1Δg State Research Articles

Spin-Restricted Descriptions of Singlet Oxygen Reactions from XMS-CASPT2 Benchmarks.

Reactions of singlet oxygen are numerous, some of which are desired but many are unwanted. Therefore, the ability to correctly predict and interpret this reactivity for complex molecular systems is essential to our understanding of singlet oxygen reactions. DFT is widely used for predicting many reactions but is not suited to degenerate electronic structures; application to isolated singlet oxygen often uses the spin-unrestricted formalism, which results in severe spin contamination. In this work, we demonstrate that spin-restricted DFT can correctly describe the reaction pathway for four prototypical singlet oxygen reactions. By careful benchmarking with XMS-CASPT2, we show that, from the first transition state onward, the degeneracy of the 1Δg state is broken due to differing interactions of the (degenerate) π* orbitals with the organic substrate; this result is well replicated with DFT. These findings demonstrate the utility of using spin-restricted DFT to explore reactions, opening the way to confidently use this computationally efficient method for molecular systems of medium to large organic molecules.

R2022: A DFT/MRCI Ansatz with Improved Performance for Double Excitations

A reformulation of the combined density functional theory and multireference configuration interaction method (DFT/MRCI) is presented. Expressions for ab initio matrix elements are used to derive correction terms for a new effective Hamiltonian. On the example of diatomic carbon, the correction terms are derived, focusing on the doubly excited 1Δg state, which was problematic in previous formulations of the method, as were double excitations in general. The derivation shows that a splitting of the parameters for intra- and interorbital interactions is necessary for a concise description of the underlying physics. Results for 1La and 1Lb states in polyacenes and 1Au and 1Ag states in mini-β-carotenoids suggest that the presented formulation is superior to former effective Hamiltonians. Furthermore, statistical analysis reveals that all the benefits of the previous DFT/MRCI Hamiltonians are retained. Consequently, the here presented formulation should be considered as the new standard for DFT/MRCI calculations.

Theoretical Study of the Excited States and Luminescent Properties of (H2O)nUO2Cl2 (n = 1–3)

The low-lying excited-state properties of the water-solvated UO2Cl2 complexes, i.e., (H2O)nUO2Cl2 (n = 1-3), below 33,000 cm-1, are investigated based on the ab initio NEVPT2 and CCSD(T) with inclusion of scalar relativistic and spin-orbit coupling effects. The simulated luminescence spectral curves agree well with the experimental spectrum in aqueous solution at -120 °C. Water coordination is found to significantly affect the character of luminescent state, which is changed from the 3Φg state in UO2Cl2 to the 3Δg state in (H2O)2,3UO2Cl2. This is distinctly different from the observed unchanged nature of luminescent state in the cases of Ar coordination to UO2Cl2 and H2O coordination to UO2F2 in the previous work. Furthermore, by combining with the theoretical results for the solvated UO2F2 system, the reason why water coordination does not remarkably change the spectral shape of UO2Cl2, as opposed to UO2F2, was explained based on the analysis of two key spectral parameters, O-U-O symmetrical vibrational frequency and U-O bond length elongation. The roles of ligand field and spin-orbit coupling in the determination of luminescent state character and spectral shape in uranyl dihalide complexes are deeply discussed and summarized. These results deepen our understanding of the luminescent properties of uranyl complexes in aqueous solution.

Photodissociation dynamics of N3.

The photodissociation dynamics of N3 + excited from its (linear) 3Σg -/(bent) 3A″ ground to the first excited singlet and triplet states is investigated. Three-dimensional potential energy surfaces for the 1A', 1A″, and 3A' electronic states, correlating with the 1Δg and 3Πu states in linear geometry, for N3 + are constructed using high-level electronic structure calculations and represented as reproducing kernels. The reference ab initio energies are calculated at the MRCI+Q/aug-cc-pVTZ level of theory. For following the photodissociation dynamics in the excited states, rotational and vibrational distributions P(v') and P(j') for the N2 product are determined from vertically excited ground state distributions. Due to the different shapes of the ground state 3A″ potential energy surface and the excited states, appreciable angular momentum j' ∼ 60 is generated in diatomic fragments. The lifetimes in the excited states extend to at least 50ps. Notably, results from sampling initial conditions from a thermal ensemble and from the Wigner distribution of the ground state wavefunction are comparable.

Singlet O2 Produced by Ultraviolet Dissociation of the β-ionone-O2 Complex.

We formed the gas-phase β-ionone-O2 complex in a supersonic expansion and then photodissociated the complex with light near 312 nm. Photodissociation resulted in the production of O2 in the a 1Δg state, which was ionized at 312 nm using (2 + 1) resonance-enhanced multiphoton ionization (REMPI). We recorded the 1O2 REMPI action spectrum and O2+ velocity map ion image following photodissociation of the complex. From the velocity map image, we determined the total recoil kinetic energy distribution from dissociation of the complex. Fitting the REMPI spectrum showed that the 1O2 product has an effective rotational temperature of about 50 K, while the recoil kinetic energy distribution was well fit with a statistical Boltzmann distribution having an effective translational temperature of 289 K. Using the average translational energy from the Boltzmann fit along with the complex dissociation energy from ab initio calculations, we determined that β-ionone was formed with an average of 2.87 eV of internal energy, which was 0.49 eV higher than previous measurements for the β-ionone triplet-state energy. Our own CCSD/cc-pVDZ//(U)MP2/cc-pVDZ calculations gave a minimum triplet-state energy of 2.04 eV. However, a large structural change occurs between the minimum singlet-ground-state geometry and the minimum triplet-excited-state geometry, and as a result, the calculated vertical energy for the triplet-state β-ionone was determined to be 3.30 eV. Comparing the ab initio and experimental results indicated that following excitation, β-ionone was formed in the triplet state but with significant internal vibrational energy. As such, complex dissociation likely proceeds following internal vibrational energy redistribution, which explains the statistical recoil kinetic energy distribution.

Mechanism of Spin-Exchange Internal Conversion: Practical Proxies for Diabatic and Nonadiabatic Couplings.

Spin-exchange internal conversion (SEIC) is a general class of reactions having singlet fission and triplet fusion as particular cases. Based on a charge transfer (CT) mediated mechanism and analytical derivation with a model Hamiltonian, we propose proxies for estimating the coupling strength in both diabatic and adiabatic pictures for general SEIC reactions. In the diabatic picture, we demonstrated the existence of a bilinear relationship between the coupling strength and molecular orbital overlap, which provides a practical way to predict diabatic couplings. In the adiabatic picture, we showed that nonadiabatic couplings can be approximated by simple functions of the wave function CT coefficients. These approaches were verified through the investigation of singlet oxygen photosensitization, where both 1Δg and 1Σg oxygen states can be competitively generated by a triplet fusion reaction. The interplay between the CT-mediated mechanism, the spatial factors of the bimolecular complex, and the electronic structure of the oxygen molecule during the reaction explains the curiously small coupling to the 1Σg state along specific incidence directions. The results from both the diabatic and adiabatic pictures provide a comprehensive understanding of the reaction mechanism, which applies to general SEIC problems.

Oxidation of 7-ethyl-2,3,5,6,8-pentahydroxy-1,4-naphthoquinone (echinochrome A) by atmospheric oxygen 1. Structure of dehydroechinochrome

The nondestructive oxidation of 7-ethyl-2,3,5,6,8-pentahydroxy-1,4-naphthoquinone by atmospheric oxygen in the ground (triplet 3Σg–) and excited (singlet 1Δg) states in different solvents (acetone, dioxane, ethanol, aqueous ethanol, water) at room temperature involves the initial formation of 7-ethyl-5,6,8-trihydroxy-2,3-dioxo-2,3-dihydro-1,4-naphthoquinone (dehydroechinochrome) accompanied by the release of an H2O2 molecule into the reaction medium. Dehydroechinochrome, being highly susceptible to hydration, successively reacts with two H2O molecules to form 7-ethyl-2,2,3,3,5,6,8-heptahydroxy-2,3-dihydro- 1,4-naphthoquinone as the relatively stable final product. In this form, it can be isolated from the mixture of reaction products.

Radioprotective Action of Low-Intensity Light into the Red Absorption Band of Endogenous Molecular Oxygen

Application of ionizing radiation in oncology (radiation therapy) is a widespread way to eliminate malignant tumors. Normal tissues are inevitable included in any radiation field, and their reliable protection is actual till now. All attempts to solve the problem are based on search of effective radioprotectors, i.e. chemical compounds of various classes, which should be entered into the patient. To date about 50,000 compounds with some radioprotection properties had been tested, but the most effective of them have been simultaneously the most toxic. Here the preliminary results of researches devoted to development of an optical technique on basis of the light-oxygen effect for the protection of women with breast cancer from side effects of the radiation therapy are presented. A low intensity emission of the semiconductor laser in a red spectral interval was used to excite a very small quantity of endogenous molecular oxygen in O2(1Δg) state. It is shown, that application of the method at occurrence of earliest signs of radiation injury allows notably reducing dangerous breaks in radiation therapy course.

Influence of a strong magnetic field on the hydrogen molecular ion using B-spline-type basis-sets**Project supported by the National Natural Science Foundation of China (Grant No. 11204389) and the Natural Science Foundation Project of Chongqing (Grant Nos. CSTC2012jjA50015 and CSTC2012jjA00012).

As an improvement on our previous work [J. Phys. B: At. Mol. Opt. Phys. 45 085101 (2012)], an accurate method combining the spheroidal coordinates and B-spline basis is applied to study the ground state 1σg and low excited states 1σu, 1πg,u,1δg,u,2σg of the in magnetic fields ranging from 109 Gs (1 Gs = 10−4 T) to 4.414 × 1013 Gs. Comparing the one-center method used in our previous work, the present method has a higher precision with a shorter computing time. Equilibrium distances of the states of the in strong magnetic fields were found to be accurate to 3∼5 significant digits (s.d.) and the total energies 6∼11 s.d., even for some antibonding state, such as 1πg, which is difficult for the one-center method to give reliable results while the field strength is B ≥ 1013 Gs. For the large disagreement in previous works, such as the equilibrium distances of the 1πg state at B = 109 Gs, the present data may be used as a reference. Further, the potential energy curves (PECs) and the electronic probability density distributions (EPDDs) of the bound states 1σg, 1πu, 1δg and antibonding states 1σu, 1πg, 1δu for B = 1, 10, 100, 1000 a.u. (atomic unit) are compared, so that the different influences of the magnetic fields on the chemical bonds of the bound states and antibonding states are discussed in detail.

The effect of gold nanoparticles on exchange processes in collision complexes of triplet and singlet oxygen molecules with excited eosin molecules

We have studied exchange processes in contact complexes of triplet eosin molecules with oxygen molecules in the triplet (3Σg−) and singlet (1Δg) states in thin polyvinylbutyral films in the presence of gold nanoparticles. Upon resonant excitation of surface plasmons in gold nanoparticles into the absorption band of eosin molecules-singlet oxygen sensitizers-we have obtained an increase in the intensity of the delayed fluorescence and an increase in the lifetime of the dye with simultaneous quenching of the luminescence of singlet oxygen. The kinetics of the delayed fluorescence of the dye as a result of singlet-triplet annihilation of triplet eosin molecules with singlet oxygen molecules has been investigated. To compare theoretical and experimental data, we have numerically simulated energy transfer processes. Rate constants of energy transfer and of singlet-triplet annihilation, as well as quenching constants of triplet states of the dye by molecular oxygen, have been calculated. Luminescence quantum yield 1Δg of polyvinylbutyral has been estimated. We have analyzed quantum-chemically electronic mechanisms of singlet-triplet annihilation of oxygen and eosin.

Spectroscopy of Ethylenedione

AbstractThe long sought‐after, intrinsically short‐lived molecule ethylenedione (OCCO) was observed and investigated by anion photoelectron spectroscopy. The adiabatic electron affinity of its quasi‐bound 3Σg− state is 1.936(8) eV. The vibrational progression with a 417(15) cm−1 frequency observed within the triplet band corresponds to a trans‐bending mode. Several dissociative singlet states are also observed, corresponding to two components of the 1Δg state and the 1Σg+ state. The experimental results are in agreement with theoretical predictions and constitute the first spectroscopic observation and characterization of this elusive compound.

Singlet–triplet separation induced by the Pseudo Jahn–Teller effect examples: The CX2 and NX2+ (X = H, Cl) molecules

A theoretical investigation with high-level multireference methods is performed to reveal the Jahn–Teller origin of the triplet and singlet bent structures for CH2, CCl2, NH2+, and NCl2+ molecules and rationalize the relative energy ordering of the lowest singlet and triplet states. At the highest-symmetry linear configurations of these triatomic systems, the Pseudo Jahn–Teller effect (PJTE) between the ground state 3Σg- and excited state 3Πu upon the bending mode, denoted as (3Σg-+3Πu)⊗πu, is the reason of producing triplet 3B1 bent structures, and the combined PJTE of (1Δg+1Πu)⊗πu and Renner–Teller effect (RTE) of 1Δg state result in the formations of singlet 1A1 and 1B1 minima. The PJTE of “Δ–Π” plays a dominant role for the instability of degenerate Delta state which is subject to RTE. The spin-crossover induced by PJTE is responsible for the reversed order of singlet and triplet ground states in CH2/NH2+ and CCl2/NCl2+. The different singlet–triplet energy separations of those molecules can be rationalized from different Jahn–Teller vibronic coupling intensity at linear configurations.

Vibrational transition spectra of H2+ in a strong magnetic field

Vibrational transition spectra of H2+ in an ultra-strong magnetic field are determined. The validity of Born—Oppenheimer approximation is analyzed based on one-center method and B-spline basis sets. It is shown that Born—Oppenheimer approximation is reliable for the investigation on the ground state and low excited states of H2+ subjected to the strong magnetic field. Furthermore, it is found that the vibrational transition spectra from 1σg, 1πu, and 1δg states lie in infrared, visible, and ultraviolet ranges with increasing magnetic field strength.

Accurate one-centre method for hydrogen molecular ion calculation using B-spline-type basis sets in strong magnetic fields

An accurate one-centre method is here applied to the calculation of the equilibrium distances and the energies for the hydrogen molecular ion in magnetic fields ranging from 109G to 4.414 × 1013 G. Both the radial and angular wavefunctions were expanded in terms of optimization B-splines. The slow convergence problem in the general one-centre method and singularities at the nuclear positions of the H+2 were solved well. The accuracy of the one-centre method has been improved in this way. We compared our results with those generated by high-precision methods from published studies. Equilibrium distances of the 1σg, u, 1πg, u, 1δg, u and 2σg states of the H+2 in strong magnetic fields were found to be accurate to three to four significant digits at least up to 2.35 × 1012 G, even for the antibonding states 1σu, 1πg and 1δu, whose equilibrium distances Req are very large.

Microwave plasma generation and filtered transport of O2 (a 1Δg)

Singlet oxygen, O2(a 1Δg), is generated using a low pressure, low power continuous microwave discharge operating at 2.45 GHz with a flow of helium seeded with 1–10% molecular oxygen. The absolute concentration of O2(a 1Δg) is measured using off-axis integrated cavity output spectroscopy to probe the Q-branch transition of the (1, 0) band of the Noxon system. In order to remove other energetic species from the flow, the post-discharge flow is passed through a coarse fritted quartz filter. The use of the quartz frit takes advantage of the substantially lower surface sticking probability of O2(a 1Δg) in comparison with other excited species on the flow. Up to 6% of the total oxygen passing through the filter remains in the a 1Δg state, and absolute densities of 2.5 × 1014 cm−3 are obtained using this method. This preparation method and transport is important in developing sources of singlet oxygen for kinetic and spectroscopic studies.

Accurate One-Centre Method for Hydrogen Molecule Ions in Strong Magnetic Field

An accurate one-centre method for the hydrogen molecule ion is tested. The slow convergence and singularities at the nuclear positions that are problems in the general one-centre method are solved well by employing the optimal radial and angular B-spline basis. Therefore, the accuracy of the one-centre method is improved observably. For the ground state of the H2+ in the free field, 7 × 10−8 accuracy is obtained, which rivals the best one-centre calculation before. As a test, the nuclear distances and the total energies of the 1σg,u 1πu, 1δg,u and 2σg states of the H22 for the magnetic held strength B = 1a.u. are also obtained. Compared to other results, five-digit accuracy at least can be arrived even for the antibonding states 1σu and 1δu, whose equilibrium distances R is very large.

Singlet Oxygen Generation Processes in Solutions of Fullerenes in Carbon Tetrachloride

Singlet molecular oxygen generation is studied by means of its NIR luminescence at light excitation by pulse, pulse‐periodic and CW sources in C60 and C70 solutions in CCl4. Singlet oxygen concentration is determined in the stationary and pulse modes of irradiation. The quenching rate constants of 1Δg state of O2 molecule by fullerene are found to be (7.2 ± 0.1)·107 M−1s−1 (C70) and < 6·104 M−1s−1 (C60). Temperature and photolytic changes in the solutions photosensibilization ability are studied using optical and ESR spectroscopy methods. A pulse liquid singlet oxygen generator is constructed and studied on the base of the C60 solution in CCl4. Production of singlet oxygen into gas phase is realized with the concentration 5·1016 cm−3.

On mechanisms of a flame velocity increase upon activation of O2 molecules in electrical discharge**This paper was presented at the Third International Symposium on Nonequilibrium Processes, Combustion, and Atmospheric Phenomena (Dagomys, Sochi, Russia, 25–29 June 2007).

The effect of the excitation of oxygen molecules to the O2(a 1Δg) and electronic states in the electrical discharge on the velocity of laminar flame propagation in the H2–O2 mixture is analysed. The experimental data on the decay of the singlet delta oxygen in the post shock region in the H2–O2 mixture and the measurements of the laminar flame velocity are used to validate the developed kinetic model. The calculations show that the excitation of O2 molecules to the a 1Δg and electronic states allows one to increase significantly (by a factor of 2.5) the velocity of flame propagation for the fuel lean hydrogen–oxygen mixture. For stoichiometric and fuel rich mixtures the increase in flame velocity due to an abundance of singlet oxygen molecules in the mixture is significantly smaller and attains a factor of 1.1.

Intensification of the oxidation of rich methane/air mixtures by O2 molecules excited to the a 1Δg state

Methane oxidation in rich CH4/air mixtures can be intensified by exciting O2 molecules to the a 1Δg state. Even small amounts of O2(a 1Δg) molecules reduce the ignition temperature and shorten the induction period. As compared to the oxidation of ordinary methane/air mixtures, oxidation in the presence of excited oxygen makes it possible to convert methane into synthesis gas (H2 + CO) at low initial temperatures of ≈600 K and atmospheric pressure and to raise the H2 and CO yields at a fixed reactor length.

Kerr effect of molecular oxygen at λ=1064 nm

We report a new measurement of the Kerr effect of molecular oxygen at λ= 1064 nm. The experimental value reported for the anisotropy of the index of refraction Δ nu l, (3.15±0.85)×10-25 m2 V-2 atm-1, is in good agreement with the value of 3.4×10-25 m2 V-2 atm-1 obtained via an ab initio calculation. We show that the dependence of the effect on the pressure is not linear because of the presence of a collision-induced absorption band around 1060 nm due to the transition from the X3 Σ- g ground state to the 1Δg state. We also give the value of the quadratic anisotropy Δnu q (-1.03±0.68)×10-25 m2 V-2 atm-2. We finally compare our ab initio theoretical and experimental results with previous existing data.