bluuebunny/crossref_metadata_2025_split huggingface dataset:
Calculation of time autocorrelation and spectral functions using locally expanded potentials
Authors: Cesar Amary , Ågren Hans 
Published: January 2005

AbstractWe present theory for autocorrelation and spectral functions and for the description of molecular motions on a multidimensional potential energy surface. The basic idea is to express the autocorrelation functions by stepwise series expansions of the molecular surface using only local information. This approach thus presents one way to avoid the dimensionality catastrophe that is associated with dynamic studies using globally calculated and fitted polyatomic energy surfaces. The autocorrelation function is obtained by solving the quantum equations of motions in a set of locally defined internal coordinates uncoupled to rotation and translation. A Fourier transform of the autocorrelation function gives a time‐evolved interpretation of the formation of the vibronic spectrum. Locally expanded and quadratically approximated Hamiltonians are assumed throughout the reaction path. The formalism implements in a direct fashion the information obtained from molecular gradient and Hessian techniques, so that the buildup of the spectrum can be followed along with the reaction walks on the molecular hypersurface. The method applies to unbound or quasi‐bound as well as to bound potentials. In the latter case, the calculation of the autocorrelation function also provides a cost‐effective way to perform Franck–Condon analyses, since, in contrast to conventional Franck–Condon methods, it avoids sum‐over‐state and basisset procedures completely. A set of 3N – 6 Cartesian coordinates are analyzed for the purpose of translational and rotational‐free walks on a locally and quadratically expanded multidimensional potential energy surface without recoursing to the standard Eckart conditions and/or the necessity of performing supplementary transformations on the gradient vector and the Hessian matrix. Demonstrations of the formalism are given by calculations of time autocorrelation and spectral functions using locally expanded potentials corresponding to N2 2IIu and O2 2IIg photoionization.
Theoretical study on the reaction mechanism of the alcoholysis of isocyanates
Authors: Tang Ming‐Sheng , Fu Xiao‐Yuan 
Published: January 2005

AbstractIn this paper, the reactions of HNCO or CH3NCO with methanol have been studied using the ab initio MO method. The geometries of the reactants and products have been optimized by the energy gradient method. The calculated results are in accordance with experiment. Using Powell's method, by the minimization of the Euclidean norm σ of the gradient, the structures of the transition state (TS) for the two reactions were obtained. The structures have been further confirmed as TS by finding that there is one and only one negative eigenvalue for their force constant matrix. The imaginary vibration mode corresponding to the TS was also discussed. The calculated activation barriers of these two reactions are 96.02 and 95.13 kJ/mol, respectively. It can be concluded that the alcoholysis reaction of isocyanate is a nucleophilic addition reaction with methanol as an electronic donor and isocyanate as an acceptor and that the hydroxyl hydrogen of CH3OH plays an important role in the reaction.
Molecular integrals of Breit interaction over Laguerre Gaussian‐type functions
Authors: Fujimura Naoki , Matsuoka Osamu 
Published: January 2005

AbstractThe Breit interaction consists of two interactions of the magnetic and the retardation terms. The molecular integrals involving the former interaction are shown to be reduced to the familiar integrals of the Coulomb repulsion. The integrals involving the latter interaction are formulated over the Laguerre Gaussian‐type functions and some advantages of the formulation are discussed. Bayman's gradient formula of the solid spherical harmonic is generalized to include the gradient of the homogeneous solid spherical harmonic.
On the evaluation of overlap integrals with exponential‐type basis functions
Authors: Homeier Herbert H. H. , Steinborn E. Otto 
Published: January 2005

AbstractThe numerical properties of a one‐dimensional integral representation [H.P. Trivedi and E. O. Steinborn, Phys. Rev. A 27, 670 (1983)] for the overlap integral of a two‐center product of certain exponential‐type orbitals (ETOs) the B functions [E. Filter and E. O. Steinborn, Phys. Rev. A 18, 1 (1978)], are examined. B functions possess a very simple Fourier transform that results in relatively simple general formulas for molecular integrals derivable using the Fourier transform method. In addition, molecular integrals for other ETOs, like the more common Slater‐type orbitals, can be written as finite linear combinations of integrals with B functions because these functions span the space of ETOs. The integrand of the integral representation mentioned above shows peculiarities requiring special quadrature methods, especially in the case of highly asymmetric charge distributions. It is shown that good results can be obtained using Möbiustransformation‐based quadrature rules well suited for the numerical integration of functions possessing a sharp peak at or near one boundary of integration [H. H. H. Homeier and E. O. Steinborn, J. Comput. Phys. 87, 61 (1990)]. The method based on Möbius‐type quadrature is compared to several other methods — based on other quadrature rules, finite sums, and infinite series representations together with convergence acceleration — to evaluate overlap integrals with B functions. The numerical results indicate that the new quadrature schemes are more efficient than are the other methods.
Dynamic stability of silacarbonyl ylide
Authors: Tachibana Akitomo , Fueno Hiroyuki , Okazaki Iwao , Yamabe Tokio 
Published: January 2005

AbstractThe structure of silacarbonyl ylide in the singlet gound state is obtained by ab initio quantum chemical calculations. This proves the recent experimental observation of silacarbonyl ylide by the photolysis of oxasilitane [W. Ando, K. Hagiwara, and A. Sekiguchi, Organometallics 6, 2270 (1987)]. The dynamic stability of the silacarbonyl ylide is studied by the TCSCF/3‐21G(*) method. The second‐order Jahn‐Teller effect is examined in connection with the active role of frontier orbitals. Configuration analysis is used to clarify characteristic electronic structure of the silylene‐carbonyl interaction.
Structure of Mn2(CO)10 and Mn2(CO)9: Insights from ab initio calculations
Authors: Veillard Alain , Rohmer Marie‐Madeleine 
Published: January 2005

AbstractOptimization of the Mn–Mn distance in Mn2(CO)10 with various basis sets of at least doublezeta quality results in Mn–Mn bond lengths in the range of 3.07–3.15 Å, 0.2–0.25 Å longer than the experimental value of 2.895 Å. Incrementing the basis set with diffuse p functions (exponent 0.0332) on the carbon atoms improves the calculated bond length to a value of 2.876 Å at the CI level, as a consequence of a charge transfer between each Mn atom and the equatorial carbonyls of the other Mn atom. For Mn2(CO)9 four structures have been studied at the SCF and CI levels with assumed geometries. The structure with a symmetric bridging carbonyl turns to be much higher in energy at the SCF level. The two structures which are purely metal–metal bonded [corresponding to the departure of an axial or equatorial carbonyl from Mn2(CO)10] are nearly degenerate in energy and more stable than the structure with a semibridging carbonyl by 5 kcal/mol at the SCF level and 10–11 kcal/mol at the CI level (seemingly at variance with the conclusions of matrix experiments that favor the semibridging structure).
Relativistic corrections in the LCGTO–local‐spin‐density method. I. Atomic bases for transition metals
Authors: Wilkin A. , Salahub D. R. 
Published: January 2005

AbstractCompact orbital GTO basis sets optimized with a nonrelativistic Hamiltonian, then decontracted, are utilized in an SCF treatment with a quasi‐relativistic scalar Hamiltonian including the mass–velocity and one‐electron Darwin operators. Ionization and excitation energies, orbital energies, and radial mean values obtained from different expansion patterns have been tested in atomic calculations for Ag and generalized for Cu and Au atoms. The one‐electron spin–orbit operator is also used in an SCF treatment. Spin–orbit coupling energies are calculated for Cu, Ag, and Au atoms.
Determination of ab initio polarizabilities of polymers: Application to polyethylene and polysilane
Authors: Champagne Benoit , André Jean‐Marie 
Published: January 2005

AbstractAn ab initio method for calculating the longitudinal linear polarizability of polymeric chains is described. This method is equivalent to an uncoupled Hartree–Fock scheme. It is applied to polyethylene and polysilane in minimal STO‐3G and extended 4‐31G basis sets. The study describes important techniques for solving the difficulties met in actual calculations: band reordering of the band structures, calculation of analytical derivatives of the energy bands ϵn(k) and LCAO coefficients cnp(k), and errors caused by the improper lattice sum truncations of the Hartree–Fock matrix.
The electronic transition energies of four alternative C60 molecules
Authors: László I. , Udvardi L. 
Published: January 2005

AbstractThe low‐lying allowed electronic transition energies of four alternative structures to the C60 cage molecule are examined. The bond turning transformation of Stone and Wales is used to generate new C60 structures with the symmetries C2v, D2h, and D6h. We have found that the C60 is indeed a truncated icosahedron as originally proposed, but the production of other alternative structures in specific experimental conditions is not excluded.
Effect of conjugated length on the calculated nonlinear second‐order optical susceptibilities of some organic molecules
Authors: Gao X. L. , Feng J. K. , Sun C. C. 
Published: January 2005

AbstractCalculations of the nonlinear second‐order optical susceptibilities (βijk) for disubstituted conjugated organic molecules are reported. The calculations are performed using the semiempirical INDO/CI method combined with a sum‐over‐states expression for βijk. The calculated results show that the second‐order static susceptibility (β0) varies as the 2.6 power of the length of the conjugated π‐electron system. Furthermore, it is found that the amount of substituent charge transfer from ground state to first excited state decreases with inccreasing chain length. A limiting value of β0 with the length of the conjugated system is discussed.
Exact nonlinear optical coefficients of quantum cell models with interacting electrons
Authors: Soos Z. G. , McWilliams P. C. M. , Hayden G. W. 
Published: January 2005

AbstractDiagrammatic valence‐bond (DVB) methods yield exact dynamic nonlinear optical (NLO) coefficients for π‐electron models of conjugated molecules and polymers. We contrast exact coefficients with mean‐field or Hückel results and discuss several qualitative differences. The length and alternation dependencies of NLO spectra for Pariser–Parr–Pople (PPP) models provide a unified treatment of even‐parity states and two‐photon processes in polyenes, polydiacetylenes, and polysilanes. We emphasize relationships between one‐ and two‐photon spectra, correlation effects involving both parallel and transverse NLO coefficients, and the nature of correlated states in polar conjugated molecules and in polymers with centrosymmetric backbones.
A dispersion formula for the second dynamic hyperpolarizability as exemplified by helium
Authors: Bishop David M. , Pipin Janusz 
Published: January 2005

AbstractA simple one‐parameter formula:
equation image
is introduced for the description of the dispersion in the second dynamic hyperpolarizability (γ), where ω = ω1 + ω2 + ω3 and ω = ω + ω + ω + ω. This formula is appropriate for both atoms and the macroscopic hyperpolarizabilities of molecules (as defined with respect to laboratory axes). Its suitability is tested for helium for five different nonlinear optical processes (including, for the first time, the ac Kerr effect). A comparison is made between the values of 𝒜 found by fitting ab initio γ values and those derived from a formula proposed by Hellwarth et al.
Theoretical study on solvation effects in chemical reactions: A vibrational coupling model
Authors: Tachikawa Hiroto , Lunell Sten , Törnkvist Christer , Lund Anders 
Published: January 2005

AbstractA vibrational coupling model to treat the solvation effects in chemical reaction rate calculations is proposed and applied to the intramolecular hydrogen transfer reaction CH3O· → ·CH2OH in the condensed phase. The effect of solvation is taken into account in two ways: (1) the solvent effect on the activation energy of the reaction is simulated by including 39 surrounding water molecules, represented by fractional charges at the assumed atomic positions, in the potential energy surface calculation; and (2) the vibrational couplings between the 10 nearest solvent molecules and the molecules constituting the reaction system are explicitly included in a vibrational frequency calculation. RRKM theory with Miller's tunneling correction included is employed to calculate the rate constants. The effect of solvation causes a significant change in the chemical reaction rate, mainly through a lowering of the activation energy. The vibrational coupling causes a slight increase of the rate constant in the tunneling region by perturbing the vibrational frequencies of the reactant and transition states, which appear in the rate‐constant expression, but has little effect at higher temperatures.
Small‐angle electron scattering by formaldehyde and ketene: Effects of electron correlation and chemical binding
Authors: Sasaki Yorihiko , Takeuchi Hiroshi , Konaka Shigehiro , Kimura Masao 
Published: January 2005

AbstractThe total (elastic plus inelastic) intensities of 51 keV electrons scattered by H2CO and H2CCO have been measured over a range of K = (4π/λ) sin(θ/2) = 1–9.5 Å−1 and compared with the theoretical intensities calculated with SCF and CI wave functions. Significant discrepancies are found between the experimental intensities and the theoretical ones based on the SCF wave functions. Most of the chemical binding and electron correlation effects observed in the total scattered intensities are reproduced by the theoretical intensities based on the CI wave functions calculated with the basis set including polarization functions on all atoms. © 1992 John Wiley & Sons, Inc.
An accurate lower‐bound calculation of the lowest triplet states of he and its isoelectronic series
Authors: Kleindienst H. , De Groot U. 
Published: January 2005

AbstractAccurate lower bounds for the nonrelativistic lowest triplet states of 4He and ∞He and the He isoelectronic series up to Z = 10 were calculated by the method of variance minimization using wave functions with logarithmic terms. Furthermore, the method of Lehmann/Goerisch was extended to self‐adjoint operators with an essential spectrum. Both methods were compared for 4He. © 1992 John Wiley & Sons, Inc.
Conformational studies of (S)idazoxan and (S)methoxyidazoxan using AM1 and PM3 semiempirical molecular orbital methods
Authors: Hariharan Sivaram , Shelver William H. 
Published: January 2005

AbstractSemiempirical calculations of the α‐2 receptor antagonist, Idazoxan, its methoxy derivative, and their respective cations and anions revealed the most stable conformers were significantly affected by electrostatic interactions. These studies showed the importance of the intramolecular electrostatic interactions between the oxygen of the benzodioxane ring and the nitrogens of the imidazoline ring in determining the global minima of these molecules. The NH bond of the imidazoline ring was attracted to the dioxane oxygen and caused the imidazoline ring to rotate to a position allowing close proximity of these groups. Favorable interactions between the oxygens and the imidazoline stabilized the axial conformer, whereas unfavorable interactions increased the stability of the equatorial isomer. AM1 and PM3 calculations sometimes predicted different global minimum conformers, indicating the need for caution in reliance on either method, particularly when hydrogen bonds play an important role in determining the structure. © 1992 John Wiley & Sons, Inc.
Conformational study of Met‐enkephalin in its zwitterionic form
Authors: Perez Juan J. , Loew Gilda H. , Villar Hugo O. 
Published: January 2005

AbstractAn extensive exploration of Met‐enkephalin in its zwitterionic form has been carried out, in order to characterize the different low‐energy conformational domains accessible to this pentapeptide. The study builds on previous studies carried out in our lab for the neutral molecule, which provided the initial geometries from which the conformational space of the charged molecule could be scanned. The initial conformations were subjected to a series of high‐ and lowtemperature molecular dynamics simulations. Snapshots along each trajectory were taken, minimized, and used as starting points in further MD trajectories until no lower‐energy conformers could be characterized. The CHARMm force field was used throughout the study for this purpose. The same search strategy was used in these studies simulating two different environmental conditions, a distance‐dependent dielectric ϵ = r and a high constant dielectric ϵ = 80. In the low dielectric environment, the formation of the salt bridge dominates the structure. In the high dielectric environment, the screening of the electrostatic interactions results in weaker intramolecular interactions. In both cases, the Gly2–Gly3 β‐turn‐type structures are preferred over the Gly3–Phe4 turns, in marked contrast to what is found for the neutral molecule. The lowest‐energy structures from both environmental conditions were reoptimized in the presence of a cluster of explicit water molecules. Reoptimization of the structures considering explicit water structures did not result in significant conformational changes for the structures characterized with the ε = r or ε = 80 environments.
Do intelligent configuration search techniques outperform random search for large molecules?
Authors: Judson R. S. , Colvin M. E. , Meza J. C. , Huffer A. , Gutierrez D. 
Published: January 2005

AbstractWe compare three global configuration search methods on a scalable model problem to measure relative performance over a range of molecule sizes. Our model problem is a 2‐D polymer composed of atoms connected by rigid rods in which all pairs of atoms interact via Lennard–Jones potentials. The global minimum energy can be calculated analytically. The search methods are all hybrids combining a global sampling algorithm with a local refinement technique. The sampling methods are simulated annealing (SA), genetic algorithms (GA), and random search. Each of these uses a conjugate gradient (CG) routine to perform the local refinement. Both GA and SA perform progressively better relative to random search as the molecule size increases. We also test two other local refinement techniques in addition to CG, coupled to random search as the global method. These are simplex followed by CG and simplex followed by block‐truncated Newton. For small problems, the addition of the intermediate simplex step improved the performance of the overall hybrid method. © 1992 John Wiley & Sons, Inc.
Investigating the possibility of simultaneously finding an electron‐hole and an electron‐pair in a molecule: Delocalization, competition of ionic vs. covalent character, and related effects in push–pull ethylenes
Authors: Karafiloglou Padeleimon , Marcos Enrique Sánchez 
Published: January 2005

AbstractThis work examines the possibility of finding an electron‐hole and an electron‐pair simultaneously in a π‐system substituted by an electron‐donating (NH2) and/or electron‐withdrawing (NO2) group. The contributions of various ionic [(+), ( ↑ ↓ )] structures are calculated from ab initio SCF–CI wave functions, using a recently developed general multielectron population analysis. The molecules studied are two monosubstituted ethylenes, aminoethylene and nitroethylene, and a disubstituted ethylene, the 2‐nitroethenamine (push–pull ethylene) in its two configurational forms. The influence of the NH2 and/or NO2 group in delocalization and ionic (vs. covalent) character of the CC double bond are investigated, along with examining the experimental chemist formalism of electron‐pair “displacements” in several resonance structures. Analysis of mutual dependence of an electron‐hole and an electron‐pair, at short and long distances, leads to the conclusion that a push–pull π‐system can stimulate the simultaneous existence of an electronhole and an electron‐pair even for nonvicinal positions. The relationship between the electronpair distributions and contributions of the corresponding structures are also examined; the effects of electronic correlation are analyzed as well. © 1992 John Wiley & Sons, Inc.
Theoretical study of the thermal unimolecular rearrangement of fluoroethylidenes
Authors: Kakkar Rita , Walia Vibha 
Published: January 2005

AbstractThe thermal unimolecular isomerization of fluoroethylidenes to the corresponding fluoroethylenes has been studied by the MNDO method. It has been shown that fluorine substitution on the carbene carbon increases the activation energy in comparison with the ethylidene rearrangement. To understand the reason for this increase in the activation energy, the charge‐transfer effects have been analyzed. Fluorine substitution at other positions does not significantly affect the activation energies. The thermodynamic parameters for the reaction have been evaluated, using vibrational and rotational spectral data calculated in this work. RRKM calculations have been performed and high‐pressure Arrhenius parameters calculated. Hydrogen–deuterium kinetic isotope effects indicate that the reaction rates are altered considerably on isotopic substitution, and the change in reaction rates depends upon the position of deuterium substitution, as well as on the number of hydrogens replaced by deuterium atoms. © 1992 John Wiley & Sons, Inc.
A molecular orbital treatment of the electronic spectra of some tryptamines
Authors: Hamed Maher M. , Abu‐Eittah Rafie H. , Mobark Zein , Abdou Mohamed M. 
Published: January 2005

AbstractThe electronic absorption spectra of tryptamine, 5‐methoxytryptamine, 6‐fluorotryptamine, N‐acetyl‐5‐hydroxytryptamine, gramine, and melatonin were investigated. The observed transitions were π–π*, and the values of band maxima and intensity reflected an extent of interaction between the indole ring and the alkylamine side chain. Molecular orbital calculations at the level of INDO/S–CI were performed on all the studied molecules. State functions and transition energies were calculated. The correspondence between the experimental and theoretical results was satisfactory. © 1992 John Wiley & Sons, Inc.
Electronic hamiltonian of diatomic molecules in the basis of coupled momenta eigenfunctions
Authors: Kruglevsky V. 
Published: January 2005

AbstractA systematic procedure has been developed to construct an electronic energy matrix for diatomics in the basis of antisymmetrized products of atomic wave functions represented as linear combinations of coupled momenta eigenfunctions. The exchange matrix element is expanded in powers of electronic interchange between atoms. General expressions of many‐electron angular coefficients have been obtained for all types of products of one‐ and two‐electron and overlap integrals in energy matrix elements. © 1992 John Wiley & Sons, Inc.