bluuebunny/crossref_metadata_2025_split huggingface dataset:
Quantum‐Chemical investigation of cation motion mechanism in modes of biological channels with phosphate groups
Authors: Strelko V. V. , Lavrinenko‐Ometzinskaja K. D. , Karaseva T. A. , Pisarzhevsky L. V. 
Published: December 2004

AbstractSpecimens of amorphous zirconium phosphates specifically formed by alkali metal cations were first studied as models of biological membranes. The mechanism of ion motion in zirconium phosphate ultrapores was investigated by the SCF MO LCAO method using a CNDO/2 approximation. Differently arranged dihydrophosphate groups were considered as models of the sorbent channel surface. Analysis of the form of potential surfaces resulting from the motion of lithium, sodium, and potassium ions near the charged centers showed that the value and position of energy barriers essentially depend on the nature of cation and the size of the pore that it penetrated into. Optimum conditions for the selectivity absorption of the ions and their further advance along the ultrapore channel are created if the cation size corresponds to the ultrapore diameter.
Influence of solvent structure on the conformation of the native DNA molecule
Authors: Frisman E. V. , Slonitsky S. V. , Veselkov A. N. 
Published: December 2004

AbstractThe investigation of the factors determining the formation and stability of the higher biopolymers structures is one of the most important trends of the development of molecular biophysics. A feature common to most macromolecular systems under physiological conditions is that they function in an aqueous environment. Thus, it is natural to assume that the peculiarities of biological macromolecules structures and their functional activity as well are closely related to the specific properties of such a unique solvent as water. The investigations of the conformational changes of biopolymer, induced by dehydration of the macromolecule, give information about the nature of the forces stabilizing its structure. The dehydration of the macromolecule in solution can be attained by addition of a nonaqueous cosolvent. Generally low‐molecular‐weight aliphatic alcohols, amides, and amines are used as a nonaqueous component. At present a vast number of experimental and theoretical data concerning the properties of water and aqueous systems are available. The specificity of water as a solvent arises primarily from the spatial hydrogen‐bonded structure. The addition of a nonaqueous component exerts changes in this structure, which evolve to the singularities of the physical characteristics of water‐nonelectrolyte mixtures. It is generally assumed that nonelectrolytes may be divided, according to their effect on the spatial water structure, largely into two basic classes: (1) the structure makers, i.e., the compounds of aliphatic alcohols type; (2) the structure breakers, i.e., the compounds of urea type. The agents belonging to the first class show a stabilizing effect in the range of low nonelectrolyte content. At a certain critical concentration, Ccrit, characteristic of each substance, the nonaqueous solute molecules leave the cavities of the spatial water structure which leads to a disruption of the latter. The agents belonging to the second class exert a structure‐breaking effect even in the range of extremely low concentrations, which arises from their high competitive ability for hydrogen bonding.
Conditions of formation of liquid crystals from biopolymer solutions
Authors: Khokhlov A. R. 
Published: December 2004

AbstractUsing the representation of a completely stiff macromolecule as a long rigid rod and the representation of a semiflexible macromolecule as a chain of freely rotating connected effective rigid rods, the conditions of formation of the liquid‐crystalline phase in the solutions of such macromolecules are obtained and the diagram of states for the corresponding phase transitions in the variables temperature‐volume fraction of polymer in the solution is constructed.
Quantum‐chemical investigation of spatial and electronic structure of verdazyl and its derivatives
Authors: Markovsky L. N. , Polumbrik O. M. , Nesterenko A. M. 
Published: December 2004

AbstractThe unrestricted HF method in the INDO approximation with optimized geometry was used to describe the electronic structure of the hetero‐ring of verdazyl and the derivatives of the latter. The results obtained are in good agreement with the experimental EPR spectra of triphenylverdazyl radical and its dipole moment. These data may be a basis for understanding and predicting the chemical properties and reactivity of verdazyl and its derivatives.
Metabolic activation of chemical carcinogens and binding of metabolites with nucleic acid bases
Authors: Nagata Chikayoshi , Kodama Masahiko , Kimura Teruyuki , Yamaguchi Tamié 
Published: December 2004

AbstractA free radical was produced enzymatically by incubating benzo(a)pyrene with liver microsomes. This radical, identified as the 6‐oxybenzo(a) pyrene radical, was sufficiently reactive to bind covalently with nucleic acid bases. Similar reactive free radicals were produced enzymatically from anthanthrene and 10‐aza‐benzo(a)pyrene, which are carcinogenic in spite of lacking so‐called “bay regions.” Hepatocarcinogens such as 3′‐methyl‐4‐dimethylaminoazobenzene and related compounds, naphthylamines, and 2‐acetylaminofluorene yield free radicals after being incubated with liver microsomes. Thus, various kinds of chemical carcinogens are found to be converted to free radicals, suggesting causal significance for the formation of free radicals in chemical carcinogenesis.
Diagram approach to character formulas for finite and compact groups
Authors: Kibler M. , Elbaz E. 
Published: December 2004

AbstractSome basic relations for the representation theory and the Wigner–Racah algebra of a finite or compact continuous group are discussed and transcribed in terms of diagrams. Special emphasis is placed on the case of a simply reducible group and all the diagrams are applicable to SU2 without any change.
Transferable integrals in a deformation‐density approach to crystal orbital calculations. I
Authors: Avery John 
Published: December 2004

AbstractIn the usual ab initio method of calculating molecular orbitals, the number of integrals to be evaluated increases as M4, where M is the number of basis functions. In this paper, an alternative method is discussed, where the computation time increases much less violently with the number of basis functions. Matrix elements of the deformation potential are evaluated by Fourier transform methods, while matrix elements of the neutral‐atom potential are evaluated by means of transferable integrals. The transferable integrals (moments of the neutral‐atom potentials) can be evaluated once and for all and incorporated as input data in computer programs. In an appendix to the paper, a general expansion theorem is discussed. This theorem allows an arbitrary spherically symmetric function to be expanded about another center.
Configuration interaction matrix elements. II. Graphical approach to the relationship between unitary group generators and permutations
Authors: Paldus J. , Wormer P. E. S. 
Published: December 2004

AbstractThe explicit expressions for the matrix elements of unitary group generators between geminally antisymmetric spin‐adapted N‐electron configurations in terms of the orbital occupancies and spin factors, given as spin function matrix elements of appropriate orbital permutations, are derived using the many‐body time‐independent diagrammatic techniques. It is also shown how this approach can be conveniently combined with graphical methods of spin algebras to obtain explicit expressions for the spin factors, once a definite coupling scheme is chosen. This method yields explicit expressions for the orbital permutations defining the spin factors. However, if desired, the explicit determination of line‐up permutations can be avoided in this approach, since they are implicitly contained in the orbital diagrams. It also clearly indicates why the geminally antisymmetric spin functions have to be used when a simple formalism is desired.
Free‐electron molecular‐orbital treatment of chalcone group of dyes
Authors: Basu Biswajit , Tiwari R. D. , Dey Arun K. 
Published: December 2004

AbstractThe position and intensity of the absorption bands of a number of chalcone dyes have been calculated on the basis of the free‐electron molecular‐orbital (FEMO) theory. The three to four absorption maxima of the chalcones are calculated by assuming the molecule to consist of two parts. The hypothetical cleavage is affected at the site of CO bond such that first and second part of the molecule contains six and eight π electrons, respectively. Each part is then treated as an independent entity and the absorption maxima obtained by the application of joint and continuity condition of Kuhn are in agreement with the experimental values. The perturbation energy correction due to the substituted functional groups with higher electronegativity than carbon have also been made in the absorption maxima. The oscillator strengths corresponding to the values of the absorption maxima have been obtained as well.
Equations‐of‐motion method: Calculation of the k lowest or highest solutions
Authors: Flament J. P. , Gervais H. P. 
Published: December 2004

AbstractThe method of optimal relaxation to determine the eigenvalues of symmetric matrices, as proposed by Shavitt, has been adapted to solve the equation‐of‐motion problem. Matrices Z and Y are obtained by one diagonalization, while matrices A and B remain unchanged. This procedure is particularly useful for high‐dimensional or nonorthogonal bases, if one needs only the lowest transition energies.
Notes on asymptotic series by H0‐strictly singular perturbations
Authors: Ingólfsson K. 
Published: December 2004

AbstractLet two strongly continuous one‐parameter semigroups of contraction operators, {Z(t); t≧ 0} and {Z0(t); t ≧ 0}, determine physical evolutions realized in the Hilbert spaces ℋ and ℋ0, respectively. We consider the mappings under Z(t) and Z0(t), when the infinitesimal generators G and G0 belong to a product class, properly defined with respect to an H0‐norm. The inversed transform of each side in the identity R(λ, G) = R(λ, –iH0)–iR(λ, –iH0)VR(λ, G), which represents a simple algebraic decomposition of the resolvent of G, converges on Ψ if and only if Ψ ∈ 𝒟(G). By iteration an asymptotic series emerges, when t → 0. Numerical considerations of this approximation in its first order may support the form of a deviation from the pure exponential decay when the semigroup is compared with a integral transform corresponding to a certain self‐adjoint H0 in ℋ0. The deviation may hardly ever be observed and it is therefore most fruitful to discuss the results inside the framework of the enveloping algebra of the semigroups.
Interaction of methane molecules
Authors: Kołos W. , Ranghino G. , Clementi E. , Novaro O. 
Published: December 2004

AbstractPotential‐energy curves for CH4·CH4 have been calculated using the SCF LCAO MO method. Six different mutual orientations of the molecules have been considered and several basis sets have been employed. An analytical potential has been fitted to the computed energy values and used in a Monte Carlo calculation to yield a spherically averaged CH4·CH4 potential. The potential, if corrected for dispersion interaction, agrees fairly well with that derived from thermophysical data. At short distances it disagrees with the potentials derived from scattering experiments which, however, are also incompatible with the experimental potential for large distances.
How good is good agreement: Evaluating the reliability of quantum‐mechanically calculated observables
Authors: Ransil Bernard J. 
Published: December 2004

AbstractThe practical value of a wave function derives from its ability to estimate or predict the chemical and physical properties of the electronic structure which it describes. Reliability is defined as a property to be evaluated on the basis of (i) the magnitude of the difference between the measured and calculated values of the observable together with (ii) the sign of the difference, (ii) the number and nature of observables correctly estimated, and (iv) the number and type of electronic structures that are correctly estimated. Systematic statistical comparison implies the existence of both internally consistent sets of wave functions for homologous series of molecules and their corresponding experimental values together with reliable error estimates. The most complete data base currently available for comparison is the spectroscopic constants for the first and second row diatomic hydrides. Utilizing appropriate statistical comparison techniques, four approximations (CEPA, PNO‐CI, GTO‐SCF, and STO‐SCF) are compared among themselves and against experimentally measured values. The CEPA approximation yields differences from experiment that approximate a normal error distribution, while the other approximations show systematic departures from experiment. Two values, ωe and ωeχe, for SH exhibit differences large enough to cast doubt upon the calculated value, the experimental value, or both.
Topological characterization of cyclic structures
Authors: Bonchev D. , Mekenyan Ov. , Trinajstić N. 
Published: December 2004

AbstractThe sum of the topological distances in the molecular graph (the Wiener number) is used for a topological characterization of the condensed polycyclic molecular systems. This topological index discriminates well the isomeric cyclic molecules. In addition, it also properly reflects their structural features. On this basis the principal points of molecular cyclicity are formulated in 15 rules.
High accuracy wave functions and eigenenergies for vibration–rotation states of diatomic molecules
Authors: Mohammad S. Noor 
Published: December 2004

AbstractA solution of the wave equation for the rotational–vibrational motion in diatomic molecules with a new exponential potential function is carried out in detail. The solution gives simple expressions for the wave functions, eigenenergies, and other related spectroscopic constants. With these expressions Franck–Condon factors for the R branches of the A 1∑‐X 1∑ band system of Ca2, have been calculated which are in excellent agreement with experiment. Various strengths and weaknesses of the present method are also discussed.
Pairwise correlated generalized valence bond model of electronic structure. VII. The b3∑ state of H2
Authors: Nyden Marc R. , Petersson G. A. 
Published: December 2004

AbstractA modification of the overlap approximation for triplet pair correlation energies is presented. The modification allows the use of either the canonical absolute overlap or the minimum absolute overlap. When applied to the b 3∑ state of H2, this approximation reproduces the observed correlation energy to within 0.0002 hartree (or ±3%) for all internuclear distances from 1.3 bohr to infinity.
Monte Carlo simulation of water solvent with biomolecules: Serine and the corresponding zwitterion
Authors: Romano Silvano , Clementi Enrico 
Published: December 2004

AbstractMonte Carlo simulation results are reported for clusters consisting of 250 water molecules surrounding serine, both in the neutral form and in two zwitterionic conformers (in order to gain some insight into conformational effects). Calculations were carried out at 300 K and using two‐body potentials obtained by means of quantum‐mechanical calculations. The spatial dependence of the average interaction energies was investigated. The solvation structure was investigated by means of radial distribution functions and probability density maps, which showed a few water molecules directly solvating the hydrophilic groups and, beyond them, a more or less rich and complex hydrogen‐bonded network of solvent molecules.
Ab initio calculations on large molecules using molecular fragments. Generalization of the molecular fragment basis at the minimum basis set level
Authors: Spangler Dale , Christoffersen Ralph E. 
Published: December 2004

AbstractBy determining basis set parameters in molecular environments using other criteria than total energy, it is shown that a generalization of the molecular fragment basis can be obtained in which calculated geometric and electronic structural properties are predicted substantially better than with other basis sets of similar size. As a first step in the development of a series of basis sets having successively greater flexibility and accuracy, several single‐zeta basis sets are created, using a two‐Gaussian contraction for each basis orbital. The best of these basis sets produced calculated geometric and electronic properties for a series of molecules that model a wide variety of organic molecules that are of better accuracy than the corresponding STO‐2G basis, and similar in most cases to STO‐3G. In addition, the basis set is shown to be applicable in either a Cartesian Gaussian basis or a floating spherical Gaussian basis.
Electron correlation and Hund's rule
Authors: Warner J. W. , Bartell L. S. , Blinder S. M. 
Published: December 2004

AbstractIt is suggested that simple electronic shielding effects induced by wave function antisymmetrization tend to govern the energy ordering of singlet and triplet terms within a two‐electron atomic configuration. This approach gives rise to the following alternating rule: For the term of greatest orbital angular momentum within a configuration, the triplet lies below the singlet. The energy ordering reverses for the term of next highest angular momentum, and continues to alternate with each change of one unit in the orbital angular momentum until the term of lowest angular momentum is reached. In an examination of over 600 energy levels of the elements and their ions, the alternating rule reliably orders singlet–triplet energy levels in some 90% of the cases.
Transferable integrals in a deformation density approach to crystal orbital calculations. IV. Evaluation of angular integrals by a vector‐pairing method
Authors: Avery John , ørmen Per‐Johan 
Published: December 2004

AbstractA general method for performing angular integrations is presented. The method depends on the fact that the integral must be invariant under rotations of the coordinate system, and it also makes use of combinatorial analysis. In most cases the method presented is computationally much faster than alternative methods of angular integration using Condon–Shortley coefficients. Applications to charge density analysis and Fourier transforms are discussed, and a general formula for the action of angular momentum projection operators on functions of the Cartesian coordinates is derived. A general angular integration formula for an m‐dimensional space is also given.
Graph‐theoretical analysis of molecular properties. Isomeric variations in nonanes
Authors: Randić Milan , Wilkins Charles L. 
Published: December 2004

AbstractPaths of length two and three appear to dominate variations in isomers of alkanes when various physical and chemical molecular properties are compared. The regularities previously observed for octanes have been fully examined for the set of 35 nonane isomers C9H20. Such examinations are facilitated by construction of appropriate grid graphs with paths of length two and three, respectively, representing molecules and connecting such points along the grid axes. By ordering structures in the two‐dimensional coordinate plane, various trends and regular changes in the relative magnitudes for the selected thermodynamic properties have been traced to molecular connectivity and topology.
On Löwdin orthogonalization
Authors: Aiken John G. , Erdos John A. , Goldstein Jerome A. 
Published: December 2004

AbstractIt is shown that the Löwdin orthogonalization gives the unique minimum for the functional ϕ measuring the least squares distance between the given orbitals and the orthogonalized orbitals. Furthermore a much stronger result is obtained, namely that ϕ has only one local minimum, which is attained at the Löwdin orthogonalization and which is global. This justifies certain computer programs that compute Löwdin orthogonalization via minimization procedures. Finally there is a discussion of replacing the least squares metric by other metrics. The Löwdin orthogonalization turns out to be optimal with respect to all the commonly encountered norms.
Improved transformation algorithm of two‐electron integrals from atomic orbital basis to a symmetry orbital basis
Authors: Takada Toshikazu , Sasaki Fukashi 
Published: December 2004

AbstractThe transformation of two‐electron integrals from an atomic orbital basis to a symmetry orbital basis is a straightforward procedure, but becomes a time‐consuming process when the system has high symmetry. In the conventional transformation algorithm, the number of multiplications is proportional to the fifth power of the basis size. To reduce the computer time, a new transformation algorithm is proposed. In this method, the number of multiplications for the transformation is proportional to the fourth power of the number of atomic orbitals in an equivalent orbital group. A new transformation program has been completed and tested against the conventional n5 method. The present method may be regarded as extension of Almlöf's method to any molecular point group. The program cuts the CPU time for the transformation to one‐third in calculations for C4H8 and NiF6. This reduction is quite significant for large systems having high symmetry.
Inclusion of hydrogen p orbitals in the semiempirical calculation of NMR parameters. I. Influence on the FPT INDO spin–spin coupling constants
Authors: Facell J. C. , Contreras R. H. 
Published: December 2004

AbstractHydrogen 2p orbitals have been introduced into the basis set to calculate the Fermi contact term of spin–spin coupling constants using the FPT INDO method. Different coupling constants show different sensitivity to these hydrogen polarization functions. Some improvements are found for molecules containing N or F. Calculations of proton‐proton geminal coupling constants give more negative results than those of FPT INDO, yielding a better agreement with experimental values. The π‐transmission mechanism is notably exaggerated.
Average fock operators
Authors: Carbó Ramon , Domingo Llorençl , Gregori Josep 
Published: December 2004

AbstractAverage procedures of SCF methods are discussed. Although average operators are known from early Hartree–Fock times, some remarks against indiscriminate use of such operator forms may still be useful. In general, unless a very particular set of average structures is chosen, a dependence of final results on the SCF starting vectors is a troublesome fact. Consequently, energies, populations and molecular parameters in general may show unpredictable behavior under various trial vectors. It is shown that variationally coherent operator forms are given by state parameters with equal values for all the active MO's used in the construction of Coulomb and exchange terms throughout the SCF cycles.
Theoretical study of substituent effects on the acidity of the methyl group: Structure of anions CH2X−
Authors: Hopkinson A. C. , Lien M. H. 
Published: December 2004

AbstractGeometries have been optimized using molecular‐orbital calculations (a) with a 4‐31G Gaussian basis set for carbanions CH2X− where X = H, CH3, NH2, OH, F, CCH, CHCH2, CHO, COCH3, CN, and NO2; and (b) with an STO‐3G basis set for methyl acetate and acetyl deprotonated methyl acetate. All the carbanions containing unsaturated substituents are planar, with a considerable shortening of the CX bond. Carbanions containing saturated substituents are pyramidal with the out‐of‐plane angle α increasing with the electronegativity of the substituent. Double‐zeta basis set calculations give proton affinities over the range 449 (for CH3CH2−) to 355 kcal/mol (for CH2NO2−), with all unsaturated anions having smaller affinities than saturated anions. The correlation of proton affinities with 1s binding energies, and with charges on both the carbon of the anion and on the acidic proton of the neutral molecule are examined.