Singlet Potential Energy Surfaces Research Articles

Computational study on mechanisms and pathways of the atmospheric C2H5O2 + IO reaction

Using QCISD(T)//B3LYP methods, mechanisms and pathways of the atmospheric C2H5O2 +IO reaction was investigated theoretically. The result indicates that the title reaction proceeds on both singlet and triplet potential energy surfaces. It is speculated that C2H5O+IOO, CH3CHO+HIO2, CH3CHO+HOIO and CH3CHO2+HOI are dominant for the C2H5O2 +IO reaction in the atmosphere. While other products including CH3CHO+HOOI, C2H5O+OIO, C2H5OIO+O(3P), C2H5OOI+O(3P), and C2H5OI+O2 are of no importances due to high barriers or unstable products. Moreover, comparisons suggest that halogen and alkyl substitution effects play minor significance in the relative stability of RO3X isomers (R=H, CH3, C2H5; X=Cl, Br, I).

Computational Study of the Mechanism, Reaction Rate and Thermochemistry of Atmospheric Oxidation of Methylamine with Singlet Oxygen

The reaction of CH₃NH₂ with O₂ on the singlet potential energy surfaces (PES) was carried out using the B3LYP, CCSD(T) and G3B3 theoretical approaches along with 6-311++G(3df,3pd) basis set. The suggested mechanism for the title reaction consists of one pre-reactive complex. From the pre-reactive complex, nine types of products, CH2NH+H2O2, CH3NH+OOH, NH2CHO+H2O, CH2NHO+H2O, NH3+HCOOH, CH2NH+H2+O2, HCHO+NH2OH, CH2NH2+HOO and CH2NH+H2OO are obtained. Six adducts are found to be thermodynamically stable with the negative value in Gibbs free energy and three products are not stable with positive value in Gibbs free energy at standard condition. NH2CHO+H2O and NH3+HCOOH with more free energy values are the thermodynamic products. CH2NH and H2O2 are the major products for the title reaction. The rate constant for the major pathway is calculated in the temperature range of 300-1000 K at atmospheric pressure at the B3LYP//6-311++G(3df,3pd) level of computation.

Mechanistic and energetic study of the atmospheric reaction of hydrosulfinyl and mercapto radicals

The gas phase reaction pathways of hydrosulfinyl, HSO, and mercapto, HS, have been studied computationally on the singlet potential energy surface. All species in the title reaction have been optimized using the MP2 method in connection with 6-311++G(3df,3pd) basis set. The single point energy calculations at the CCSD(T) level have been carried out on the optimized geometries of the MP2 computational level using aug-cc-pVTZ basis set. Five types of products, H2S+3SO, S2O+H2, H2O+3S2, trans-HSSOH and trans-HSOSH, are formed from HSO and HS reaction. trans-HSOSH adduct is directly generated from the reactants after passing two transition states. Other products have been obtained through changing the singlet reactant complex defined as CR. All products have enough stability in thermodynamic approach. trans-HSSOH and H2O+3S2 products with higher stability are defined as thermodynamic products. The kinetic investigations show that the trans-HSOSH adducts is obtained as the main product. Also, the rate constants show a non-Arrhenius behavior in the temperature range of 300–2500K.

Reaction Dynamics of O((3)P) + Propyne: II. Primary Products, Branching Ratios, and Role of Intersystem Crossing from Ab Initio Coupled Triplet/Singlet Potential Energy Surfaces and Statistical Calculations.

The mechanism of the O((3)P) + CH3CCH reaction was investigated using a combined experimental/theoretical approach. Experimentally the reaction dynamics was studied using crossed molecular beams (CMB) with mass-spectrometric detection and time-of-flight analysis at 9.2 kcal/mol collision energy. Theoretically master equation (ME) simulations were performed on a potential energy surface (PES) determined using high-level ab initio electronic structure calculations. In this paper (II) the theoretical results are described and compared with experiments, while in paper (I) are reported and discussed the results of the experimental study. The PES was investigated by determining structures and vibrational frequencies of wells and transition states at the CASPT2/aug-cc-pVTZ level using a minimal active space. Energies were then determined at the CASPT2 level increasing systematically the active space and at the CCSD(T) level extrapolating to the complete basis set limit. Two separate portions of the triplet PES were investigated, as O((3)P) can add either on the terminal or the central carbon of the unsaturated propyne bond. Minimum energy crossing points (MECPs) between the triplet and singlet PESs were searched at the CASPT2 level. The calculated spin-orbit coupling constants between the T1 and S0 electronic surfaces were ∼25 cm(-1) for both PESs. The portions of the singlet PES that can be accessed from the MECPs were investigated at the same level of theory. The system reactivity was predicted integrating stochastically the one-dimensional ME using Rice-Ramsperger-Kassel-Marcus theory to determine rate constants on the full T1/S0 PESs, accounting explicitly for intersystem crossing (ISC) using the Landau-Zener model. The computational results are compared both with the branching ratios (BRs) determined experimentally in the companion paper (I) and with those estimated in a recent kinetic study at 298 K. The ME results allow to interpret the main system reactivity: CH3CCO + H and CH3 + HCCO are the major channels active on the triplet PES and are formed from the wells accessed after O addition to the terminal and central C, respectively; (1)CH3CH + CO and C2H3 + HCO are the major channels active on the singlet PES and are formed from the methylketene and acrolein wells after ISC. However, also a large number of minor channels (∼15) are active, so that the system reactivity is quite complicated. The comparison between computational and experimental BRs is quite good for the kinetic study, while some discrepancy with the CMB estimations suggests that dynamic non-ergodic effects may influence the system reactivity. Channel specific rate constants are calculated in the 300-2250 K and 1-30 bar temperature and pressure ranges. It is found that as the temperature increases the H abstraction reaction, whose contribution is negligible in the experimental conditions, increases in relevance, and the extent of ISC decreases from ∼80% at 300 K to less than 2% at 2250 K.

Excited-State Proton Transfer and Intramolecular Charge Transfer in 1,3-Diketone Molecules.

The photophysical signature of the tautomeric species of the asymmetric (N,N-dimethylanilino)-1,3-diketone molecule are investigated using approaches rooted in density functional theory (DFT) and time-dependent DFT (TD-DFT). In particular, since this molecule, in the excited state, can undergo proton transfer reactions coupled to intramolecular charge transfer events, the different radiative and nonradiative channels are investigated by making use of different density-based indexes. The use of these tools, together with the analysis of both singlet and triplet potential energy surfaces, provide new insights into excited-state reactivity allowing one to rationalize the experimental findings including different behavior of the molecule as a function of solvent polarity.

Reaction Dynamics of O((3)P) + Propyne: I. Primary Products, Branching Ratios, and Role of Intersystem Crossing from Crossed Molecular Beam Experiments.

We performed synergic experimental/theoretical studies on the mechanism of the O((3)P) + propyne reaction by combining crossed molecular beams experiments with mass-spectrometric detection and time-of-flight analysis at 9.2 kcal/mol collision energy (Ec) with ab initio electronic structure calculations at a high level of theory of the relevant triplet and singlet potential energy surfaces (PESs) and statistical calculations of branching ratios (BRs) taking into account intersystem crossing (ISC). In this paper (I) we report the results of the experimental investigation, while the accompanying paper (II) shows results of the theoretical investigation with comparison to experimental results. By exploiting soft electron ionization detection to suppress/mitigate the effects of the dissociative ionization of reactants, products, and background gases, product angular and velocity distributions at different charge-to-mass ratios were measured. From the laboratory data angular and translational energy distributions in the center-of-mass system were obtained for the five competing most important product channels, and product BRs were derived. The reactive interaction of O((3)P) with propyne under single collision conditions is mainly leading to the rupture of the three-carbon atom chain, with production of the radical products methylketenyl + atomic hydrogen (BR = 0.04), methyl + ketenyl (BR = 0.10), and vinyl + formyl (BR = 0.11) and the molecular products ethylidene/ethylene + carbon monoxide (BR = 0.74) and propandienal + molecular hydrogen (BR = 0.01). Because some of the products can only be formed via ISC from the entrance triplet to the low-lying singlet PES, we infer from their BRs an amount of ISC larger than 80%. This value is dramatically large when compared to the negligible ISC reported for the O((3)P) reaction with the simplest alkyne, acetylene. At the same time, it is much larger than that (∼20%) recently observed in the related reaction of the three-carbon atom alkene, O((3)P) + propene at a comparable Ec. This poses the question of how important it is to consider nonadiabatic effects and their dependence on molecular structure for this kind of combustion reactions. The prevalence of the CH3 over the H displacement channels is not explained by invoking a preference for the addition on the methyl-substituted acetylenic carbon atom, but rather it is believed to be due to the different tendencies of the two addition triplet intermediates CH3CCHO (preferentially leading to H elimination) and CH3COCH (preferentially leading to CH3 elimination) to undergo ISC to the underlying singlet PES. It is concluded that the main coproduct of the CO forming channel is singlet ethylidene ((1)CH3CH) rather than ground-state ethylene. By comparing the derived BRs with those very recently derived from kinetics studies at room temperature and 4 Torr we obtained information on how BRs vary with collision energy. The extent of ISC is estimated to remain essentially constant (∼85%) with increasing Ec from ∼1 to ∼10 kcal/mol. The present experimental results shed light on the mechanism of the title reaction at energies comparable to those involved in combustion and, when compared with the results from the statistical calculations on the ab initio coupled PESs (see accompanying paper II), lead to an in-depth understanding of the rather complex reaction mechanism of O + propyne. The overall results are expected to contribute to the development of more refined models of hydrocarbon combustion.

ChemInform Abstract: Regulation of Nonadiabatic Processes in the Photolysis of Some Carbonyl Compounds

AbstractReview: 83 refs.

Isomer-Specific Chemistry in the Propyne and Allene Reactions with Oxygen Atoms: CH3CH + CO versus CH2CH2 + CO Products.

We report direct experimental and theoretical evidence that, under single-collision conditions, the dominant product channels of the O((3)P) + propyne and O((3)P) + allene isomeric reactions lead in both cases to CO formation, but the coproducts are singlet ethylidene ((1)CH3CH) and singlet ethylene (CH2CH2), respectively. These data, which settle a long-standing issue on whether ethylidene is actually formed in the O((3)P) + propyne reaction, suggest that formation of CO + alkylidene biradicals may be a common mechanism in O((3)P) + alkyne reactions, in contrast to formation of CO + alkene molecular products in the corresponding isomeric O((3)P) + diene reactions, either in combustion or other gaseous environments. These findings are of fundamental relevance and may have implications for improved combustion models. Moreover, we predict that the so far neglected (1)CH3CH + CO channel is among the main reaction routes also when the C3H4O singlet potential energy surface is accessed from the OH + C3H3 (propargyl) entrance channel, which are radical species playing a key role in many combustion systems.

Mechanistic and kinetic study on the reaction of ozone and trans-2-chlorovinyldichloroarsine

Singlet and triplet potential energy surfaces for the atmospheric ozonation of trans-2-chlorovnyldichloroarsine (lewisite) are investigated theoretically. Optimizations of the reactants, products, intermediates and transition states are carried out at the BHandHLYP/6-311+G(d,p) level. Single point energy calculations are performed at the CCSD(T)/6-311+G(d,p) level based on the optimized structures. The detailed mechanism is presented and discussed. Various possible H (or Cl)-abstraction and C (or As)-addition/elimination pathways are considered. The results show that the As-addition/elimination is more energetically favorable than the other mechanisms. Rice–Ramsperger–Kassel–Marcus (RRKM) theory is used to compute the rate constants over the possible atmospheric temperature range of 200–3000 K and the pressure range of 10−8–109 Torr. The calculated rate constant is in good agreement with the available experimental data. The total rate coefficient shows positive temperature dependence and pressure independence. The modified three-parameter Arrhenius expressions for the total rate coefficient and individual rate coefficients are represented. Calculation results show that major product is CHClCHAs(OOO)Cl2 (s-IM3) at the temperature below 600 K and O2 + CHClCHAsOCl2 (s-P9) play an important role at the temperature between 600 and 3000 K. Time-dependent DFT (TD-DFT) calculations indicate that CHCl(OOO)CHAsCl2 (s-IM3) and CHOAsCl2 (s-P5) can take photolysis easily in the sunlight. Due to the absence of spectral information for arsenide, computational vibrational spectra of the important intermediates and products are also analyzed to provide valuable evidence for subsequent experimental identification.

The substituent effect of the pentafluorophenyl groups on ruthenium-porphyrin-catalyzed intramolecular amidation of sulfamate ester: A DFT study

The substituent effect of the pentafluorophenyl (PhF5) groups on the Ru-porphyrin complex-catalyzed intramolecular reaction was investigated with density function theory calculations. The interested reaction steps are the unsubstituted and substituted Ru-porphyrin complexes-mediated nitrene insertion into one of the CH bonds on a certain sulfamate ester. For both of the unsubstituted and substituted reactant complexes, the singlet state has a close energy to the triplet state. The substitution of the PhF5 groups leads to a lowered reaction barrier on both of the singlet and triplet potential energy surfaces (PESs). The barrier lowering corresponds to an increased reaction rate by 2 orders of magnitude on both PESs. The singlet PES has a predominant contribution to the overall reaction rate compared to the triplet one for both of the unsubstituted and substituted cases. The results from the natural bond orbital analysis show that the PhF5 groups are highly negative in charge, and the group substituting leads to an increased electron deficiency of the nitrene N atom. This increase can account for the increase of the reaction rate on both of the singlet and triplet PESs.

A comprehensive understanding of the chemical vapour deposition of cadmium chalcogenides using Cd[(C6H5)2PSSe]2 single-source precursor: a density functional theory approach

BackgroundThe phosphinato complexes of group IIB are of great interest for their potential toward technological applications. A gas phase mechanistic investigation of the chemical vapour deposition of cadmium chalcogenides from the decomposition of Cd[(C6H5)2PSSe]2, as a single source precursor is carried out and reported herein within the framework of density functional theory at the M06/LACVP* level of theory.ResultsThe results reveal that the activation barriers and the product stabilities on the singlet potential energy surface (PES) favour CdS decomposition pathways, respectively. However, on the doublet PES, the activation barriers favour CdS while the product stabilities favour CdSe decomposition pathways, respectively. Contrary to the previously reported theoretical result for Cd[(iPr)2PSSe]2, CdSe decomposition pathways were found to be the major pathways on both the singlet and the doublet PESs, respectively.ConclusionExploration of the complex gas phase mechanism and a detailed identification of the reaction intermediates enable us to understand and optimise selective growth process that occur in a chemical vapour deposition.Graphical abstractStructure of Cd[(C6H5)2PSSe]2 single-source precursor

Role of Carbon-Addition and Hydrogen-Migration Reactions in Soot Surface Growth.

Using density functional theory and master equation modeling, we have studied the kinetics of small unsaturated aliphatic molecules reacting with polycyclic aromatic hydrocarbon (PAH) molecules having a diradical character. We have found that these reactions follow the mechanism of carbon addition and hydrogen migration (CAHM) on both spin-triplet and open-shell singlet potential energy surfaces at a rate that is about ten times those of the hydrogen-abstraction-carbon-addition (HACA) reactions at 1500 K in the fuel-rich postflame region. The results also show that the most active reaction sites are in the center of the zigzag edges of the PAHs. Furthermore, the reaction products are more likely to form straight rather than branched aliphatic side chains in the case of reacting with diacetylene. The computed rate constants are also found to be independent of pressure at conditions of interest in soot formation, and the activation barriers of the CAHM reactions are linearly correlated with the diradical characters.

气相中Th活化C<sub>2</sub>H<sub>4</sub>的自旋禁阻反应机理

用密度泛函理论中的UB3LYP方法，研究了单重态和三重态势能面自旋禁阻反应Th(6d27S2) + C2H4→ThH3-CCH的微观机理。找到了两条反应通道，对其中涉及的两态反应(TSR)进行了分析。进而运用Hammond假设和Yoshizawa等的内禀坐标单点垂直激发计算的方法找出了一系列势能面交叉点[crossing points (CPs)]，并作了相应的讨论。通过计算，讨论了势能面交叉和可能的自旋翻转过程。用Harvey等的方法优化出最低能量交叉(MECP)，进一步讨论了Th与C2H4的反应中不同势能面之间的“系间窜越”(ISC)的可能性。最后，对前线分子轨道MECP2做了简单的讨论，这些理论结果可以作为进一步理论研究和实验的指导。 Using UB3LYP method in density functional theory, the micro mechanism of the singlet and triplet potential energy surfaces of the three spin forbidden reactions Th(6d27S2) + C2H4→ThH3-CCH was investigated. Two reaction channels were found, and the two state reaction (TSR) was analyzed. Then we used the method of single point vertical excitation calculation of intrinsic coordinates Hammond and Yoshizawa’s hypothesis, found a series of potential energy surface crossing point of [crossing points (CPs)], and made the corresponding discussion. By calculation, the crossover of potential energy surface and the spin flip process are discussed. Using Harvey and other methods to optimize the minimum energy crossover (MECP), the possibility of the “inter system channeling” (ISC) between different potential energy surfaces in the reaction of Th and C2H4 is further discussed. In the end, we make a brief discussion on the frontier molecular orbital MECP2, which can be used as a guide for further theoretical research and experiment.

Theoretical Study on the Atmospheric Reaction of HS with HO<sub>2</sub>: Mechanism and Rate Constants of the Major Channel

The mechanism for the biradical reaction of HS with HO2 is investigated at the CCSD(T)/6-311++ G(3df,2pd)//B3LYP/6-311+G(2df,2p) level on both the singlet and triplet potential energy surfaces, along with rate constant calculations of the major channel. The results show that there are eight reaction channels involved in the HS + HO2 reaction system. The major channel R1 of the title reaction occurs on the triplet potential energy surfaces, and includes two pathways: Path 1 (R → IM1 → TS1 → P1(O2 + H2S)) and Path 1a 701 Acta Phys. -Chim. Sin. 2016 Vol.32

Regulation of nonadiabatic processes in the photolysis of some carbonyl compounds.

Carbonyl compounds studied are confined to acetyl halide (CH3COCl), acetyl cyanide (CH3COCN), acetyl sulfide (CH3COSH), acetaldehyde (CH3CHO), and methyl formate (HCOOCH3). They are asymmetrically substituted, but do not follow the well-known Norrish type I reactions. Each compound ejected in an effusive beam at about 300 K is commonly excited to the (1)(n, π*)CO lower state; that is, a nonbonding electron on O of the C[double bond, length as m-dash]O group is promoted to the antibonding orbital of π*CO. The photolysis experiments are conducted in the presence of Ar gas and the corresponding fragments are detected using time-resolved Fourier-transform Infrared (FTIR) emission spectroscopy. The enhancement of the collision-induced internal conversion or intersystem crossing facilitates the dissociation channels via highly vibrational states of the ground singlet (So) or triplet (T1) potential energy surfaces. In this manner, an alternative nonadiabatic channel is likely to open yielding different products, even if the diabatic coupling strength is strong between the excited state and the neighboring state. For instance, the photodissociation of CH3COCl at 248 nm produces HCl, CO, and CH2 fragments, in contrast to the supersonic jet experiments showing dominance of the Cl fragment eliminated from the excited state. If the diabatic coupling strength is weak, dissociation proceeds mainly through internal conversion, such as the cases of CH3COCN and CH3COSH. The photodissociation of CH3COCN at 308 nm has never been reported before, while for CH3COSH matrix-isolated photodissociation was conducted that shows a distinct spectral feature from the current FTIR method. The CH3CHO and HCOOCH3 molecules belong to the same type of carbonyl compounds, in which the molecular products, CO + CH4 and CO + CH3OH, are produced through both transition state and roaming pathways. Their products are characterized differently between molecular beam and current FTIR experiments. For instance, the photodissociation of HCOOCH3 at 248 nm yields CO with the vibrational state v ≥ 4, in contrast to the molecular beam experiments producing CO at v = 1. The photodissociation of CH3CHO at 308 nm intensifies a low energy component in the CH4 vibrational distribution, thus verifying the transition state pathway for the first time.

Theoretical Mechanism Study on the Reaction of FOO Radical with NO

The mechanism for the reaction of radical FOO with NO was investigated theoretically. The electronic structure information for the singlet potential energy surfaces (PES) was obtained at the MP2/6-311+G(2df) level of theory, and the single-point energies were refined by the CCSD(T)/6-311+G(2df) level. The rate constants as well as the pressure and temperature dependence of various product channels for the reaction are predicted. The calculated results show that starting from the energy-rich intermediateFOONOtp(IM1), at room temperature 298 K and at 1 Torr, FNO is the exclusive product which is consistent with the experimental results. The present results will be useful to gain a deep insight into the reaction mechanism of FOO + NO reaction.

Photostability of Hydroxocobalamin: Ultrafast Excited State Dynamics and Computational Studies.

Hydroxocobalamin is a potential biocompatible source of photogenerated hydroxyl radicals localized in time and space. The photogeneration of hydroxyl radicals is studied using time-resolved spectroscopy and theoretical simulations. Radicals are only generated for wavelengths <350 nm through a mechanism that involves competition between prompt dissociation and internal conversion. Characterization of the lowest-lying singlet potential energy surface provides insight into the photochemistry of hydroxocobalamin and other cobalamin compounds.

Possible interstellar formation of phosphorus analogue of hydrazoic acid: A computational study on the reaction between HN and PN

Using computational calculations we have shown the reaction between the NH and NP. We have analysed both the singlet and triplet potential energy surfaces for the reaction and found that in both the surfaces, the reaction is exothermic in nature. Based on this, we suggest that the species like HNNP, HNPN and HPNN (Phosphorous analogues of the Hydrazoic acid) can possibly be detected in the interstellar medium. We hope these molecules may be able to act as precursors for many phosphorus containing prebiotic molecules. We have also found that many isomers have the signature NN linkage and suggest that they can be viewed as potential candidates as reservoir for the N2 in interstellar medium. Also, we have shown how unlike the N2 which has very large dissociation energy and zero dipole moment; its analogue NP shows some interesting chemistry owing to its comparatively low dissociation energy and small dipole moment.

The structure and aromaticity of 1,4 and 1,5 dihydro diazocine, dioxocine, hydro oxazocine: ab initio CCSD(T) singlet potential energy surface study

The search for a minima on the ab initio CCSD(T) singlet potential energy surface of 1,4- and the hitherto unknown 1,5-dihydro diazocine, oxazocine, and dioxocine was performed. The calculations predict planar 1H,4H-1,4-diazocine and 4H-1,4-oxazocine, and nonplanar 1,4-dioxocine as the lowest energy isomers that were in agreement with experimental observations. Energy lowest structures of 5H-1,5-oxazocine, 1H,5H-1,5-diazocine, and 1,5-dioxocine were planar. The calculations confirm the aromaticity of 1H,4H-1,4-diazocine and 4H-1,4-oxazocine, and the nonaromatic nature of 1,4-dioxocine and foresee the aromaticity of the 1,5-compounds. The 1H,5H-1,5-diazocine was thermally more stable than the 1H,4H-1,4-diazocine. In the case of dioxocines and oxazocines the 1,4-species are more stable than the 1,5-species.

Mechanisms and kinetics of reaction CHClBr•+NO2

The reaction CHClBr•+NO2 was investigated via quantum chemical methods and kinetic calculations. The reaction mechanism on the singlet potential energy surface(PES) was considered by B3LYP method, and the energies were calculated at the CCSD(T) and CASPT2 levels of theory. The rate constants and the ratios of products were obtained by utilizing VTST and RRKM methods over wide temperature and pressure ranges. Our results indicate that carbon-to-nitrogen approach via a barrierless process is preferred in the initial association of CHClBr• and NO2. The dominant product is BrNO+CHClO(P1), which agrees well with the experimental observation. P2(ClNO+CHBrO) and P3(HNO+CBrClO) may also have minor contributions to the reaction. The calculated overall rate constants are independent of pressure and consistent with the experimental data, which can be fitted with the following equation over the temperature range of 200—1500 K: k(T)=2.31×10–15 T 0.99exp(771/T). Compared with reaction CH2Br•+NO2, reaction CHClBr•+NO2