Triplet Potential Energy Surfaces Research Articles

Theoretical investigations on the atmospheric reaction of n-C3H7O2 with ClO radicals

As a key intermediate during propane oxidation, n-propyl peroxy radicals (n-C3H7O2) play significant roles in the atmosphere. In this study the reaction of n-C3H7O2 with chlorine monoxide ClO was studied using quantum chemistry methods. Result shows that addition–elimination, substitution and abstraction mechanisms were located on the singlet and triplet potential energy surfaces (PESs). On the singlet PES, the final products of C2H5CHO + HClO2 and C2H5CHOO + HOCl will be feasible with low or moderate barriers. While on the triplet PES all barriers are too high and can be negligible to the overall reaction. Moreover, it is presumed that the title reaction might be potential source of C2H5CHOO in the atmosphere, especially in the marine boundary layer (MBL) with high ClO concentrations.

Photoisomerization of (Cyanomethylene)cyclopropane (C5H5N) to 1-Cyano-2-methylenecyclopropane in an Argon Matrix.

Broad-band ultraviolet photolysis (λ > 200 nm) of (cyanomethylene)cyclopropane (5) in an argon matrix at 20 K generates 1-cyano-2-methylenecyclopropane (7), a previously unknown compound. This product was initially identified by comparison of its infrared spectrum to that predicted by an anharmonic MP2/6-311+G(2d,p) calculation. This assignment was unambiguously confirmed by the synthesis of 1-cyano-2-methylenecyclopropane (7) and observation of its authentic infrared spectrum, which proved identical to that of the observed photoproduct. We investigated the singlet and triplet potential energy surfaces associated with this isomerization process using density functional theory and multireference calculations. The observed rearrangement of compound 5 to compound 7 is computed to be endothermic (3.3 kcal/mol). We were unable to observe the reverse reaction (7 → 5) under the photochemical conditions.

Study on Formation of Interstellar C7H from Reactions C4 + C3H2 and C4H + C3H.

We interrogated C7H produced from reactions C4 + C3H2/C4H + C3H → C7H + H using both translational and photoionization spectroscopy. Reactants C3H, C3H2, C4, and C4H were synthesized in two crossed beams of 1% C2H2/He ignited by pulsed high-voltage discharge. The individual contributions of reactions C4 + C3H2 and C4H + C3H to product C7H were evaluated as 17:83 from reactant concentrations in both molecular beams. The translational energy distribution, the angular distribution, and the photoionization efficiency curve of product C7H were unraveled. C7H was identified as the most stable linear isomer by its photoionization efficiency curve that features two ionization thresholds corresponding to separate transitions to singlet and triplet states of l-C7H+. The quantum-chemical calculations indicate that the associations of C4 with C3H2 and C4H with C3H incur no entrance barriers, and the most favorable exit channel leads to product l-C7H + H. It is the first time demonstrating that C7H is producible from reactions 1,3C4 + 1C3H2 and 2C4H + 2C3H on the lowest-lying singlet and triplet potential energy surfaces of 1,3C7H2. This work implies that the reactions of C4 + C3H2 and C4H + C3H might have contributions to interstellar C7H to some extent as compared with the C + C6H2 reaction commonly adopted in an astrochemical model.

Unraveling the interaction between singlet state atomic oxygen O(1D) and water: toward the formation of oxywater and hydrogen peroxide.

We performed high-level quantum mechanical calculations to explore the interaction of atomic oxygen in the ground triplet state, O(3P), and the excited singlet state, O(1D), with water. We reported the potential energy curves for a few lowest electronic states when an atomic oxygen approaches the oxygen of a water molecule. Our results predict the formation of a singlet oxywater species as the product of O(1D) and H2O which lies about 149.33 kJ mol-1 below the total energy of a singlet oxygen atom and a water molecule. Our calculations predict that an O(3P) atom interacting with a water molecule forms a triplet oxywater complex with a shallow minimum on the triplet potential energy surfaces. We examined the transition of the singlet state oxywater species to hydrogen peroxide through the unimolecular reaction pathway, a (1,2)-hydrogen shift. We reported the structural properties, vibrational frequencies, and dipole moments of oxywater species, the transition state, and hydrogen peroxide. We also reported the energy barrier for the transition, and we provided an estimate for the respective reaction rate constant. In addition, we investigated the impact of solvents on the reaction pathway using an implicit solvation model of water. We predict that a singlet state oxywater species has a longer lifetime in a water environment than in the gas phase.

Insight into the reactions of n-, iso-, sec- and tert-Butylperoxy radicals with hydroperoxyl radical in the Atmosphere: Reaction Mechanism, thermodynamic analysis and kinetics

Butylperoxy radical (C4H9O2) is recognized today as an important atmospheric intermediate which formed by the oxidation of butane (C4H10), playing a crucial role in the cycling of carbon. This work gives a theoretical investigation of the reaction mechanism, thermodynamic analysis and kinetics of the reactions between HO2 and four isomers (n-, iso-, sec- and tert-Butylperoxy radicals) of C4H9O2, which has been computed using the CCSD(T)/aug-cc-pVDZ//B3LYP/6-311G(d,p) level of theory. As a result, the above four reactions proceed on both singlet and triplet potential energy surfaces have been demonstrated. Additionally, the formation of alkylhydroperoxides and 3O2via triplet hydrogen abstraction from the four reactions are the major reaction channels while other singlet product channels are negligibly small. Besides, rate coefficients for the title reactions were estimated using the transition state theory (TST) over the temperature range 258 – 378 K, and these results exhibit a negative temperature dependence.

Singlet and Triplet Pathways Determine the Thermal Z/E Isomerization of an Arylazopyrazole-Based Photoswitch.

Understanding the thermal isomerization mechanism of azobenzene derivatives is essential to designing photoswitches with tunable half-lives. Herein, we employ quantum chemical calculations, nonadiabatic transition state theory, and photosensitized experiments to unravel the thermal Z/E isomerization of a heteroaromatic azoswitch, the phenylazo-1,3,5-trimethylpyrazole. In contrast to the parent azobenzene, we predict two pathways to be operative at room temperature. One is a conventional ground-state reaction occurring via inversion of the aryl group, and the other is a nonadiabatic process involving intersystem crossing to the lowest-lying triplet state and back to the ground state, accompanied by a torsional motion around the azo bond. Our results illustrate that the fastest reaction rate is not controlled by the mechanism involving the lowest activation energy, but the size of the spin-orbit couplings at the crossing between the singlet and the triplet potential energy surfaces is also determinant. It is therefore mandatory to consider all of the multiple reaction pathways in azoswitches in order to predict experimental half-lives.

DFT investigations on the mechanisms and kinetics for the DMS + O3 reaction

AbstractThe potential energy surface (PES) for the reaction of ozone with dimethyl sulfide (DMS) was calculated at the CCSD(T)/6‐311++G(3df,2pd)//M06‐2X/6‐311++G(d,p) levels of theory. Result shows that on the singlet PES the addition–elimination mechanism is dominant, and H‐abstraction mechanism is less competitive. The major channel starts from the addition of ozone and DMS leading to a weak intermediate IM1, which decomposes subsequently to DMSO and 1O2 via a barrier around 38.8 kJ/mol. With a barrier of 64.0 kJ/mol, the formation of HO3 + CH3SCH2 via H‐abstraction mechanism is subdominant. Besides, DMSO + 1O2 can take place further reactions to produce several products. The substitution mechanism was located on the triplet PES, however, with a rather high barrier it is negligible. Furthermore, the rate constants for the two channels leading to DMSO + 1O2 and HO3 + CH3SCH2 were calculated from 200 to 1000 K. The total rate constant is 1.13 × 10‐20 cm3·molecule‐1·s‐1 at 298 K and 1 atm, in good agreement with previous experimental data. The overall rate constants are positive temperature dependent in the whole temperature range.

Mechanistic Understanding and Reactivity Analyses for the Photochemistry of Disubstituted Tetrazoles

The photolysis of tetrazoles has undergone extensive research. However, there are still some problems to be solved in terms of mechanistic understanding and reactivity analyses, which leaves room for theoretical calculations. Herein, multiconfiguration perturbation theory at the CASPT2//CASSCF level was employed to account for electron correction effects involved in the photolysis of four disubstituted tetrazoles. Based on calculations of vertical excitation properties and evaluations of intersystem crossing (ISC) efficiencies in the Frank-Condon region, the combination of space and electronic effects is found in maximum-absorption excitation. Two types of ISC (1ππ* → 3nπ*, 1ππ* → 3ππ*) are determined in disubstituted tetrazoles, and the obtained rates follow the El-Sayed rule. Through mapping three representative types of minimum energy profiles for the photolysis of 1,5-, and 2,5-disubstituted tetrazoles, a conclusion can be drawn that the photolysis of tetrazoles exhibits reactivity characteristic of bond-breaking selectivity. Kinetic evaluations show that the photogeneration of singlet imidoylnitrene operates predominately over that in the triplet state, which can be confirmed by a double-well model in the triplet potential energy surface of 1,5-disubstituted tetrazole. Similar mechanistic explorations and reactivity analyses were also applied to the photolysis of 2,5-disubstituted tetrazole to unveil fragmentation patterns of nitrile imine generation. All computational efforts allow us to better understand the photoreactions of disubstituted tetrazoles and to provide useful strategies for regulating their unique reactivity.

Controlling the Triplet Potential Energy Surface of Bimetallic Platinum(II) Complex by Constructing Structure-Property Relationship: A Theoretical Exploration.

For phosphorescent materials, managing the triplet potential energy surface stands for controlling the phosphorescence quantum yield. However, due to the complexity and variability, the triplet potential energy surface can be managed with difficulty. In this work, a series of bimetallic Pt(II) complexes, namely Pt-1, Pt-1-1, Pt-1-2, Pt-2, Pt-3-5, and Pt-6-7, are employed as models to construct a relationship between the structures and triplet potential energy surfaces, aiming to achieve meaningful information to manage the triplet potential energy surface. On the basis of the results, it is observed that the triplet potential energy surface has an intimate connection with the structures of bimetallic Pt(II) complexes. In the case of the primordial Pt(II) complex, the triplet potential energy surface consists of two minimal points, illustrating various properties, which can largely affect the phosphorescence quantum yield. Once the intramolecular steric hindrance, restriction effect, and metallophilic interaction (Pt-Pd/Pd-Pd) are employed by tailoring the structures of primordial Pt(II) complexes, the triplet potential energy surface can be reconstructed via one minimal point-charactered short metal-metal distance, resulting in different photophysical properties. The relationship between the triplet potential energy surface and structure is essentially unveiled from the structural and electronic viewpoints. The conclusions originated from the structural and electronic investigations can be regarded as indicators to accurately and expediently predict the triplet potential energy surfaces of bimetallic Pt(II) complexes. The results presented here are helpful in addressing the designed strategies as they show that the triplet potential energy surfaces of bimetallic Pt(II) complexes can be properly tuned.

Reactions O(3P, 1D) + HCCCN(X1Σ+) (Cyanoacetylene): Crossed-Beam and Theoretical Studies and Implications for the Chemistry of Extraterrestrial Environments.

Cyanoacetylene (HCCCN), the first member of the cyanopolyyne family (HCnN, where n = 3, 5, 7, ...), is of particular interest in astrochemistry being ubiquitous in space (molecular clouds, solar-type protostars, protoplanetary disks, circumstellar envelopes, and external galaxies) and also relatively abundant. It is also abundant in the upper atmosphere of Titan and comets. Since oxygen is the third most abundant element in space, after hydrogen and helium, the reaction O + HCCCN can be of relevance in the chemistry of extraterrestrial environments. Despite that, scarce information exists not only on the reactions of oxygen atoms with cyanoacetylene but with nitriles in general. Here, we report on a combined experimental and theoretical investigation of the reactions of cyanoacetylene with both ground 3P and excited 1D atomic oxygen and provide detailed information on the primary reaction products, their branching fractions (BFs), and the overall reaction mechanisms. More specifically, the reactions of O(3P, 1D) with HCCCN(X1Σ+) have been investigated under single-collision conditions by the crossed molecular beams scattering method with mass spectrometric detection and time-of-flight analysis at the collision energy, Ec, of 31.1 kJ/mol. From product angular and time-of-flight distributions, we have identified the primary reaction products and determined their branching fractions (BFs). Theoretical calculations of the relevant triplet and singlet potential energy surfaces (PESs) were performed to assist the interpretation of the experimental results and clarify the reaction mechanism. Adiabatic statistical calculations of product BFs for the decomposition of the main triplet and singlet intermediates have also been carried out. Merging together the experimental and theoretical results, we conclude that the O(3P) reaction is characterized by a minor adiabatic channel leading to OCCCN (cyanoketyl) + H (experimental BF = 0.10 ± 0.05), while the dominant channel (BF = 0.90 ± 0.05) occurs via intersystem crossing to the underlying singlet PES and leads to formation of 1HCCN (cyanomethylene) + CO. The O(1D) reaction is characterized by the same two channels, with the relative CO/H yield being slightly larger. Considering the recorded reactive signal and the calculated entrance barrier, we estimate that the rate coefficient for reaction O(3P) + HC3N at 300 K is in the 10-12 cm3 molec-1 s-1 range. Our results are expected to be useful to improve astrochemical and photochemical models. In addition, they are also relevant in combustion chemistry, because the thermal decomposition of pyrrolic and pyridinic structures present in fuel-bound nitrogen generates many nitrogen-bearing compounds, including cyanoacetylene.

A combined crossed molecular beam and theorerical study of the O(3P,1D) + acrylonitrile (CH2CHCN) reactions and implications for combustion and extraterrestrial environments.

Acrylonitrile (CH2CHCN) is ubiquitous in space (molecular clouds, solar-type star forming regions, and circumstellar envelopes) and is also abundant in the upper atmosphere of Titan. The reaction O(3P) + CH2CHCN can be of relevance in the chemistry of the interstellar medium because of the abundance of atomic oxygen. The oxidation of acrylonitrile is also important in combustion as the thermal decomposition of pyrrolic and pyridinic structures present in fuel-bound nitrogen generates many nitrogen-bearing compounds, including acrylonitrile. Despite its relevance, limited information exists on this reaction. We report a combined experimental and theoretical investigation of the reactions of acrylonitrile with both ground 3P and excited 1D atomic oxygen. From product angular and time-of-flight distributions in crossed molecular beam experiments with mass spectrometric detection at a collision energy, Ec, of 31.4 kJ mol-1, we have identified the primary reaction products and determined their branching fractions (BFs). Theoretical calculations of the relevant triplet and singlet potential energy surfaces (PESs) were performed to interpret the experimental results and elucidate the reaction mechanism. Adiabatic statistical calculations of product BFs for the decomposition of the main triplet and singlet intermediates have been carried out. Combining the experimental and theoretical results, we conclude that the O(3P) reaction leads to two main product channels: (i) CH2CNH (ketenimine) + CO (dominant with a BF of 0.87 ± 0.05), formed via efficient intersystem crossing from the entrance triplet PES to the underlying singlet PES, and (ii) HCOCHCN + H (minor, with a BF of 0.13 ± 0.05), occurring adiabatically on the triplet PES. Our study suggests the inclusion of this reaction as a possible destruction pathway of CH2CHCN and a possible formation route of CH2CNH in the interstellar medium. The O(1D) + CH2CHCN reaction mainly leads to the formation of CH2CNH + CO adiabatically on the singlet PES. This result can improve models related to the chemistry of interstellar ice and cometary comas, where O(1D) reactions can play a role. Overall, our results are expected to be useful for improving the models of combustion and extraterrestrial environments.

Near-exact nuclear gradients of complete active space self-consistent field wave functions.

In this paper, we study the nuclear gradients of heat bath configuration interaction self-consistent field (HCISCF) wave functions and use them to optimize molecular geometries for various molecules. We show that HCISCF nuclear gradients are fairly insensitive to the size of the "selected" variational space, which allows us to reduce the computational cost without introducing significant errors. The ability of the HCISCF to treat larger active spaces combined with the flexibility for users to control the computational cost makes the method very attractive for studying strongly correlated systems, which require a larger active space than possible with a complete active space self-consistent field. Finally, we study the realistic catalyst, Fe(PDI), and highlight some of the challenges this system poses for density functional theory (DFT). We demonstrate how HCISCF can clarify the energetic stability of geometries obtained from DFT when the results are strongly dependent on the functional. We also use the HCISCF gradients to optimize geometries for this species and study the adiabatic singlet-triplet gap. During geometry optimization, we find that multiple near-degenerate local minima exist on the triplet potential energy surface.

The Reaction of HO2 and CH3O2: CH3OOH Formed from the Singlet Electronic State Surface

High-level coupled-cluster calculations in combination with two-dimensional master equation simulations were used to study the HO2 + CH3O2 reaction, which plays an important role in the oxidation of methane and hydrocarbons in the Earth’s atmosphere and low-temperature combustion. The main reaction pathways taking place on the lowest-lying triplet and singlet potential energy surfaces (PES) were characterized. Interestingly, methyl hydroperoxide (CH3OOH), the sole product, could be produced from both the triplet and singlet PESs, with a ratio of roughly 9:1. Formaldehyde is not made as a primary product, but can be formed via secondary chemistry. The formation of methyl tetraoxide (MTO) from the singlet PES is unimportant. The calculated reaction rate coefficients were found to be practically pressure-independent for p ≤ 760 Torr and can be given by k(T)=2.75×10−13×e+1.75 kcal mol−1/RT (in cm3/s), an expression useful for kinetics modeling over the range T = 200–800 K. The rate constant has a slight negative Arrhenius energy dependence of about −1.75 kcal mol–1, falling about a factor of 30 from 200 K to 800 K.

Investigate the Relationship between Structure and Triplet Potential Energy Surface to Control the Phosphorescence Quantum Yield of Platinum(II) Complex: A Theoretical Investigation.

Triplet potential energy surfaces are extremely important for phosphors because they are closely related to radiative and nonradiative decay processes. In this article, the correlations between the strctures and the triplet potential energy surfaces for Pt(II) complexes are investigated in detail with the help of density functional theory (DFT). The calculated results indicate that triplet hypersurface minima with different configurations, i.e., planar and bent, rely on the geometries of the platinum(II) complex. A bent configuration could cause an obvious decrease in the phosphorescence quantum yield, and an unusual low-lying triplet excited-state decay route is proposed. In addition, the extension of π-conjugation and addition of suitable substituents, for example arylboron, are promising strategies for changing the triplet hypersurface to achieve the minimum with a planar configuration, leading to a high phosphorescence quantum yield. Moreover, to predict the triplet hypersurface, a useful and simple strategy has been put forward. In our study, the relationship between the structure and the lowest-lying triplet potential energy surface of a Pt(II) complex is constructed, which is significant and meaningful for controlling the phosphorescence quantum yield to design high-performance phosphorescent materials used in the field of organic light-emitting diodes (OLEDs).

A DFT study on the reaction mechanisms of the oxidation of ethylene mediated by technetium and manganese oxo complexes.

The oxidation of ethylene catalyzed by manganese and technetium oxo complexes of the type MO3L (M = Tc, Mn, and L = O-, Cl-, F-, OH-, Br-, I-) on both singlet and triplet potential energy surfaces (PESs) have been studied. All molecular structures were stable on the singlet PES except for the formation of the dioxylate intermediate for the MnO3L (L = O-, Cl-, F-, OH-, Br-, I-) catalyzed pathway. Frontier molecular orbital calculations showed that electrons flow from the HOMO of ethylene into the LUMO of the metal-oxo complex for all complexes studied except for MO3L (M = Tc, Mn, and L = O-) where the vice versa occurs. In the reaction of both TcO3L and MnO3L (L = O-, Cl-, F-, OH-, Br-, I-) with ethylene, it was observed that the formation of the dioxylate intermediate along the [3 + 2] addition pathway on the singlet reaction surface is both kinetically and thermodynamically favorable over its formation via the [2 + 2] pathway. Furthermore, it was observed that TcO4- and MnO4- catalyzed pathways exclusively form diols on the singlet PES. The formation of epoxides on the singlet surface is kinetically favorable through the [2 + 1] and [2 + 2] channel for the MnO3L (L = F-, Cl-, Br-, I-, OH-) and TcO3L (L = F-, Cl-, Br-, I-, OH-) catalyzed surfaces respectively. In all cases, the TcO3L complexes were found to be polar compared to the MnO3L complexes. The MnO4- (singlet) and MnO3F (singlet) are the best catalysts for the exclusive formation of the diols and epoxides respectively.

Theoretical investigation of triplet potential energy surfaces for (C^C*) cyclometalated platinum(ii) complexes and the corresponding control strategies

Triplet potential energy surfaces, for phosphorescent materials, play a predominant role in determining the radiative and non-radiative decay processes.

Quantum chemical study of the reaction paths and kinetics of acetaldehyde formation on a methanol-water ice model.

Acetaldehyde (CH3CHO) is ubiquitous in interstellar space and is important for astrochemistry as it can contribute to the formation of amino acids through reaction with nitrogen containing chemical species. Quantum chemical and reaction kinetics studies are reported for acetaldehyde formation from the chemical reaction of C(3P) with a methanol molecule adsorbed at the eighth position of a cubic water cluster. We present extensive quantum chemical calculations for total spin S = 1 and S = 0. The UωB97XD/6-311++G(2d,p) model chemistry is employed to optimize the structures, compute minimum energy paths and zero-point vibrational energies of all reaction steps. For the optimized structures, the calculated energies are refined by CCSD(T) single point computations. We identify four transition states on the triplet potential energy surface (PES), and one on the singlet PES. The reaction mechanism involves the intermediate formation of CH3OCH adsorbed on the ice cluster. The rate limiting step for forming acetaldehyde is the C–O bond breaking in CH3OCH to form adsorbed CH3 and HCO. We find two positions on the reaction path where spin crossing may be possible such that acetaldehyde can form in its singlet spin state. Using variational transition-state theory with multidimensional tunnelling we provide thermal rate constants for the energetically rate limiting step for both spin states and discuss two routes to acetaldehyde formation. As expected, quantum effects are important at low temperatures.

Theoretical study on the corrosion of thorium nuclear fuel by water: The effect of two-state reaction mechanism

The title reaction was investigated by DFT calculations. This process realizes in two steps. In the first step, singlet and triplet potential energy surfaces cross each other. We found that two-state reaction (TSR) mechanism is involved, enabling singlet product to be favored energetically. Then, ThO continues to react with H2O to form ThO2. Only the activation energy of singlet addition–elimination channel matches well with experimental reports. This work illustrates the occurrence of TSR mechanism in the corrosion of actinide nuclear fuel and further suggests a potential yet crucial impact on related fields. It deserves particular attention.

Two-state reaction guaranteed high efficiency hydrogen production in Sn+H2O reaction: A preliminarily theoretical study based on single molecule model

The Sn + H2O reaction is important in both hydrogen production through solar thermochemical redox cycles and investigations like the treatment of nuclear reactors or flame inhibition. Based on single molecule model, this work systematically explores its possible reacting channels in different spin multiplicities at CCSD(T)//DFT level of theory to reveal the underlined mechanism for its reported efficient hydrogen production. It is found that the singlet and triplet potential energy surfaces cross each other during the water attacking process, which makes the hydrogen production channel in the singlet state energetically favored. Quantitative calculations about the possibility of surface crossing and spin inversion with respect to minimum energy crossing point, spin-orbit coupling coefficient and intersystem crossing probability confirm that the optimal reacting pathway involves the two-state reaction scenario. This special reactive pattern makes hydrogen production not only possible but also efficient. Analysis of the equilibrium constant of reactive channels and their variation with temperature reveals the performance of two-state reaction channel agrees well with reported data range and nontrivial temperature dependence.

Computational study on the reaction collisions for the state-to-state process of N(4 S)+NO(2Π)→O(3 P)+N2(X 1Σ+ g)

Quasi-classical trajectory method has been employed in the theoretical calculations for the state-to-state processes of the reaction N(4 S)+NO(2Π)→O(3 P)+N2(X 1Σ+ g) over a temperature range of 300 K ≤ T ≤ 10,000 K on Global triplet potential energy surface (PES) for N2O system. The thermal rates calculated for the N + NO→O + N2 reaction show a great agreement with the previous theoretical and experimental results. The results obtained herein fill the gaps in the existing data for the state-to-state processes in the N2O system and could be applied to large temperature ranges relevant to the model of hypersonic gas flows. In addition, the vibrational relaxation rate coefficients of the non-reactive collision between N atom and NO molecule in the low vibrational excited state were calculated and reported.