Showing papers on "Potential energy surface published in 1970"

Quantum Mechanics of the H+H2 Reaction: Exact Scattering Probabilities for Collinear Collisions

Abstract: The H + H2 reaction is very important in theoretical chemical dynamics (1-4). A model that is often used to study this reaction is to restrict the atoms to lie on a nonrotating line throughout the collision and to consider that the system is electronically adiabatic, i.e., it remains the lowest electronic state throughout the collision. This reduces the problem to scattering of three particles on a potential energy surface which is a function of two linearly independent coordinates. This model has been studied classically (5-8), and Mortensen and Pitzer (9) have calculated exact quantum mechanical reaction probabilities at five relative translational energies E0. In this Communication, we present some results of our more extensive exact calculations on this model of the H + H2 reaction and show their consequences for the validity of approximate theories of chemical reactions. For the cases considered here, the assumption of electronic adiabaticity causes very little error (10).

ESR Study of the Kinetic Isotope Effect in the Reaction of H and D Atoms with CH4

Abstract: ESR atom detection has been employed to determine the kinetic isotope effect involved in the reactions: H+CH4→H2+CH3, D+CH4→HD+CH3.New data were obtained for Reaction (1) leading to a slightly revised value for k1 of (6.25 ± 2.00) × 1013exp[− (11 600 ± 150) / RT] in units of cm3 mole−1·sec−1. Assuming that the ratio of pre‐exponential factors for Reactions (1) and (2) is 1.38 we compute a value for k2 of (4.5 ± 2.0) × 1013exp[− 11 100 ± 150) / RT] cm3 mole−1·sec−1. The observed kinetic isotope effect is compared with those calculated from absolute rate theory using the semiempirical Sato potential energy surface, or that obtained from the empirical bond‐energy–bond‐order (BEBO) method. These two methods give equally good predictions of the value of k1 / k2. Application of a one‐dimensional tunnel correction results in poorer agreement between theory and experiments.

Diabatic and adiabatic processes in photochemistry

Abstract: Photochemical reactions may be classified as adiabatic or as diabatic whether the chemical change occurs on the same potential energy surface or not. In the adiabatic cases deexcitation occurs either in the reactant (I) or in the product (II) while in the diabatic case it occurs in between them (III). The potential energy surfaces of polyatomic systems are discussed in relation to photochemistry and the possible conditions for the different behaviour of photo-reactions are formulated. Experimental evidence for the appearance of excited products in photoreactions in solution is given. I. CLASSIHCATION OF PHOTOCHEMICAL REACTIONS Thermal reactions with rare exceptions occur adiabatically on the lowest potential energy surface. An equivalent statement for photochemical reactions is less easy. They start in one of the electronically excited states of a reactant but end with the products in their ground states. This corresponds to a transition from a higher potential surface of the system to the lowest one. So, in principle, all photoreactions include diabatic processes. If one regards, however, electronic excitation and deexcitation as physical rather than chemical processes one may classify a reaction as adiabatic or as diabatic whether the chemical change takes place on the same potential surface or not. This leads us to a distinction between three different classes of photoreactions in which deexcitation occurs (I) in the reactant(s), or (II) in the product(s), or (III) anywhere in between on the reaction path. Schematically, these classes may be characterized as follows:

Geometry changes in excited states

Abstract: A brief discussion of the experimental methods of obtaining geometrical information about the equilibrium geometry of molecules in their electronically excited states is followed by a sampling of geometrical information available at present. The significance of simple orbital and symmetry arguments exemplified by Walsh diagrams is stressed. The geometrical tendencies of alkane excited states are studied. We find an excited state of methane that is planar. The lowest excited state of ethane should have longer C—H bonds and a shorter C—C bond than the ground state, and should also prefer an eclipsed conformation. Excited cyclopropane breaks one C—C bond. The lowest n, it" state of benzophenone is computed to be more planar than the ground state, and apparently does not become pyramidal at the carbonyl group. The isocyanide—cyanide rearrangement potential surface cautions us not to assume necessarily that excited state reaction is facilitated by a geometry change bringing closer the geometry of reactant to that of product. Every state of a molecule may be represented by a potential energy surface. An excited state surface is one such surface, the ground state of an isomer another. The excited state surface potentially differs from that of the ground state as much as the latter does from the surface of an isomer of very different geometry. It is not surprising therefore that upon electronic excitation molecules may adopt equilibrium geometries very different from those in their ground states. I would like to report on some of these geometry changes here, with particular emphasis on the interaction between theory and experiment in this area. Our primary source of experimental information on geometry changes remains the elucidation of the rotational fine structure of electronic transitions. The theoretical procedures leading from the observed high resolution spectrum to the moments of inertia of the molecule are summarized in the classic work of Herzberg'. These procedures rank among the highest achievements of theoretical chemistry and physics. The kind of information that emerges from spectroscopic studies is illustrated by the equilibrium bond length, the only free geometrical parameter, in various states of C2, listed in Table 12 Each molecular orbital may be classified as bonding (o, ire) or antibonding (at, itt), and these tendencies may be graded. Occupation of bonding orbitals decreases the equilibrium internuclear separation, occupation of antibonding orbitals increases this separation. These arguments, now so

Investigation of the Quenching of Excited Mercury Atoms by the Alkanes

Abstract: The quenching of Hg(63P1), Hg(63P0), and Hg(61P1) by the alkanes was investigated by a theoretical model based on the formation of a relatively long‐lived (Hg*HR) complex on a potential energy surface, most likely a polar one, which leads to the quenching of Hg* primarily by C–HHg bond rupture. The decomposition of the (Hg*HR) complex via C–HHg bond scission was treated as a unimolecular decomposition. The potential‐energy surfaces associated with the reaction coordinate were defined by thermochemical and spectroscopic data, semiempirical calculations and, whenever not critical, by reasonable estimates. The unimolecular decomposition rate constants were calculated making use of the Rice–Ramsperger–Kassel–Marcus (RRKM) theory. This approach correlates the nature and magnitude of the C–H vs C–D isotope effects and the large variation in quenching efficiencies of the various alkanes satisfactorily with the C–HHg bond strength, zero point energy, and structural effects (vibrational degrees of freedom). Conseq...

Structure of the potential energy surface at large deformations.

Abstract: The two-center shell model has been generalized to the shape of two overlapping spheroids with equal mass. In this model shell corrections have been calculated and the potential energy surface of two heavy nuclei has been investigated. The influence of fragment shells in the model gives rise to structure in this surface which supports assumptions of earlier models for the scission point.

Kinetic isotope effects in the reactions of bromine atoms with CH4 and CD4

Abstract: The competitive reactions of Br atoms with CH4 and CD4 were studied over the temperature range of 562° to 637°K. Over this temperature interval, the kinetic isotope effect, kH/kD, varied from 3.05 to 2.47 for the reactions The rate constant ratio kH/kD, expressed in Arrhenius form, was found to equal (1.10 ± 0.05) exp (1030 ± 60/RT). A comparison is presented between the experimental result and the result obtained theoretically from absolute rate theory using the London-Eyring-Polanyi-Sato (LEPS) method of constructing the potential energy surface of the reaction. The agreement between theory and experiment is very poor, and this is believed to arise from the highly unsymmetrical nature of the potential energy surface involved in these reactions. A comparison is also presented between the kH/kD values obtained in the Br + CH4–CD4 experiments and the available data on the corresponding Cl + CH4–CD4 reactions.