bluuebunny/crossref_metadata_2025_split huggingface dataset:
Interaction between polymers: Dispersion energy of two infinite linear chains
Authors: Beleznay F. , Suhai S. , Ladik J. 
Published: December 2004

AbstractUsing the newly developed discrete variant of the k · p perturbation theory for linear chains an expression is derived for the dispersion interaction between two infinite chains valid for both insulating and metallic polymers. It is shown that the formally divergent dispersion energy is also finite for metallic systems, but it may be considerably enhanced in polymers with partly filled bands as compared with insulting ones. The accuracy of the proposed k · p expansion is investigated on model chains through comparisons with direct ab initio Hartree–Fock crystal orbital calculations. Applying second order perturbation theory an explicit formula in terms of band parameters is given for the numerical calculation of the dispersion energy.
Relativistic theory of binding energies of heavy positive ions
Authors: Marini U. , Marconi Bettolo , March N. H. 
Published: December 2004

AbstractThe self‐consistent relativistic Thomas–Fermi theory of heavy positive ions with N electrons and nuclear charge Ze is shown to lead to a chemical potential μ which has the scaling property

with ϵ = α2Z2, α being the fine structure constant. Combining this with the Layzer–Bahcall expansion for the total energy E(Z, N), namely,

it is proved that the coefficients Enm (N) at large N have the asymptotic behavior Nn–2m/3#1/3. The corresponding result for the scaling of the relativistic Thomas–Fermi energy is

Scaling properties of the higher order terms in Enm (N) and E(Z, N) are also proposed.
Electron correlation described by extended geminal models: The EXGEM2 and EXGEM3 models
Authors: Røeggen I. 
Published: December 2004

AbstractThe general extended geminal model is reviewed and two new approximate models EXGEM2 and EXGEM3 are introduced. In a test calculation on water using a double‐zeta basis set, the approximate models recover 93.4% of the full CI correlation energy defined within the same basis set. A test calculation on the neon atom demonstrates that the performance of the models in comparison with the full CI, will improve as the basis set is increased. It is suggested that for basis sets of moderate and large size, and which include polarization functions, the extended geminal models are likely to recover 95%–97% of the correlation energy obtainable by the full CI.
Quantum chemical calculation of phosphorescence microwave double resonance spectra
Authors: Minaev B. F. , Muldahmetov Z. M. , Irgibaeva I. S. , Tlepbergenov T. O. , Kizhner D. M. 
Published: December 2004

AbstractLifetimes of individual spin sublevels, vibronic intensities of phosphorescence, zero field splitting, hyperfine and nuclear quadruple tensors for the lowest triplet state, g factors, and dipole nature of microwave transitions have been calculated on the bases of CNDO and INDO methods taking into account spin–orbit, spin–spin, vibronic, and hyperfine interactions by perturbation theory. The results are in qualitative agreement with phosphorescence microwave double resonance data. Influence of intermolecular interaction on the zero field splitting are also investigated.
Coherent multiphoton absorption: Dependency on the relative phase of two successive pulses
Authors: Besnainou Sylvette , Stone James , Diels Jean‐Claude 
Published: December 2004

AbstractWhen a multilevel system excited by a laser pulse absorbs a cascade of photons, a second pulse of same frequency, equal duration but dephased by π takes it back to its initial state. This property can be inferred by studying the laser field system interaction matrix, using the rotating wave approximation.
Towards a “Chemical” Hamiltonian
Authors: Mayer I. 
Published: December 2004

AbstractAn analysis of the LCAO Hamiltonian is performed in terms of a “mixed” formulation of the second quantization for nonorthogonal orbitals, compressing the different interactions to one‐ and two‐center terms as far as possible by performing appropriate projections. For this purpose an operator of atomic charge is also introduced, the expectation values of which are the Mulliken gross atomic populations on the individual atoms. The LCAO Hamiltonian is decomposed into terms having different physical meaning and significance: (i) sum of effective atomic Hamiltonians; (ii) the electrostatic interactions in the point‐charge approximation; (iii) the electrostatic effects connected with the deviation of the actual charge distribution from the pointlike one; (iv) two‐center overlap effects; (v) finite basis (“counterpoise”) correction terms related to the individual atoms; and (vi) similar finite basis correction terms with respect to the two‐center interactions. Only terms of types (i) to (iv), containing no three‐ or four‐center integrals, are considered as having physical significance. Based on the analysis of the Hamiltonian, an energy partitioning scheme is developed, and explicit expressions are given for one‐ and two‐center (and basis extension) components of the SCF energy. The approach is also applied to the problem of intermolecular interactions, and an explicit formula is given permitting calculation of the “counterpoise” part of the supermolecule energy by properly taking into account that it depends not only on the extension of the basis, but also on the occupation of the additional orbitals in the intervening molecule—a factor completely overlooked in the usual scheme of calculations.
Some remarks on the hydrogen atom
Authors: Grinberg Horacio , Marañon Julio , Vucetich Hector 
Published: December 2004

AbstractThe path integral for the Green's function involving the Coulomb potential in combination with the Kustaanheimo‐Stiefel transformation is used to generate the atomic orbitals of the nonrelativistic hydrogen atom as various combinations of the product of one‐dimensional isotropic harmonic oscillator wave functions. The use of the transformation is justified, by connecting the homogeneous space with the quotient space in the Feynman quantization formalism.
Incidence of the relativistic corrections in electronic properties of 5d compounds: MS–Xα Calculations on hexachloroiridates (III and IV) and tungsten trioxide
Authors: Chermette H. , Pertosa P. , Goursot A. , Penigault E. 
Published: December 2004

AbstractMultiple‐scattering–Xα (MS–Xα) calculations have been performed on IrCl, IrCl, and WO clusters. Relativistic calculations have been performed by the inclusion of the approximation of Wood and Boring in the MS–Xα method, i.e., mass–velocity and Darwin terms are taken into account self‐consistently, while the effects of the spin–orbit operator are estimated by first‐order perturbation theory. The strong contraction of s and p orbitals can be seen on selected displays of radial wave functions and significant changes in the energy diagrams between the nonrelativistic and the relativistic calculations can be obtained. The comparison with photoemission spectra clearly shows the necessity of including relativistic corrections in the calculations. The calculated spin–orbit parameters are in agreement with experiment for core levels, while a strong dependence of the spin–orbit parameters on the one electron energies is pointed out; this dependance is unconnected from the atomic contributions to the molecular orbitals.
Routes to full CI through extrapolation in multireference configuration calculations for electronic structures
Authors: Burton Peter G. 
Published: December 2004

Abstract“Full CI” energy extrapolation techniques for use with MRDCI (or MCSCF‐CI) wave function calculations are illustrated for (H2)2 which is “pathological” [C. F. Jackels and I. Shavitt, Theor. Chim Acta 58, 81 (1981)]. Our analysis suggests that this extrapolation should be fundamentally more reliable than cluster correction formula results, through the application of a double internal consistency scaling of systematic error estimates, despite explicit treatment of only a small fraction of the configuration list of a full CI. The results of extrapolation suggest that the van der Waals interaction for H2–H2 at R = 6.5a0 in the planar T configuration is less than 200 μH greater than our semiempirical estimate for this interaction strength. Although the role played by the limited basis (80 functions) is not assessed by the present calculations, it appears possible that absolute errors from extrapolation from 16255 SAF results to estimate the full CI value (912464 SAF's) are as small as 0.5 kJ/mol.
Theoretical studies on the conformation of saccharides. V. Hydration of the acetal segment in glycosides
Authors: Tvaroška Igor , Kožár Tibor 
Published: December 2004

AbstractHydration sites of the acetal segment were studied in five of the most stable conformers of 2‐methoxy‐tetrahydropyran (MTHP) as the first step in the determination of the hydration scheme of glycosides. The intramolecular geometries of a supermolecule formed with MTHP and water were calculated by a PCILO quantum‐chemical method. The hydration sites determined can be classified into two groups: (a) individual sites, in which water interacts with one oxygen only, and (b) bridging sites, in which water interacts with both oxygens. The interaction energies of the individual sites are approximately 22 kJ mol−1, and 26 and 29 kJ mol−1 in the bridging sites. An increase of the number of water molecules in the hydration shell of MTHP showed that monohydration of the glycosidic linkage oxygens was most advantageous. Despite of the fact that the hydration shell have various structures in the individual MTHP conformers, the obtained results indicate that the hydration does not operate against the anomeric or exoanomeric effects, i.e., it does not influence the equilibrium of the MTHP conformers in favor of the trans arrangements of a glycosidic bond. Therefore, the experimentally observed stabilization of the trans positions in aqueous solutions should be considered as a result of influence of water being a dielectric continuum.
Conformational and MO studies of hydroxy‐2‐aminotetralins
Authors: Kocjan D. , Šolmajer T. , Hodošček M. , Hadži D. 
Published: December 2004

AbstractAttempting to explain the differences in the pharmacological profiles of the isomeric monohydroxy‐and dihydroxy‐2‐aminotetralins (DHAT) which are potent dopaminergic agonists we have calculated the conformational energies of 2‐aminotetralin and its N,N‐dipropyl derivative using the QCFF/Pi and PCILO methods. Molecular electrostatic potential (MEP) maps based on ab initio (STO‐3G) wave functions were computed for both dihydroxytetralins. Root‐mean‐square (rms) deviations from steric congruence between the enantiomeric 5,6‐ and 6,7‐DHAT based either on atomic centers or on the minima in MEP near the putative points of attachment to the receptor are small, but may nevertheless be sufficient to cause differences in activity on subtypes of the dopamine receptor. N,N‐dipropyl substitution influences the conformational energies of the skeleton and the preferences in the orientation of the propyl groups in the isomeric DHAT may be important for the interaction with the receptor. The HOMO energies of the isomeric HAT and DHAT do not correlate with their potencies.
Spectroscopy with sound on superfluid 3He B
Authors: Lee D. M. 
Published: December 2004

AbstractCollective modes of superfluid 3He B have been studied by means of sound attenuation and sound velocity studies. A new sound attenuation peak has been observed in superfluid 3He B at ħω ≈ Δ(T). This peak has been identified with the real squashing mode. Pulse time of flight methods has been used in these studies. At high sound amplitudes, interesting transient effects have been observed which have been tentatively identified with solitonlike behavior.
Generator coordinate treatment of some model periodic systems
Authors: Kauder L. R. , Ray A. K. , Trickey S. B. 
Published: December 2004

AbstractThe generator coordinate method (GCM) is a powerful variational technique invented for studying nuclear collective motion and extended in recent times by Lathouwers, Van Leuven, and Laskowski et al. to problems of molecular electronic structure and vibrations. In preparation for the application of GCM to electron correlation and lattice dynamical problems in solids, we have studied linear periodically bounded systems consisting of one‐ and two‐mass anharmonic chains of both finite and infinite length. Preliminary results for the variational energy and vibrational spectra from some simple intrinsic functions are presented.
β‐Lactam ring stability and antibacterial potency: A novel eigenvalue problem
Authors: Davies R. H. , Morris T. R. 
Published: December 2004

AbstractLow inoculum potency data in vitro for 16 clinical β‐lactam antibiotics have been analyzed, and a physical model for interpreting the results of a number of bacterial strains has been derived. An analytic criterion for performing a unitary transformation on the potency data is developed following the identification of a physical vector present within the data which is attributable to an activation energy required for the transport of the β‐lactam into a biological membrane. This vector has inverse slope relations in Gram positive and Gram negative bacteria and provides the basis for the analytic criterion for the unitary transformation. Compounds with similar potency spectra which differ only in the absolute magnitude of their effect will possess similar transport properties. It is shown that a slow rate of membrane entry for the β‐lactam has overriding consequences on differences in fast rates of binding to the target enzymes and to β‐lactamases, and a second primary vector is established directly from the biological data related to the ease of β‐lactam ring opening. This vector offers precise evidence for testing the solvational and theoretical requirements for predicting the biological stability of novel β‐lactam ring compounds.
Quantum chemical studies on the conformational structure of bacterial peptidoglycan. V. PCILO Calculations on β(1 → 4) linked disaccharide of N‐acetyl‐glucosamine and N‐acetyl muramic acid
Authors: Yadav Janardan Singh 
Published: December 2004

AbstractThe glycan moiety of the bacterial peptidoglycan consists of alternatingly β(1 → 4) linked disaccharides of N‐acetylglucosamine (NAG) and its 3‐O‐D lactic derivative, N‐acetyl β‐D‐muramic acid (NAM). PCILO conformational energy calculations have been carried out for NAG–NAM and NAM–NAG disaccharides to see whether or not the glycan strands possess a chitinlike structure as suggested by earlier workers. In agrement with recent experimental findings, the present results also suggest that the chitinlike structure is energetically disallowed. Furthermore, the bulky N‐acetyl substituents at C2 positions of the two sugar molecules are found to be relatively less important in stabilizing mutual orientations of the two pyranosyl rings.
Electron densities and chemical bonding in TiC, TiN, and TiO derived from energy band calculations
Authors: Blaha Peter , Schwarz Karlheinz 
Published: December 2004

AbstractThe chemical bonding in the interesting class of refractory transition metal compounds is illustrated for TiC, TiN, and TiO. Self‐consistent augmented plane wave (APW) calculations are already available for these compounds. Using the respective potentials we have repeated the band calculations on a finer k grid with the linearized APW method to obtain accurate densities of states (DOS). These DOS can be divided into local partial contributions to characterize the bonding. Further information can be obtained from a decomposition of the metal d DOS into t2g and eg symmetry components. These partial local DOS are compared with the LCAO counterpart and give a first picture of the chemical bonding in these compounds. The electron densities corresponding to the occupied valence states are obtained from the LAPW calculations. They provide further insight into characteristic trends in the series from TiC to TiO: around the nonmetal site the density shows increasing localization; around the metal site the deviation from spherical symmetry changes from eg to t2g. These effects can be traced back to the three types of valence bands. Electron density plots of characteristic band states (all energies of a selected k point in the Brillouin zone) will be shown. These plots can describe the different types of bonding occurring in these systems.
Sequential isomerism of activated complexes: Interfering versus noninterfering intermediate
Authors: Slanina Zdeněk 
Published: December 2004

AbstractA unimolecular process is considered that was treated experimentally as a concerted one; however, subsequent quantum‐chemical analysis has proved that the process is realized via an intermediate (double sequential isomerism of activated complexes). The intermediate is supposed to contribute directly to values of the primary observed data (e.g., spectral absorbances). Within this approach of interfering intermediate any degree of this interference is allowed including the limit case of noninterfering intermediate serving as a reference standard. A technique is suggested enabling correct comparison of the quantum‐chemical outputs with the experimental overall values for rate constants and activation enthalpy and entropy. The technique employs mere amount of information currently obtainable from quantum‐chemical study of a rate process (i.e., representation of potential energy hypersurface by its stationary points and activated‐complex theory). Properties of the derived formulae are illustrated with model examples. The results are important for meaningful comparison of experimental and theoretical data in the case of processes with sequential isomerism of activated complexes disclosed only theoretically.
Adsorption‐complex isomerism and quantum‐chemical studies in heterogeneous catalysis
Authors: Slanina Zdeněk 
Published: December 2004

AbstractThe present quantum‐chemical studies of the interaction of gases with solid surfaces indicate frequently the existence of several different structures of the model cluster complex with the adsorbed system. The usual approach to this adsorption‐complex isomerism consists of selection of the potential energy‐lowest member of the set of isomers. A weighting treatment is presented within which all the isomeric structures (or the part of them that is active in the respective experimental technique) can contribute to the values of observables. The treatment suggested requires information amount which corresponds to the usual capabilities of today's numerical quantum chemistry, i.e., concept of localized adsorption is employed. In this way correct theoretical equivalents of experimental data can be obtained. Two types of the isomerism are considered, viz., site‐caused isomerism and adsorbate‐caused isomerism. It is shown that the treatment in the latter case can, under acceptable assumptions, be reduced to the recently described technique for the site‐caused isomerism. An illustrative example is presented based on the literature data about the water–silica interaction. The results are important for correct comparison of theoretical and observed data in the field of heterogeneous catalysis and sorption.
Structure–activity relationship and the mechanism of analgesia of the analgesic drug 3‐methyl‐fentanyl and its analogs
Authors: Changying Chen , Lemin Li 
Published: December 2004

AbstractThe semiempirical self‐consistent field MO (CNDO/2) calculations have been undertaken for 13 compounds involving 3‐methyl‐fentanyl and its analogs. Various quantum chemical indices were obtained. The relationships between the biological activity and quantum chemical indices of molecules of these compounds were investigated and it was found that there are remarkable relationships between the activity and some important quantum chemical indices such as the α‐electron density of β substituent and the electron density of piperidyl nitrogen. The nature of interaction between the analgesics' molecule and the receptor was discussed on the basis of the relationships between the activity and quantum chemical indices and it was proposed that the β‐substituent functions not only as a hydrophobic group, but also its electronic property has important effect on the activity. It may act as an electron acceptor and form acceptor–donor complex with receptor through the charge transfer process. The piperidyl nitrogen interacts with receptor forming quaternary ammonium ion and acting as positive charge center, and its electron density also has an important effect on the activity. The amide oxygen and nitrogen may be other active centers which function as donors when they interact with the receptor. The influence on activity through introduction of the hydroxyl group to the β‐position of the 1‐substituent of the piperdyl ring has also been discussed.
Quantum chemical structure–activity relationships on β‐carbolines as natural monoamine oxidase inhibitors
Authors: Martin M. , Sanz F. , Campillo M. , Pardo L. , Perez J. , Turmo J. , Aulló J. M. 
Published: December 2004

AbstractThe electron density and the molecular electrostatic potential of the β‐carbolines are studied using ab initio STO‐3G wave functions. The analysis was done from the point of view of a previous model built with monoamine oxidase substrates and irreversible inhibitors. The results confirm the usefulness of the model and make it possible to propose new precision to the molecular electrostatic potential patterns needed to have monoamine oxidase inhibitory activity.
Variation in allocation of time, water and energy in Hoopoe Larks from the Arabian Desert
Authors: Tieleman B. I. , Williams J. B. , Visser G. H. 
Published: December 2004

Summary

Patterns of resource allocation in different times of the year can provide insights into the effects of simultaneous environmental constraints on reproduction and survival of desert birds. Field metabolic rate (FMR), water influx rate (WIR) and patterns of time allocation of Hoopoe Larks (Alaemon alaudipes Desfontaines) were investigated during spring, when larks were rearing offspring, and during summer, presumably the harshest time for desert birds, in the Arabian Desert.
FMR was 45% below allometric predictions in spring (1·83 times basal metabolic rate, BMR) and 58% below predictions in summer (1·37 times BMR). WIR was 37% lower than predicted during the nestling period and 55% lower than predicted during summer.
Activity patterns during spring and summer were bimodal, with birds foraging in the early morning and late afternoon and resting in the shade or attending the nest during midday. Foraging time was similar in spring (32·0% of total daytime) and summer (33·6% of total daytime), despite higher energy and water intakes for parents and offspring in spring, suggesting that food availability was higher in spring. The non‐foraging time was entirely spent resting in summer, while in spring half of this time was allocated to caring for offspring and the other half to resting in the shade. Hoopoe Larks fed their offspring 55 times per day and shaded their chicks about 5 h per day to protect them from solar radiation.
High Tas combined with low and unpredictable food availability in deserts may constrain reproduction by limiting the amount of time, water and energy that parents can devote to rearing offspring.