Showing papers on "Pi interaction published in 1997"

Separation of S-wave pseudoscalar and pseudovector amplitudes in π−P↑ → π+π−n reaction on polarized target

Abstract: A new analysis of S-wave production amplitudes for the reaction π−p↑ → π+π−n on a transversely polarized target is performed. It is based on the results obtained by the CERN-Cracow-Munich collaboration in the ππ energy range from 600 MeV to 1600 MeV at 17.2 GeV/c π− momentum. Energy-independent separation of the S-wave pseudoscalar amplitude (π exchange) from the pseudovector amplitude (a 1 exchange) is carried out using assumptions much weaker than those in all previous analyses. We show that, especially around 1000 MeV and around 1500 MeV, the a1 exchange amplitude cannot be neglected. The scalarisoscalar ππ phase shifts are calculated using fairly weak assumptions. Below the KK threshold we find two solutions for the π — π phase shifts, for which the phases increase slower with the effective π — π mass than the P-wave phases. Both solutions are consistent with a broad f 0(500) but only one is similar to the well-known “down” solution. We find also the third solution (with a somewhat puzzling behavior of inelasticity) which exhibits a narrow f 0(750) claimed by Svec. All the solutions undergo a rapid change at the KK threshold. Above 1420 MeV the phase shifts increase with energy faster than those obtained without the polarized-target data. This phase behavior as well as an increase of the modulus of the a1-exchange amplitude can be due to the presence of the f 0(1500).

Through-Bond Orbital Coupling, the Parity Rule, and the Design of “Superbridges” Which Exhibit Greatly Enhanced Electronic Coupling: A Natural Bond Orbital Analysis

Abstract: An ab initio molecular orbital study of through-bond (TB) orbital interactions has been carried out on several series of diene hydrocarbons, 2(n)−17(n), in which the double bonds are covalently attached to a variety of rigid saturated hydrocarbon bridges with lengths, n, ranging from four to 17 C−C σ bonds. The resulting TB π+,π- and π*+,π*- splitting energies, ΔE(π) and ΔE(π*), respectively, were obtained at the HF/3-21G level of theory. The distance dependence of ΔE(π) and ΔE(π*) for each type of diene was fitted to the respective exponential decay profiles, ΔE(π) = A exp(−βhn) and ΔE(π*) = B exp(−βen). It was found that both βh and βe were dependent on the nature of the hydrocarbon bridge. For example, βh is found to range from 0.6 per bond for 3(n) to only 0.05 per bond for 7(n) and 8(n). The βh values for the polynorbornane bridge dienes, 2(n), and the hybrid norbornane−bicyclo[2.2.0]hexane bridge dienes, 3(n), are notably larger than that for the divinylalkanes, 4(n), and Natural Bond Orbital (NBO) ...

Spectroscopy and molecular structure of efficient laser dyes: vibronic spin-orbit interactions in heterocyclics.

Abstract: Some characteristics of selected laser dyes are reviewed, and the effect of heterocyclic atom substitutions on triplet (π, π*) transitions is studied experimentally. Specifically, the intensities (oscillator strengths f) of the lowest-energy singlet–singlet (S–S) and triplet–triplet (T–T) transitions of anthracene and some of its heterocyclic analogs were measured. It is shown that this type of substitution of carbon atoms by heteroatoms results in a considerable reduction of intensity of T–T transitions. This observation is important to laser dye technology. The effect is explained by the existence of an efficient vibronic coupling mechanism between (n, π*) and (π, π*) triplet states in heteroatom molecules. Some general guidelines for how to find efficient laser dyes are proposed.

Analytical one-particle approach to the π electronic structure of heterocyclic polymers

Abstract: The one-electron description of heterocyclic five-membered ring polymers is derived on the basis of the Su–Schrieffer–Heeger–Huckel type Hamiltonian which accounts for the electronic interaction of the heteroatom p-orbital lone pair with the π band structure of the carbon backbone An explicit form of the fifth order equation, the solutions of which determine the dispersion relations for five π electron bands and closed expressions of molecular orbitals, is obtained The main accent is put on the gross π electronic polymer structure It is shown that there exists one-to-one correspondence between the structure considered as a function of basic system parameters [such as the electron on-site energies at carbon (C) and heteroatom (X), and resonance integrals associated with C–C and X–C bonds] and zeros of the Green function of polymer building blocks—monomers This interrelation is expressed in the form of certain equations for the monomer Green function matrix elements, which predict the values of system p

Electronic structure of axially ligated rhodium carboxylates: π back-bonding revisited

Abstract: Fenske-Hall molecular orbital calculations have been applied to a new, structurally isolable analog of the intermediate involved in catalytic rhodium carboxylate carbenoid transformations. Results from the structural characterization of axially ligated rhodium acetate phenylisonitrile complexes have been utilized in approximate molecular orbital calculations. The results from the calculations suggest that a significant degree of π back-bonding exists between the metal and isonitrile fragments which, by analogy, implies that π back-bonding should also exist in the rhodium carbenoid intermediate. Sensitivity of the Fenske-Hall method to the magnitude of π back-donation in these complexes was gauged through calculations involving different phenylisonitrile groups in which the π back-bonding ability was modulated through derivativization. The reliability of the Fenske-Hall method was evaluated through a comparison to a high-level calculation.