Showing papers on "Potential energy surface published in 1996"

Potential Energy Surface for the Benzene Dimer. Results of ab Initio CCSD(T) Calculations Show Two Nearly Isoenergetic Structures: T-Shaped and Parallel-Displaced

Abstract: It is proposed that the benzene dimer has two almost isoenergetic structures. This has been confirmed by very high level ab initio calculations. MP2, MP4, and CCSD(T) calculations with various basis sets up to the augmented correlation consistent polarized double-zeta (aug-cc-pvdz) one were performed for the sandwich (S), T-shaped (T), and parallel-displaced (PD) structures of the benzene dimer. MP2 strongly overestimates the stabilization energy and leads to incorrect global minimum. Also, the MP4 predictions differ from the CCSD(T) ones. At the CCSD(T) level the T structure was the most stable structure with stabilization energy of about 2.3 kcal/mol. The PD structure at the same level was found to be slightly less stable (by 0.2−0.3 kcal/mol). The energy difference between these structures is sensitive to the theoretical level applied, and we estimated that both structures are similarly stable. The absolute value of stabilization energy is believed to be close to the genuine value; this conclusion was ...

Density Functional Theory Prediction of the Relative Energies and Isotope Effects for the Concerted and Stepwise Mechanisms of the Diels−Alder Reaction of Butadiene and Ethylene

Abstract: Density-functional theory has been applied to the study of the mechanism of the Diels−Alder reaction of butadiene and ethylene. Both synchronous concerted and two-step diradical mechanisms were studied at the Becke3LYP/6-31G* level. The lowest energy stepwise pathway has a free energy of activation 7.7 kcal/mol above that of the concerted path. Spin correction of the spin-contaminated diradical transition structure energy reduces this energy difference to 2.3 kcal/mol. A study of the H2 potential energy surface suggests that the spin-projection procedure overcorrects the energies of diradical species; the diradical energies likely fall between the corrected and uncorrected values. Thus, the free energy of concert for the Diels−Alder reaction is predicted to be between 2.3 and 7.7 kcal/mol, in excellent agreement with thermochemical estimates. Energies of reaction and geometries of the reactants and product are in good agreement with available experimental results. Calculated secondary kinetic isotope effe...

A general method for constructing multidimensional molecular potential energy surfaces from ab initio calculations

Abstract: A general interpolation method for constructing smooth molecular potential energy surfaces (PES’s) from ab initio data are proposed within the framework of the reproducing kernel Hilbert space and the inverse problem theory. The general expression for an a posteriori error bound of the constructed PES is derived. It is shown that the method yields globally smooth potential energy surfaces that are continuous and possess derivatives up to second order or higher. Moreover, the method is amenable to correct symmetry properties and asymptotic behavior of the molecular system. Finally, the method is generic and can be easily extended from low dimensional problems involving two and three atoms to high dimensional problems involving four or more atoms. Basic properties of the method are illustrated by the construction of a one‐dimensional potential energy curve of the He–He van der Waals dimer using the exact quantum Monte Carlo calculations of Anderson et al. [J. Chem. Phys. 99, 345 (1993)], a two‐dimensional potential energy surface of the HeCO van der Waals molecule using recent ab initio calculations by Tao et al. [J. Chem. Phys. 101, 8680 (1994)], and a three‐dimensional potential energy surface of the H+3 molecular ion using highly accurate ab initio calculations of Rohse et al. [J. Chem. Phys. 101, 2231 (1994)]. In the first two cases the constructed potentials clearly exhibit the correct asymptotic forms, while in the last case the constructed potential energy surface is in excellent agreement with that constructed by Rohse et al. using a low order polynomial fitting procedure.

An accurate multireference configuration interaction calculation of the potential energy surface for the F+H2→HF+H reaction

Abstract: A three dimensional potential energy surface for the F+H2→HF+H reaction has been computed using the internally contracted multireference configuration interaction (MRCI) method with complete active space self‐consistent field (CASSCF) reference functions and a very large basis set. Calibration calculations have been performed using the triple‐zeta plus polarization basis set employed in previous nine‐electron full CI (FCI) calculations of Knowles, Stark, and Werner [Chem. Phys. Lett. 185, 555 (1991)]. While all variational MRCI wave functions yield considerably larger barrier heights than the FCI, excellent agreement with the FCI barrier height and the exothermicity was obtained when the Davidson correction was applied (MRCI+Q). The convergence of the barrier height and exothermicity, spectroscopic constants of the HF and H2 fragments, and the electron affinity of the fluorine atom with respect to the basis set has been carefully tested. Using the largest basis sets, which included 5d, 4f, 3g, and 2h func...

A refined H3 potential energy surface

Abstract: In evaluating some low temperature (T<1000 K) thermal rate coefficients for inelastic rotational excitation of H2 by H atoms, Sun and Dalgarno have found a marked sensitivity to the potential energy surface adopted for the calculations. We have investigated the origin of the discrepancies between previous H3 potential energy surfaces and have developed a refined surface which addresses these concerns. New quasiclassical trajectory calculations of cross sections for low energy rotational excitation are reported. The refined surface is based on 8701 ab initio energies, most newly computed for this purpose. It has the same functional form as our earlier (BKMP) surface, but since the fit of the parameters is more fully constrained than for any previous surface it is a more accurate representation. The refined surface matches the ab initio energies with an overall rms error of 0.27 mEh (i.e., 0.17 kcal/mol) and a maximum absolute deviation of 6.2 mEh (for a very compact high energy equilateral triangle conformation). For ‘‘noncompact’’ conformations (no interatomic distance smaller than 1.15 bohr), the rms error is 0.18 mEh and the maximum absolute deviation is 1.7 mEh. The refined surface is compared critically to four previous surfaces, including the DMBE surface of Varandas et al., in several respects: Legendre expansion coefficients; the interaction region for low energy rotational excitation; near the collinear saddle point; near conical intersections of the ground and first excited state surfaces; the van der Waals well; and compact geometries. We have also compared new first excited state ab initio energies for 1809 conformations with corresponding predictions from the DMBE surface.

Structure of Water Clusters. The Contribution of Many-Body Forces, Monomer Relaxation, and Vibrational Zero-Point Energy

Abstract: The vibrationally averaged properties of small water clusters from the dimer to the hexamer are discussed. The potential energy surface used contains explicit many-body terms which allows the non-pairwise interactions to be considered. The ground vibrational states are calculated accurately using a diffusion quantum Monte Carlo algorithm which gives vibrationally averaged rotational constants in good agreement with experiment. The many-body forces cause a destabilization of the more closed structures, and there is a significant variation in the intermolecular zero-point energies for different structures. Cyclic structures are easily the most stable for the trimer and tetramer; in the latter case, this is probably due to the three- and four-body forces. The cyclic pentamer is also probably the structure with the highest dissociation energy when all effects are considered. For the hexamer, a noncyclic cagelike structure is found to be most stable and its stability is due to a relatively low zero-point energy.

Quantum mechanical angular distributions for the F+H2 reaction

Abstract: Quantum mechanical integral and differential cross sections have been calculated for the title reaction at the three collision energies studied in the 1985 molecular beam experiment of Lee and co‐workers, using the new ab initio potential energy surface of Stark and Werner (preceding paper). Although the overall agreement between the calculated and experimental center‐of‐mass frame angular distributions is satisfactory, there are still some noticeable differences. In particular, the forward scattering of HF(v′=3) is more pronounced in the present calculations than it is in the experiment and the calculations also predict some forward scattering of HF(v′=2). A comparison with the quasiclassical trajectory results of Aoiz and co‐workers on the same potential energy surface shows that the forward scattering is largely a quantum mechanical effect in both cases, being dominated by high orbital angular momenta in the tunneling region where the combined centrifugal and potential energy barrier prevents classical trajectories from reacting. The possible role of a reactive scattering resonance in contributing to the quantum mechanical forward scattering is also discussed in some detail.

Transition-State Spectroscopy of Cyclooctatetraene

Abstract: The 351-nanometer photoelectron spectrum of the planar cyclooctatetraene radical anion (COT·-) shows transitions to two electronic states of cyclooctatetraene (COT). These states correspond to the D4h 1A1g state, which is the transition state for COT ring inversion, and the D8h 3A2u state. The electron binding energy of the 1A1g transition state is 1.099 ± 0.010 electron volts, which is lower by 12.1 ± 0.3 kilocalories per mole than that of the 3A2u state. The photoelectron spectrum shows that the singlet lies well below the triplet in D8h COT and confirms ab initio predictions that the molecule violates Hund's rule. Vibrational structure is observed for both features and is readily assigned by use of a simple potential energy surface.

Thermal rate constants of the N 2 +O→NO+N reaction using ab initio 3 A″ and 3 A' potential energy surfaces

Abstract: Theoretical determinations of the thermal rate constants and product energy distributions of the N2+O→NO+N reaction, which plays a crucial role in hydrocarbon air combustion and high temperature air chemistry, are carried out using a quasiclassical trajectory method. An analytical fit of the lowest 3A′ potential energy surface of this reaction based on the CCI ab initio data is obtained. The trajectory study is done on this surface and an analytical 3A″ surface proposed by Gilibert et al. [J. Chem. Phys. 97, 5542 (1992)]. The thermal rate constants computed from 3000 to 20 000 K are in good agreement with the available experimental data. In addition, the dependence of the rate constant on the N2 internal state is studied. It is found that a low vibrational excitation can reduce the rate constant of this reaction by a factor of 3. Also, we investigate the effect of the N2 vibrational state on the product NO vibrational distribution, and it is found that at low N2 vibrational states, the NO vibrational dist...

A global H2O potential energy surface for the reaction O(1D)+H2→OH+H

Abstract: A global, single‐valued ground‐state H2O potential surface for the reaction O(1D)+H2→OH+H has been constructed from a new set of accurate ab initio data using a general multidimensional interpolation method. The ab initio calculations are of the multireference, configuration interaction variety and were carried out using augmented polarized triple zeta basis sets. The multidimensional method is formulated within the framework of the reproducing kernel Hilbert space theory. The H2O potential is expressed as a many‐body sum of a single one‐body term, three two‐body terms, and a single three‐body term. The one‐body term is the dissociation energy to the three‐atom limit 2H(2S)+O(3P). The two‐body terms are two O–H and one H–H adiabatic diatomic potentials of lowest energy. Each diatomic term is obtained by interpolating a discrete set of ab initio data using a one‐dimensional, second‐order, distancelike reproducing kernel. The three‐body term is obtained by interpolating the difference of the H2O ab initio d...

A six dimensional quantum study for atom–triatom reactions: The H+H2O→H2+OH reaction

Abstract: A time‐dependent wave packet method has been developed to study atom–triatom ABC+D→AB+CD reactions in full six dimensions (6D). The approach employs a body‐fixed coupled angular momentum basis for three angular coordinates, and three 1D bases for three radial coordinates. It permits the calculation of diatom AB vibrational state resolved total reaction probability for an initial rovibrational state of the triatom ABC. The approach is applied to study the H+H2O→H2+OH reaction on the modified Schatz–Elgersman potential energy surface. Initial state‐selected total reaction probabilities are presented for initial ground and several vibrationally excited states of H2O for total angular momentum J=0, along with the final OH vibrational state distributions. We also report the cross sections for reaction from the initial ground vibrational and the first bending excited states of H2O. Comparisons are made between our results and those from other theoretical calculations and experiments.

How Does Fe+ Activate C−C and C−H Bonds in Ethane? A Theoretical Investigation Using Density Functional Theory†

Abstract: The potential energy surface (PES) corresponding to the reaction of the iron cation with ethane, which represents a prototype of the activation of C−C and C−H bonds in alkanes by transition metal cations, has been investigated employing the recently suggested hybrid density functional theory/Hartree−Fock method (B3LYP) combined with reasonably large one-particle basis sets. The performance of this computational approach has been calibrated against experimentally known Fe+−R binding energies of fragments R relevant to the [Fe,C2,H6]+ PES and against the relative energies of the possible exit channels. Both the C−C and C−H bond activation branches of the PES are characterized by a low barrier for the first step, the insertion of the iron cation into a C−C and C−H bond, respectively. Rate determining are the second steps which in the C−C bond activation branch corresponds to an [1,3]-H shift leading to a complex between FeCH2+ and methane. Along the C−H activation reaction coordinate, no transition state cor...

The potential energy surface of H2 16O

Abstract: We report here a new determination of the H216O potential energy surface from experimental data. The calculations have been carried out by means of the very accurate and highly efficient method proposed and applied to H216O in a previous paper [Polyansky, Jensen, and Tennyson, J. Chem. Phys. 101, 7651 (1994)]. This previous work has been significantly improved by inclusion of additional terms in the analytical expression used to represent the potential energy surface. Previously, 1600 rotation‐vibration term values for H216O were fitted with a standard deviation of 0.36 cm−1. With the extended model of the present work, this standard deviation could be improved to 0.25 cm−1. With the extended model and the new fitted potential function we have calculated a data set comprising 3200 term values, all of which can be compared with experimentally derived values. The standard deviation for this data set is 0.6 cm−1. The data set contains rotationally excited energy levels for all the 63 vibrational states which have been characterized by high resolution spectroscopy. The potential energy function obtained in the present work improves drastically the agreement with experiment for the highly excited local mode stretching states above 20 000 cm−1. For the vibrational band origins of these states, the highest of which is measured at 25 118 cm−1, our previous fitted potential produced discrepancies of more than 100 cm−1. These deviations are reduced to less than 1 cm−1 by the potential energy function of the present work. We show that no significant improvement of the fit can be obtained by extending the analytical expression for the potential energy by further high‐order terms. An analysis of the residuals shows that at the level of accuracy achieved, the major contribution to the error originates in the neglect of nonadiabatic correction terms in the Born‐Oppenheimer kinetic energy operator. We conclude that any further improvement of the potential energy surface requires that such correction terms be included in the Hamiltonian. With the present potential, reliable extrapolations towards higher rotational and vibrational energies can be carried out, and we expect that such calculations can be very helpful in the assignment of experimental spectra involving highly excited states.

Proton-Coupled Electron Transfer Reactions: Evaluation of Rate Constants

Abstract: A recent theory [Cukier, R I J Phys Chem 1995, 99, 16101] that predicts the rate of a proton-coupled electron transfer (PCET) reaction is developed further In PCET, the electron and proton may transfer consecutively, electron transfer (ET) followed by proton transfer (PT), designated as ET/PT, or they may transfer concertedly, in one tunnel event, designated as ETPT Since the proton charge is coupled to the solvent dipoles in a fashion similar to the electron−solvent coupling, the effect of solvation on the shape of the proton potential energy surface must be known in order to evaluate the PCET rate constant We show how dielectric continuum theory can be used to obtain the proton-solvated surfaces that are dependent on whether the electron is in its initial or final state The proton will affect the PCET rate via Franck−Condon factors between the proton surfaces for the initial and final electron states The proton energy levels will also influence the activation energy for the PCET process The r

Direct Dynamics Calculation for the Double Proton Transfer in Formic Acid Dimer

Abstract: The dynamics of the double proton transfer in formic acid dimer (FAD) complex has been studied by the direct dynamics approach with variational transition state theory using multidimensional semiclassical tunneling approximations High-level ab initio quantum mechanical calculations were performed to estimate the energetics of the double proton transfer Dimerization energies and the barrier height have been calculated at the G2* level of theory, which yields −142 and 894 kcal mol-1, respectively A quantum mechanical potential energy surface has been constructed using the AM1 Hamiltonian with specific reaction parameters (AM1-SRP) which are obtained by adjusting the standard AM1 parameters to reproduce the energetics by high-level ab initio quantum mechanical calculation The minimum energy path has been calculated on this potential energy surface and other characteristics of the surface were calculated as needed The two protons are transferred synchronously, so the transition state possesses D2h symm

The C7H6 Potential Energy Surface Revisited: Relative Energies and IR Assignment

Abstract: The geometries and force fields of phenylcarbene (PC) and cycloheptatrienylidene (CHT) in their singlet and triplet electronic states as well as of cycloheptatetraene (CHTE) and bicyclo[4.1.0]heptatriene (BCT) and the transition states for the formation and decay of the latter were evaluated by various methods. Relative single point energies were calculated at the CCSD(T)/cc-pVDZ//BLYP/6-31G* level. Finally, the effects of extending the basis set to triple-ζ quality were estimated by (R)MP2 calculations and carried over proportionally to CCSD(T). These calculations show that CHTE which has a strongly distorted allenic structure is the most stable species on that part of the C(CH)6 surface which was examined in the present study, followed by planar 3PC. The strained BCT is found to be nearly degenerate in energy with 1PC, but the high activation energy for its formation from 1PC together with the low activation energy for ring-opening to CHTE suggests that this species cannot persist under the experimental...

Structure, Dynamics, and Thermodynamics of Clusters: Tales from Topographic Potential Surfaces

Abstract: Theoretical studies of atomic and molecular clusters often seek to explain structure, dynamics, and thermodynamics in terms of the underlying potential energy surface and the form of the interparticle interaction. One specific example from each of these categories is considered here; the overall approach can be summarized as global analysis of potential surfaces. Changes in the most favorable cluster morphology can be qualitatively understood as a function of the range of the interparticle forces. Thermodynamic properties can be calculated from a representative sample of local minima on the potential energy surface. However, prediction of dynamics requires not only knowledge of minima but also transition states and reaction pathways.

Potential energy surface and quasiclassical trajectory studies of the n(2d)+h2 reaction

Abstract: We present dynamical studies of the CN+H{sub 2} reaction based on an empirical potential energy surface that is derived from high quality {ital ab} {ital initio} calculations. The {ital ab} {ital initio} calculations, which use a multireference configuration interaction method with large correlation consistent basis sets, indicate that the linear HHCN barrier is about 4.3 kcal/mol above CN+H{sub 2}, and that there is no reaction path which connects CN+H{sub 2} to the stable intermediate H{sub 2}CN, although there is a path for dissociation of H{sub 2}CN to H+HCN. The empirical surface is written as a sum of two-, three-, and four-body terms, with the two- and three-body terms for HCN based on an accurate global surface that describes both the HCN and HNC force fields. The four-body terms are developed so as to describe the HHCN linear saddle point and the H{sub 2}CN minimum accurately, as well as dissociation of H{sub 2}CN into HCN+H, and the ridge which separates the abstraction and H{sub 2}CN dissociation pathways. Other features of the potential surface, such as the HCNH {ital cis} and {ital trans} minima, and the pathways leading to the formation of HNC+H are also described, though less accurately. Three different choicesmore » for the HHCN saddle point properties are considered. We find that the surface which matches the {ital ab} {ital initio} barrier energy most accurately gives rate constants that are too low. Much better agreement is obtained using a 3.2 kcal/mol barrier. The trajectory results show typical dependence of the CN+H{sub 2} reactive cross sections on initial translational energy and initial vibration/rotation state, with CN behaving as a spectator and H{sub 2} playing an active role in the reaction dynamics. Analysis of the H+HCN products indicates that both the C-H stretch and bend modes are significantly excited, with bend excitation showing strong sensitivity to the saddle point properties and to reagent translational energy.« less

The effect of the range of the potential on the structure and stability of simple liquids: from clusters to bulk, from sodium to

Abstract: For systems with sufficiently short-ranged interparticle forces, such as some colloidal systems and perhaps , the liquid phase can be thermodynamically unstable. By analysing the effect of the range of the interatomic forces on the multidimensional potential energy surfaces of bulk material and clusters, a microscopic view of this phenomenon is provided. Structural analysis of the minima on the potential energy surface provides evidence for the polytetrahedral character of the liquid phase, and allows us to examine the evolution of the phase-like forms of clusters to the bulk limit. We find that essentially bulk-like liquid structure can develop in clusters with as few as 55 atoms. The effect of the range of the potential on the thermodynamics is illustrated by a series of simulations of 55-atom clusters. For small clusters bound by long-ranged potentials the lowest energy minimum has an amorphous structure typical of the liquid-like state. This suggests an explanation for the transition from electronic to geometric magic numbers observed in the mass spectra of sodium clusters.

Carbene Rearrangements Unsurpassed: Details of the C7H6 Potential Energy Surface Revealed

Abstract: The rearrangement of phenylcarbene (1) to 1,2,4,6-cycloheptatetraene (3) has been studied theoretically, using SCF, CASSCF, CASPT2N, DFT (B3LYP), CISD, CCSD, and CCSD(T) methods in conjunction with the 6-31G*, 6-311+G*, 6-311G(2d,p), cc-pVDZ, and DZd basis sets. Stationary points were characterized by vibrational frequency analyses at CASSCF(8,8)/6-31G* and B3LYP/6-31G*. Phenylcarbene (1) has a triplet ground state (3A‘‘) with a singlet−triplet separation (ΔEST) of 3−5 kcal mol-1. In agreement with experiment, chiral 3 is the lowest lying structure on this part of the C7H6 potential energy surface. Bicyclo[4.1.0]hepta-2,4,6-triene (2) is an intermediate in the rearrangement of 1 into 3, but it is unlikely to be observable experimentally due to a barrier height of only 1−2 kcal mol-1. The enantiomers of 3 interconvert via the 1A2 state of cycloheptatrienylidene (4) with an activation energy of 20 kcal mol-1. The “aromatic” 1A1 state, previously believed to be the lowest singlet state of 4, is roughly 10 kc...

Amino groups in nucleic acid bases, aniline, aminopyridines, and aminotriazine are nonplanar: Results of correlated ab initio quantum chemical calculations and anharmonic analysis of the aniline inversion motion

Abstract: The amino group nonplanarity in nucleic acid bases, aniline, aminopyridines, and aminotriazine was investigated by ab initio methods with and without inclusion of correlation energy utilizing medium and extended basis sets. For all the systems studied, the amino group was found to be nonplanar and the coupled cluster method [CCSD(T)] ‘‘nonplanarities’’ and inversion barriers slightly higher than their second‐order many‐body perturbation‐theory (MP2) counterparts. To assess the reliability of the calculations, inversion splittings for aniline and aniline‐ND2 were evaluated by solving a two‐dimensional vibrational Schrodinger equation for the large‐amplitude inversion and torsion motions, while respecting the role of small‐amplitude C–N stretching and H–N–H bending motions. Because a large number of points is required for the description of the aniline potential energy surface, the Hartree–Fock (HF) method with 6‐31G* basis set was utilized. The vibrational calculations were performed within the framework of the semirigid bender Hamiltonian of Landsberg and Bunker. Excellent agreement between experimental and theoretical inversion‐torsion frequencies for fundamental, overtone, and combination modes was found, which gives strong evidence for the adequacy of the theoretical model used in general, and potential energy surface in particular. Similarity between the HF, MP2, and CCSD(T) aniline inversion barriers and amino group nonplanarities gives us confidence that the MP2 and CCSD(T) inversion barriers and amino group nonplanarities of the DNA bases, aminopyridine, and aminotriazine, are close to the actual values which are still experimentally unknown.

Kinetics of the C2H3 + H2 ⇄ H + C2H4 and CH3 + H2 ⇄ H + CH4 Reactions

Abstract: The kinetics of the reactions C2H3 + H2 → H + C2H4 (1) and CH3 + H2 → H + CH4 (2) have been studied in the temperature ranges 499−947 K (reaction 1) and 646−1104 K (reaction 2) and He densities (6−18) × 1016 atoms cm-3 by laser photolysis/photoionization mass spectrometry. Rate constants were determined in time-resolved experiments as a function of temperature. Ethylene was detected as a primary product of reaction 1. Within the above temperature ranges the experimental rate constants can be represented by Arrhenius expressions k1 = (3.42 ± 0.35) × 10-12 exp(−(4179 ± 67 K)/T) cm3 molecule-1 s-1 and k2 = (1.45 ± 0.18) × 10-11 exp(−(6810 ± 102 K)/T) cm3 molecule-1 s-1. Experimental values of k2 are in agreement with the available literature data. The potential energy surface and properties of the transition state for reactions (1, −1) were studied by ab initio methods. Experimental and ab initio results of the current study were analyzed and used to create a transition state model of the reaction. The resul...

Theoretical Study of H+ Translocation along a Model Proton Wire

Abstract: The mechanism of proton translocation along linear hydrogen-bonded water chains is investigated. Classical and discretized Feynman path integral molecular dynamics simulations are performed on protonated linear chains of 4, 5, and 9 water molecules. The dissociable and polarizable water model PM6 of Stillinger and co-workers is used to represent the potential energy surface of the systems. The simulations show that quantum and thermal effects are both important because the height of the barriers opposing proton transfer are strongly coupled to the configuration of the chain, which is, in turn, affected by the presence of an excess proton. For characterization of the quantum effects, the energy levels of the hydrogen nucleus located at the center of a protonated tetrameric water chain are calculated by solving the Schroedinger equation for an ensemble of configurations which were generated with path integral simulations. Analysis shows that the first excitation energies are significantly larger than the th...

The unimolecular dissociation of HCO. II. Comparison of calculated resonance energies and widths with high‐resolution spectroscopic data

Abstract: We present a theoretical study of the unimolecular dissociation resonances of HCO in the electronic ground state, X1A′, using a new ab initio potential energy surface and a modification of the log‐derivative version of the Kohn variational principle for the dynamics calculations. Altogether we have analyzed about 120 resonances up to an energy of ≊2 eV above the H+CO threshold, corresponding to the eleventh overtone in the CO stretching mode (v2=11). The agreement of the resonance energies and widths with recent stimulated emission pumping measurements of Tobiason et al. [J. Chem. Phys. 103, 1448 (1995)] is pleasing. The root‐mean‐square deviation from the experimental energies is only 17 cm−1 over a range of about 20 000 cm−1 and all trends of the resonance widths observed in the experiment are satisfactorily reproduced by the calculations. The assignment of the states is discussed in terms of the resonance wave functions. In addition, we compare the quantum mechanical state‐resolved dissociation rates ...

Interconversions of Phenylcarbene, Cycloheptatetraene, Fulvenallene, and Benzocyclopropene. A Theoretical Study of the C7H6 Energy Surface

Abstract: Ab initio calculations at the G2(MP2,SVP) and B-LYP/6-311+G(3df,2p)+ZPVE levels have been used to examine the potential energy surface of C7H6. Fulvenallene (6) is the most stable C7H6 isomer considered in this study. 1-Ethynylcyclopentadiene (9A), benzocyclopropene (10), and 1,2,4,6-cycloheptatetraene (4) lie 12, 29, and 49 kJ mol-1, respectively, above 6. Phenylcarbene (1) is calculated is to have a triplet (3A‘‘) ground state, 16 kJ mol-1 more stable than the singlet state (1A‘). Interconversion of 1 and 4 is predicted to have a moderate activation barrier, with the involvement of a stable bicyclic intermediate (bicyclo[4.1.0]hepta-2,4,6-triene, 2). However, 2 is found to lie in a shallow potential energy well with a small barrier (8 kJ mol-1) to rearrangement to 4. At the G2(RMP2,SVP)//QCI level, the 3A2 and 3B1 triplet states of cycloheptatrienylidene (33) are predicted to lie very close in energy. The singlet “aromatic” cycloheptatrienylidene (13, 1A1) is found to be a transition structure interconv...

High-Pressure Mass Spectrometric Investigations of the Potential Energy Surfaces of Gas-Phase SN2 Reactions

Abstract: High-Pressure Mass Spectrometric (HPMS) experiments have been carried out to probe the details of the double minimum potential energy surface for gas-phase SN2 reactions. The well depths and entropy changes associated with the formation of entrance and exit channel electrostatic complexes for the chloride and bromide adducts of methyl, ethyl, isopropyl, and tert-butyl chlorides and bromides have been determined from the temperature dependence of the equilibrium constants for adduct formation. In the cases of “symmetric” complexes associated with identity SN2 reactions, there is an increase in well depth as the size and, therefore, polarizability of the alkyl group increases. Concomitant with this is an increase in the magnitude of the negative entropy change for complex formation which is the result of an increase in the frequency of the intermolecular mode(s) of the complex arising from the increased bond strength. The data for the unsymmetrical adducts for the non-identity SN2 reactions show the same pa...

Quantum state‐to‐state reaction probabilities for the H+H2O→H2+OH reaction in six dimensions

Abstract: A time‐dependent wave packet method has been employed to calculate the state‐to‐state reaction probability for the H+H2O(0,0,0)→H2(v1,j1)+OH(v2,j2) reaction for J=0 and initial nonrotating H2O on the modified Schatz–Elgersman potential energy surface in full six dimensions (6D). Starting from a wave packet for an atom–triatom asymptotic state in atom–triatom Jacobi coordinates, we transfer the wave packet to diatom–diatom Jacobi coordinates after the wave packet moves into the interaction region. Propagation is then carried out in the diatom–diatom Jacobi coordinates until the reaction flux measured in the diatom–diatom asymptotic region is converged.

An improved potential energy surface for the h2cl system and its use for calculations of rate coefficients and kinetic isotope effects

Abstract: We present a new potential energy surface (called G3) for the chemical reaction Cl + H2 → HCl + H. The new surface is based on a previous potential surface called GQQ, and it incorporates an improved bending potential that is fit to the results of ab initio electronic structure calculations. Calculations based on variational transition state theory with semiclassical transmission coefficients corresponding to an optimized multidimensional tunneling treatment (VTST/OMT, in particular improved canonical variational theory with least-action ground-state transmission coefficients) are carried out for nine different isotopomeric versions of the abstraction reaction and six different isotopomeric versions of the exchange reaction involving the H, D, and T isotopes of hydrogen, and the new surface is tested by comparing these calculations to available experimental data. The theoretical data are also used to investigate the equilibrium constant and the branching ratio for the reverse reaction, and calculations of...

An analytical six-dimensional potential energy surface for dissociation of molecular hydrogen on Cu(100)

Abstract: A six‐dimensional (6D) potential energy surface (PES) describing the molecule–surface interaction in the dissociative chemisorption system H2+Cu(100) is presented. The PES is based on slab calculations performed using the generalized gradient approximation (GGA) of density functional theory (DFT). To allow the use of the PES in dynamics calculations which can test the validity of the DFT/slab approach by comparing with available experiments on dissociative chemisorption, the PES was fit to an analytical form. The fit used describes the orientational dependence of the molecule–surface interaction above the high symmetry sites upto second order in spherical harmonics. The barriers to dissociation calculated for H2 approaching with its molecular axis parallel to the surface are all located in the exit channel. Also, for different impact sites and orientations, the height and the distance to the surface associated with the barrier correlate well with the chemisorption energy of the H‐atoms in the sites to whi...