bluuebunny/crossref_metadata_2025_split huggingface dataset:
Molecular valence calculations based on stepwise approximations for orthogonality and core‐valence interactions
Authors: Pakkanen Tapani A. , Lindblad Marina , Laitinen Risto S. 
Published: December 2004

AbstractAn approximate method for the calculation of the electronic structure of molecules has been developed. The principal approximations involve freezing of the atomic core orbitals, description of the core‐valence interactions by density matrix expansion techniques, and the enforcement of the core‐valence orthogonality by auxiliary bases and projection operator techniques. The general principle in the approximations is their stepwise execution. Each approximation can be separately relaxed and tested for reliability. The developed computational techniques have been described. A detailed analysis for the approximations is given for LiH. Other molecular test calculations include HCl, H2S, H2Se, and H2S2 molecules. The results demonstrate the suitability of the method for molecular studies.
Fluorescence lifetime distribution of folded and unfolded proteins
Authors: Gratton E. , Silva N. , Mei G. , Rosato N. , Savini I. , Finazzi‐Agro A. 
Published: December 2004

AbstractTime‐resolved fluorescence of single tryptophan proteins have demonstrated the complexity of protein dynamic and protein structure. In particular, for some single tryptophan proteins, their fluorescence decay is best described by a distribution of fluorescence lifetimes rather than one or two lifetimes. Such results have provided further confirmation that the protein system is one which fluctuates between a hierarchy of many conformational substates. With this scenario as a theoretical framework, the correlations between protein dynamic and structure are investigated by studying the time‐resolved fluorescence and anisotropy decay of holo and apo human superoxide dismutase (HSOD) at different denaturant concentrations. As a function of guanidine hydrochloride (GdHCl), the width of the fluorescence lifetime distribution of HSOD displays a maximum which is not coincident with the fully denatured form of HSOD at 6.5M GdHCl. Furthermore, the width of the fluorescence lifetime distribution for the fully denatured forms of holo and apo HSOD is greater than that of the native forms.
A semiempirical study of the reaction of the hemimercaptal of methylglyoxal and glutathione at the active center of glyoxalase I
Authors: Ventura Oscar N. , Cubas María L. 
Published: December 2004

AbstractAn AM1 and MNDO–PM3 semiempirical study of the reaction mechanism of glyoxalase I is presented. According to some authors, this enzyme serves as a detoxifying agent, transforming ketoaldehydes to hydroxiacids. According to others, however, the enzyme serves as a growth‐control agent, destroying 2‐oxopropanal (MG), which might be involved in some, mostly unknown, biological pathways. Following a previous study on the conformation of the hemimercaptal of MG and glutathione (HSG) done using molecular mechanics, we study here the possible reaction path of this substrate at a model reaction site in the enzyme. First, the active center of the enzyme was modeled using a Zn2+ cation with several ligands. Second, its complex with the hemimercaptal of MG and HSG (SAG) was calculated using semiempirical methods. The reaction proceeds from the substrate to the enediol intermediate in two steps: In the first one, a proton from the activated carbon in SAG is captured by a base–which we model in this work by HO−, CH3COO−, and imidazole anion. Our calculations show that the likeliness of the reaction is enhanced if imidazole is used as the base instead of an acid conjugate base or a free hydroxide anion. Moreover, we found a stable intermediate carbanion–described for the first time as an intermediate in this mechanism–whose existence is supported by some of the available experimental evidence. In the second step of the mechanism, a proton is transferred from a Zn‐bound water to the intermediate carbanion, giving the enediol species, originally postulated as the intermediate in this mechanism, and leaving a hydroxide anion bound to the Zn2+ complex. Comparing this reaction with the same process when no metal complex is present–as if the reaction occurred in aqueous solution–it is shown that the metal ion activates water, facilitating the proton transfer. Our calculations point out the existence of two potential energy barriers to be overcome in the process: the need of the metal center to be coordinated to water as a prerequisite for the second proton transfer to take place, and the fine tuning caused in the first proton transfer by the very nature of the base that initially captures the proton. No significant differences in the qualitative characteristics of the reaction were found in using AM1 or PM3 as the method of calculation. © 1992 John Wiley & Sons, Inc.
Benchmark values for two‐center Coulomb integrals over slater‐type orbitals
Authors: Jones Herbert W. 
Published: December 2004

AbstractThe Löwdin α‐function method, in which displaced orbitals are expanded in an infinite series of spherical harmonics, is implemented for Slater‐type orbitals using a commercial computer algebra program, Mathematica, The program, which is included, generates a C matrix with integer elements that characterizes our approach to multicenter molecular integrals. The general two‐center, two‐electron Coulomb repulsion integral is produced analytically with a finite number of terms. Each Coulomb formula may be evaluated to arbitrary precision, since Mathematica works with integer arithmetic. Hence, cancellation errors can be overcome. © 1993 John Wiley & Sons, Inc.
A quantum monte carlo simulation of the two‐dimensional H2 molecule
Authors: Bianchi R. , Bressanini D. , Cremaschi P. , Mella M. , Morosi G. 
Published: December 2004

AbstractThe problems connected with the simulation by the diffusion quantum Monte Carlo method of atomic and molecular systems in the two‐dimensional (2D) space have been investigated on the 1 2S and 2 2S states of the hydrogen atom and on the 1 1S and 1 3S states of the helium atom, assuming r−1 Coulomb interactions between the particles. The potential surface of the 1Σg+ state of the 2D hydrogen molecule has been calculated in the range 0.1–1.5 bohr: A dissociation energy of 1.2703 Hartrees is found at the equilibrium distance of 0.3639 bohr. © 1994 John Wiley & Sons, Inc.
A vibrational Hamiltonian model for triatomic molecules based on the Kratzer and Poschl Teller potentials
Authors: Requena Alberto , Alacid Mercedes , Bastida Adolfo , Zúñiga Jośe 
Published: December 2004

AbstractA Hamiltonian model to describe molecular vibrations of triatomic molecules is proposed. The Hamiltonian is based on the use of the Kratzer potential variable for the stretching motions and a perturbed Poschl Teller potential for the bending one. The perturbation and variational treatments to compute the vibrational energies of this Hamiltonian can be developed using a zero‐order system that includes part of the couplings between the stretching and bending motions. All the matrix elements involved in these calclations can be then evaluated in closed form. A numerical application to the HCN molecule is made. © 1994 John Wiley & Sons, Inc.
Fast and stable algorithm for the analytical computation of two‐center Coulomb and overlap integrals over Slater‐type orbitals
Authors: Hierse W. , Oppeneer P. M. 
Published: December 2004

AbstractProceeding from analytical expressions for two‐center kernel functions that we derived recently, we present new analytical formulas for the two‐center Coulomb and overlap integrals over Slater‐type orbitals. These formulas are of an exceptionally simple analytical structure and high numerical efficiency. An especially important point is that for the most frequently needed ranges of discrete quantum numbers, the formulas are completely stable in the cases of nearly equal scaling parameters or vanishing interatomic distances, except for one particular case of the Coulomb integral. No special asymptotic formulas are needed any more to compute the two‐center integrals over Slater‐type orbitals in these case. Furthermore, a largely recursive formulation makes the integral evaluation very economical and fast. In particular, we assess the numerical performance of a new kind of angular momentum recurrences that we have proposed in a previous article [W. Hierse and P.M. Oppeneer, J. Chem. Phys. 99, 1278 (1993)]. © 1994 John Wiley & Sons, Inc.
Charge‐exchange in the nonadiabatic Na/W gas‐surface reaction using a diabatic representation
Authors: Kristyan Sandor , Olson John A. 
Published: December 2004

AbstractA theoretical description of near‐resonant charge exchange is presented for gas‐surface collisions, based on the eikonal formalism, short wavelength approximation, and diabatic representation. The dynamics of the charge transfer is analyzed for the Na/W system. Accurate model diabatic potential surfaces are used to compute the charge exchange during scattering and desorption. The Pade method is applied to approximate the charge exchange probability. The derivations, developments, and comparisons of approximations of the charge‐exchange probability are discussed. These approximations are useful in experimental surface analysis to estimate the integral value of diabatic coupling between the atomic and ionic states. The frequency of oscillation during the charge‐exchange process is also discussed. Model diabatic potentials in analytic forms for the Na/W system are reported. © 1995 John Wiley & Sons, Inc.
Properties of the atomic form factor: Applications and shell structure
Authors: Gálvez F. J. , Porras I. 
Published: December 2004

AbstractA relationship between the atomic form factor, F(k), and its first derivative, both at the origin, is presented. A new function f(k), related in a simple way to F(k), has been studied and some applications have been performed. They led us to find lower bounds to F(k) for all k and to other quantities such as the charge density at the origin, ρ(0), and radial expectation values. Finally, interesting effects on the Laplacian of f(k) due to the atomic shell structure were found. © 1995 John Wiley & Sons, Inc.
Decay of charged stabilized jellium clusters
Authors: Vieira Armando , Brajczewska Marta , Fiolhais Carlos 
Published: December 2004

AbstractThe stabilized jellium model is a simple modification of the jellium model, which more realistically describes simple metals of high density, such as Al, Ga, Pb, etc. We analyzed the fragmentation processes of charged spherical Al clusters in the framework of the stabilized jellium model. Kohn‐Sham calculations of the parents and daughters, using the local density approximation, have been made. We evaluated the dissociation energies of Al, Al, and Al, with N=1‐30 atoms, in all possible decay channels. We discuss the most favorable decay channels, which are ruled by the shell structure (magic numbers of valence electrons in the parents and the daughters) oscillations around an average trend given by a liquid drop model. We compare our calculations with others and with the available experimental data. © 1995 John Wiley & Sons, Inc.
Density functional theory of born couplings: Consequences for electron flow in Jahn–Teller molecules and superconductors
Authors: Sukumar N. 
Published: December 2004

AbstractThe dynamics of Jahn–Teller systems has recently been discussed in terms of generalized electronic charge and current densities in nuclear‐coordinate space. The introduction of the electronic phase as a function of both electronic and nuclear coordinates, in addition to the electronic density, was a crucial component of this formulation. Here, a densitybased treatment of Born couplings is derived from first‐principles quantum mechanics beyond the Born–Oppenheimer approximation. Because of the degenerate electronic configuration of a Jahn–Teller molecule, there are an infinite number of ways in which the charge distribution can be oriented for the same energy, leading to a vanishing bond hardness for the molecule in the symmetric nuclear configuration. Further, the moving nuclear framework serves as the perturbation necessary to define the orientation of the charge density, leading to unhindered rotation of the charge cloud. This leads to the dynamical Jahn–Teller problem, namely, the coupling of electronic and nuclear motions through the Born coupling terms. Applications to superconductivity theory are discussed. © 1995 John Wiley & Sons, Inc.
A density functional theory formulation of the reaction field model of solvent effects
Authors: Cemtreras Renato , Pérez Patricia , Aizman Arie 
Published: December 2004

AbstractIt is possible to reformulate the reaction field (RF) model of continuum solvent effects, by considering an approximate expression describing the energy changes from one ground state to another, in the frame of density functional theory (DFT). The energy functional for an arbitrary electronic system coupled to a spin‐independent electrostatic external perturbation is used to derive the well‐known Born expression giving the electrostatic component of the solvation energy of an atomic ion. The approximate RF–DFT model is illustrated for a series of representative singly positive and negatively charged atomic ions. A Kohn–Sham (KS)‐like formalism is then proposed to compute solvation energies within a self‐consistent field scheme. The extension of the RF‐DFT model to molecular systems is also outlined. © 1995 John Wiley & Sons, Inc.
Stationary point structure and energetics: Density functional study including solvent effects on the tautomerization of formamide and 2‐pyridone
Authors: Adamo Carlo , Lelj Francesco 
Published: December 2004

AbstractSolvent effects on the thermodynamics and kinetics of some tautomerization reactions, chosen as the most representative of this wide class of reactions, are analyzed. Solvent effects have been modeled by the self‐consistent reaction field approach that we have implemented in the ADF package. We investigated the structure and energetics of the saddle points for the formamide/formamidinic acid and 2‐pyridone/2‐hydroxypyridine tautomerizations. Our results, carried out both at local and gradient corrected levels, are in good agreement with previous post‐Hartree–Fock computations in predicting the structure and the energy of the investigated saddle points. On the other hand, our computations show that solvent effects play a relevant role on the energetics of the reactions. In particular, the interconversion of 2‐hydroxypyridine into 2‐pyridone is sensibly favored in aqueous solution, being that the latter tautomers are stabilized by solvent interactions. This latter result is in agreement with experimental findings. © 1995 John Wiley & Sons, Inc.
Corrosion behaviour of stainless steel implants in biological media
Authors: Hofmann J. , Michel R. , Holm R. , Zilkens J. 
Published: December 2004

AbstractThe corrosion behaviour of orthopaedic metal implants was studied in an animal experiment in which 30 rabbits had stainless steel plates implanted after a varization osteotomy of the tibiae of their hind legs for one year. ESCA was used for the investigation of the surfaces of the inplants. The removed plates were covered by an easily destroyable 3–5 nm thick oxidic layer which contained Cr(III), Fe(II), Fe(III), Mo(VI) and Mo(IV), but no Ni. This passive layer (and possibly also the nearest metal layer below it) is strongly enriched in Cr. Additionally, corrosion experiments with radioactivated implants in Ringer's solution demonstrated that the passivation of the stainless steel is accompanied by marked selective solution of the steel constituents. The observed solubilities (Co > Ni > Fe > Mo > Cr) are inversely correlated with the abundance of the respective element in the passive layer. The trace element composition of the tissues was measured by instrumental neutron activation analysis. It turned out that the actual burdening of the tissues surrounding the implants by corrosion products is exhibited by strong enrichments of the elements Cr, Fe, Co, Ni and Mo relative to their normal concentrations. While Cr, Co, Ni and Mo are abundant in fairly constant ratios in the contact tissues, the excess of Fe is influenced by biological regulation mechanisms. The extent to which the distribution patterns of steel‐specific elements in animal (and human) tissues can be explained by the dissolution of the steel during the passivation process is discussed.
X‐ray photoelectron spectroscopic studies on ZnO/Bi2O3 varistors
Authors: Sainkar S. R. , Badrinarayanan S. , Sinha A. P. B. , Date S. K. 
Published: December 2004

AbstractThe intergranular layer and its microstructure, proposed earlier to behave non‐linearly in the ZnO/Bi2O3 varistor ceramic system, has been studied by XPS. Our results clearly indicate the formation of an intergranular layer rich in Bi2O3 separating the semiconducting ZnO grains. With increase in sintering temperature, XPS results show the enrichment of the intergranular layer (Bi2O3 phase) without an additional phase being formed. The non‐linear coefficient (α) in the current–voltage characteristic increases smoothly up to 1300°C. Afterwards, there is a loss of Bi ions from the Bi2O3 enriched intergranular phase resulting in the decrease of α, and in the characteristic XPS peak intensity.
A study of oxygen‐implanted Si0.5Ge0.5 alloy by XPS and thermodynamic analysis
Authors: Castle J. E. , Liu H. D. , Saunders N. 
Published: December 2004

AbstractA thermodynamic study of oxygen‐implanted Si0.5Ge0.5 alloy based on an XPS analysis is reported for the first time. The alloy was grown by molecular beam epitaxy (MBE) and was implanted with high‐dose oxygen ions (up to 1.8 × 1018 cm−2) at an energy of 200 keV. The changes of Si, Ge and O activities with the concentration of oxygen were calculated using the THERMO‐CALC program, and chemical potentials of these elements throughout the oxygen‐buried region were plotted out according to the chemical state information obtained from XPS depth profiling results.Transport during implantation and in subsequent annealing follows the gradient of chemical potential: elemental Si migrates into the oxygen‐rich layer to form SiO2 by the replacement of Ge in Ge oxides. This leads to a decrease of the total free energy for the system of Si, Ge and O in the sample. The description of the system in terms of activities and chemical potential enables true diffusion profiles to be used in place of phenomenological descriptions such as ‘snowplough’ or ‘pile‐up’.
Surface structural studies of polyethylene, polypropylene and their copolymers with ToF SIMS
Authors: Lianos L. , Quet C. , Duc Tran Minh 
Published: December 2004

AbstractStatic secondary ion mass spectrometry (SIMS) was applied for probing structural differences such as unsaturation and branching in the case of simple aliphatic hydrocarbon polymers (polyethylene, polypropylene and their copolymers). The parameter introduced for quoting surface amounts of branching is the percentage of C6 to C8 cluster emission relative to the sum of all C2 to C8 clusters. The ratio of the total emission of hydrogen‐deficient fragments to the sum of all C2 to C8 clusters is found to be a good pointer for unsaturation, especially for probing vinyl bonds. While applied successfully to polymers of the same family, these pointers cannot provide structural information on polyethylene/polypropylene copolymer surfaces in a straightforward way. The chemical surface composition of these copolymers is correlated to the percentage of the m/z 69 fragment.
The effect of cooling on the development of composition profiles in Cu–Pt alloys under Ar+ ion bombardment
Authors: Li Chunfei , Kurokawa Akira , Asahata Tatsuya , Shimizu Ryuichi 
Published: December 2004

AbstractThe effect of cooling on the development of the altered layer in a Cu‐Pt alloy under Ar+ ion bombardment has been studied through the accurate determination of the compositional profiles in the near‐surface region by sequential ISS‐AES measurements and by a new method. Although the ISS‐AES measurements have not revealed any noticeable difference between the composition profiles at about −150°C and room temperature, a significant difference has been observed using the new method based on He+‐ISS monitoring of the surface composition under bombardment of mixed (Ar+ + He+) ions, followed by abrupt switching to bombardment with only He+ ions. The measurement procedures are described in detail, and the results obtained by both the techniques are discussed.
Versuche zur Darstellung von Blei(IV)‐azid‐Verbindungen
Authors: Möller Heinz 
Published: December 2004

AbstractWährend die Umsetzung von Bleidioxyd mit wässeriger Stickstoffwasserstoffsäure unter heftiger Zersetzung ohne Bildung von Blei(IV)‐azid verlief, wurden aus Mennige und Bleisesquioxyd rote Lösungen mit einem wesentlichen Gehalt an Blei(IV)‐azid erhalten. Die Zusammensetzung bei Verwendung von 1 n HN3 entspricht der Formulierung PbN9 bis PbN10. Bei Erhöhung der Azid‐Ionen‐Konzentration wurde die Zusammensetzung PbN12 fast erreicht. Diese Blei(IV)‐azid‐Lösungen zeigen Selbstzerfall unter Stickstoffentwicklung, Entfärbung und Abscheidung von Blei(II)‐azid. Sie haben stark oxydierende Eigenschaften. Auch durch Umsetzung von Salzen des vierwertigen Bleis mit HN3 bzw. angesäuerten Azidlösungen wurden rote, Blei(IV)‐azidhaltige Lösungen erhalten. – Versuche zur Darstellung fester Blei(IV)‐azid‐Verbindungen wurden durch Umsetzung von Ammoniumhexachloro‐plumbat mit Natriumazid und Aufnahme in organischen, carbonylhaltigen Lösungsmitteln mit nachfolgender Ausfällung durchgeführt. Die erhaltenen dunkelroten, kristallinen Produkte waren unbeständig und explosiv.
Lösungsmitteleinschlüsse bei organischen Berylliumsalzen. I. Berylliumoxyacetat und Äthanol
Authors: Hardt Horst Dietrich 
Published: December 2004

AbstractBerylliumoxyacetat erfährt in Berührung mit wasserfreien Äthanol eine teilweise Abspaltung von Essigsäureanhydrid, wobei sich höher‐basische Be‐Acetate bilden. Diese schließen, zusammen mit unverändertem Oxyacetat, bemerkenswerte Mengen des Lösungsmittels ein und bilden, wie vorläufig angenommen werden muß, ein intermolekular vernetztes Gerüst, welches bei der thermischen Zersetzung einen Rückstand von Berylliumoxyd hinterläßt.
Zur Kenntnis des Sπ
Authors: Schenk Peter W. , Thümmler Ursus 
Published: December 2004

AbstractMit Hilfe einer auf dem nukleophilen Angriff der Schwefel‐Schwefel‐Bindung durch Triphenylphosphin beruhenden jodometrischen Methode wurden Sπ und Sλ in Schwefel‐Kohlenstofflösungen nebeneinander quantitativ bestimmt. Durch Vergleich der so erhaltenen Werte mit den durch Eindunsten der Lösungen und Extraktion der Rückstande mit CS2 für den entstandenen Sμ erhaltenen Werten, ergibt sich, daß Sx mit einer Umwandlungsausbeute von 55%in Sμ übergeht. Dies steht in Übereinstimmung mit früheren Befunden, in denen wir auf Grund von kryoskopischen Bestimmungen in CS2 für S, eine Molekelgröße von Sπ gefunden und zur Deutung der Schmelzpunktsanomalie des monoklinen Schwefels eine etwa 60proz. Umwandlung des Sπ in Sμ angenommen hatten.
Beiträge zur Chemie der Bor‐Azide. II. Zur Kenntnis von Diphenylborazid
Authors: Paetzold Peter I. 
Published: December 2004

AbstractDiphenylborazid bildet sich aus Ph2BCl und LiN3 in Benzol als eine unter Luftausschluß beständige Flüssigkeit, die oberhalb 100° N2 abspaltet und dabei in Tetraphenyldibordinitrid übergeht.
Some new acido (thiosulphato or chlorothiosulphato)‐amino (propylenediamine or ethylene‐bis‐biguanide) cobalt(III) complexes
Authors: Sen Gupta Nihar Ranjan 
Published: December 2004

AbstractThe preparations and properties of the thiosulphatocobalt(III) co‐ordination compounds Na[Co(Pn)2(S2O3)2], [Co(Pn)2(S2O3)Cl] and [Co2{Et(BigH) 2}2(S2O3)3] as well as of the cyanosalt [Co(Pn)2(CN)2]2S2O3 are described (Pn = propylene diamine; Et(BigH)2 = ethylene‐bis‐biguanide).