bluuebunny/crossref_metadata_2025_split huggingface dataset:
Attractive‐dominant and repulsive‐dominant hydrocarbon reactions
Authors: George Philip , Bock Charles W. , Trachtman Mendel , Brett Alistair M. 
Published: December 2004

AbstractThe difference between the expectation values of the total electronic kinetic energy operator (ΔEK), and the operators accounting for the Coulombic interactions between the electrons and nuclei (ΔVen), between all pairs of electrons (ΔVee), and between all pairs of nuclei (ΔVnn) for the product and reactant species in a wide variety of hydrocarbon reactions are calculated using single determinant basis set data reported in the literature. Following Allen, their contributions to ΔET, the difference between the corresponding total molecular energies and thus the reaction heat, are grouped together as a repulsion energy term, ΔErep = ΔEK + ΔVee + ΔVnn, and an attraction energy term ΔEattr = ΔVen. For all but 2 of the 71 individual reactions considered in this paper, the experimental reaction heat at 0°K corrected for zero‐point energy contributions, (ΔH)zpe, is the result of near compensation between far larger ΔErep and ΔEattr terms, in sharp contrast to the much smaller ΔErep and ΔEattr terms which are characteristic of many molecular rotation processes. By matching the sign of (ΔH)zpe with that of ΔErep or ΔEattr, as the case may be, the reactions are classified as attractive‐dominant or repulsive‐dominant (46 in the former class and 23 in the latter), a property which is independent of the direction in which the reaction is written. The sign and magnitude of ΔVee, ΔVnn, and ΔVen and reaction category are discussed in relation to the various kinds of structural change involved in going from reactants to products. For the vast majority of reactions, the numerical relationship ΔVee ≈ ΔVnn has been found to hold to within a few percent.
Comparison of UHF and PHF methods for hyperfine structure calculations
Authors: Lunell Sten 
Published: December 2004

AbstractThe validity of two models used in comparisons of the UHF and PHF methods in hyperfine structure calculations is investigated for the lowest 2P state of Li. An improved model for the angular splitting of the core orbitals is proposed and shown to give an accurate description of both the UHF and the PHF wave functions.
Invariance properties of the multipole expansion with respect to the choice of the coordinate system
Authors: Stolarczyk Leszek Z. , Piela Lucjan 
Published: December 2004

AbstractThe physical interpretation of intermolecular interactions is usually based on the well‐known multipole expansion of the inverse of the interparticle distance. The interaction energy is then interpreted as a sum of terms arising from the interaction of various multipole moments of both systems. It is supposed that the interaction energy calculated via the truncated multipole expansion generally depends on the choice of local coordinate systems through the coordinate dependence of the multipole moments. In this paper we prove that each term of the multipole expansion given in the form ∑k = 1 Ck/Rk is invariant with respect to identical translations and arbitrary rotations of the local coordinate systems. The invariant form of the convergence criterion of the multipole expansion is given and discussed.
Improved uncoupled Hartree–Fock (IUCHF) perturbation methods and bounds for the second‐order energy in coupled Hartree–Fock perturbation theory
Authors: Vogler H. 
Published: December 2004

AbstractDifferent kinds of improved uncoupled Hartree–Fock methods are proposed for the calculation of second‐order perturbation energies. Using these methods inequalities are derived for the error of the uncoupled procedure with geometric approximation.
Continuum contribution to polarizabilities and scaling
Authors: Canuto Sylvio , Goscinski Osvaldo 
Published: December 2004

AbstractThe role of the continuum, so important in ground‐state properties, is known to be less important and may even be negligible in the computation of polarizabilities and other higher negative moments of the oscillator strength distribution. This can be rationalized in terms of the solutions of the inhomogeneous equation generating the moments as an alternative to the dominating first term in an eigenfunction expansion. This leads in a natural form to the approximation Fϕ0(1 + w(r)) for the perturbed wave function where 1 + w(r) represents a nonuniform scaling of the unperturbed function ϕ0.
Realization of the elementary biological function as a localized electron subsystem interaction process
Authors: Ananchenkov V. I. , Leontyev V. B. 
Published: December 2004

AbstractWhile regarding the elementary biological function as a series of elementary acts resulting in either chemical bond formation (breakage) or a changed molecular subsystem state, we would like to focus attention on some of the most essential and general features of this process in terms of the Von Neumann theory of self‐reproducing automata.
Electron mechanism of nonequilibrium molecular vibration excitation in biological systems
Authors: Kovarskii V. A. , Chaikovskii I. A. , Popov E. A. 
Published: December 2004

AbstractThe electron mechanism of the nonequilibrium vibration excitation is considered in multiatom molecules with the condition that the bottleneck effect takes place. The quantum‐statistical theory states that if the simplest enzyme reaction is proposed on the basis of the vibrational energy recuperation idea, the vibrons emitted in producing the free product and enzyme activate the first stage of the formation of the Michaelis–Menten complex and the multiphonon feedback mechanism is realized. Resulting solutions are applied to explain some biochemical and genetic effects.
Conformational studies of double‐helical polynucleotides by the method of pair‐wise potential functions
Authors: Il'Icheva I. A. , Tumanyan V. G. , Kister A. E. , Dashevsky V. G. 
Published: December 2004

AbstractDouble‐helical polynucleotide conformations, poly(dA)·poly(dT), poly(d(A‐T))·poly(d(T‐A))·poly(dG)·poly(dC), and poly(d(G‐C))·poly(d(C‐G)) are analyzed by the atom–atom potential method. The energy optimization is carried out in the space of eight independent geometric parameters using analytical procedures for the constraints, taking into account the flexibility of the β‐D‐deoxyribose rings. At the first stage, the full screening of atomic partial charges was assumed. The structures of the calculated B and the A forms of DNA are characterized by low energy and absence of short contacts; the dihedral angles are near the average values in the monomers. With the typical energy difference of 3–5 kcal/mol nucleotide pairs in all cases, the B form is more preferable as compared to the A form. At the final step the effect of the Coulomb term is evaluated for poly(dA)·poly(dT) using various values of the effective dielectric constant (ϵ = 28, 24, 20, 18, 14, 12, 10, 8, 6, 4, and 1). If ϵ ⩽24, the energy optimization leads A to B. We discuss the stereochemical details of the intermediate conformations on the A–B path and hypothesize the nature of stability of the A and the B forms and the mechanism of the A–B transition.
Study of pair correlation in three‐electron atoms
Authors: Ho Fat D. , Sanders Frank C. 
Published: December 2004

AbstractPair correlation in the ground state of the Li isoelectronic sequence is studied through four approximate wave functions which incorporate inter‐ and intrashell pair correlation. Of these functions, two possess symmetry appropriate to a three‐electron system, while two do not. The functions are not variational functions in the usual sense. They are instead fixed linear combinations of products of orbitals and pair functions for the appropriate states of two‐electron atoms. They are considered here as zero‐order approximations to the exact wave functions, and the corresponding zero‐order Hamiltonians are obtained. The simplest of these functions is improved by the introduction of a screening parameter for the “outer” electron. This latter function is found to be a satisfactory compromise between accuracy and simplicity and is proposed for study via higher‐order perturbation theory.
Behavior of the electron density near an impurity
Authors: Adawi I. 
Published: December 2004

AbstractThe behavior of the electron density n(r) and potential energy V(r) near the origin, where an impurity of charge Z is located, is studied using the Lindhard dielectric theory of the free‐electron gas. The leading odd terms in the power‐series expansion of n(r) and V(r) are obtained. It is shown that the derivative n′(0) = −2Zn0/a0, where n0 is the free‐electron gas density and a0 is the Bohr radius.
New self‐consistent theory of atoms: Toward a unified theory of atoms and atomic aggregates
Authors: Ogawa Tohru 
Published: December 2004

AbstractA variational theory is proposed where the distance dependence of two‐center integrals is semianalytically given and the outer‐shell orbitals are eigenfunctions of the one‐particle Schrödinger equations. It is useful in the attempt of unifying the theory of atoms and atomic aggregates such as molecules, microclusters, crystals, etc., especially in the problems of predicting their stable structures. The purpose of this theory is not to derive lower ground‐state energies but to obtain some useful wave functions for the explanation of some tendencies of the atomic aggregates in the periodic table.
Formulation of the LCAS MS SCF method within the Gaussian basis set
Authors: Roszak Szczepan , Chojnacki Henryk 
Published: December 2004

AbstractWithin the presented LCAS MS (linear combination of atomic spinors–molecular spinors) SCF formalism both large and small components of the spinor radial parts have been expanded within the Gaussian basis set. The respective expressions for matrix elements as well as for one‐ and two‐electron integrals are given.
Vertical ionization potential in hydrogen molecule with many‐body Green's function method
Authors: Uehara Hiroshi , Itagaki Takuma 
Published: December 2004

AbstractThe many‐body Green's function method is applied to the vertical ionization potential of the hydrogen molecule. The ionization potential is calculated iteratively by expanding the self‐energy part up to third order. The effects of higher‐order correlation corrections and nondiagonal self‐energy elements on the solutions of the Dyson equation are examined with some techniques and approximations, by means of which a Koopmans' defect of 97.7% of the accurate value is obtained.
Temperature effect on the vapor‐phase exciplexes of cyano‐substituted anthracenes
Authors: Hirayama Satoshi 
Published: December 2004

AbstractThe emission spectra and lifetimes of the vapor‐phase exciplexes of four cyano‐substituted anthracenes with N,N‐dimethylaniline (DMA) as a donor are examined over a wide temperature range. The activation energies associated with the exciplex dissociation are calculated to be of the order of 10 kcal/mol. The entropy change in forming the exciplexes is discussed in relation to the efficiency of the exciplex formation. For various donors other than DMA, preliminary results on how they interact with excited 9,10‐dicyanoanthracene are also given.
Vibrational spectra of molecules in the Hartree–Fock dielectric screening approach
Authors: Van Camp P. E. , Van Doren V. E. , Devreese J. T. 
Published: December 2004

AbstractTraditionally, the calculation of the vibrational spectra of molecules involves at one point or another a numerical differentiation procedure. Such a method has some serious drawbacks both in efficiency and in accuracy. In this paper, an alternative method based on linear response theory is presented. The second derivative of the ground‐state energy is expressed in terms of the electron density response matrix by means of perturbation theory. The unperturbed wave functions are obtained from the Hartree–Fock equation. First‐order perturbation theory applied to this equation leads to the Hartree–Fock linear response. As an illustration of this method the vibrational frequency of a H2 molecule is calculated. The result is 1.348 × 1014 Hz as compared to the experimental value of 1.319 × 1014 Hz. This method is also applicable in the calculation of the phonon dispersion curves of solids.
Moment expansion method to calculate vibrational frequencies of molecules and solids
Authors: Devreese J. T. , Van Camp P. E. , Van Doren V. E. 
Published: December 2004

AbstractThe vibrational properties of a quantum system are determined by the density response matrix. In linear response theory this quantity is connected to the polarizability matrix, which can be expressed in terms of a double summation over one‐particle energies and wave functions. In has been shown that this expression is not useful in the calculation of vibrational frequencies because of the very slow convergence of the summation in terms of the unoccupied states. In this paper, a different but equivalent expression is presented using a continued fraction. The resulting expression contains only one summation over the occupied states, solving in this way all the problems connected with the sum‐over‐states expression of the polarizability matrix. The elimination of all the unoccupied states via the use of the moment formula turns out to be a crucial step in the solution of the problem of the first‐principles calculation of the vibrational spectra of molecules and solids.
Metal bridge effect: V. Quantum chemical studies of some metal chelates
Authors: Fischer‐Hjalmars I. , Henriksson‐Enflo A. 
Published: December 2004

AbstractMetal chelates [(C2H2X2)2M]n with ligating atoms X = NH, O, S and central atoms M = Be2+, Mg2+, Ni2+, Zn2+ (n = 0, ± 2), and M = Li+, Cu+ (n = ± 1, −3), have been studied in our Laboratory for some years by ab initio calculations. In this article it is shown that certain features of the complexes are the same for all the different metals in our series. These features include a drastic reduction of the energy gap between unoccupied and occupied orbitals, when electrons are added to the positively charged complexes. This change of the energy gap is shown to be an effect of the ligand dimer [(C2H2X2)2]n. But this dimer can only exist when a positive ion, e.g., a metal ion, forms a bridge between the two monomers. The reduced energy gap implies a strong bathochromic shift of the electronic spectrum and low electrical resistivity.
Hydrophile–hydrophobe equilibrium of some prostaglandins—CNDO/SW study
Authors: Grigoras S. , Rusu I. , Pausescu E. , Medesan A. , Moldoveanu S. 
Published: December 2004

AbstractThe problem of the existence of a prostaglandin (PG) receptor is still controversial, but it seems to be clear that the main action of the PGs concerns the interface of the cellular membrane (which has a predominantly lipid character) and the extracellular liquid (which mainly consists of water). Also especially in the case of the exogenous PGs, the transport of these substances in the organism in influenced by the hydrophilic‐hydrophobic properties of the reactive molecules. These properties can be estimated by MO studies using a solvation model and a semiempirical method of calculation. This paper is concerned with the molecules of PGA1 and PGE1. MO studies were performed for these molecules with the CNDO/SW method in order to calculate ionization potentials, dipole moments, and molecular polarizabilities. With these computed molecular properties, the electrostatic and van der Waals interactions with water and octanol molecules are evaluated and compared within the framework of the original model. This study allows the differentiation of the hydrophilic–hydrophobic character of the PGs investigated and the comparison of the results with experimental data.
Solvent effects on CD and MCD spectra of phenylalanine and its related compounds
Authors: Komiyama Tomoko , Miwa Makoto 
Published: December 2004

AbstractSolvent effects and pH effects on CD and MCD spectra of phenylalanine and its derivatives were observed. CD spectra of N‐acetyl‐L‐phenylalanine ethyl ester in various solvents were analyzed using the rotamer population determined by NMR. The magnitude of optical activity due to each rotamer was determined for two CD bands. The signs of Cotton effect of rotamers I and II are positive and that of rotamer II is negative. The optical activity is greater in polar than in nonpolar solvents. MCD spectra of phenylalanine reveal a remarkable pH effect. In the alkaline pH range it is of the toluene type, from which it turns to a weak spectrum in the acid pH range.
Fluorescence spectra and their temperature dependence of amorphous anthracene films doped with naphthacene
Authors: Maruyama Yusei , Takamiya‐Ichikawa Keiko 
Published: December 2004

AbstractFluorescence spectra and their temperature dependence of amorphous anthracene films doped with naphthacene were observed. An amorphous film was prepared by vacuum evaporation technique on a quartz plate cooled down to 30–40°K. The obtained results were discussed in relation to the structural change in the amorphous film.
On the localization of defects
Authors: Calais Jean‐Louis 
Published: December 2004

AbstractThe range in ordinary space of a defect in a solid, polymer, or a large molecule is an important parameter in many physical and chemical situations like, e.g., substitutional and interstitial impurities in ionic crystals, deep and shallow energy levels in semiconductors, core holes in systems studied by photoelectron spectroscopy, and hydrogen in metals. Both the electronic structure and the lattice of the host molecule or solid are normally influenced by the defect, and it is desirable to treat, as far as possible, these two aspects together. An explicit procedure for investigating the degree of localization is proposed that is based on the direct calculation of Wannier functions and partitioning technique.
Exciton transfer in the two‐dimensional antiferromagnet (C2H5NH3)2MnCl4
Authors: Kojima N. , Ban T. , Tsujikawa I. 
Published: December 2004

AbstractExciton transfers in the two‐dimensional and spin‐canted antiferromagnet (C2H5NH3)2MnCl4 were studied by investigating the absorption spectra 6A1g → 4T2g(4D) of Mn2+. A magnon side band (exciton–magnon simultaneous excitation) which has an anomalous shape with the cutoffs at the low‐energy as well as the high‐energy sides was observed. This anomalous shape could be reproduced in calculation as magnon side band, considering both the intersublattice and intrasublattice exciton transfers. From the analysis of this band shape, the magnitudes of the intersublattice and intrasublattice exciton transfers are estimated to be 10.2 and 6.8 cm−1, respectively. In the case of simple antiferromagnets without spin canting, the intersublattice exciton transfer process is forbidden because of the spin angular momentum. However, in the case of (C2H5NH3)2MnCl4, the prohibition of the intersublattice exciton transfer is removed by the canted‐spin arrangement.
Single‐determinantal open‐ and closed‐shell FSGO calculations for some diatomic molecules
Authors: Pakiari A. H. , Linnett J. W. 
Published: December 2004

AbstractOpen‐shell single‐determinantal calculations are reported here for the molecular species H2, Li2+, N2, O2 (triplet), O22−;, O2−, O22+, O2+, and F2; corresponding closed‐shell calculations are reported for the species H2, N2, O2 (singlet), O22−, O22+, and F2. The floating spherical Gaussian orbital (FSGO) method was employed. The calculated trend in bond lengths of isonucleic diatomic molecules is in agreement with experiment. For heteronucleic diatomic molecules, however, the experimental trend in bond lengths is not obtained; in this connection, the effect of lone pairs on bond length is discussed. The dissociation energies of H2 and Li2+ are evaluated. The energy gap between the triplet and singlet states of the oxygen molecule is calculated to be 8.96 eV compared to the experimental value of 4.54 eV.
Computer‐generated formulas for overlap integrals of slater‐type orbitals
Authors: Jones Herbert W. 
Published: December 2004

AbstractUsing a modified form of Sharma's method for the expansion of a Slater‐type orbital in spherical harmonics about a displaced center, a general expression for the overlap integral between two orbitals is derived that is equivalent to that given by Sharma. By use of a simple kind of “computer algebra” this expression is developed here into a formula that is equivalent to earlier known ones and may have computational advantages when the generated formula coefficients are stored. This method is capable of extension to more complicated integrals.
Simple relationship between total molecular correlation energies and LMO sizes
Authors: Peterson M. R. , Poirier R. A. , Daudel R. , Csizmadia I. G. 
Published: December 2004

AbstractAs an alternative to the density functional approach to estimating total molecular correlation energies, the equation Ecorr=−0.06593 ∑ R, where Ri=〈r2〉 and i runs over the localized molecular orbitals was fitted to 25 STO‐3G data points with a root‐mean‐square error of 0.025 hartree. Other more general equations were tried but no significant improvement was observed in the fit.
The graph center concept for polycyclic graphs
Authors: Bonchev Danail , Balaban Alexandru T. , Randić Milan 
Published: December 2004

AbstractWhile the concept of the graph center is unambiguous (and quite old) in the case of acyclic graphs, an attempt has been made recently to extend the concept to polycyclic structures using the distance matrix of a graph as the basis. In this work we continue exploring such generalizations considering in addition to the distance matrix, self‐avoiding walks or paths as graph invariants of potential interest for discriminating distinctive vertex environments in a graph of polycyclic structures. A hierachy of criteria is suggested that offers a systematic approach to the vertex discrimination and eventually establishes in most cases the graph center as a single vertex, a single bond (edge), or a single group of equivalent vertices. Some applications and the significance of the concept of the graph center are presented.
Thermal dissociation of 1,2‐dioxethane. I. Charge density distribution along the reaction path and in different HF solutions
Authors: Hilal Rifaat 
Published: December 2004

AbstractA charge density distribution study, based on ab initio SCF–6‐31G wave functions, on the optimized geometry of 1,2‐dioxethane indicates its high instability which results from (i) the high ring strain, (ii) the large amount of charge contained on the nonbonded sides of the oxygen nuclei, and (iii) the weak binding character of the charge density in the OO bond region. The redistribution of the charge density when 1,2‐dioxethane undergoes change in its nuclear configuration has been discussed. This redistribution of charge along the reaction path (the dissociation of 1,2‐dioxethane to formaldehyde products) shows clearly the preceding nature of the electron cloud. The characteristic features of the charge density distribution of the two Hartree–Fock solutions to which the SCF procedure converges have been analyzed and discussed.