bluuebunny/crossref_metadata_2025_split huggingface dataset:
A comparison of two group theoretical models of reaction mechanisms on potential surfaces
Authors: Mezey Paul G. 
Published: December 2004

AbstractThe collection of families of equivalent reaction paths within the low lying regions of potential energy hypersurfaces can be characterized in terms of a group, the fundamental group of reaction mechanisms. This group is compared to the family of all general reaction mechanisms on a complete potential energy hypersurface (considering all nuclear configurations of a fixed set of nuclei). This latter family also has an algebraic structure (a groupoid) that contains a group for every reactant. This groupoid and its subgroups are defined in terms of sequences of open neighborhoods of critical points (representing chemical structures) on the hypersurface, and provide a complete algebraic description of all general reaction mechanisms. This approach simplifies the intractable problem of all possible reaction paths on a surface to a well defined algebraic problem of a discrete set of reaction mechanisms. The relevance of the approach to computer based quantum chemical synthesis planning is pointed out.
Applicability of nondegenerate many‐body perturbation theory to quasi‐degenerate electronic states. II. A two‐state model
Authors: Wilson S. , Jankowski K. , Paldus J. 
Published: December 2004

AbstractIn reply to Kaldor's [Int. J. Quantum Chem. XX, XXX (1985)] criticism of our study of simple four‐electron models, in which the degree of quasi‐degeneracy can be continuously varied, by the finite‐order nondegenerate many‐body perturbation theory, we examine in more detail a simple two‐state model that the used to substantiate his claim that “the low order sum of the perturbation series is not very meaningful” in view of its divergence. It is shown that in contrast to Kaldor's claim, the partitioning used increases the radius of convergence of the considered perturbation series and is in principle capable to make it convergent. It is also shown that the convergence of the series is not very essential and that even divergent series can provide useful estimate of the exact result, particularly when the resummation techniques, such as Padè approximants or continued fractions, are employed. Finally, the shortcomings of the existing multi‐reference perturbation approaches, which Kaldor advocates, are pointed out.
On the evaluation of the integral 〈pu|∂/∂ν|s〉 occurring in covalency reduction factor
Authors: Jaiswal P. S. , Lahiry S. 
Published: December 2004

AbstractExplicit expressions for integrals of the form 〈pu|∂/∂ν|s〉 occurring in expression of covalence reduction factors, introduced by Stevens and Tinkham, have been derived for various types of atomic functions. Numerical values of the integral for some common ligands are given. Misconceptions regarding the sign and the magnitude of this integral existing in the literature have been clarified.
Molecular‐orbital treatment of some heterocycles with three hetero‐atoms and their benzo‐derivatives
Authors: Abu‐Eittah Rafie H. , Hamed Maher M. , Nigm Abdullah , El‐Azhary Adel 
Published: December 2004

AbstractMolecular‐orbital calculations, using the INDO procedures, were carried out on the tautomers of 1, 2, 4‐triazole and its benzo‐derivative as well as on 1, 2, 5‐oxadiazole and its benzo‐derivative. Calculations of the transition energy as well as the dipole moment helped to predict the predominant tautomer in an equilibrium mixture. The correspondence between the calculated and observed data is satisfactory. The results of calculations indicated some differences in the electronic structures of the tautomers of a compound.
Quasirelativistic methods
Authors: Karwowski J. , Kobus J. 
Published: December 2004

AbstractA comparative study of quasirelativistic equations used in atomic structure calculations has been performed. A uniform derivation of all the equations is presented, and some of their specific features are discussed in detail. Electron density distributions, orbital energies, and expectation values of rn obtained with different methods are compared with the ones resulting from the Schrödinger and Dirac equations. The most accurate are found to be the equations of Wood and Boring and of Barthelat, Pelissier, and Durand. (They reproduce almost exactly the Dirac electron densities and expectation values.) The simplest, though least accurate, equation is proposed by us. It gives the relativistic energy corrections with about 6% accuracy and retains exactly the form of the nonrelativistic Schródinger equation. Consequently, its application in analytical SCF‐CI calculations does not require any additional integral calculation.
The difluoroethylenes: A reconsideration of the “Cis effect”
Authors: Heaton M. M. , El‐Talbi M. R. 
Published: December 2004

AbstractAb initio molecular orbital calculations have been carried out on all three isomers of difluoroethylene with geometry optimization. The calculations were done with a “double‐zeta” basis set. After correction of the SCF energies for the effect of electron correlation, the 1,1‐isomer is shown to be 8 kcal/mole lower in energy than the cis, which is 1 kcal/mole more stable than the trans. As the stabilization of the isomers increases, the distance between the fluorine atoms decreases: trans, 3.57 Å; cis, 2.77 Å; 1,1‐isomer 2.20 Å. A simple explanation for these trends is based upon electrostatics and the small size and high electronegativity of the fluorine atom. As the fluorine atoms come closer together, the destabilization due to nuclear repulsions and electron repulsions is offset by the increased stabilizing electrons‐nuclei attractions.
On bond orders and valences in the Ab initio quantum chemical theory
Authors: Mayer I. 
Published: December 2004

AbstractThe concept of bond order and valence indices calculated at the ab initio level is discussed in connection with their close relationship to the nonclassical exchange effects in bonding. An improved definition of bond order and free valence indices is given for the open‐shell SCF (UHF) case, and the generalization of the bond order and valence indices to correlated wave functions is also introduced.
New global constraints on electronic energy hypersurfaces
Authors: Mezey Paul G. 
Published: December 2004

AbstractImproved global energy bounds — valid for the entire electronic energy hypersurfaces of a variety of polyatomic molecular systems — are proposed. The new constraints are applicable to a larger class of molecules and are tighter than the constraints proposed earlier. The new global bounds are easily applicable using readily available energy values of atoms and atom‐ions. The actual evaluation of these constraints typically involves only “back‐of‐the‐envelope” calculations, providing both upper and lower bounds for a complete energy hypersurface, even for very large molecules. Such global energy bounds are of some importance in theoretical studies of chemical reactions and conformational changes. The proposed bounds are likely to find some practical applications in computer‐based quantum‐chemical synthesis planning, using multidimensional potential surfaces.
Computer‐generated formulas for four‐center integrals over slater‐type orbitals
Authors: Jones Herbert W. 
Published: December 2004

AbstractA triple infinite sum of formulas is generated by a computer algebra implementation of the Löwdin α function method (supplemented by a C matrix with integer elements that characterize an orbital) for a fourcenter two‐electron repulsion integral over 1s Slater‐type orbitals, with screening constants of 1. In this example, one orbital is placed at the origin and the others at equal distances along the x, y, and z axes. Surprisingly few terms are needed for five figure accuracy. There should be no problem in generalizing these methods to all cases of four‐center integrals.
Reality and complexity in asymptotic expansions for eigenvalues and eigenfunctions, with application to the JWKB connection‐formula problem
Authors: Silverstone Harris J. 
Published: December 2004

AbstractAsymptotic expansions occur widely in quantum physics. The Rayleigh‐Schrödinger perturbation theory for hydrogen in an electrostatic field (the LoSurdo—Stark effect) is one example. The 1/R expansion for the hydrogen molecule ion H is a second. The quantum defect theory and the JWKB method are two more. It is not so widely known that the sum of such real asymptotic expansions may be complex, while the sum of complex asymptotic expansions may be real. The key to this nonintuitive behavior is Borel summation. By examining a simple example related to the exponential integral, the nature of this real‐iscomplex, complex‐is‐real phenomenon is made simple. Then special application is made to derive and clarify the connection formulas (to all orders) in the JWKB method.
On the irreducible tensor operators and the unitarily invariant decomposition of hermitian operators
Authors: Zeng Zonghao , Sun Chia‐Chung 
Published: December 2004

AbstractIn this paper, the unitarily invariant decomposition of Hermitian operators is performed by means of irreducible tensor operators to give the explicit expression of the coupling coefficients for [1m] X [r‐n] → [2s, 1t] with respect to the group structure

with the Gel'fand chain of subgroups

.
Open‐Shell coupled‐cluster method: Variational and nonvariational calculation of ionization potentials
Authors: Haque Azizul , Kaldor Uzi 
Published: December 2004

AbstractA hermitian, variational open‐shell coupled‐cluster method is described and applied to the calculation of H2O and N2 ionization potentials in the T ≈ T2 approximation. A nonvariational calculation is also carried out, with the inclusion of T1 and T3 in addition to T2. Both methods give fair agreement with experiment when only T2 is taken into account. T3, which is included at present in the nonvariational scheme only, has a considerable effect on the results and gives good agreement with experiment.
Neutral organic lewis acids of π type
Authors: Cammi Roberto , Ghio Caterina , Tomasi Jacopo 
Published: December 2004

AbstractThe interaction of some neutral acids of π type, bearing appropriate unsymmetrical substitutions at the CC group with some selected bases (H2O, NH3, OH−), is compared with that of the parent compound of a new set of neutral π acids, bearing symmetrical substitutions at the CC group with the same bases. The analyses of the interaction energy, performed according to two decomposition schemes, with and without the counterpoise corrections, make clear the similarity of symmetrically and unsymmetrically substituted neutral organic acids.
A structural rule of polyhedral boranes and heteroboranes
Authors: Tang Au‐Chin , Li Qian‐Shu 
Published: December 2004

AbstractIn this paper we propose a topological rule to account for both the simple and conjuctopolyhedral boranes and heteroboranes. The calculated results are in good agreement with the real molecular formulas. Furthermore, we have also made some quantum chemistry calculations to verify and discuss the rule.
Quantum mechanical treatment of biopolymers as solids: Possible implications for carcinogenesis
Authors: Ladik J. , Otto P. , Bakhshi A. K. , Seel M. 
Published: December 2004

AbstractAfter briefly reviewing the results of ab initio band structure calculations on periodic polynucleotides, it is shown how these band structures can be corrected for correlation (quasi particle band structures). Some new results on the electronic structure of aperiodic polypeptides (having up to four different amino acids as components) obtained with the help of the matrix block negative factor counting technique are then presented. They are supplemented by similar results calculated for nonperiodic nucleotide base stacks. Some plausible long‐range effects for the activation of oncogenes through carcinogens are then reviewed. Finally, the preparation of much better defined samples of biopolymers (on which reliable physical measurements could be performed) is discussed.
High‐resolution photoelectron spectrum of ozone
Authors: Cvitaš T. , Klasinc L. , Kovač B. 
Published: December 2004

AbstractThe photoelectron spectra of ozone have been recorded and the first three electronic band systems reassigned on the basis of observed vibrational structure and calculations reported in the literature. The systems X̃,Ã, and B̃ at 12.75, 13.03, and 13.57 eV are assigned as 2A1, 2A2, and 2B2, respectively.
Weak intermolecular interactions: Statics and dynamics
Authors: Zahradník Rudolf , Hobza Pavel 
Published: December 2004

AbstractThis contribution consists of four parts: energy calculations, physical (mainly spectroscopic) characteristics, and static and dynamic aspects of interactions leading to the formation of van der Waals species.The main, first, part includes specific comments on computational procedures for systems of different size ([number of atoms, number of electrons]: small [4, 10], medium [dozens, hundreds], large [103, 104]) and colloid systems (considering also supermolecular structures). Rigorous and simplified methods of molecular quantum mechanics can be used with the first and second groups, respectively; for larger systems, only empirical potentials and the methods of the physics of a continuum are available. The transferability of empirical potential parameters is critically examined. The role of temperature, Gibbs energy, and entropy is mentioned together with the ensemble theory. The search for stationary points on potential energy surfaces (PES) and analytical fits to PES are reviewed briefly.The second part is an outline of what is expected from computational chemistry to meet the needs of spectroscopists. The third section deals with selection rules, equilibria and the rates of processes involving van der Waals species, and the role of these species in common chemical reactivity. The crucial role of entropy is mentioned in connection with hydrophobic phenomena, entropy‐driven processes, and partitioning of substances between water and a nonpolar phase.In the final section, the role of computer experiments (molecular dynamics, Monte Carlo) is pointed out. Some shortcomings and promising features of these techniques are summarized.
The lower valence states of linear polyenes: Locations and identifications
Authors: Mc Diarmid Ruth 
Published: December 2004

AbstractThe experimental transition energy to the valence Ag state of butadiene is used to validate a subset of the theoretical calculations that have been carried out on this molecule. The validated calculated Ag transition energies of these calculations are then compared with the experimental Bu transition energies to determine the relative state ordering in isolated, unsubstituted butadiene, hexatriene, and octatetraene. The Ag origins are concluded to lie below the Bu origins for all polyenes except butadiene. Experimental confirmation of this conclusion is presented. The extreme breadth of the Ag transition, 1.0 eV, is noted. Some possible implications of this breadth are discussed.
Adiabatic coordinate separation and large N‐dimensional limit in two‐electron ions
Authors: Goscinski Osvaldo , Mujica Vladimiro 
Published: December 2004

AbstractThe 1/N expansion is used to generate semiclassical effective potentials for two‐electron ions. These potentials can be considered an improvement over the conventional ones computed by Lin and Fano for the full two‐electron coulomb potential insofar as they incorporate some of the quantum mechanical effects. No falling into the nucleus is allowed. Furthermore, the minima of these effective potentials are in surprising agreement with those computed by Fano, Macek, and Lin. This would indicate that the collective variable R, associated with N = 6, leads to a much smaller expansion parameter N, making the adiabatic separation more justified. The role and significance of these broken symmetry solutions with geometric localization are discussed as well as generalizations. The procedure is semiclassical in the sense that the kinetic energy becomes negligible by having a large effective mass.
Bond critical points in the electronic structures of binary hydrides
Authors: Edgecombe K. E. , Boyd R. J. 
Published: December 2004

AbstractThe bond critical points of the binary hydrides formed by the elements of the first two rows of the periodic table have been calculated. Particular attention has been paid to the basis‐set dependence of the bond critical points at the experimental equilibrium geometries, or where necessary at model geometries. With the exception of H2S, stepwise extension of the basis set leads to a smooth convergence of the bond critical points to a set of values which appear to converge to the Hartree–Fock limit. For H2S it is shownb that the position of the bodn critical point is not only more sensitive to the presence of polarization functions in the basis set, but depends strongly on the orbital exponents of the polarization functions. Extensive optimizations of the exponents of the polarization functions have been carried out with the (12s9p/5s) basis set for second‐row hydrides. The effects of contracting the Huzinaga basis sets have been examined.
Electronic structure and chemical bonding of the dimer of bis(η5‐cyclopentadienyl)ytterbium methyl
Authors: Xu(K. H.Hsu) Guang‐Xian , Ren Jingqing 
Published: December 2004

AbstractThe electronic structure, charge distribution, and chemical bonding of the dimer of bis‐(η5‐cyclopentadienyl)ytterbium methyl have been studied by an unrestricted INDO program made applicable for the lanthanoid compounds [1]. The bond order, composition, and the nature of the bridge bonds have been discussed. The existence of weak interaction between two ytterbium atoms was shown.
A finite expansion method for the calculation and interpretation of molecular electrostatic potentials
Authors: Rabinowitz James R. , Namboodiri Krishnan , Weinstein Harel 
Published: December 2004

AbstractBecause it is useful to have the molecular electrostatic potential as an element in a complex scheme to assess the toxicity of large molecules, efficient and reliable methods are needed for the calculation and characterization of these potentials. A multicenter multipole expansion of the molecular electron charge density calculated with a limited Gaussian basis set is shown here to have only a finite number of nonzero terms from which the molecular electrostatic potential can be calculated. The discrete contributions to the electrostatic potentials from the terms of this expansion provide a physically meaningful decomposition of the potential and a means for its characterization. With pyrrole as an example, the electrostatic potential calculated from this finite expansion of the electron density is compared to that obtained from exact calculations from the same wave function. Good agreement is obtained at distances greater than 1.5 A from any atom in the molecule. In contrast, rearrangement of the terms into an expansion corresponding only to Mulliken atomic charges and dipoles yields a decomposition that produces electrostatic potentials which agree less well with the exact potential. This discrepancy is attributable to the neglect of terms due to higher moments.
On optimization procedures based upon the atomic thomas‐fermi energy
Authors: Ten Hoor Marten J. 
Published: December 2004

AbstractFor the total atomic Thomas‐Fermi (TF) energy many expressions in terms of the kinetic and potential energy contributions can be given. Thirty of these expressions exhibit either a maximum or a minimum if some variational approximation to the TF function is used. Three of these expressions, to note E, G, and J (see text) have been used in an optimization procedure, in which four two‐parameter and two three‐parameter approximate TF functions have been considered. One‐parameter functions cannot be optimized, as the one parameter must be fixed to ensure proper normalization. It is found that optimization of E and G give reasonable and similar results, whereas the results of optimization of J are generally not very impressive. Where possible, expectation values of the type 〈rn〉 (with n = −1, 1, 2, and 3) have been calculated from ten approximate TF functions. A new estimate of the exact atomic TF energy, as well as of the derivative of the TF function at the origin, has been obtained.
Model universal coulomb hole function for many‐electron systems
Authors: Gritsenko O. V. , Bagaturjants A. A. , Kazansky V. B. 
Published: December 2004

AbstractIn the framework of the homogeneous electron gas theory we give a model function Gc of the Coulomb hole that can be considered as an approximate universal correlation function for many‐electron systems. The function Gc reflects the right asymptotic behavior of the correlation function of an electron gas in high and low density limits and enables one to reproduce experimental correlation energies of a number of atoms of the first and second periods in a local density approximation with the relative error 0.3–4.2%. The estimate of contributions of electrons with parallel or antiparallel spins into correlation energy shows that in the domain of densities typical for atoms of the first and second periods, the Coulomb correlation of electrons with parallel spins is in high extent suppressed by the Fermi correlation.
An MNDO study of the reaction of methanol with fulminic acid and acetonitrile oxide
Authors: Sharma Krishan K. , Aggarwal Anil K. 
Published: December 2004

AbstractThe reaction pathway of fulminic acid (HCNO) and acetonitrile oxide (CH3CNO) with methanol as a nucleophile (RCNO + CH3OH → RC(OCH3)NOH) and the formation of H‐bonded complex with methanol have been studied using the MNDO method. MNDO‐SCF calculations were performed with complete geometry optimization using the Davidon–Fletcher–Powell method. The reaction pathways were studied by varying all the bond lengths, the bond angles and the twist angles, using the distance C3O2(R) between the carbon of the 1,3‐dipoles and the oxygen of the methanol molecule as the reaction coordinate. The reaction is exothermic and proceeds in two steps. The first step is the formation of a five‐centered hydrogen‐bonded complex (INT) and is the rate‐determining step of the reaction. The second step involves the rearrangement of the H‐bonded complex to the product, and this step requires a very small amount of activation energy. Thus, there is an intermediate on the reaction pathway, and therefore, the reaction is stepwise. Acetonitrile oxide is less reactive (activation energy 34.59 kcal/mol) relative to fulminic acid (activation energy 28.91 kcal/mol).
Tautomerism of uracil, cytosine, isocytosine, and some of their thio‐derivatives
Authors: Leś Andrzej , Ortega‐Blake Iván 
Published: December 2004

AbstractWe present an ab initio study of tautomerism in uracil, cytosine, isocytosine, and some of their derivatives, mainly those due to thio substitution. We advance gas phase enthalpies for the tautomerization processes. We observe that the substitution of atoms not directly involved in the tautomerism has no appreciable effect on the enthalpy values. On the other hand, thio substitution of the oxygen involved in the tautomerism produces a general and small shift favoring the thiol isomer in aromatic complexes. We compare the theoretical predictions with the experimental evidence available and find them to agree.