Absolute configuration reassignment of two chromanes from Peperomia obtusifolia (Piperaceae) using VCD and DFT calculations

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Comparative vibrational spectra of pilosine and epiisopilosine crystals

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Temperature-dependent Raman spectra of K0.2Na0.8NbO3 ceramics

We performed temperature-dependent Raman scattering studies on K0.2Na0.8NbO3 ceramics and compared the results with those for NaNbO3. The wavenumbers associated with NbO6 vibrations suggest the existence of two phase transitions, as occurs with pure NaNbO3 ceramics. Although the disorder on the Na/K site does not change either the room temperature phase of K0.2Na0.8NbO3 or the sequence of phase transitions compared with NaNbO3, it changes the temperature of the lowest phase transition and strongly modifies the temperature of the antiferroelectric --> new phase II phase transition. Additionally, the linewidth analysis shows that the orientational mechanism is the dominant contribution to linewidth, although the anharmonic contribution is increased, when compared with NaNbO3, owing to the random distribution of potassium in the sodium niobate matrix. Copyright (C) 2004 John Wiley Sons, Ltd.

Q-ANALOG REALIZATION OF NUCLEON PAIRING

A q-deformed analogue of zero-coupled nucleon pair states is constructed and the possibility of accounting for pairing correlations examined. For the single orbit case, the deformed pairs are found to be more strongly bound than the pairs with zero deformation, when a real-valued q parameter is used. It is found that an appropriately scaled deformation parameter reproduces the empirical few nucleon binding energies for nucleons in the 1f7/2 orbit and 1g9/2 orbit. The deformed pair Hamiltonian apparently accounts for many-body correlations, the strength of higher-order force terms being determined by the deformation parameter q. An extension to the multishell case, with deformed zero-coupled pairs distributed over several single particle orbits, has been realized. An analysis of calculated and experimental ground state energies and the energy spectra of three lowermost 0+ states, for even-A Ca isotopes, reveals that the deformation simulates the effective residual interaction to a large extent.

Two-phase separation observed in thermally treated pellets of ClO4- doped poly(3-methylthiophene) from electron spin resonance measurements

A pressed pellet of CIO (-)(4) poly (3-methylthiophene) (P3MT) was heated for two hours at 85 degrees C and suddenly dropped in liquid nitrogen. A change was observed around 220 K in the Electron Spin Resonance (ESR) spectra when the sample was slowly cooled from room temperature. ESR line asymmetry parameter (A/B) showed two spatially separated phases. One was identified as a small metallic-like phase. The other phase, the larger one, makes a transition to a semiconducting Charge-Density Wave (CDW) state.

Ghost poles in the nucleon propagator in the linear sigma model approach and its role in pi N low-energy theorems

Complex mass poles, or ghost poles, are present in the Hartree-Fock solution of the Schwinger-Dyson equation for the nucleon propagator in renormalizable models with Yukawa-type meson-nucleon couplings, as shown many years ago by Brown, Puff and Wilets (BPW), These ghosts violate basic theorems of quantum field theory and their origin is related to the ultraviolet behavior of the model interactions, Recently, Krein et.al, proved that the ghosts disappear when vertex corrections are included in a self-consistent way, softening the interaction sufficiently in the ultraviolet region. In previous studies of pi N scattering using ''dressed'' nucleon propagator and bare vertices, did by Nutt and Wilets in the 70's (NW), it was found that if these poles are explicitly included, the value of the isospin-even amplitude A((+)) is satisfied within 20% at threshold. The absence of a theoretical explanation for the ghosts and the lack of chiral symmetry in these previous studies led us to re-investigate the subject using the approach of the linear sigma-model and study the interplay of low-energy theorems for pi N scattering and ghost poles. For bare interaction vertices we find that ghosts are present in this model as well and that the A((+)) value is badly described, As a first approach to remove these complex poles, we dress the vertices with phenomenological form factors and a reasonable agreement with experiment is achieved, In order to fix the two cutoff parameters, we use the A((+)) value for the chiral limit (m(pi) --> 0) and the experimental value of the isoscalar scattering length, Finally, we test our model by calculating the phase shifts for the S waves and we find a good agreement at threshold. (C) 1997 Elsevier B.V. B.V.

CORRELATIONS AMONG LOW-ENERGY OBSERVABLES OF THE 4-NUCLEON SYSTEM

Using a simple effective nucleon-trinucleon interaction we predict and study certain new correlations which are expected to exist between low-energy s-wave spin-isospin singlet observables of the four-nucleon system when various potential models yielding essentially identical two- and three-nucleon observables are employed in four-nucleon calculations. The study of such correlations using different realistic potential models is expected to yield information about the nucleon-nucleon and three-nucleon interactions.

Properties and one-step synthesis of (2-acetylpyridine)tetraammineruthenium(II), [Ru-II(2-acpy)(NH3)(4)](2+) and tetraammine(2-benzoylpyridine)ruthenium(II), [Ru-II(NH3)(4)(2-bzpy)](2+) Redox potentials, UV-Vis and NMR spectra

A more direct and efficient route to the syntheses of [Ru(NH3)(4)(X-Y)](BF4)(2), where X-Y can be 2-acetylpyridine (2-acpy) or 2-benzoylpyridine (2-bzpy), based on the reactions of [RuCl(NH3)(5)]Cl-2 with these ortho-substituted azines is described. The [Ru(2-acpy)(NH3)(4)](BF4)(2) and [Ru(NH3)(5)(2-bzpy)](BF4)(2) complexes have a molar conductance of 328 and 292 Ohm(-1) cm(2) mol(-1), respectively, corresponding to a 1:2 species in solution. These complexes showed two intense absorption bands around 620-650 and 380 nm, the energies of which are solvent dependent, decreasing with the increase of the Gutman's donor number of the solvent, and were assigned as metal-to-ligand charge transfer (MLCT). The complexes have oxidation potentials (Ru-II/III) of +0.380 V vs. Ag/AgCl (2-acpy) and +0.400 V vs. Ag/AgCl (2-bzpy), and reduction potentials (X-Y0/-) of -1.10 V vs. Ag/AgCl (2-acpy) and -0.950 V vs. Ag/AgCl (2-bzpy) on CF3COOH/NaCF3COO at pH=3.0, scan rate 100 mV s(-1), [Ru]=1.0x10(-3) mol l(-1). Both processes show a coupled chemical reaction. Upon oxidation of the metal center, the MLCT absorption bands are bleached and restored upon subsequent reduction. In order to confirm the structure of the complexes a detailed LH NMR investigation was performed in d(6)-acetone. Further confirmation of the structure was obtained by recording the N-15 NMR spectrum of [Ru(NH3)(4)(2-bzpy)](2+) in d(6)-DMSO using the INEPT pulse sequence improving the sensitivity of N-15 by polarization transfer from the protons to the N-15. The Nuclear Overhauser Effect (NOE) experiments were made qualitatively for [Ru(NH3)(4)(2-acpy)](2+), and showed that H-6 of the pyridine is close to a NH3 proton, which should then be in a cis position, and, hence, confirming that acpy is acting as a bidentate ligand. (C) 1999 Elsevier B.V. Ltd. All rights reserved.

Electronic spectra, ESR and crystal structure of tetrahedral halocuprates(II). The existence of cations (2tbpo·H+) with a very short hydrogen bond

Two compounds [2tbpo·H+)2[CuCl4]= (yellow) and (2tbpo·H+)2[CuBr4]= (dark purple) (tbpo = tribenzylphosphine oxide) have been prepared and investigated by means of crystal structure, electronic, vibrational and ESR spectra. The crystal structure of the (2tbpo·H+)2[CuCl4]= complex was determined by three-dimensional X-ray diffraction. The compound crystallizes in the space group P42/n with unit-cell dimensions a = 19.585(2), c = 9.883(1)Å, V = 3790 (1)Å3, Z = 2, Dm = 1.303 (flotation) Dx = 1.302 Mg m-3. The structure was solved by direct methods and refined by blocked full-matrix least-squares to R = 0.053 for 2583 observed reflections. Cu(II) is coordinated to four chlorides in a tetrahedral arrangement. Tribenzylphosphine oxide molecules, related by a centre of inversion, are connected by a short hydrogen bridge. Chemical analysis, electronic and vibrational spectra showed that the bromide compound is similar to the chloride one and can be formulated as (2tbpo·H+)2[CuBr4]=. The position of the dd transition bands, the charge transfer bands, the ESR and the vibrational spectra of both complexes are discussed. The results are compared with analogous complexes cited in the literature. © 1983.

NiBr2 complexes with triphenylarsine oxide (Ph3AsO). Two new interesting structures

The synthesis and crystal structure of two complexes resulting from interaction between NiBr2 and triphenylarsine oxide (Ph3AsO) is described. Green and orange complexes can be obtained from the blue, probably tetrahedral complex [NiBr2(Ph3AsO)2], depending on the solvents used for recrystallization. NiBr2·4[(C6H5)3AsO]·8H2O (green): M = 1650.2, P21/c, a = 13.731(2), b = 16.267(3), c = 17.647(2) Å, β = 112.04(1)°, V = 3651.4 Å3, Z = 2, Dx = 1.501 g cm-3, CuKα, λ = 1.54184 Å, μ = 38.67 cm-1, R = 0.039, 3741 unique reflections, 3203 with I > 3σ(I). NiBr2·4[(C6H5)3AsO]·3|2(C6H5CH3)·H2O (orange): M = 1663.7, P1, a = 12.647(8), b = 13.953(5), c = 22.853(6) Å, α = 90.91(3), β = 96.70(4), γ = 111.16(4)°, V = 3727.4 Å3, Z = 2, Dx = 1.482 g cm-3, MoKα, λ = 0.71073 Å, μ = 30.48 cm-1, R = 0.087, 8600 unique reflections, 4293 with I > 3σ(I). In the green complex the Ni(II) ion is sited on a center of symmetry and is octahedrally coordinated to six water molecules, hydrogen bonded to the Ph3AsO molecules and to the bromide anions forming a second coordination sphere in a nearly octahedral arrangement. In the orange complex the cation is pentacoordinated with the four oxygen atoms of the Ph3AsO ligands forming the basis of a tetragonal pyramid and with one Br- anion in the apical position. The absorption spectrum of the orange complex is compared with the spectra of other Ni(II) square pyramidal complexes described in the literature. © 1984.

Dimensional versus cut-off renormalization and the nucleon-nucleon interaction

The role of dimensional regularization is discussed and compared with that of cut-off regularization in some quantum mechanical problems with ultraviolet divergence in two and three dimensions with special emphasis on the nucleon-nucleon interaction. Both types of renormalizations are performed for attractive divergent one- and two-term separable potentials, a divergent tensor potential, and the sum of a delta function and its derivatives. We allow energy-dependent couplings, and determine the form that these couplings should take if equivalence between the two regularization schemes is to be enforced. We also perform renormalization of an attractive separable potential superposed on an analytic divergent potential.

Two-body Dirac equation approach to the deuteron

The two-body Dirac(Breit) equation with potentials associated to one-boson-exchanges with cutoff masses is solved for the deuteron and its observables calculated. The 16-component wave-function for the Jπ = 1+ state contains four independent radial functions which satisfy a system of four coupled differential equations of first order. This system is numerically integrated, from infinity towards the origin, by fixing the value of the deuteron binding energy and imposing appropriate boundary conditions at infinity. For the exchange potential of the pion, a mixture of direct plus derivative couplings to the nucleon is considered. We varied the pion-nucleon coupling constant, and the best results of our calculations agree with the lower values recently determined for this constant.

Relativistic quark spin coupling effects in the nucleon electromagnetic form factors

We investigate the effect of different forms of relativistic spin coupling of constituent quarks in the nucleon electromagnetic form factors. The four-dimensional integrations in the two-loop Feynman diagram are reduced to the null-plane, such that the light-front wave function is introduced in the computation of the form factors. The neutron charge form factor is very sensitive to different choices of spin coupling schemes, once its magnetic moment is fitted to the experimental value. The scalar coupling between two quarks is preferred by the neutron data, when a reasonable fit of the proton magnetic momentum is found. (C) 2000 Elsevier Science B.V.

Nucleon-nucleon interaction: Central potential and pion production

We show that the tail of the chiral two-pion exchange nucleon-nucleon potential is proportional to the pion-nucleon (πN) scalar form factor and discuss how it can be translated into effective scalar meson interactions. We then construct a kernel for the process NN → πNN, due to the exchange of two pions, which may be used in either three-body forces or pion production in NN scattering. Our final expression involves a partial cancellation among three terms, due to chiral symmetry, but the net result is still important. We also find that, at large internucleon distances, the kernel has the same spatial dependence as the central NN potential and we produce expressions relating these processes directly.

Antinucleon spectra in the Dirac equation with scalar and vector Wood-Saxon potentials

We analyze here the spin and pseudospin symmetry for the antinucleon spectra solving the Dirac equation with scalar and vector Wood-Saxon potentials. In relativistic nuclear mean field theories where these potentials have large magnitudes and opposite signs we show that contrary to the nucleon case where pseudospin interaction is never very small and cannot be treated perturbatively, for antinucleon systems this interaction is perturbative and an exact pseudospin symmetry is possible. This result manifests the relativistic nature of the nuclear pseudospin symmetry. © 2009 American Institute of Physics.