bluuebunny/crossref_metadata_2025_split huggingface dataset:
Spectres de masse des composés organiques. 3e Communication.. Perte de méthyle dans la fragmentation des alcanes
Authors: Liardon R. , Gäumann T. 
Published: December 2004

AbstractThe reaction mechanism for the loss of methyl and methane from the parent ion is investigated for n‐alcanes. By using different deuterated species, mainly hexanes, it is shown that these two reactions proceed by two paths, one being the simple split of a C‐C‐bond, whereas the other consists in an internal rearrangement followed by a loss of a C‐atom from within the chain. This process is becoming the main process for this class of fragmentation as the hydrocarbon chain is getting longer. A small internal isotope effect has also been found.
Magnetische Protonenresonanz‐Spektroskopie der Bufadienolide (steroidartigen Krötengifte)
Authors: Gsell L. , Tamm Ch. 
Published: December 2004

AbstractThe NMR. spectra of nearly all bufadienolides (steroidal toad poisons) of known structure (aglycones and acetyl derivatives) were measured (some in various solvents) and evaluated. The results obtained allow the application of the principle of the self‐consistent chemical shift values (‘rule of additivity’) for the calculation of the chemical shifts of the 18‐ and 19‐ methyl groups. The frequencies of resonance and the coupling of the protons of the 17β‐pentadienolide ring and of the α‐protons of the functional groups were analysed. For the determination of the NMR. spectra of polyhydroxylated bufadienolides and cardenolides d6‐dimethylsulfoxide proved to be the most suitable solvent. In this solvent not only the nature but also the relative positions of several hydroxyl groups can be recognized, the rule of additivity being also applicable.On the basis of the data recorded the position and stereochemistry of functional groups in steroidal toad poisons of unknown structure can be determined with a high degree of probability.
Dopaldehyd: erstmalige Isolierung aus einer Pflanze in Form seines (Z)‐Enol‐(E)‐Kaffeesäureesters
Authors: Arihara Shigenobu , Rüedi Peter , Eugster Conrad Hans 
Published: December 2004

AbstractDopaldehyd (homoprotocatechu aldehyd; 3,4‐dihydroxyphenyl acetaldehyd), in form of its (Z)‐enol‐(E)‐caffeic ester, has been detected for the first time as a natural occurring compound. It is a constitutent of the yellow pigments of the glands on the leaves of Plectranthus caninus ROTH (Labiatae).
Skeletal muscle wasting and contractile performance in septic rats
Authors: Minnaard R. , Drost M. R. , Wagenmakers A. J. M. , van Kranenburg G. P. , Kuipers H. , Hesselink M. K. C. 
Published: December 2004

AbstractWe investigated the temporal effects of sepsis on muscle wasting and function in order to study the contribution of wasting to the decline in muscle function; we also studied the fiber‐type specificity of this muscle wasting. Sepsis was induced by injecting rats intraperitoneally with a zymosan suspension. At 2 h and at 2, 6, and 11 days after injection, muscle function was measured using in situ electrical stimulation. Zymosan injection induced severe muscle wasting compared to pair‐fed and ad libitum fed controls. At 6 days, isometric force‐generating capacity was drastically reduced in zymosan‐treated rats. We conclude that this was fully accounted for by the reduction of muscle mass. At day 6, we also observed increased activity of the 20S proteasome in gastrocnemius but not soleus muscle from septic rats. In tibialis anterior but not in soleus, muscle wasting occurred in a fiber‐type specific fashion, i.e., the reduction in cross‐sectional area was significantly smaller in type 1 than type 2A and 2B/X fibers. These findings suggest that both the inherent function of a muscle and the muscle fiber‐type distribution affect the responsiveness to catabolic signals. Muscle Nerve, 2005
Electronic structures and spectra of halogenated purines and pyrimidines III.σ‐Electronic structure and some comments on the mechanism of the xanthine oxidase reaction of 2‐halopurines and related analogs
Authors: Song Pill‐Soon 
Published: December 2004

AbstractThe extended HMO (EHMO) and Pople‐Segal SCF‐MO‐CNDO/2 calculations on purine, 8‐oxopurine, 2‐oxopurine, 2‐fluoropurine, and 2‐chloropurine indicate significant polarizations of the σ cores. It is shown that the polarizations of both σ and π frameworks are mutually opposing in some cases. The results from the two methods are compared for these complex biomolecules. It is found that the EHMO calculations tend to over‐polarize the σ and π frameworks. However, the CNDO shows anomalous π‐electron densities on N(7) and C(8) of the purine rings, and the reason for this anomaly is not certain.The application of the results to the xanthine oxidase system indicates that the substrate molecules are subject to a specific orientation on the enzyme surface to counteract the electronic reactivity, in support of the previous prediction based on the π‐electron calculations. The CNDO results appear to be more satisfactory than the EHMO in this respect.
On partitioning the overlap charge density in self‐consistent charge molecular‐orbital calculations
Authors: Hillier Ian H. , Wyatt John F. 
Published: December 2004

AbstractAlternatives to the Mulliken method of partitioning the overlap charge density in self‐consistent charge molecular‐orbital calculations are examined. It is found that more realistic methods may lead to significantly different self‐consistent charges, and different ordering of the electronic energy levels.
Une theorie adiabatique de la relaxation spin‐reseau
Authors: Levy Joseph 
Published: December 2004

AbstractAn adiabetic theory of spin lattice relaxation is proposed. It is shown that if one takes account of the adiabatic adaptation of the electrons to the motion of the ionic complex the motor of the relaxation is not the crystalline potential modulation but the fluctuation of the effective magnetic field produced by the complex near the paramagnetic system.
Wellenmechanische absolutrechnung am HeHHe+
Authors: Swanstrøm P. , Janoschek R. , Preuss H. 
Published: December 2004

AbstractThe wave function of the HeHHe+ molecule has been calculated by means of the GENERAL SCF–MO–LC(LCGO) PROGRAM SYSTEM, taking all four electrons into account. The calculations were carried out for a number of linear equidistant, linear non‐equidistant, and bent nuclear arrangements. The minimum energy of −5.7930 a.u. was found for a linear equidistant configuration with a HeH distance of 0.939 Å. The corresponding ionization energy was 37.9 eV. An estimation of the energy of formation of HeHHe+ from HeH+ and He based on SCF‐calculations on HeH+ and He gave 7.9 kcal/mole. The frequencies of the normal vibrations were calculated.
A double self‐consistent‐field method for electron correlation: Applications to LiH, H2O, and cyclopropane
Authors: Liakus A. , Gallup G. A. 
Published: December 2004

AbstractA computational procedure which introduces some electron correlation into quantum mechanical calculations is described. Diverse applications are given. Among these, the method gives values of 65.6 kcal/mole for the dissociation energy of LiH, 62.0 kcal/mole for the ring‐opening energy of cyclopropane, and 9.123 × 155 dynes/cm for the O–H stretching force constant of H2O. These results, and others given, suggest that our procedure has great potential and provides a very economical method for introducing electron correlation into a wave‐function.
A basis set investigation for the oxygen 1s ionization potential in H2O
Authors: Ågren H. , Svensson S. , Wahlgren U. I. 
Published: December 2004

AbstractThe basis set dependency of the core ionization potential of water was investigated for atomic optimized Cartesian Gaussian basis sets. Basis set parameters optimized for oxygen yielded better IP's than basis set parameters optimized for fluorine. Not much was gained by using different parameters for the ground state and for the 1s hole state.
A numerical integration scheme for iterative calculations on atomic systems
Authors: Blakemore M. , Evans G. A. , Hyslop J. 
Published: December 2004

AbstractA recursive numerical integration scheme based on the method of Clenshaw and Curtis is proposed for the efficient implementation of the variation–iteration procedure for the computation of approximate energies and wave functions for atomic systems. Extensive numerical tests are carried out to assess the accuracy and efficiency of the method and inaccuracies in some earlier calculations are pointed out.
Floating spherical gaussian orbital open‐shell calculations on the four‐electron H4 system
Authors: Huang Hsing Hua , Pakiari Ali H. 
Published: December 2004

AbstractFloating spherical Gaussian orbital (FSGO) open‐shell calculations have been made to determine the potential energy surface of planar square and rectangular arrangements of the four‐electron system H4. This surface is discussed in relation to the bimolecular isotope exchange reaction H2+D2‐→ 2HD. The changes in energy and geometry accompanying the coplanar approach of two hydrogen molecules interacting chemically have also been investigated. Calculations on the electronic energies of planar T‐shaped and kite arrangements of H4 of various sizes show that it is unlikely that these configurations can serve as transition states for the exchange reaction. However, the energy curve for linear configurations of H4 (HHH … H), calculated as a function of the H3 … H distance with the symmetric linear H3 (H‐H‐H) unit fixed at the internuclear distance of 1.9080 a.u., is found to have a deep minimum (−1.9176 a.u.) at an r(H3 … H) distance of 1.5846 a.u. The overall results suggest that the following mechanism for the exchange reaction, H2+H2→H2+H+H→H3+H→H+H2+ H→H2+H2 could be advantageous as it requires a barrier height of 0.1604 a.u. which is significantly lower than that calculated from the saddle point energy (0.1950 a.u.). However, the problem of reconciling this with the experimental activation energy of 0.0685 a.u. still remains.
Orthogonal trajectories of the electron density
Authors: Collard Kathleen , Hall George G. 
Published: December 2004

AbstractFunctions of several variables, such as the electron density and the nuclear Born–Oppenheimer potential, may be analysed very effectively using orthogonal trajectories. In particular the behaviour of these trajectories at critical points of the functions is investigated in detail. The use of orthogonal trajectories in the virial partitioning of Bader and in the definition of reaction coordinates is extended. A chemical interpretation of the critical points is suggested. The relevance of catastrophe theory is illustrated.
On some specific properties of systems endowed with dipole–dipole interactions
Authors: Knežević G. , Marinković M. , TošIć B. 
Published: December 2004

AbstractSpecific properties of systems with dipole–dipole interactions in the one‐dimensional structures leading to the violation of the statistical equivalent of the continuity equation resulting in the creation of qualitatively new excitations are analyzed. Those excitations are the result of the interaction between the initial quasiparticle currents. The structures discussed may be of importance in biophysics, and the biophysical aspects of the obtained results are considered. In comparison with other mechanisms, the exciton mechanism is shown to be of a very specific nature as its transport properties are sharply dependent upon the type of exciting light bounded within the range of temperatures covered by living organisms. This can result in an accumulation of interface excitations on a specific lattice molecule and cause its qualitative transformations with corresponding biological consequences.
PCILO calculations on hydrogen bonded complexes. Dimers involving second row atoms
Authors: Weller Th. 
Published: December 2004

AbstractThe PCILO method was used to study systems with the following types of intermolecular hydrogen bonds, involving 2nd row atoms: N…︁HS, S…︁HS, N…︁HCl, Cl…︁HF, Cl…︁HCl, P…︁HN, P…︁HP, P…︁HF, S…︁HF.In all the cases calculated, the PCILO method yields an accurate qualitative description of the stabilization energies and to some extent quantitatively good agreement with reliable ab initio and experimental results. The corresponding equilibrium distances are in almost all cases smaller (up to 0.6 Å) than the ab initio values of the same compounds.
A new algorithm for the symmetric (Löwdin) orthonormalization
Authors: Scharfenberg Peter 
Published: December 2004

AbstractA new method for the numerical calculation of (inverse) square roots of real symmetric matrices is proposed. Simplicity of the algorithm, good numerical accuracy, short computational time and the possibility to solve the problem in an (n, n) array are some of its features; in addition a new theoretical view on this problem is presented.
On the convergence of one‐centre partial wave treatments for diatomic molecules. II. Heteronuclear one‐electron diatomic molecules using the 1sσ, 2sσ, 3sσ, 2pσ, 3pσ, and 3pσ, states of HeH++ as models
Authors: Ali M. K. , Meath W. J. 
Published: December 2004

AbstractExact separated atom nuclei and centre of nuclear charge centred partial wave solutions for the Schrödinger equation are obtained for the 1sσ, 2sσ, 3sσ, 2pσ, 3dσ, and 3pσ states of HeH++ as a function of the internuclear separation R and the number of partial waves used to represent the wave functions for the molecules. If the expansion centre is chosen appropriately one‐centre techniques are in general very efficient for these Coulomb dominated interactions relative to molecules like H+2(1sσg) which have a large electron exchange contribution to their interaction energy. In general the centre of nuclear charge is not the most suitable expansion centre for heteronuclear molecules for most values of R.
Quantum‐mechanical theory for transport across biological membranes
Authors: Hosur Ramakrishna V. 
Published: December 2004

AbstractA general quantum‐mechanical description of molecular transport across biological membranes has been developed. The one‐dimensional Schrödinger equation for the motion of ions and molecules across the biological membrane has been solved using reasonable potential barriers. Two types of potential profiles for such transports have been considered: a flat barrier proposed by Hall et al. for lipids and an oscillatory potential proposed by Lauger for lipids and by us in this paper for intrinsic proteins. The general behavior of the transmission coefficient as a function of energy and the variation of the permeability as a function of structural changes in the membrane have been discussed. Finally, the importance and areas of applicability of such a theory have been pointed out.
A generalized formula for the energies of alternant molecular orbitals. II. Heteronuclear molecules
Authors: Tyutyulkov N. N. 
Published: December 2004

AbstractUsing the method of alternant molecular orbitals (AMO), it is shown that the energies of AMOS (Ekσ) for an arbitrary heteronuclear alternant system, having a singlet ground state, are connected with the energies of MOS (ek(k)) obtained by means of the conventional Hartree–Fock (HF) method (SCF‐LCAO‐MO‐PPP) via the formula:

In the general case, the determination of the correlation corrections δi,kσ is connected with the solving of a complicated system of integral equations, which is considerably simplified if the Hubbard approximation is accepted for the electron interaction.The energy spectrum of a chain with two atoms in the elementary cell (AB)n is considered as an example. It is shown that if nontrivial solutions exist (δi,kσ ≠ 0), the correlation correction for AMOS of different spin are different (δi,kσ ≠ δi,kβ), from which it follows, that the width of the energy gap ΔE∞ for AMOS with different spin is different: ΔE∞,α ≠ ΔE∞,β.
Finite nucleus model for the Fermi contact interaction
Authors: Hannesson Thorsteinn , Blinder S. M. 
Published: December 2004

AbstractA model for the Fermi contact interaction is proposed in which the charge and magnetic moment of the nucleus are uniformly distributed within a sphere of radius r0. This leads to a Schrödinger equation, which is solvable without perturbation theory. In the mathematical limit r0 → 0, the usual Coulomb plus delta function potential is obtained. It is shown that the magnetic perturbation energy goes to zero for a repulsive delta function, to ‐∞ for an attractive delta function. A projected application is to computation of nuclear spin–spin coupling constants in molecules, particularly HD.
Two‐parameter exponential‐type basis functions for atomic calculations
Authors: Höjer Germund 
Published: December 2004

AbstractIt is found that ordinary STOs fall off too fast in the atomic region in many cases. A new type of basis set, which is more adaptable to the rather different requirements of the various atomic orbitals in an atom, is developed. The suggested functional form χ(r) = Nrn‐1 exp{−α[(βr + 1)1/2 − 1]} contains the STOs as a limiting case. Calculations on a series of atoms from H to Zn show that the new basis gives better results than STOs for equal basis set size. The necessary integrals do not present any problem to evaluate.
Ab initio molecular fragment calculations with pseudopotentials: Hydrocarbon calculations of double‐zeta quality
Authors: Gáspár R. , Gáspár R. 
Published: December 2004

AbstractPseudopotential theory is introduced into the ab initio FSGO molecular fragment method. A theoretical background for the pseudopotential fragment description and a method for large molecule formation is presented. Core‐valence electron separation is achieved at both levels of the calculations with the resulting simplification of the molecular calculations. Using pseudopotentials of double‐zeta quality a detailed description of pseudopotential molecular fragments CH4 (tetrahedral) and CH3 (planar) is presented. Applications of the pseudo‐FSGO molecular fragment method to hydrocaroons are discussed. The results are compared to those of the original FSGO method and experiment.
Symmetry properties of graphs of interest in chemistry. II. Desargues–Levi graph
Authors: Randić Milan 
Published: December 2004

AbstractThe Desargues–Levi graph represents important chemical transformations: (1) isomerization routes for some carbonium ion rearrangements, (2) isomerization of trigonal bipyramidal structures, and (3) some pseudorotations of octahedral complexes. The symmetry properties of this graph have not been fully investigated in the past. Using the concept of the smallest binary code, all permutations which form the symmetry operations in the graph are registered. The resulting symmetry group can be represented as the direct product of S5 (the full symmetric permutation group on five objects) and Ci (the inversion in the center). There are 14 classes belonging to the following partitionings: 120(1), 1826(1), 1428(1), 1236(1), 123262(1), 2 63(2), 2244(2), 210(3), 54(1), and 102(1). The total of 240 symmetry operations are distributed among the above 14 classes as follows: 1, 10, 15, 20, 20, 20, 20, 30, 30, 15, 10, 1, 24, and 24, respectively. Since partitioning cannot uniquely characterize a class, it is suggested that the distance between vertices in a cycle be introduced as an additional parameter to discriminate among classes having identical partitioning. Also, a suggestion to a generalization of the Mulliken notation for irreducible representations of the point molecular groups valid for more versatile symmetry groups of graphs is indicated.
Improved interaction potential for alkali halide molecules
Authors: Mohammad S. Noor 
Published: December 2004

AbstractAn improved interaction potential has been devised for diatomic alkali halide molecules. This potential, in addition to similar attraction terms as in the Rittner potential, includes a new exponential for the short‐range repulsion. The constant m in the exponential is seen to be well expressible in terms of the parameters of the Rittner potential. The new potential is also correlated with different properties, as for example, effective charges, effective radii, effective principal quantum numbers, etc., of the combining ions. Various spectroscopic constants, viz., the ionic dissociation energy Di, the vibrational–rotational coupling constant αe, the vibrational anharmonicity constant ωexe, as well as two second‐order spectroscopic constants γe and βe have been calculated for this and for the Rittner potential. From comparisons between these two potentials, the new one has been observed better than the other.
Quadrupole moments, dipole quadrupole A and quadrupole C polarizabilities by means of perturbation theory
Authors: Espinoza LL. , Toro A. , Fuentealba P. 
Published: December 2004

AbstractQuadrupole moments, dipole quadrupole polarizability A, and quadrupole polarizability C values have been obtained for a set of organic molecules, in both the ground and first singlet excited states, by means of Rayleigh–Schrödinger perturbation theory. Computations for π systems have been carried out using the PPP‐SCF procedure. Results are discussed through the paper and compared with available experimental and theoretical information. Quadrupole moments are shown to be very sensitive to the quality of the wave functions, and the A and C polarizabilities are highly dependent on geometries.
Electronic structure, spectra, and mechanism of photodimerization of pyrimidine bases
Authors: Srivastava S. K. , Mishra P. C. 
Published: December 2004

AbstractVarious electronic indices calculated by the CNDO/s‐CI method for uracil, 5‐fluorouracil, thymine, and cytosine and the Woodward‐Hoffmann rule applied to the photodimerization of these molecules lead us to the following conclusions: (i) decrease of the C5C6 bond order consequent to excitation is generally correlated with the photodimerization reaction for all the biological pyrimidines; (ii) excited pyrimidine molecules may be nonplanar and the excitation would be delocalized over both the molecules which photodimerize; (iii) electrostatic interactions may play a significant role in the initial stages of the photodimerization reaction; (iv) singlet precursors to the stable triplet photodimers may exist; and (v) electron density does not, in general, increase in the C5C6 bond in the excited state as compared to the ground state, contradicting the proposals of some earlier workers. Hybrid population densities and σ‐bond orders have been calculated for the first time for the molecules. Fluorine in 5‐fluorouracil perturbs the hybrid populations as compared to those of uracil mainly in its vicinity.
Definition of invariant molecular tensors in an origin change. I. General formalism
Authors: Duprez‐Quesnoit C. , Tillieu J. 
Published: December 2004

AbstractAmong the tensors used in the published literature to characterize a phenomenon of nonlinear optics, some are not invariant under a change of origin. In this paper, we show that this invariance can be obtained in a general manner, if the densities of electric and magnetic polarization are not separated but replaced by a density of magnetoelectric polarization whose quantum calculation is performed for a molecular crystal.
Use of molecular properties in the one‐dimensional model of nonadiabatic transitions
Authors: Macías A. , Riera A. 
Published: December 2004

AbstractA systematic analysis of a diabatization method proposed in a recent publication is presented for the one‐dimensional linear model of nonadiabatic transitions, and the method is extended to the exponential model. The asymptotic behavior of molecular properties is studied, and the usefulness of simple properties like the kinetic energy and multipole moments is examined in detail. The method is easy to use, and provides a reliable a posteriori procedure to deal with the existence of strong radial couplings between adiabatic states, when a program to calculate the radial couplings exactly is not available.