The calculation of fluorine hyperfine interactions in π-electron radicals

Q factors have been calculated for fluorine hyperfine interaction in π-electron radicals using an independent-electron model for the σ orbitals. It is shown that the off-diagonal elements give an important contribution to fluorine coupling constants. Combining these Q factors with calculated π-electron spin densities of fluoro-nitrobenzenes gives good agreement with experimental coupling constants.     

Estimation of hyperfine coupling constants of 29Si nuclei in the radical anions of trimethylsilyl-substituted π-electron systems

An attempt has been made to relate the coupling constant a _Si of a ²⁹Si nucleus in the radical anions of nine trimethylsilyl derivatives of π-electron systems to the spin populations ρ _Si and of the silicon and the substituted carbon centre , respectively. For the parameter Q _CSi of the relation an absolute value of circa 20 gauss is found, whereas Q _Si appears not to be significantly different from zero. The coupling constants a _Si calculated by means of this relation agree moderately well with the experimental values (standard error circa 0·6 gauss). An HMO model in which the trimethylsilyl substituents are treated as pseudo-π-centres, with the parameters h _Si = -1·5 and k _CSi = 0·55, reproduces satisfactorily the spin distribution in the corresponding radical anions.     

Optimum parametrization in SCF π-electron theory

An attempt has been made to obtain ground-state charge distributions (as reflected by proton and carbon-13 magnetic shielding constants), ionization potentials, and pi-electron excitation energies for a number of substituted benzenes. A standard SCF-LCAO-MO method was employed, with the total-spin-differential overlap approximation. Excitation energies were obtained by the method of configuration interaction using all singly-excited configurations. Approximations were introduced so that only two parameters had to be specified for each atom. Since the two-centre repulsion integrals introduced by Mataga gave the best results for benzene, they were used throughout. Heteroatom parameters were chosen empirically, with the help of McWeeny's self-consistent perturbation theory, to give the best overall results for monosubstituted compounds. In general the calculated results were very reasonable.     

Models of πNN interactions

A NN model inspired by Quantum Chromodynamics is presented. The model gives an accurate fit to the most recent Arndt NN phase shifts up to 1 GeV and can be applied to study intermediate- and high-energy nuclear reactions.Invited talk to the symposium Mesons and Light Nuclei IV, Bechyn, Czechoslovakia, September 5–10, 1988.This work is supported by the U.S. Department of Energy, Nuclear Physics Division, under contract W-31-109-ENG-38.     

The π-electron spectra of the benzene N-heterocyclics

The π-electron spectra of the nitrogen heterocyclics are discussed on the basis of a perturbation applied to the spectra of the corresponding aromatic hydrocarbons. The frequency of the first π→π band in pyridine is the same as that of the corresponding band in benzene, because of the accidental cancellation of a first-order blue shift by a second-order red shift. The first-and second-order shifts are related to two perturbation parameters, and these are obtained from experiment by examining the spectra of the benzene heterocyclics. These parameters can then be satisfactorily related to one another by assuming a perturbation field of the form exp (?2ηr)/r and by allowing for the C-N bond being shorter than the C-C bond in benzene.     

Production of Λ hypernuclei in π+ A interactions

The production of Λ hyperons in π⁺ A interactions is studied on the basis of the intranuclear-cascade model, and the excitation-energy, momentum, angular-momentum, charge, and mass distributions of the ensemble of product hypernuclei are determined. The decay of excited hypernuclei via particle emission and fission is considered. It is shown that, for the production of heavy Λ hypernuclei, the optimal momentum of the incident π⁺ meson is about 1 GeV/c. The cross section of the channel in which heavy-nucleus decay induced by a π⁺ meson is accompanied by the emission of a K ⁺ meson is estimated, and it is proposed to use the reaction (π⁺, K ⁺ f) in measurements of the fission barrier in hypernuclei and of their level density.     

Theory for position of muonium in α-quartz and associated hyperfine interaction

The electronic structures and hyperfine interactions of muonium and hydrogen in -quartz are investigated by the unrestricted Hartree-Fock cluster procedure. The muonium is found to be trapped near the center of the line joining two silicon atoms. On including vibrational effects, the muon hyperfine constant comes out as 1.09 times that for free muonium, this ratio being larger than unity and smaller than for protons in trapped hydrogen, both features being in agreement with experiment.Briefly reported in Abstract at the American Physical Society meeting in New Orleans, March 1988. See Bull. Am. Phys. Soc. 33 (1988) 770.     

Calculation of orbital magnetic moments of π-electron terms of cyclic molecules in a collective-motion model

The magnetic moments of levels of ring -electron chains along a cyclic core of a molecule are calculated for various values of the form factor of the periodic potential of the -core field. The cases of short-range (flexible chain) and long-range (rigid chain) strong interelectron repulsion are considered. The magnetic moments of the flexible-chain levels are practically independent of the form factor. The magnetic moments of the rigid-chain levels are less than the corresponding moments of the flexible chain, but at certain values of the form factor resonance occurs and the rigid-chain moments approach those of the flexible chain. An anomalously large Zeeman splitting of the upper electron levels of large aromatic molecules is predicted.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 37–42, October, 1976.     

The effect of bond length variations in molecular orbital calculations of π-electron spectra—Aniline

The effect of bond length changes on ground and excited state energies is examined. The results are applied to aniline.     

Mössbauer hyperfine interactions in thermally treated iron-yttrium oxide systems

Iron-yttrium mixed oxides in varying ratios (YFe 91, 31, 11, 12, 13, and 19) annealed to different temperatures (850, 1000 and 1250 C) have been investigated by Mössbauer spectroscopy down to 85 K. The Mössbauer spectra are complex in nature, showing up to three magnetically split hyperfine patterns in iron-rich (Y Fe 11, 12, 13 and 19) samples; these three components are attributed to characteristic YFeO₃, Y₃Fe₅O₁₂ and hematite. The yttrium-rich samples (YFe 91, 31) show only one six-line spectrum of YFeO₃.     

Scaling of semi-inclusive spectra of π−mesons frompp interactions

An analysis of experimental data for 6–400 GeV/c shows that a two-parameter dependence of spectra on collision energy \((\sqrt s )\) and the multiplicity of negative particles (n) reduces to a single-parameter dependence on some function of these variables, which can be well approximated by a simple expressions/n.     

Excited-state absorption and third-order optical nonlinearities in symmetric π-electron organic molecules

Excited-State Absorption (ESA), Two-Photon Absorption (TPA) and the third-order polarizability (;,, – ) have been investigated for a model dichloride derivative of a symmetrically substituted benzylidene analine (SBAC), using a multielectron configuration-interaction procedure. The calculations indicate that SBAC exhibits ESA across the visible region of the spectrum, but that it is not as extensive as for molecules such as the phthalocyanines. The magnitude of the third-order polarizability is dominated by resonance enhancement from a very strongA _g B _u one-photon absorption. The calculated off-resonance value for (;,, – ) suggests that SBAC is a potential candidate for ultrafast switching applications.     

Ultra-violet absorption spectra and π-electron structures of nitromethane and the nitromethyl anion

The ultra-violet absorption spectrum of nitromethane was measured under various conditions. Besides the weak band at 270 mμ, a strong band, which may be regarded as due to the longest wavelength π→π* transition band, was observed at 198 mμ in the gaseous state. Further, the absorption spectrum of nitromethane was measured in aqueous solutions with several different ph values, and a strong band was observed at 233 mμ. From the fact that the pK_a value evaluated on the basis of the ph dependence of the absorption intensity is equal to that obtained electrometrically, it was concluded that the 233 mμ band is to be ascribed to the nitromethyl anion (H₂C^-NO₂). This band was found to shift to 238 mμ in alcoholic KOH solution.

The π-electron structure of the anion was studied theoretically by taking into account configuration interaction in terms of the ground, charge transfer, and locally excited configurations. It is shown that the 233 mμ band of the anion may be interpreted as an intramolecular charge-transfer absorption involving a large electron transfer from CH₂ ^- toward NO₂. It is suggested that electron donating substituent groups like NH₂, OH, and CH₂ ^- should cause the 198 mμ band of the nitro group to shift toward shorter wavelengths, in marked contrast to the case of substituted benzene molecules like aniline and phenol.     

Open beauty production in high energy π−-tungsten interactions

We present a study of \(B\bar B\) meson pair production inπ ^? interactions at 140, 194 and 286 GeV incident pion energy. At 286 GeV, where we have the best statistics, we find a model-dependent \(B\bar B\) production cross-section \(\sigma _{BB} = 14_{ - 6}^{ + 7} nb/nucleon\) .