Showing papers on "Potential energy surface published in 1989"

Theoretical Methods for Rovibrational States of Floppy Molecules

Abstract: Theoretical calculation of rotation-vibration energy levels of polyatomic molecules is a topic with a long history, characterized by a close, symbiotic relationship with molecular spectroscopy on one side and quantum chem­ istry on the other. What brings them together is the notion of the potential energy surface, which plays a central role in our understanding of the molecular structure and dynamics. In the case of polyatomic molecules, the experimental spectra cannot be inverted directly to yield potential surfaces [see Ref. (1) for some recent efforts within the semiclassical SCF approach], but they do provide a stringent test for the theoretically obtained potential surfaces and observables derived from them. These surfaces, usually from ab initio calculations, seldom meet the standards of spectroscopic accuracy, especially if more extended, high-energy regions are of interest. The only practical way available to test and improve them is by comparing the calculated and the experimental spectra, and minimizing the difference between the two. The subject of the theoretical treatment of coupled molecular vibrations has undergone a real renaissance in the past decade. Significant conceptual advances have been made, particularly concerning highly vibrationally and

A study of the singlet and triplet states of vinylidene by photoelectron spectroscopy of H2C=C−, D2C=C−, and HDC=C−. Vinylidene–acetylene isomerization

Abstract: The X 1A1, a 3B2, and b 3A2 states of vinylidene are observed in the ultraviolet (351.1–364.0 nm) photoelectron spectra of X 2B2 H2CC−, X 2B2 D2CC−, and X 2A’ HDCC−. The X 1A1 state exhibits vibrational structure well above the barrier for isomerization to acetylene. A strict lower bound to the lifetime of the singlet state against rearrangement is τ>0.027 ps, with an estimate of τ≊0.04–0.2 ps based on a simulation of the line shapes including rotational broadening. A vibrational analysis of the singlet and lower triplet state bands provides vibrational frequencies and estimates of the changes of molecular geometries between the anion and the neutral species. A qualitative potential energy surface for the CH2 rock mode, which closely corresponds to the reaction coordinate for isomerization, is extracted from the experimental data. The adiabatic electron affinity is EA(X 1A1 H2CC)=0.490±0.006 eV and the triplet term energies are T0(a 3B2 H2CC)=2.065±0.006 eV and T0(b 3A2 H2CC)=2.754±0.020 eV. Exp...

New theoretical concepts for understanding organic reactions

Abstract: Multidimensional theoretical stereochemistry and conformational potential energy surface topology.- Some practical suggestions for optimizing geometries and locating transition states.- Reaction topology and quantum chemical molecular design on potential energy surfaces.- Topology of molecular shape and chirality.- Adiabatic and diabatic surfaces in the treatment of chemical reactivity. I. Theory.- Adiabatic and diabatic surfaces in the treatment of chemical reactivity. II. An illustrative application to the Diels Alder reaction.- A qualitative valence bond model for organic reactions.- Solvent effects on potential energy surfaces and chemical kinetics.- Modifications of potential energy surfaces by solvation and catalysis.- Computational tests of potential energy surfaces from dynamical properties.- Dynamical formulation of transition state theory: variational transition states and semiclassical tunneling.- Theoretical models for reaction dynamics in polyatomic molecular systems.- Practical applications of new theoretical concepts in organic chemistry.

Supercomputer algorithms for reactivity, dynamics and kinetics of small molecules

Abstract: Recent Advances in Electronic Structure Theory and their Influence on the Accuracy of Ab initio Potential Energy Surfaces.- Modern Electronic Structure Calculations: The Accurate Prediction of Spectroscopic Band Origins.- Potential Energy Surfaces of Several Elementary Chemical Reactions.- Calculation and Characterization of Reaction Valleys for Chemical Reactions.- Computed Potential Energy Surfaces for Chemical Reactions.- An Ab initio Study on the Coordination of Formaldehyde, Carbon Dioxide, Dinitrogen and Related Molecules to Iron(0) and Nickel(0) Fragments.- Kinetic Paths from the Hyperspherical Perspective: Ab initio Potential Energy Surface for the O(3P)+H2 Reaction.- Exact Quantum Results for Reactive Scattering Using Hyperspherical (APH) Coordinates.- Computational Strategies and Improvements in the Linear Algebraic Variational Approach to Rearrangement Scattering.- How Variational Methods in Scattering Theory Work.- Quantum Dynamics of Small Systems Using Discrete Variable Representations.- Finite Element Calculations of Scattering Matrices for Atom-Diatom Reactive Collisions. Experiences on an Alliant FX/8.- Investigations with the Finite Element Method. The Collinear H + H2, F + H2 and Ne + H2+ Reactions.- Calculation of Multichannel Eigenvalues and Resonances.- Accurate Determination of Polyatomic Infrared Spectra.- The Calculation of Ro-Vibrational Spectra Using Supercomputers.- Approximate Quantum Techniques for Atom Diatom Reactions.- Approximate Quantum Mechanical Calculations on Molecular Energy Transfer and Predissociation.- Temperature-Dependent Rate Constants for Ion-Dipole Reactions: C+(2P) + HCl(X1?+).- Classical Path Approach to Inelastic and Reactive Scattering.- Intramolecular Energy Transfer in HC and HO Overtone Excited Molecules.- Classical Trajectory Studies of Gas Phase Reaction Dynamics and Kinetics Using Ab initio Potential Energy Surfaces.- Quasiclassical Calculations for Alkali and Alkaline Earth + Hydrogen Halide Chemical Reactions Using Supercomputers.- Dynamics of the Light Atom Transfer Reaction: Cl + HCl ? ClH + Cl.- The Modeling of Complex Gas Phase Reactions: From Expert Systems to Supercomputers.

A theoretical study of the dissociation of H2/Cu

Abstract: We present calculations for the dissociative adsorption of hydrogen molecules on a Cu surface as a function of initial translational energy and vibrational quantum state. Classical, semiclassical, and fully quantum calculations are performed and the results compared. The potential energy surface was based upon a total energy calculation for H2 on a small Cu cluster and has been previously employed in dynamical simulations. Our results show that for low primary beam energies, dissociation occurs primarily via tunneling through the activation barrier in the vibrational coordinate. Populating the initial vibrational states is shown to enhance reactivity, but not simply by a total energy shift. By changing the hydrogen isotope it is shown that tunneling effects can persist up to quite high molecular masses. This occurs because the activation barrier lies in the vibrational coordinate, where the reduced mass of the molecule determines the dynamics.

Accurate determination of a potential energy surface for CD3H

Abstract: We present in this paper an application of the Lanczos algorithm to the fitting of a potential energy surface from the experimental spectrum. This method presents two definite improvements on the conventional approach: (i) the Lanczos algorithm allows to treat ultra‐large basis sets (120 000 states in this calculation) and (ii) it is possible to tune selectively the calculation to specific components of the spectrum (e.g., a given combination band nνi+mνj) thus reducing considerably the complexity of the fit. This method has been applied to the CD3H molecule, considering all the vibrational degrees of freedom. Converged line positions have been obtained for high overtones of the C–H stretching mode (up to 16 000 cm−1). The accuracy of the fitted surface is demonstrated by direct comparison of experimental and calculated spectroscopical parameters.

Effective medium potentials for molecule–surface interactions: H2 on Cu and Ni surfaces

Abstract: A new approximate method is developed for the calculation of the adiabatic potential energy surface for a molecule outside a metal surface. It is computationally fast enough to be useful in simulations of the dynamics of adsorbing and desorbing molecules. The method is characterized by the fact that the functional form of the total energy expression is derived from density functional theory, that each of the terms entering can be given a precise physical interpretation, and that most of the parameters entering can be calculated, within the local density approximation. The method is explicitly derived for H2 outside metal surfaces and the applicability is illustrated for H2 adsorbing on various Cu and Ni surfaces. Although very approximate, the calculated potentials seem to include a number of features observed experimentally: Ni is more active in dissociating H2 than Cu, and open surfaces are more active than close‐packed ones. Moreover, the method is simple enough that one can contemplate studying variat...

Femtosecond real‐time probing of reactions. III. Inversion to the potential from femtosecond transition‐state spectroscopy experiments

Abstract: Femtosecond transition-state spectroscopy (FTS) of elementary reactions [M. Dantus, M. J. Rosker, and A. H. Zewail, J. Chem. Phys. 87, 2395 (1987)] provides real-time observations of photofragments in the process of formation. A classical mechanical description of the time-dependent absorption of fragments during photodissociation [R. Bersohn and A. H. Zewail, Ber. Bunsenges. Phys. Chem. 92, 373 (1988)] forms the basis for the present scheme for relating observations to the potential energy surface. A direct inversion scheme is presented that allows the difference in the two relevant excited-state potential curves to be deduced from observed transients at different probe wavelength tunings. In addition, from the shape and dependence of the transients on pump wavelength, information on the lower of the two potential curves (i.e., that of the dissociating molecule) is obtained. The methodology is applied to the experimental FTS data (Dantus et al.) on the CN photofragment from the ICN photodissociation.

Highly correlated single‐reference studies of the O3 potential surface. I. Effects of high order excitations on the equilibrium structure and harmonic force field of ozone

Abstract: The potential energy surface of ozone in the vicinity of the equilibrium geometry is investigated by single‐reference many‐body perturbation theory (MBPT) and coupled‐cluster (CC) methods. As expected from the known inadequacies of the independent‐particle picture of O3, analysis of the CCSDT‐1 wave function reveals considerable mixing between the [core⋅⋅⋅]4b226a211a22 and [core⋅⋅⋅]4b226a212b21 configurations. Smaller, but still significant, contributions come from other configurations involving redistribution of electrons within the out‐of‐plane π orbital framework. As expected, the equilibrium structure and harmonic force field computed at the SCF level of theory are in considerable error. When allowance is made for electron correlation effects, the discrepancies between theory and experiment for the equilibrium structure and totally symmetric force field are significantly reduced, and the MBPT(4), CCSD, CCSD+T(CCSD) and CCSDT‐1 results are in reasonable agreement with accepted values. Asymmetric stretc...

Intramolecular dynamics of van der Waals molecules: An extended infrared study of ArHF

Abstract: The near‐infrared spectrum of ArHF prepared in a slit supersonic expansion is recorded with a difference frequency infrared laser spectrometer. By virtue of the high sensitivity of the technique, and the lack of appreciable spectral congestion at the 10 K jet temperature, we observe 9 of the 11 vibrational states with energies below the Ar+HF(v=1, j=0) dissociation limit. These include (1000), the lowest bound HF (v=1) state, the singly, doubly, and quadruply van der Waals stretch excited states (1001) (1002), and (1004), both the Σ bend (1200) and Π bend (111e,f 0), and the multiply excited, Π bend plus van der Waals stretch (111e,f 1). Two Ar+HF(v=0) states, (0000) and (0001), are also characterized. This spectroscopic information is quite sensitive to the Ar+HF potential energy surface away from the equilibrium configuration, and thus provides a rigorous test of trial potential energy surfaces. Excellent agreement is obtained between experiment and the predictions of a recently reported Ar+HF(v=1) pote...

Vibrational spectrum, dipole moment function, and potential energy surface of the CH chromophore in CHX3 molecules

Abstract: The Fermi‐resonance overtone spectra of the CH chromophore up to about 18 000 cm−1 are evaluated by variational vibrational calculations for the CHX3 molecules trideuteromethane (CHD3), trifluoromethane (CHF3), chloroform (CHCl3) and 1,1,1,3,3,3‐hexafluoro‐2‐trifluoromethylpropane [(CF3)3CH]. Using appropriate model potential functions in a normal coordinate subspace, one can derive parameters for the CH chromophore potential and empirical dipole moment functions. For CHD3 and CHF3 ab initio (SCF‐CI and vibrational variational) calculations are presented, the results of which compare well with the experiments and for CHD3 also with previous (MRD‐CI) ab initio results. For all cases an accurate similarity transformation to the equivalent tridiagonal form of the effective hamiltonian can be made and the corresponding spectroscopic parameters agree with previous results. Comparison is also made with results from an internal coordinate model Hamiltonian.

Relaxed potential energy surfaces of maltose

Abstract: Experimentally observed solution conformations of carbohydrate molecules might correspond to a dynamical average of several interconverting conformers in solution. In order to understand and describe more precisely molecular flexibility and motions, new computational routes have to be envisaged. Compared to conventional approaches where sugar residues are treated as rigid, the optimization of all the internal parameters—i.e., bond angles, valence angles, and all torsional angles—is an important step toward more realistic information. Here we report the calculations of potential energy surfaces where all the internal coordinates of the molecules were “relaxed” and minimized through an extensive molecular mechanics scheme. For this work, a prototypical carbohydrate system, the disaccharide α-maltose, was selected. The inclusion of the relaxed principle into conformational description of maltose does not generally alter the overall shape of the allowed low-energy regions, or the position of the local minima. However, flexibility within the ring plays a crucial role. Its principle effect is the lowering of energy barriers to conformational transitions about the glycosidic bonds, permitting pathways among the low-energy minima. This occurs with retaining the overall 4C1 conformation of the glucose residues. The torsional angles corresponding to the orientations of the hydroxyl groups, especially the primary hydroxyl ones, display stable arrangements separated by energy barriers. They create subpopulations of stable conformers and it has not been possible to take into consideration interconversion of one subpopulation to another one. A “synthetic” relaxed potential energy surface is proposed, which can provide a realistic starting base for further investigation of solution behavior of dynamic simulations.

Analysis of the potential energy surface of Ar–NH3

Abstract: The combination of supermolecular Mo/ller–Plesset treatment with the perturbation theory of intermolecular forces is applied in the analysis of the potential energy surface of Ar–NH3. Anisotropy of the self‐consistent field (SCF) potential is determined by the first‐order exchange repulsion. Second‐order dispersion energy, the dominating attractive contribution, is anisotropic in the reciprocal sense to the first‐order exchange, i.e., minima in one nearly coincide with maxima in the other. The estimated second‐order correlation correction to the exchange effect is nearly as large as a half ΔESCF in the minimum and has a ‘‘smoothing’’ effect on the anisotropy of e(20)disp. The model which combines ΔESCF with dispersion energy (SCF+D) is not accurate enough to quantitatively describe both radial and angular dependence of interaction energy. Comparison is also made between Ar–NH3 and Ar–PH3, as well as with the Ar dimer.

Theoretical studies of the hydrogen peroxide potential surface. 1. An ab initio anharmonic force field

Abstract: Ab initio GVB + 1 + 2 calculations, employing a (4s3p2d1f/3s2p) basis set, have been used to characterize the ground-state potential energy surface of hydrogen peroxide. An anharmonic force field that is quartic in the nontorisonal degrees of freedom and higher order in the torsion is reported. A second-order perturbation theory calculation of the vibrational energy levels using the ab initio force field yields fundamental frequencies that are within 25 cm{sup {minus}1} (3%) of experiment. A reaction path analysis of the torsional mode is also presented. The predicted tunneling splitting between the lowest pair of torsional states is 11.214 cm{sup {minus}1}. Also calculated are tunneling splittings for excited vibrational states for both H{sub 2}O{sub 2} and D{sub 2}O{sub 2}.

Photodissociation dynamics of water in the second absorption band. II. Ab initio calculation of the absorption spectra for H2O and D2O and dynamical interpretation of ‘‘diffuse vibrational’’ structures

Abstract: We calculated the absorption spectra of H2O and D2O in the second absorption band around 128 nm using a two‐dimensional ab initio potential energy surface for the B(1A1) electronic state. Nonadiabatic coupling to the lower states A and X and the vibrational degree of freedom of the OH fragment are completely neglected. Despite these limitations the agreement with the measured spectra is very satisfactory. The overall shape, the width, and the energetical position of the maximum are well described. Most important, however, is the reproduction of the diffuse vibrational structures superimposed on the broad background. It is demonstrated that this structure is not caused by pure bending‐excitation in the B state with associated bending quantum numbers ν’2=1,2,3,... as originally assumed. Because the equilibrium HOH bending angle and the equilibrium H–OH distance are very different in the ground and in the excited state, the main part of the spectrum and especially the diffuse structures occur at high ene...

Numerical implementation of reactive scattering theory

Abstract: Algorithms for the effective calculation of reactive scattering probabilities are developed and tested on the hydrogenic atom–diatom system described by the Siegbahn–Liu–Truhlar–Horowitz potential energy surface. A three‐dimensional finite element procedure is designed from a description in terms of hyperspherical coordinates. The Wigner–Eisenbud R‐matrix theory is used for a recursive procedure which admits control with limits on the hyperradial propagation inward from an asymptotic region and for a symmetry preserving transformation to arrangement channel Jacobi coordinates.

Reaction rates of H(H2), D(H2), and H(D2) van der Waals molecules and the threshold behavior of the bimolecular gas‐phase rate coefficient

Abstract: We study low‐energy quantal phenomena in the rearrangement of three‐atom systems composed of H and D. All calculations are carried out on the double many‐body expansion potential energy surface for the hydrogen trimer. The unimolecular rearrangements of van der Waals molecules, such as D⋅⋅⋅H2→HD⋅⋅⋅H, are studied as a model for the exchange transfer reaction in condensed phases, and the gas‐phase bimolecular reactions, such as D+H2→HD+H, are studied to probe the limiting low‐temperature threshold behavior, which is compared to that predicted by quantum mechanical threshold laws. The reaction rates are studied down to temperatures of 10−3 K. We also predict the spectroscopic tunneling shift on the lowest energy levels of the H⋅⋅⋅H2 complex.

Spectroscopy and photodissociation dynamics of H2O: Time‐dependent view

Abstract: We present a study of H2O photodissociation dynamics in the first excited state. The absorption and emission spectra are calculated quantum mechanically using a time‐dependent wave packet propagation technique. The excited state potential energy surface is obtained from ab initio data generated by Palma et al., while the ground state is a model by Reimers and Watts cast in a simple analytical form and constructed by fitting to infrared data. The variation of the transition dipole moment is taken into account by using an analytical functional form which is adjusted to fit the low resolution experimental emission spectrum. The calculated absorption spectrum is very similar to both the experimental one and results obtained with other methods. The high resolution emission spectrum is calculated for the first time and shows excellent agreement with the recently measured experimental spectrum by Hudson and co‐workers. Dynamics on both the ground and excited states are examined.

Fluxionality and stereochemistry of 5-coordinate Cu2+ complexes. The potential energy surface and spectroscopic implications

Abstract: Cu2+ ions in five-coordination (Cl−1, NH3, NCS−, etc.) generally stabilize an elongated square pyramid, which is slightly preferred to a compressed trigonal bipyramid. This stereochemical behaviour can be understood by considering the vibronic interaction between the A′1 ground state and the first excited E′ state via the e′ vibrations in D3h symmetry (pseudo-Jahn-Teller coupling), in combination with an E′ ⊗ e′ interaction (Jahn-Teller coupling in the excited state), leading into the lower C2v (C4v) and Cs symmetries. The adiabatic ground state potential surface is calculated in a semi-empirical model on the basis of available spectroscopic data. The minima at the points, which characterize the elongated C2v (C4v) geometry, are rather flat and can be shifted to any other point of the potential surface by steric ligand and/or geometric packing influences. The CuCl53− square pyramids in [Co (NH3)6] CuCl5 undergo a pseudorotation to (dynamically averaged) trigonal bipyramids at 280 K, with the Cl− ligands along the threefold axis remaining fixed in space. In contrast the nuclear displacements along the e′ coordinates occur in an unrestricted manner above 285 K in case of the Cu(NH3)52+ square pyramids in [Cu(NH3)5]Br2 (“Berry rotation”). The wavefunctions, g-tensor components and d-d transition energies have been derived as functions of the e′ distortion coordinates and were explicitly calculated for the CuCl53− model case.

Analytical gradients for unrestricted Hartree–Fock and second order Mo/ller–Plesset perturbation theory with single spin annihilation

Abstract: Unrestricted Hartree–Fock (UHF) wave functions and Mo/ller–Plesset perturbation theory (UMPn) based on a single spin‐unrestricted reference determinant can contain significant contamination from unwanted spin states. This contamination may lead to large distortions of the potential energy surface, particularly at the UMPn level. A simple approximation to projected UMPn theory (PMPn) improves the shape of the potential energy surface significantly. Formulas for analytical gradients of the PUHF and approximate projected UMP2 energies with single annihilation have been derived and programmed. This code has been applied to the optimization of transition states for H+C2H4, H+CH2O and H+C2H2 at the PMP2/6‐31G* level.