Energy Surface For Reaction Research Articles

A Crossed Molecular Beam Study of the Reaction O(1D) + HI → IO + H

The angular and velocity distributions of the IO product from the reaction O(1D) + HI have been obtained in crossed beam experiments at collision energies, Ec, of 4.7 and 13.6 kcal/mol. The product center-of-mass angular distribution is found to be nearly backward−forward symmetric, with forward scattering favored. The forward to backward scattering ratio increases with increasing Ec, from which it is deduced that the reaction occurs via an intermediate complex having a lifetime comparable to its rotational period. Within the “osculating” model for chemical reactions, the complex lifetime is estimated to be 4 and 1 ps at low and high Ec, respectively. A large fraction (0.55 at low Ec and 0.46 at high Ec) of the total available energy was found to be released into product translational energy. Comparison with the dynamics of the analogous reactions O(1D) + HCl and O(1D) + HBr is carried out: similarities as well as differences are noted and discussed in terms of the potential energy surfaces for reaction. From comparison of signal intensities and the previously estimated lower limits to the branching ratio of the relative cross sections for XO + H and OH + X channels in the O(1D) + HX (X = Cl, Br) reactions, it is concluded that the H-displacement pathway accounts for a significant fraction of the overall reaction also in the case of O(1D) + HI.

Potential Energy Surfaces for Chemical Reactions: An Analytical Representation from Coarse Grained Data with an Application to Proton Transfer in Water

In this paper we address the issue of how to represent the potential energy surfaces that arise in chemical reactions from coarse grained electronic structure calculations. Using a reductionistic method based on the reaction surface model, we develop a computational protocol which combines tensor product spline fitting with bivariate interpolation. This approach is particularly useful when one wishes to retain a high degree of accuracy for a few special degrees of freedom. An application of the procedure has been developed for the transfer of a proton between two water molecules. Starting from MP2/6-311G** calculations on H5O2+ dimer, we construct the global potential energy surface governing the proton transfer as well as the pattern of charge distributions. In order to study large reactive systems embedded in an external medium, we show how a less demanding procedure can be implemented. It rests on a minimum coupling approach of second-order Taylor expansions of the potential about quasi-stationary poin...

Systematic Study of the Potential Energy Surface for the Base-Induced Elimination Reaction of Fluoride Ion with Ethyl Fluoride Using Density Functional Theory

The potential energy surface for the reaction between fluoride ion and ethyl fluoride was explored using density functional theory and a high-level ab initio procedure (G2+). A wide variety of functionals and basis sets were systematically examined. Four reactions with known thermochemistry were used as benchmarks, and while the experimental results could be reproduced, no single functional or class of functionals consistently outperformed the others. Transition structures and an ion−molecule complex, F-··CH3CH2F, are discussed. The DFT barriers are consistently smaller and looser than those obtained with high-level ab initio calculations. Moreover, the anti elimination transition state is unusually sensitive to the choice of functional and basis set; in many cases the transition structure does not appear to exist on the potential energy surface. The origin of this problem may reside in the large charge inhomogeneities characteristic of transition states and/or an inadequate description of the exchange interaction. This shortcoming, clearly, demands a great deal more attention.

The role of solute - solvent interactions in the dynamics of unimolecular reactions in compressed solvents

The role of the solvent in the liquid phase dynamics of unimolecular processes involving small energy barriers is studied by ultrafast laser spectroscopy. Different regions of the interaction potential between the solvent environment and the reactant are characterized by investigating photoisomerization reactions of trans-stilbene and related compounds in liquid n-alkanes and n-alkanols over a wide pressure range. By varying the physical properties of the solvent environment in such a controlled way, aspects of the reactant - solvent interaction which dominate the different density regimes are identified. In particular, the kind of friction acting along the reaction path, the role of solvent-induced changes of the effective potential energy surface for reaction, and the coupling of dielectric solvent shell relaxation and motion along the reaction coordinate are addressed.

ChemInform Abstract: Potential Energy Surfaces for Chemical Reactions at Solid Surfaces

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Solvent and nuclear dynamic effects on intermolecular electron transfer

An ultrafast intermolecular electron transfer (ET) from electron donating solvent to an excited dye molecule was found. A temperature-dependent non-exponential time dependence was observed in aniline, and a temperature-independent single exponential process for Nile blue (160 fs) and oxazine 1 (260 fs) was observed in N,N-dimethylaniline. The solvation times of solvent anilines were obtained by dynamic Stokes shift measurements. The rate of ET in some systems was observed to be much greater than the solvation time of anilines. The dynamic behavior was simulated by the 2-dimensional potential energy surface for reaction, taking into account of the effects of both solvent reorientation and nuclear motion of reactants.

Potential Energy Surfaces for Chemical Reactions at Solid Surfaces

Many-body potential energy surfaces (PESs) for describing atomic interactions in gas-solid and surface reaction dynamics are reviewed in this work. Initial PESs from the 1960s-1970s were restricted to a diatomic molecule interacting with a solid surface. Since the 1980s, a multitude of many-body reactive PESs, their parameterization, and their applications have been reported in the literature. Although we mention most of the PESs in general, we have chosen to describe only those that either have had general utility or have had staying power, i.e. they have been used widely by other research groups. The potentials discussed in the most detail are the Stillinger-Weber and Tersoff Si PESs, the Brenner hydrocarbon PES, and the embedded-atom method (EAM) style potentials for metals. We conclude that although these PESs have been used successfully in large-scale computer simulations, further development is needed in many-body PESs. In particular, the development of new functional forms for multicomponent reactive systems is required.

Reactions of cyanogen radical with alkanes and an explanation for negative temperature dependence of rate constants

Reactions of cyanide radicals with alkanes have been investigated by ab initio methods. It is found that the potential energy surface for reaction of ĊN with a primary CH bond in methane has a small positive barrier while reactions of ĊN with a secondary and a tertiary CH bond in alkanes are barrierless at the correlated level. A simple explanation for the obtained negative temperature dependence of rate constants for reactions of Ċ with a secondary and a tertiary CH bond in alkanes are given in terms of the collision theory of bimolecular reactions. It is shown that for barrierless reactions the negative temperature dependence of the rate constants is attributed to the variation of the pre-exponential factor with temperature.

Potential Energy Surfaces for Chemical Reactions at Solid Surfaces

Potential Energy Surfaces for Chemical Reactions at Solid Surfaces

Ab initio study on the methanol-water cation radical potential energy surface

Potential energy surface for reaction of CH3OH+· and H2O was investigated at MP2/3-21G*(O) and MP2/6-31G* levels. Six minima and four saddle points were localized. The three deepest minima are of H-bond type while the fourth one possesses the three-electron hemibond.

State specific reactions of Ba(1S) and Ba(1D2) with water and methanol

The reactions of Ba(6s2 1S0) and Ba(6s5d 1D2) with water and methanol were studied under single collision conditions using crossed molecular beams. Reaction of ground state Ba(1S)+H2O led to dominant formation of BaO+H2 at all collision energies studied (12–30 kcal/mol). Although the reaction Ba(1S)+H2O→BaOH+H could also be observed at collision energies above the endoergicity of the reaction (13 kcal/mol), it remained a minor channel even at the highest collision energy. Electronic excitation of the Ba atom reactant to the metastable 1D2 state led to a large enhancement in reactivity, but the products were BaOH+H rather than BaO+H2. The dominance of BaO+H2 from ground state Ba(1S) even at collision energies nearly equal to the Ba(1D) excitation energy of 11 395 cm−1 (∼32 kcal/mol) indicates that the observed reaction state specificity results from participation of different potential energy surfaces for reactions of ground and excited state atoms. Collisions of Ba(1S) and Ba(1D) with methanol led only to formation of BaOCH3+H. We observed no BaOH+CH3 or BaO+CH4, despite the fact that they are the more thermodynamically favorable channels. Although reactivity with methanol was strongly enhanced by electronic excitation of the incident Ba atom, no reaction was observed from Ba(1S) or Ba(1D)+dimethyl ether (CH3OCH3). These observations strongly suggest that the reactions leading to radical formation involve H-atom migration.

A global strategy for determining reaction paths

The aim of this paper is to give a well-parallelizable curve variational method of finding Fukui's IRC and locating saddle points or other stationary points on potential energy surfaces of chemical reactions, based upon Mezey's theory on catchment regions of the gradient field in mass weighted coordinates.

Use of AM1 and PM3 methods for the investigation of energies and structures of compounds O=PXYZ, S=PXYZ

At the present time quantum mechanical calculations are widely used in the study of structure and physicochemical properties of molecules. With complex compounds it is necessary to use semi-empirical methods. However, none of the well-known procedures (CNDO/2, MINDO/3, MNDO) are any use for calculating the enthalpy of formation (AHf) or potential energy surfaces for reactions of phosphoorganic molecules of the type O=PXYZ and S=PXYZ (X, Y, Z = OR, F, NR2, CH3) , which play an important role in organic and bioorganic chemistry. Thus, although the semi-empirical scheme MNDO [I] reproduces experimental data for organic molecules with reasonable accuracy it does not give satisfactory results for the AH of phosphorus compounds (V) (errors are of the order of i0 kcal/mole) due to inadequacy of parametrization (see sample [2]).

ChemInform Abstract: Features of the Potential Energy Surface for Reactions of OH with CH2O.

ChemInform Abstract: Features of the Potential Energy Surface for Reactions of OH with CH2O.

An empirical valence bond model for constructing global potential energy surfaces for chemical reactions of polyatomic molecular systems

An empirical valence bond (EVB) model is proposed for constructing reactive potential energy surfaces of polyatomic molecular systems. Specifically, it is shown how the exchange potential V{sub 12}(q) of the EVB model can be chosen so that the EVB potential V(q) {triple bond} 1/2(V{sub 11}(q) + V{sub 22}(q)) - ((1/2(V{sub 11}(q) - V{sub 22}(q))){sup 2} + V{sub 12}(q){sub 2}){sup 1/2} exactly reproduces the transition-state geometry, energy, and force constant matrix obtained by an independent ab initio calculation. (Here q denotes all the 3N-6 nuclear coordinates of the N-atom system, and V{sub 11} and V{sub 22} are empirical diabatic, i.e., nonreactive, potential functions that describe the reactant and product regions of the potential surface, respectively.) Application of the overall prescription to a variety of two-dimensional test potential surfaces shows that this version of the EVB model provides an excellent description of reactive potential surfaces for a wide variety of situations.

Features of the potential energy surface for reactions of hydroxyl with formaldehyde

The authors use ab initio multiconfiguration self-consistent-field (MCSCF) and multireference configuration interaction (MRCI) methods to investigate the potential energy surface for reactions of hydroxyl radical with formaldehyde. The appropriate minima, transition states, and products for hydrogen abstraction from formaldehyde and OH addition to the carbon or oxygen are located at the MCSCF level of theory with a double-{zeta} plus polarization (DZP) quality basis set. Dynamical correlation effects on the energetics are determined by single-point MRCI calculations at the MCSCF stationary points also with a DZP basis set.

The mass weighting problem of potential energy surfaces for chemical reactions: dissociation and isomerisation pathways of HCN

The mass-weighting problem is discussed in the context of the reaction pathways of a chemical rearrangement. Some simple differential geometrical relations are used to connect different coordinate systems and achieve an independent definition of a reaction path from the coordinate system. In order to describe the procedure in more detail, it is applied to the molecule hydrogen cyanide (HCN) and its different reaction paths. The action of mass weighting on these paths is illustrated.

Symmetry and periodicity of potential surfaces: a test for multicenter interactions

Symmetry and periodicity of potential energy surfaces of chemical reactions and conformational changes are determined by the symmetry properties of the nuclear frameworks of all possible nuclear configurations of the given overall stoichiometry. For example, a mirror plane of a nuclear configuration implies a mirror plane of the potential surface (or that of the potential energy hypersurface in higher dimensions), and a local rotational symmetry of substituents implies a translational symmetry, that is, periodicity of the potential surface, if the latter is defined in terms of the usual bond length/bond angle internal coordinates. Such symmetry relations on potential surfaces are rather trivial consequences of molecular symmetry properties; however, when taken collectively for entire domains of nuclear configurations, they lead to nontrivial conclusions. Whereas symmetry properties and energy contents of individual conformations can be studied locally within limited domains of the potential surface, a global analysis of the potential surface may reveal significantly more. In this note, some consequences of the above approach are explored, and a simple test is proposed for the detection and evaluation of the importance of multicenter interactions in conformers related to one another by bond rotations.

The MNDO-85 system of programs for calculating the electronic structure, physicochemical properties, and reactivity of molecular systems by the MDNO, MNDOC, and AM1 semiepirical methods

The MNDO method has been used in the last few years with growing frequency for the investigation of organic and organometallic compounds and is gradually displacing other semiempirical calculation schemes. The MNDOC method was developed on the basis of the MNDO method and makes it possible to take into account electron correlation in the framework of perturbation theory. It was specifically designed for the study of excited states and calculations of the potential energy surfaces of chemical reactions. A new modification of the MNDO method, known as the AM1 method, now exists. These methods have been used to simulate the atomic parameters of H, C, N, O, Li, Be, B, F, Al, Si, P, S, Cl, Br, Sn, and I. This paper briefly assesses the uses of MNDO and its two modifications.

ChemInform Abstract: A Theoretical Study of Atomic Sulfur Reactions with Alkanes, Alkenes, and Alkynes.

AbstractThe results of an MNDO study of the potential energy surface for reaction of Sulfur in its ground S(3P) and first excited state S(′ D) with alkanes, alkenes, and alkynes are reported.