g). This indicates that the formation of the complex proceeds via a reactive state of molecular oxygen. The higher ²A″ electronic curves correlate with the metal ²P excited state, and the oxygen binding is found to be less favorable. The present results are shown to have an important bearing on the experimentally known catalytic properties of oxygen adsorbed on silver surfaces.     

Oxygen Quenching of nπ* Triplet Phenyl Ketones: Local Excitation and Local Deactivation

We have studied the charge‐transfer‐induced deactivation of nπ* excited triplet states of benzophenone derivatives by O₂(³Σ), and the charge‐transfer‐induced deactivation of O₂(¹Δ_g) by ground‐state benzophenone derivatives in CH₂Cl₂ and CCl₄. The rate constants for both processes are described by Marcus electron‐transfer theory, and are compared with the respective data for a series of biphenyl and naphthalene derivatives, the triplet states of which have ππ* configuration. The results demonstrate that deactivation of the locally excited nπ* triplets occurs by local charge‐transfer and non‐charge‐transfer interactions of the oxygen molecule with the ketone carbonyl group. Relatively large intramolecular reorganization energies show that this quenching process involves large geometry changes in the benzophenone molecule, which are related to favorable Franck‐Condon factors for the deactivation of ketone‐oxygen complexes to the ground‐state molecules. This leads to large rate constants in the triplet channel, which are responsible for the low efficiencies of O₂(¹Δ_g) formation observed with nπ* excited ketones. Compared with the deactivation of ππ* triplets, the non‐charge‐transfer process is largely enhanced, and charge‐transfer interactions are less important. The deactivation of singlet oxygen by ground‐state benzophenone derivatives proceeds via interactions of O₂(¹Δ_g) with the Ph rings.     

Molecular orbital theory of the hydrogen bond. 25. Water–uracil complexes in excited n → π states

Hydrogen bonding of uracil with water in excited n → π* states has been investigated by means of ab initio SCF -CI calculations on uracil and water–uracil complexes. Two low-energy excited states arise from n → π* transitions in uracil. The first is due to excitation of the C₄? O group, while the second is associated with excitation of the C₂? O group. In the first n → π* state, hydrogen bonds at O₄ are broken, so that the open water–uracil dimer at O₄ dissociates. The “wobble” dimer, in which a water molecule is essentially free to move between its position in an open structure at N₃? H and a cyclic structure at N₃? H and O₄ in the ground state, collapses to a different “wobble” dimer at N₃? H and O₂ in the excited state. The third dimer, a “wobble” dimer at N₁? H and O₂, remains intact, but is destabilized relative to the ground state. Although hydrogen bonds at O₂ are broken in the second n → π* state, the three water–uracil dimers remain bound. The “wobble” dimer at N₁? H and O₂ changes to an excited open dimer at N₁? H. The “wobble” dimer at N₃? H and O₄ remains intact, and the open dimer at O₄ is further stabilized upon excitation. Dimer blue shifts of n → π* bands are nearly additive in 2:1 and 3:1 water:uracil structures. The fates of the three 2:1 water:uracil trimers and the 3:1 water:uracil tetramer in the first and second n → π* states are determined by the fates of the corresponding excited dimers in these states.     

Multiple-State Emissions from Neat,Single-Component Molecular Solids: Suppression of Kasha's Rule

Three rigid and structurally simple heterocyclic stilbene derivatives, (E)-3H,3′H-[1,1′-biisobenzofuranylidene]-3,3′-dione, (E)-3-(3-oxobenzo[c] thiophen-1(3H)-ylidene)isobenzofuran-1(3H)-one, and (E)-3H,3′H-[1,1′-bibenzo[c] thiophenylidene]-3,3′-dione, are found to fluoresce in their neat solid phases, from upper (S₂) and lowest (S₁) singlet excited states, even at room temperature in air. Photophysical studies, single-crystal structures, and theoretical calculations indicate that large energy gaps between S₂ and S₁ states (T₂ and T₁ states) as well as an abundance of intra and intermolecular hydrogen bonds suppress internal conversions of the upper excited states in the solids and make possible the fluorescence from S₂ excited states (phosphorescence from T₂ excited states). These results, including unprecedented fluorescence quantum yields (2.3–9.6 %) from the S₂ states in the neat solids, establish a unique molecular skeleton for achieving multi-colored emissions from upper excited states by “suppressing” Kasha's rule.     

First-principles study of O2 adsorption and dissociation on the CuCr2O4 (100) surface

The adsorption and dissociation of molecular oxygen on spinel CuCr₂O₄ (100) surface were carried out by first-principles calculations based on density functional theory (DFT). The calculated results indicate that the Cr site is most favorable for atomic oxygen adsorption, with an adsorption energy of 402.8 kJ/mol. For molecular oxygen adsorption, there are three types of favorable interaction modes: O₂ forms bonds with the Cu site or O₂ binds to two Cr sites or O₂ interacts with both Cu and Cr sites simultaneously. The lowest activation energy (E_a = 35.4 kJ/mol) was found through exploring possible reaction pathways for O₂ dissociation. The relationship between E_a and reaction enthalpy (ΔH) for O₂ dissociation adsorption reactions fits Brønsted-Evans-Polanyi (BEP) behavior.     

Multiple‐State Emissions from Neat,Single‐Component Molecular Solids: Suppression of Kasha's Rule

Three rigid and structurally simple heterocyclic stilbene derivatives, (E)‐3H,3′H‐[1,1′‐biisobenzofuranylidene]‐3,3′‐dione, (E)‐3‐(3‐oxobenzo[c] thiophen‐1(3H)‐ylidene)isobenzofuran‐1(3H)‐one, and (E)‐3H,3′H‐[1,1′‐bibenzo[c] thiophenylidene]‐3,3′‐dione, are found to fluoresce in their neat solid phases, from upper (S₂) and lowest (S₁) singlet excited states, even at room temperature in air. Photophysical studies, single‐crystal structures, and theoretical calculations indicate that large energy gaps between S₂ and S₁ states (T₂ and T₁ states) as well as an abundance of intra and intermolecular hydrogen bonds suppress internal conversions of the upper excited states in the solids and make possible the fluorescence from S₂ excited states (phosphorescence from T₂ excited states). These results, including unprecedented fluorescence quantum yields (2.3–9.6 %) from the S₂ states in the neat solids, establish a unique molecular skeleton for achieving multi‐colored emissions from upper excited states by “suppressing” Kasha's rule.     

Spin-orbit coupling effects in the gas-phase reaction Se + O2

The potential energy surface sections of the ground and excited states of the SeOO (C _s ) superoxo complex have been constructed on the basis of spin-orbit coupling calculations. The activation barriers to the forward and reverse reactions??superoxide SeOO ? peroxide Se(O₂) ? dioxide SeO₂??have been calculated. Changes in the spectral and spin properties of oxygen (zero-field splitting) and possible channels of formation of active forms of ¹O₂ oxygen (¹?? _g , ¹?? _g ⁺ ) upon dissociation of excited states of selenium oxo complexes have been scrutinized.     

Supramolecular Control of Singlet Oxygen Generation

Singlet oxygen (¹O₂) is the excited state electronic isomer and a reactive form of molecular oxygen, which is most efficiently produced through the photosensitized excitation of ambient triplet oxygen. Photochemical singlet oxygen generation (SOG) has received tremendous attention historically, both for its practical application as well as for the fundamental aspects of its reactivity. Applications of singlet oxygen in medicine, wastewater treatment, microbial disinfection, and synthetic chemistry are the direct results of active past research into this reaction. Such advancements were achieved through design factors focused predominantly on the photosensitizer (PS), whose photoactivity is relegated to self-regulated structure and energetics in ground and excited states. However, the relatively new supramolecular approach of dictating molecular structure through non-bonding interactions has allowed photochemists to render otherwise inactive or less effective PSs as efficient ¹O₂ generators. This concise and first of its kind review aims to compile progress in SOG research achieved through supramolecular photochemistry in an effort to serve as a reference for future research in this direction. The aim of this review is to highlight the value in the supramolecular photochemistry approach to tapping the unexploited technological potential within this historic reaction.     

Effect of the Media on the Quantum Yield of Singlet Oxygen (O2(1Δg)) Production by 9H‐Fluoren‐9‐one: Solvents and Solvent Mixtures

We have investigated the effect of a series of 18 solvents and mixtures of solvents on the production of singlet molecular oxygen (O₂(¹Δ_g), denoted as ¹O₂) by 9H‐fluoren‐9‐one (FLU). The normalized empirical parameter E derived from E_T(30) has been chosen as a measure of solvent polarity using Reichardt's betaine dyes. Quantum yields of ¹O₂ production (Φ_Δ) decrease with increasing solvent polarity and protic character as a consequence of the decrease of the quantum yield of intersystem crossing (Φ_ISC). Values of Φ_Δ of unity have been found in alkanes. In nonprotic solvents of increasing polarity, Φ_ISC and, therefore, Φ_Δ decrease due to solvent‐induced changes in the energy levels of singlet and triplet excited states of FLU. This compound is a poor ¹O₂ sensitizer in protic solvents, because hydrogen bonding considerably increases the rate of internal conversion from the singlet excited state, thus diminishing Φ_Δ to values much lower than those in nonprotic solvents of similar polarity. In mixtures of cyclohexane and alcohols, preferential solvation of FLU by the protic solvent leads to a fast decrease of Φ_Δ upon addition of increasing amounts of the latter.     

Closed and open-shell atomic oxygen on silver: two distinct patterns of the O<Subscript>1s</Subscript> binding energy and X-ray absorption O <Emphasis Type="Italic">K</Emphasis>-edge spectra as revealed by density functional theory

The electronic structure of atomic oxygen adsorbed species is studied by means of the density functional theory in the context of the ethylene epoxidation on the silver surface. The adsorbed oxygen species are modeled by the Ag₂O molecule either in its closed (¹A₁) or open-shell states (³B₁ and ¹B₁). In both open-shell states the 1s level appears to be lower than that in ¹A₁ by about 2 eV. This is apparently a sequence of the separation of electron pair, occupying the *-type highest occupied molecular orbital (HOMO), decreasing the electron density at the oxygen center. Such variation of the O_1s level for closed and open-shell Ag₂O states seems to explain the X-ray photoelectron spectroscopy (XPS) data concerning two distinct atomic oxygen species on silver surface having the O_1s binding energy of about 528 and 530 eV, called nucleophilic and electrophilic oxygen, respectively. The X-ray absorption O K-edge spectra (XANES) calculated for two types of the Ag₂O states by means of multiple-scattered-X-based approach appears to be in a qualitative agreement with those experimentally recorded for nucleophilic and electrophilic oxygen.     

Defects and characteristics of the structure and properties of aluminum oxides

Based on IR spectroscopy data, it was established that nonstoichiometry defects in the structure of aluminum oxides were components of the Wannier-Mott exciton states and included the Al-O, Al-Al, O₂, O ₂ ⁺ , O ₂ ^? , O ₂ ^2? , O₃, and O_2n isolated oscillators in the ground and electronically excited states. It was shown that their presence manifested itself by thermoemission of molecular oxygen singlet forms, excess heat capacity, and anomalous diamagnetism at elevated temperatures.     

Construction and applications of symmetrized valence bond wave functions

A method constructing symmetry-adapted bonded Young tableau bases is proposed, based on the symmetry properties of bonded tableaus and the projection operator associated with a point group. Several examples including the ground states and π excited states of O₃⁻, O₃, O₃⁺, and C₃⁻ are shown for instruction to construct the symmetrized valence bond (VB) wave function. Excitation energies of transitions from the ground states to π excited states of O₃⁻, C₃H₅, and C₃⁻ are calculated with an optimized symmetrized valence bond wave function in the σ–π separation approximation. Good agreement between the VB and experimental excitation energies is observed. The bonding features of the ground state and the first π excited singlet and triplet states for S₃ are discussed according to bonding populations from VB calculations. Both the singlet-biradical and the dipole structures have significant contributions to the ground state X ¹A₁ of S₃, while the excited state 1 ¹B₂ is essentially composed of the dipole structures, and the 1 ³B₂ excited state is comprised from a triplet-biradical structure. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 66 : 1–7, 1998     

Electronically excited oxygen states in exoemission and heterogeneous catalytic oxidation reactions

The role played by electronically excited oxygen in exoemission and heterogeneous catalysis is considered. The results obtained in a study of thermally stimulated exoemission from the surface of Al₂O₃ and SiO₂ are compared with data of temperature-programmed desorption of singlet oxygen O₂(¹Δ_g) from the surface of Al₂O₃ and HZSM-5 zeolites with different SiO₂/Al₂O₃ ratios. The role played by electronically excited oxygen states in heterogeneous catalysis is discussed on the basis of our own and literature data. Thermally stimulated exoemission after the action of an electron flow is considered taking into account electron-stimulated desorption and the available data on electron bombardment in catalysis.     

Density functional study of O2 adsorption on (100) surface of γ‐uranium

Chemisorption of the oxygen molecule on the (100) surface of γ‐uranium was investigated using the generalized gradient approximation to Density Functional Theory. Dissociative adsorptions of O₂ are found to be significantly favored compared to molecular adsorptions. Interstitial adsorptions of molecular oxygen are less probable, as no bound states are found in this case. Only after dissociation of O₂ is atomic oxygen diffusion through the surface possible. The O 2p orbitals are found to hybridize with U 5f bands, and some of the U 5f electrons become more localized. A significant charge transfer from the first layer of the uranium surface to the oxygen atoms is found to occur, making the bonding partly ionic. For the most favored site, the dissociative chemisorption energy is ～9.5 eV, which indicates a strong reaction of uranium surface with oxygen. Spin polarization does not have a significant effect on the chemisorption process. For most of the sites and approaches, chemisorption configurations are almost same for both spin‐polarized and non‐spin‐polarized cases. For the most favored chemisorption sites of oxygen on uranium, paramagnetic adsorption is slightly stronger, by 0.304 eV, compared to magnetic adsorption. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005