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Recently, surface‐hopping ab initio molecular dynamics (SH‐AIMD) simulations have come to be used to discuss the mechanisms and dynamics of excited‐state chemical reactions, including internal conversion and intersystem crossing. In dynamics simulations involving intersystem crossing, there are two potential energy surfaces (PESs) governing the motion of nuclei: PES in a spin‐pure state and PES in a spin‐mixed state. The former gives wrong results for molecular systems with large spin‐orbit coupling (SOC), while the latter requires a potential gradient that includes a change in SOC at each point, making the computational cost very high. In this study, we systematically investigate the extent to which the magnitude of SOC affects the results of the spin‐pure state‐based dynamics simulations for the hydride MH2 (M = Si, Ge, Sn, Pb) by performing SH‐AIMD simulations based on spin‐pure and spin‐mixed states. It is clearly shown that spin‐mixed state PESs are indispensable for the dynamics simulation of intersystem crossing in systems containing elements Sn and Pb from the fifth period onward. Furthermore, in addition to the widely used Tully's fewest switches (TFS) algorithm, the Zhu‐Nakamura (ZN) global switching algorithm, which is computationally less expensive, is applied to SH for comparison. The results from TFS‐ and ZN‐SH‐AIMD methods are in qualitative agreement, suggesting that the less expensive ZN‐SH‐AIMD can be successfully utilized to investigate the dynamics of photochemical reactions based on quantum chemical calculations. [ABSTRACT FROM AUTHOR]

3‐Carene is an important potential biofuel with properties similar to the jet‐propellant JP‐10. Its thermal decomposition and combustion behavior is to date unknown, which is essential to assess its quality as a fuel. A combined experimental and kinetic modeling study has been conducted to understand the initial decomposition of 3‐carene. The pyrolysis of 3‐carene was investigated in a jet‐stirred quartz reactor at atmospheric pressure, at temperatures varying from 650 to 1050 K, covering the complete conversion range. The decomposition of 3‐carene was observed to start around 800 K, and it is almost complete at 970 K. Online gas chromatography shows that primarily aromatics are generated which suggests that 3‐carene is not a good fuel candidate. The potential energy surface for the initial decomposition pathways determined by KinBot shows that a hydrogen elimination reaction dominates, giving primarily cara‐2,4‐diene. Next to this molecular pathway, radical pathways lead to aromatics via ring opening. The kinetic model was automatically generated with Genesys and consists of 2565 species and 9331 reactions. New quantum chemical calculations at the CBS‐QB3 level of theory were needed to calculate rate coefficients and thermodynamic properties relevant for the primary decomposition of 3‐carene. Both the conversion of 3‐carene and the yields of the primary products (ie, benzene and hydrogen gas) are well predicted with this kinetic model. Rate of production analyses shows that the dominant pathways to convert 3‐carene are hydrogen elimination reaction and radical chemistry. [ABSTRACT FROM AUTHOR]

The adiabatic mechanism of the reaction of trichloroethylene with O(3P), exploring the various O‐atom addition and H‐atom abstraction channels, is theoretically studied at the MP2/6‐311++G(2d, 2p), MP2/aug‐cc‐pVTZ, CCSD/6‐31G(d), G3, and CBS‐QB3 levels of theory. From a kinetic point of view, the addition to the less substituted carbon atom of the double bond is more favorable than the addition to the more substituted carbon. Such O‐atom addition reactions are favored over the one possible hydrogen‐abstraction reaction. Calculations of the present study showed that five products are obtained: HCCl + C(O)Cl2 (P1), Cl + ClC(O)CHCl (P2), H + ClC(O)CCl2 (P3), Cl + HC(O)CCl2 (P4), and CH(O)Cl + CCl2 (P5). The products P2 and P4 are found to be the most favored ones. The kinetic calculations of rate constant in the range of 285–395 K are performed at the CBS‐QB3 level of theory and are in conformity with the experimental outcomes. [ABSTRACT FROM AUTHOR]

A globally correct potential energy surface (PES) for the molecular ion is presented. The Born-Oppenheimer (BO) ab initio grid points of Pavanello et al. [J. Chem. Phys.136, 184303 (2012)] are refitted as BOPES75K, which reproduces the energies below dissociation with a root mean square deviation of 0.05 cm−1; points between dissociation and 75,000 cm−1 are reproduced with the average accuracy of a few wavenumbers. The new PES75K+ potential combines BOPES75K with adiabatic, relativistic and quantum electrodynamics (QED) surfaces to provide the most accurate representation of the global potential to date, overcoming the limitations on previous high accuracy PESs near and above dissociation. PES75K+ can be used to provide predictions of bound rovibrational energy levels with an accuracy of approaching 0.1 cm−1. Calculation of rovibrational energy levels within PES75K+ suggests that the non-adiabatic correction remains a limiting factor. The PES is also constructed to give the correct asymptotic limit making it suitable for use in studies of the prototypical chemical reaction. An improved dissociation energy for is derived as cm−1. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • A potential energy surface for the lowest 2A′ state of LiHO+ is constructed. • The ab initio energy points are extrapolated to the complete basis set limit. • The O + LiH+ reaction are investigated using the quasiclassical trajectory method. • The reaction mechanism ruling the O + LiH+ reaction changes with collision energy. Abstract A potential energy surface (PES) for the lowest 2A′ state of LiHO+ is constructed using ab initio calculations. The MRCI method with AVQZ and AV5Z basis sets is employed during the calculations. The artificial neural network is utilized to fit the analytical PES based on 28,552 energy points which are extrapolated to the complete basis set limit. The reaction dynamics of the O + LiH+ ⟶ Li+ + HO reaction is investigated by means of quasi-classical trajectory method. The dynamics results show that the reaction mechanism dominating the title reaction changes from indirect to direct mechanism with increasing collision energy. [ABSTRACT FROM AUTHOR]

This paper investigates the simulation of a modified Hamilton–Jacobi equation in order to provide a classical wavefront-based formulation of reaction dynamics. Here, the wavefront is interpreted as minimizing a particular action functional. The method is rooted in the geometric optics and an eikonal equation is typically used for describing the light wave propagation. We have introduced a general formulation for reaction force surface (RFS) and a host of several force-based properties, which offer a reinvigorated understanding of the occurrence of a reaction event at molecular level. The calculation is performed on a model potential energy surface representing the hydrogen transfer reaction in malonaldehyde. We have investigated the application of a generalized Hamilton-Jacobi (GHJ) equation and fast marching method (FMM) in order to deliver a new wave-based approach for studying chemical reaction dynamics. Our approach introduced several new concepts, such as reaction action surface (RAS), reaction force surface (RFS), reaction force field vectors (RFFV), reaction path force (RPF) and reaction path force constant (RPFC), which are often overlooked in chemical reaction dynamics. We have calculated these properties so as to provide an alternate force-based understanding of chemical reaction with specific example of the hydrogen transfer reaction in Malonaldehyde. [ABSTRACT FROM AUTHOR]

A novel global potential energy surface for H2S+(X 4A″) based on accurateab initiocalculations is presented. Energies are calculated at the multi-reference configuration interaction level with Davidson correction using aug-cc-pVQZ basis set plus core-polarisation high-exponent d functions. A grid of 4552 points is used for the least-square fitting procedure in the frame of a many-body expansion. The topographical features of the new potential energy surface are here discussed in detail. Such a surface is then employed for dynamic studies of the S(4S) + H2(X 1Σ+g) →SH+(X 3Σ−) + H(2S) reaction using the quasi-classical trajectory method. State specific trajectories are calculated, for both ground and ro-vibrationally excited initial states of H2(X 1Σ+g). Corrections to the zero point energy leakage of the classical calculations are also presented. Calculated reaction cross sections and rate constants are here reported and compared with available literature. [ABSTRACT FROM PUBLISHER]

A dynamics study of the collisions between the cyano radical and methane is performed at twelve collision energies, between 0.5 and 10.0 kcal mol. Using quasi-classical trajectory calculations on an analytical full-dimensional potential energy surface (PES-2017), the results were compared with the previous theoretical study, based on the EVB surface, and with the scarce experimental data. Reaction probability increases with collision energy, reasonably simulating the concave-down shape of the line-of-centres model, but in contrast with the EVB results. Both products, CH( v) and HCN( v , v , v ), present a cold rotational distribution, associated with linear transition-state structures. While the CH( v) product appears practically in its vibrational ground state, the HCN( v , v , v ) product appears vibrationally excited in all modes, especially bending mode, v , which reproduces the experimental evidence. Our results show forward-backward product scattering distribution at all collision energies (clearly low collision energies), which differ from the EVB results, where backward scattering is observed at low energies. Finally, when vibrational and rotational restrictions were included in the theoretical analysis, the experimental conditions of future velocity map imaging experiments were simulated, anticipating the dynamics behaviour of the title reaction. Product correlated dynamics properties were reported for comparison with future experiments. In these conditions, the present theoretical results acquire a predictive character and stimulate future experiments. [ABSTRACT FROM AUTHOR]

Using computational calculations, we have revisited the potential energy surface (PES) of the reaction between OH and SO2, which is believed as the rate-limiting step in the atmospheric formation of H2SO4. In this work, we report for the first time the presence of a pre-reaction hydrogen-bonded complex between OH and SO2 in the reaction PES. Based on this finding, it has been shown that the reaction can be considered as a two-step process in which the first step is the formation of the pre-reaction complex and the second step is the transformation of this complex to the product. It was observed that due to the presence of this pre-reaction complex as a potential well in the reaction PES, the barrier height got increased by around two-fold for the second step. Based on this observation, it has been proposed that the kinetics of the reaction is going to be affected. Also based on the analysis of the geometries of this pre-reaction complex and the transition state, it has been argued that the step involving the transformation of this pre-reaction complex to the product via the transition state is going to be the slowest step as this transformation involves large structural changes of the stationary points involved. We report for the first time the presence of a pre-reaction hydrogen-bonded complex between OH and SO2 in the reaction potential energy surface (PES). Due to the presence of this pre-reaction complex as a potential well, the barrier height was increased by around two-fold for the next step, i.e., the transformation of this complex to the product via the transition state. This affects the kinetics of the reaction. [ABSTRACT FROM AUTHOR]

Using the first-principles density functional theory approach, a detailed gas phase dehydrogenation analysis has been carried out for the monomeric as well as the dimeric units of lithium hydrazinidoborane (LiN 2 H 3 BH 3 ). Atoms in molecules formalism (AIM) has been employed in order to get bonding features along the desired reaction pathways. The exploration of potential energy surfaces from different structural isomers of both monomeric and dimeric hydrazinidoborane (LiHB) suggests that all the dehydrogenation reactions are endothermic in nature. Hydrogen generation from the dimeric lithium hydrazinidoborane provides an idea about the effect of neighbouring molecules on the dehydrogenation process occurring in the solid state. The lithium ion plays the central role in transferring the hydride in the dehydrogenation of both the monomeric and dimeric form of hydrazinidoborane. In summary our theoretical calculations are expected to shed light on the decomposition mechanism of alkali metal substituted hydrazine-borane type of compounds. [ABSTRACT FROM AUTHOR]

The potential energy surface (PES) is just a theoretical construct based on the Born-Oppenheimer approximation, but it underlies various phenomena, including molecular vibrations, collisional ionizations, and chemical reactions. This account describes how a new idea for global reaction route mapping (GRRM), which had seemed to be impossible for chemical systems with more than three atoms, was born and has been developed during the course of the study of the PES. GRRM has pioneered new fields of chemistry. Furthermore, techniques for GRRM are still developing, and GRRM is further extending its application to various areas of chemistry and chemical physics. [ABSTRACT FROM AUTHOR]

Rate constants for the reactions of C2H6, C2H5D and C2D6with.CCl3. for the production of CHCl3and CDCl3(k1,k2,k3andk4) were computed using variational transition state theory coupled with hybrid-meta density functional theory (MPWB1K) over the temperature range of 200–2900 K. The ground-state vibrational adiabatic potential was plotted for all channels. Small- and large-curvature tunnelling were determined to include quantum effects in the calculation of rate constants. Harmonic vibrational frequencies along the reaction path were calculated in curvilinear coordinates with scaled frequencies. Anharmonicity was included in the lowest-frequency torsion. The position of formation and dissociation of bonds was specified using the variation in harmonic vibrational frequencies along the reaction path. Representative tunnelling energy and the thermally averaged transmission probability at 298 K (P(E)exp ( − ΔE/RT)) were determined for the reactions in which tunnelling is important. The kinetic isotope effect was used to calculate the considerable contributions of tunnelling and vibration. The expressions for rate constants were determined using nonlinear least-square fitting over the temperature range of 200–2900 K. [ABSTRACT FROM AUTHOR]

Formic acid is a typical molecule that is involved in a lot important solar energy conversion processes. We perform first-principles calculations on the molecular mechanism of its photocatalytic decomposition reaction (PCD) on the anatase TiO 2 (101) surface. We find that the reaction barrier is sensitively dependent on the adsorption structure of the molecule. The one-step PCD of the monodentate formic acid has a lower barrier than that of bidentate formate. Coadsorbed water molecules can transform the formate from a bidentate to a monodentate configuration which greatly lower its decomposition barrier. Water molecule can also induce the spontaneous dissociation of the formic acid molecule. The monodentate dissociated formic acid is stabilized by the hydrogen bonds which will slightly enhance the barrier for its photodecomposition. However, the reaction rate can be further enhanced if the hydrogens are removed (for example, by oxygen molecules). Therefore, using coadsorbate and deliberately introducing and removing hydrogen bonds can be two strategies to tailor the photoreaction rate of the molecules. [ABSTRACT FROM AUTHOR]

The title reaction has a great deal of intriguing characters and provides energy for a chemical laser, and of much theoretical and experimental interest. Accurate description of the F + H2 → HF + H and F + HD → HF/DF + D/H reaction within the Born-Oppenheimer approximation presents a great challenge to the capabilities of modern quantum chemistry methods. The FHH system, which has strongly coupled eleven electrons, requires elaborate quantum chemistry calculations in the determination of potential energies with high accuracy. Moreover, a quantum reaction dynamics calculation for the F + H2/HD reaction requires much computational effort due to its large exothermicity. After more than half a century's endeavor, now we are capable of describing the F + H2/HD reaction close to spectroscopic accuracy. In this article, a brief review of the development of the potential energy surfaces and quantum reaction dynamics of the title reaction and its isotope substitutes is presented. Some of the results by quantum reactive scattering theory using the latest several typical potential energy surfaces are given, along with the corresponding experimental measurements, to illustrate the current stage of our capability on describing the F + H2/HD reaction. © 2015 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Much progress has been achieved in the developments of accurate and efficient algorithms for quantum dynamics calculations in the past two decades. Based on the reliable potential energy surfaces (PESs) constructed with state-of-the-art ab initio calculations, quantum reactive scattering calculations for the atom-diatom collisions can be performed accurately at the state-to-state level. In this review, we described the general strategies of constructing a PES and the Chebyshev real wave packet method briefly, and summarized our recent studies on the PESs and state-to-state reaction dynamics for the reactions of the atom N with radicals OH, CH, and C2. Such barrierless complex-forming reactions are very important for understanding low-temperature reaction dynamics and still difficult to treat accurately. © 2014 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

In the present study, the density functional method with recently developed M06 functionals has been used to study the reaction of CF3OCH2CH3 with the OH radical. All possible hydrogen abstraction and displacement reaction channels have been modeled. The minimum energy path on the respective potential energy surface and energetics were calculated at the M06-2X/6-311++G(d,p) level of theory. Two different reaction mechanisms were considered: ( i) reactant and product complexes called the complex mechanism and ( ii) the direct mechanism (reactant → transition state → product). Tunneling corrections were made using the Eckart unsymmetrical potential. The overall rate constant calculated by the complex mechanism ( keff = 1.8 × 10−13 cm3 molecule−1 s−1) has been found to be in good agreement with the experimentally determined value (1.5 ± 0.25 × 10−13 cm3 molecule−1 s−1), while the rate constant calculated by the direct mechanism ( kD = 7.6 × 10−14 cm3 molecule−1 s−1) is about two times lower than the experimental value. The theoretical studies show that hydrogen atom abstraction from the -CH2- site is the most favorable reaction pathway and the reaction involves prereactive and product complexes before leading to stable product formation. [ABSTRACT FROM AUTHOR]

The intrinsic reaction coordinate (IRC) approach has been used extensively in quantum chemical analysis and prediction of the mechanism of chemical reactions. The IRC gives a unique connection from a given transition structure to local minima of the reactant and product sides. This allows for easy understanding of complicated multistep mechanisms as a set of simple elementary reaction steps. In this article, three topics concerning the IRC approach are discussed. In the first topic, the first ab initio study of the IRC and a recent development of an IRC calculation algorithm for enzyme reactions are introduced. In the second topic, cases are presented in which dynamical trajectories bifurcate and corresponding IRC connections can be inaccurate. In the third topic, a recent development of an automated reaction path search method and its application to systematic construction of IRC networks are described. Finally, combining these three topics, future perspectives are discussed. © 2014 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Two applications of the perimetric coordinates to triatomic potential energy surfaces are explored. The first is the depiction of level surfaces, providing a global visualization of the potential energy surface. In this representation, the relation among the stationary points and the dissociation channels are easily seen. These level surfaces are shown to be the natural generalization of the well-known two-dimensional level curves of restricted potential energy surfaces; they also allow us to understand the relation of the two-dimensional graphs with the global potential energy surface. Case studies for [Cl, H], [H, O, Cl] and [O] potential energy surfaces are discussed in detail. The second application of the perimetric coordinates is the construction of global potential energy surfaces by means of cubic splines, thus avoiding the restrictions related to the triangular inequality. The procedure is straightforward, and a careful analysis of the influence of the interpolation knots on the quality of the potential energy surface was carried out. The potential energy surfaces constructed by such procedure were able to reproduce known dynamical features of the following chemical reactions: Cl + H → H + HCl, F + HD → HF(DH) + D(H) and O(D) + HCl → Cl + OH (H + OCl). [ABSTRACT FROM AUTHOR]

The lowest three adiabatic potential energy surfaces (1¹ A′, 1¹ A″ and 1³A″) of the HNS molecular system have been developed using ab initio MRCI+Q-level electronic structure calculations with a complete-basis-set approach in order to understand the NS production mechanisms from the S(³P)+NH(X³Σ)→NS(X²Π)+H(²S) and N(4S)+SH(X²Π)→NS(X²Π)+H(²S) reactions. The results of time-independent quantum reactive scattering calculations show that the reactions proceed with the contribution of both direct and HNS/HSN complex-forming mechanisms on all the three potential energy surfaces. It is found that the reaction dynamics is not entirely statistical and cannot be described with a simple capture theory despite that the reactions are barrierless with large exoergicity. [ABSTRACT FROM AUTHOR]

Reaction of chlorine (Cl) radical with heterocyclic saturated ether, tetrahydrofuran has been studied. The detailed reactivity and mechanism of this reaction is analyzed using hybrid density functional theory (DFT), B3LYP and BB1K methods, and aug-cc-pVTZ basis set. To explore the mechanism of the reaction of tetrahydrofuran with Cl radical, four possible sites of hydrogen atom (H) abstraction pathways in tetrahydrofuran were analyzed. The barrier height and rate constants are calculated for the four H-abstraction channels. The BB1K calculated rate constant for α-axial H-abstraction is comparable with the experimentally determined rate constant. It reflects that α-axial H-abstraction is the main degradation pathway of tetrahydrofuran with Cl radical. DFT-based reactivity descriptors are also calculated and these values describe α-axial H-abstraction as the main reaction channel. [ABSTRACT FROM AUTHOR]

Application of Hamilton–Jacobi (HJ) equation to reaction systems which involve energy barrier(s) leading to the product is relatively new. Such problems are described by a new class of HJ equation, called the generalised HJ equation. This new HJ equation renders an anisotropic propagation for the wave front. In this paper, we describe the adaptation of the fast marching method (FMM) and the generalised HJ equation to understand a new class of reaction process where the energy barrier does not lead to the product; instead, a new class of states are detected along the reaction path of such reactions. These states are valley-ridge inflection point, branching point and potential energy ridge. Such reactions are characterised as bifurcation reactions. We have identified a new classical wave front, called the reaction action front (RAF) which distinctly separates the reaction system into a reactant zone and a product zone connected by a third zone, called ‘neck’. The RAF is an important tool to understand the bifurcation reaction and the associated reaction paths. We have also introduced a convenient way to compute the reaction path force (RPF) using the FMM. The RPF for a bifurcation reaction significantly differs from the reactions with energy barrier, and so, the RPF provides vital information about the occurrence of branching of a path. The method has been tested for the isomerisation reaction of methoxy radical (H3C) to hydroxymethylene radical (H2ĊOH). [ABSTRACT FROM AUTHOR]

Highlights: [•] Reactions of CF3C(O)OCH2CF3 with OH radicals and Cl atoms are investigated. [•] Two conformers relatively close in energy have been identified. [•] Reaction profiles are modeled including the formation of pre-reactive and post-reactive complexes. [•] Calculated rate constant values are in good agreement with the experimental results. [•] Bond dissociation energy and are also reported. [Copyright &y& Elsevier]

A new six-dimensional (6D) global potential energy surface (PES) is proposed for the full range description of the interaction of the [ABSTRACT FROM AUTHOR]

Highlights: [•] High level quantum chemical calculations are performed for two SN2 reactions. [•] The calculated gradient reaction pathways for reactions have an unusual behavior. [•] An unusual saddle point of index two lies on the gradient reaction path. [•] VRI points have been detected by using Newton trajectories for the reaction path. [•] An infinite flow of gradient lines emanates at three equivalent product minima. [ABSTRACT FROM AUTHOR]

The complex triplet potential energy surface for the reaction of the triplet oxygen atom O(P) with cis-2-butene is investigated at the CBS-QB3 level of theory. The different possible isomerization and dissociation pathways, including both O-additions and H-abstractions, are thoroughly studied. Our calculations show that as found for the trans-2-butene reaction, in the high-pressure limit, the major product is CHCHC(O)H + CH (P1), whereas in the low-pressure limit the most thermodynamically stable product forms CHCO + CHCH (P4). The experimental negative activation energy reported for the addition step is very well reproduced at the CBS-QB3 level of theory. Various thermodynamic and kinetic values of interest for these reactions are predicted for the first time. A discussion on the negative activation energy for the addition step of the trans- and cis-2-butene reactions with O(P) focussing on the addition reactant complexes is presented. [ABSTRACT FROM AUTHOR]

We introduce a new tool for reaction pathway analysis based on a chemical bonding density descriptor obtained from the higher resolution Poincaré–Hopf relation. To remove the degeneracy of the Poincaré–Hopf relationship for critical points of the electron density, we introduce the concepts of bonding cardinality and characterise the topology of rings and cages. This allows us to relate the changes in the relative energetic stability along an isomerisation reaction pathway with changes in the topology of the electron density. To demonstrate the utility of this approach, we consider the complete circuit of the reaction pathway connecting the various Möbius and Hückel structures in Au and/or Pd bis-metallated octaphyrins. Where the topological feature ‘cages’ exist in adjacent topologies, the degeneracy in the Poincaré–Hopf relationship is completely removed even for differences in relative energies of only 0.1 kcal/mol. Metallation of the Möbius and Hückel structures was found to increase the topological complexity and bonding density. Combining Pd and Au nuclei in the octaphyrin complexes reduced the smoothness of the potential energy surface. The presence of long, weak C–H contacts correlates in all cases with the lowest energy enantiomers and the most negative nuclear-independent chemical shift values of the reaction pathways in Au and/or Pd bis-metallated octaphyrins. [ABSTRACT FROM AUTHOR]

Density functional theory (DFT) with relativistic corrections of zero-order regular approximation (ZORA) has been applied to explore the reaction mechanisms of ethane dehydrogenation by Zr atom with triplet and singlet spin-states. Among the complicated minimum energy reaction path, the available states involves three transition states ( TS), and four stationary states ( 1) to ( 4) and one intersystem crossing with spin-flip (marked by ⇒): Zr + CH → Zr-CH-CH ( 1) → TS → ZrH-CH-CH ( 2) → TS ⟹ ZrH-CH = CH ( 3) → TS → ZrH-CH = CH ( 4). The minimum energy crossing point is determined with the help of the DFT fractional-occupation-number (FON) approach. The spin inversion leads the reaction pathway transferring from the triplet potential energy surface (PES) to the singlet's accompanying with the activation of the second C-H bond. The overall reaction is calculated to be exothermic by about 231 kJ mol. Frequency and NBO analysis are also applied to confirm with the experimental observed data. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

The complex potential energy surface for the reaction of triplet oxygen atom, O(P), with trans-2-butene is investigated at the MP2, PMP2, CCSD(T), CBS-4M and CBS-QB3 levels of theory. Both the O-addition and the two H-abstraction channels are thoroughly investigated. As expected, the calculations show that the O-addition is the most favourable process from a kinetic point of view. Various isomerization and dissociation pathways are explored following the addition of O(P) to trans-2-butene. Our calculations show that in the high-pressure limit, the major products are CHCHC(O)H + CH (P1); whereas in the low-pressure limit the most thermodynamically stable products form: CHCO + CHCH (P4). A more realistic prediction of reaction products is an intermediate case, being P1 more abundant followed by P4, in agreement with experimental data. The CBS-QB3 Gibbs free energies for the O-addition are smaller than those obtained at the CBS-4M level of theory, but the same general trends are obtained by these methods. The reaction and activation enthalpies, calculated at the CBS-QB3 level of theory, are in very good agreement with the experimental results available. [ABSTRACT FROM AUTHOR]

The Valley-Ridge inflection (VRI) points are related to the branching of a reaction valley or reaction channel. These points are a special class of points of the potential energy surface (PES). They are also special points of the Valley-Ridge borderline of the PES. The nature of the VRI points and their differences with respect to the other points of the Valley-Ridge borderline is analyzed using the Löwdin's partitioning technique applied to the eigenvalue equation of the Hessian matrix. Eigenvalues and eigenvectors of the Hessian are better imaginable than the former used adjoint matrix. [ABSTRACT FROM AUTHOR]

Abstract: Nitrate radical (NO3) is reported to be one of the initial decomposition products of RDX, while it has never been detected in experiment studies. This study for the first time theoretically investigated the reactions of H X CN (X =1,2) with NO3 by computational chemistry methods. The calculated results demonstrate that the barrier height of H abstraction reaction of H2CN with NO3 is close to 0kcalmol−1, implying that the reaction may be responsible for the disappearance of NO3 during the decomposition process of RDX. The study encourages further experimental investigation in future. [Copyright &y& Elsevier]

The potential energy surface (PES) of a chemical reaction contains Valley-Ridge inflection points (VRI) if-an often occurring phenomenon-the reaction path branches. In this paper, we introduce a new direct search method to detect these VRI points. It is based on an inductive execution of Gauss-Newton steps. For the first time, we were able to find not only singular nonsymmetric VRI points, but also whole VRI manifolds on the PES of HCN. [ABSTRACT FROM AUTHOR]

The direct aldol reaction between acetone and 4-nitrobenzaldehyde catalyzed by sterically hindered l-prolinamide derivative (64 atoms) of the (11 S,12 S)-9,10-dihydro-9,10-ethanoanthracene-11,12-diamine molecule has been investigated using density functional theory at the B3LYP/6-31G(d) level of theory. The reliability of the B3LYP/6-31G(d) calculations to elucidate the reaction mechanism and estimate activation and reaction energies was confirmed by energy calculation of the net reaction with full geometry optimizations of the reactants and product at the B3LYP/6-311++G(2d,2p) as well as B2PLYP/def2-TZVPP levels with correction to Van der Waals interaction. The calculations reveal that the l-prolinamide derivative catalyzes the reaction according to a multistep enamine mechanism with highly activated C-C bond and/or enamine formation in the proposed mechanism. The final elementary reaction - the C-N bond cleavage in the chiral diol adduct - is accompanied by a very large barrier, which may inhibit further progression of the reaction. The origin of the enantioselectivity and the corresponding reaction paths to a chiral product were unambiguously identified. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Multireference single and double configuration interaction (MRCI) calculations including Davidson (+Q) or Pople (+P) corrections have been conducted in this work for the reactants, products, and extrema of the doublet ground state potential energy surface involved in the N( 2D) + CH4 reaction. Such highly correlated ab initio calculations are then compared with previous PMP4, CCSD(T), W1, and DFT/B3LYP studies. Large relative differences are observed in particular for the transition state in the entrance channel resolving the disagreement between previous ab initio calculations. We confirm the existence of a small but positive potential barrier (3.86 ± 0.84 kJ mol−1 (MR-AQCC) and 3.89 kJ mol−1 (MRCI+P)) in the entrance channel of the title reaction. The correlation is seen to change significantly the energetic position of the two minima and five saddle points of this system together with the dissociation channels but not their relative order. The influence of the electronic correlation into the energetic of the system is clearly demonstrated by the thermal rate constant evaluation and it temperature dependance by means of the transition state theory. Indeed, only MRCI values are able to reproduce the experimental rate constant of the title reaction and its behavior with temperature. Similarly, product branching ratios, evaluated by means of unimolecular RRKM theory, confirm the NH production of Umemoto et al., whereas previous works based on less accurate ab initio calculations failed. We confirm the previous findings that the N( 2D) + CH4 reaction proceeds via an insertion-dissociation mechanism and that the dominant product channels are CH2NH + H and CH3 + NH. © 2012 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Abstract: Based on the ground state HN2(12A′) potential energy surface (PES), the effects of collision energy and the rotational excitation of the reagent on the product alignment and the orientation for the title reaction are investigated by using quasi-classical trajectory (QCT) method. The distribution of polar angle P(θ r ) at collision energies 2–40kcal/mol with v = 0 and j = 0 level has been presented. The three angle distribution functions P(θ r ), P(), P(θ r , ) and four generalized polarization dependent differential cross sections (PDDCSs) at collision energy of 10kcal/mol with different rotational excitation are plotted. The results show that the product rotational angular momentum j′ is not only aligned, but also oriented along the direction perpendicular to the scattering plane. It has been found that the three angle distributions are sensitive to initially rotational excitation. The HHL mass combination and the property of the PES are used to explain the found results. [Copyright &y& Elsevier]

The Onsager reaction field model was applied to 1-hydroxy-9,10-anthraquinone derivatives dissolved in binary solvent mixtures of methanol/water. Each solution was placed in a cavity surrounded by a continuous dielectric constant medium, and then the quantum chemical descriptors of the solute were calculated. Multiple linear regression was used to perform the QSPR model calculations in order to predict the acidity constants of the solutes. The resultant hexa-parametric model was able to predict the acidity constants within absolute average deviations less than 1.82 %. The proposed model suggests that the acidic property of the solute is controlled by solvation processes through coulombic interactions as well as orbital energies. It was found that the solvent lowered the excited orbital energy states of all 1-hydroxy-9,10-anthraquinone derivatives. The model proposes that an effective solvation cavity needs more methanol molecules than water in order to increase the solvation energy in the solution. [ABSTRACT FROM AUTHOR]

Transition state theory was introduced in the 1930s to account for chemical reactions. Central to this theory is the idea of a potential energy surface (PES). It was assumed that quantum mechanical computation, when it became possible, would yield such surfaces, but for the time being they would have to be constructed empirically. The approach was very successful. Nowadays, quantum mechanical ab initio electronic structure calculations are possible and from their results PESs can be constructed. Such surfaces are now widely used in the explanation of chemical reactions in place of the traditional empirical ones. It is argued here that theoretical basis of such PESs is not quite as clear as is usually assumed and that, from a quantum mechanical perspective, certain puzzles remain. [ABSTRACT FROM AUTHOR]

An in-depth theoretical study is carried out at the B3LYP/6-311G(d,p), M062X/aug- cc-pVDZ and CCSD(T)/6-311++G(3df,2dp) (single-point) levels as an attempt to explore the mechanism of the little-understood ion-molecule reaction between NH+ and CH2O. Various possible reaction pathways are taken into account. It is shown that six dissociation products, including P 1 (2N + CH2OH+), P 2 (4N + CH2OH+), P 3 (3NH + CH2O+), P 4 (NH2 + HCO+), P 5 (NH [ABSTRACT FROM AUTHOR]

The potential energy surface for the reaction of a typical molecular system composed of N atoms is defined uniquely by 3 N-6 coordinates. These coordinates can be defined by the Cartesian coordinates of the atomic centers (minus overall translation and rotation), or a set of internally defined coordinates such as bond stretches, angle bends, and torsions. By applying principal component analysis to the geometries along a reaction path, a reduced set of coordinates, d ≪ 3 N-6, can be obtained. This reduced set of coordinates can reproduce the changes in geometry along the reaction path with chemical accuracy and may help improve the efficiency of reaction path optimization algorithms. [ABSTRACT FROM AUTHOR]

To identify the transition state accurately and efficiently on a high-dimensional potential energy surface is one of the most important topics in kinetic studies on chemical reactions. We present here an algorithm to search the transition state by so-called force reversed method, which only requires a rough reaction direction instead of knowing the initial state and final state. Compared to the nudged elastic band method and the dimer method that require multiple images, the present algorithm with only single image required saves significantly the computational cost. The algorithm was implemented in the first-principle periodic total energy calculation package and applied successfully to several prototype surface processes such as the adsorbate diffusion and dissociation on metal surfaces. The results indicate that the force reversed method is efficient, robust to identify the transition state of various surface processes. [ABSTRACT FROM AUTHOR]

The stereodynamics of the title reaction on the ab initio1A′ potential energy surface (PES) (B. Bussery-Honvault, P. Honvault, and J.-M. Launay. 2001. J. Chem. Phys. 115: 10701) at a collision energy of 16 kJ/mol have been studied using quasi-classical trajectory (QCT) method. Vector properties including angular momentum alignment parameters and four polarization-dependent differential cross sections (PDDCS) of the product CH are presented. Furthermore, the influence of reagent vibrational and rotational excitations on the product vector properties have also been studied in the present work. The calculated results indicate that the angle distributions of the CH product are mainly dominated by backward-forward scattering. [ABSTRACT FROM AUTHOR]

Density functional theory calculations have been performed to explore the potential energy surfaces of CO bond activation in CO2 molecule by gas‐phase Nb atom and Nb+ cation for better understanding the reaction mechanism of second‐row metal with CO2. The minimum‐energy reaction path is found to involve the spin inversion in the different reaction steps. This potential energy curve‐crossing dramatically affects the reaction energetic. The present results show that the mechanism is insertion‐elimination mechanism along the CO bond activation reaction. All theoretical results not only support the existing conclusions inferred from early experiment but also complement the pathway and mechanism for this reaction. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011 [ABSTRACT FROM AUTHOR]

The CH3OH with NH(3Σ-) and NH2(2B1) reactions are key processes in methanol combustion. Optimized geometries and frequencies have been computed at UMP2 and UB3lYP levels. Energy values are improved using UQCISD(T), G3MP2, and BMC-CCSD methods using UMP2/6-311+G(d,p) optimized structures. For the two reactions, hydrogen abstraction and SN2 substitution mechanisms have been investigated. Due to high barriers, the SN2 substitution pathways play negligible roles. Methyl hydrogen abstraction channels are preferred to that of hydroxyl hydrogen, and the final products of NH3 with CH2OH are major. [ABSTRACT FROM AUTHOR]

The stereodynamics of the title reaction based on the ground 2A′ potential energy surface (PES) has been investigated using the method of the quasi-classical trajectory (QCT) at different collision energies (23 kcal/mol, 35 kcal/mol and 46 kcal/mol). The vector properties of the angular momentum (described by the distribution of K - J′P(θr), the dihedral angle distribution of K - K′ - J′P(φr) and the angular distribution P(θr, ϕr)) and the four PDDCSs [(2π/σ)(dσ00/dωt), (2π/σ)(dσ20/dωt), (2π/σ)(dσ22+/dωt), (2π/σ)(dσ21-/dωt)] of the product LiF at each collision energy have been presented, respectively. Further, the collision energy effects on the behavior of the product LiF have been discussed and studied. [ABSTRACT FROM AUTHOR]

Abstract: The structure, energies, electronic spectra and thermal trans–cis isomerization of azobenzene (AB) attached non-covalently to the sidewall of CNT(8,8) were calculated using the ONIOM(B3LYP/6–31 + G*:UFF) level of theory. We found that some subtle differences existed between the structures of the attached AB and the isolated one. The combination of AB and CNT(8,8) favored in energy because of their negative interaction energies. The attached trans-AB (TAB) was more stable than the physisorbed cis-AB (CAB). The electronic spectrum calculations showed that the absorption wavelengths of the attached CAB were red-shifted for the lowest three singlet states. The trans–cis isomerization process of the AB attached non-covalently to the CNT(8,8) involved mainly the bending of CNN angle. The π–π interaction between AB and CNT(8,8) was found to restrain the trans-to-cis isomerization of AB, but had little influence on the cis-to-trans backward reaction. [ABSTRACT FROM AUTHOR]

Density functional theory (DFT) calculations have been performed to explore the potential energy surfaces of C-O bond activation in CO molecule by gas-phase Mocation and Mo atom, in order to better understanding the mechanism of second-row metal reacting with CO. The minimum energy reaction path is found to involve the spin inversion in the different reaction steps. This potential energy curve-crossing dramatically affects reaction exothermic. The present results show that the reaction mechanism is insertion-elimination mechanism along the C-O bond activation branch. All the theoretical results not only support the existing conclusions inferred from early experiment, but also complement the pathway and mechanism for this reaction. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

The potential energy surfaces (PESs) were investigated for di-π-methane rearrangement of 4-phenyl-4H-pyran at CASSCF(12,12)/6-31G(d) and multireference second-order perturbation theory. The three-step mechanism proposed by Zimmerman was confirmed for the reaction from the excited triplet state. The minimum energy path indicated that the initial σ-bond formation takes place from the 3(π,π*) state localized at the C2ᆖC3 olefinic moiety to give a primary phenyl-bridged biradical (3BR1). This biradical is immediately converted to a 1,3-biradical (3BR2). The intersystem crossing to 1BR2 can take place in the vicinity of the minimum energy crossing point due to the spin-orbit coupling interaction. 1BR2 is rather different from 3BR2 in geometrical and electronic structures and is regarded as a biradicaloid with zwitterionic character. The subsequent ring-closure of 1BR2 is almost barrierless and yields the expected product, 6-endo-phenyl-2-oxabicyclo[3.1.0]hexene. These results provide deeper insight into the previous experimental observations regarding the photochemical reactions of related compounds. [Copyright &y& Elsevier]

Based on a previous potential energy surface describing the H + CCl reaction, a new analytical surface named PES-2010 was developed modifying both the functional form to give it more flexibility, and the calibration process in which exclusively theoretical information was used. Thus, the surface is completely symmetric with respect to the permutation of the four methane chlorine atoms, and no experimental information is used in the process. For the kinetics, the thermal rate constants were calculated using variational transition-state theory with semiclassical transmission coefficients over a wide temperature range, 300-2,500 K. The theoretical results reproduce the experimental variation with temperature. The influence of the tunneling factor is small, since the abstraction reaction involves the motion of a heavy particle (a chlorine atom) that cannot easily tunnel through the reaction barrier. The coupling between the reaction coordinate and the vibrational modes shows qualitatively that the HCl stretching mode in the products appears vibrationally excited. The dynamics study was performed using quasi-classical trajectory calculations, including corrections to avoid the zero-point energy problem. First, we found that the HCl( ν′, j′) product mostly appears with small rotational energy and vibrational population inversion. Second, the state-specific scattering distributions show backward scattering, which becomes more noticeable as the HCl( ν′) vibrational state increases. Unfortunately, no experimental dynamics data are available for the title reaction, but the comparison with the kinematically similar and well-studied H + Cl reaction shows good agreement, indicative of similar mechanisms. These kinetics and dynamics results seem to indicate that the potential energy surface is adequate to describe this reaction, and the reasonable agreement with experiment lends further confidence to this new surface. [ABSTRACT FROM AUTHOR]

The intermediates [Si,O,C,O] of the Si + CO reaction have been studied in detail using high level ab iniitio methods. Both singlet and triplet [Si,O,C,O] species are characterized structurally and energetically. On the singlet potential energy surface (PES), the vdw-OSi-CO isomer and in the triplet PES, the bent-SiOCO isomer is found to be thermodynamically as well as kinetically most stable species. All possible isomerization transition states (TS) are located on both singlet and triplet potential surfaces. On the triplet surface, the stability of the trans-OSiCO isomer is comparable with that of the bent-SiOCO isomer. A non-planar cis-SiOCO isomer is located on the triplet PES, which is predicted for the first time. Heats of formation at 0 K (Δ H°, 0 K) for all singlet and triplet species are computed using G3B3, G3MP2, and CBS-Q theories. The discrepancy between G3B3 and the other two methods for the heat of formation value for triplet trans-OSiCO is discussed. The PESs for singlet as well as triplet species with their dissociation asymptotes are explored at the CCSD(T)/6-311G(d,p)//MP2/6-311G(d,p) level of theory. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2011 [ABSTRACT FROM AUTHOR]

The mechanisms for the reaction of CHSSCH with OH radical are investigated at the QCISD(T)/6-311++G(d,p)//B3LYP/6-311++G(d,p) level of theory. Five channels have been obtained and six transition state structures have been located for the title reaction. The initial association between CHSSCH and OH, which forms two low-energy adducts named as CHS(OH)SCH (IM1 and IM2), is confirmed to be a barrierless process, The SS bond rupture and HS bond formation of IM1 lead to the products P1(CHSH + CHSO) with a barrier height of 40.00 kJ mol. The reaction energy of Path 1 is −74.04 kJ mol. P1 is the most abundant in view of both thermodynamics and dynamics. In addition, IMs can lead to the products P2 (CHS + CHSOH), P3 (HO + CHS + CHS), P4 (CH + CHSSOH), and P5 (CH + CHSSO) by addition-elimination or hydrogen abstraction mechanism. All products are thermodynamically favorable except for P4 (CH + CHSSOH). The reaction energies of Path 2, Path 3, Path 4, and Path 5 are −28.42, −46.90, 28.03, and −89.47 kJ mol, respectively. Path 5 is the least favorable channel despite its largest exothermicity (−89.47 kJ mol) because this process must undergo two barriers of TS5 (109.0 kJ mol) and TS6 (25.49 kJ mol). Hopefully, the results presented in this study may provide helpful information on deep insight into the reaction mechanism. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2011 [ABSTRACT FROM AUTHOR]

Abstract: A multi-coordinate expansion of potential energy surfaces has been used to perform quantum dynamical calculations for reactions showing double-minimum potentials. Starting from the transition state, a fully automated algorithm for exploring the multi-dimensional potential energy surface represented by arbitrary internal or normal coordinates allows for an accurate description of the relevant regions for vibrational dynamics calculations. An interface to our multi-purpose quantum-dynamics program MrPropa enables routine calculations for simple chemical reactions. Illustrative calculations involving potential energy surfaces obtained from explicitly-correlated coupled-cluster calculations, CCSD(T)-F12a, are provided for the tunneling splittings in the isotopologues of hydrogen peroxide and for reaction dynamics based on the enantiomeric inversion of PHDCl. [Copyright &y& Elsevier]