Indirect 13C?13C couplings in benzene and the contribution of their anisotropies to the direct couplings in the oriented molecule

The indirect ortho and para carbon-carbon coupling constants in benzene were determined utilizing the deuterium isotope effect on the carbon chemical shifts in 1-d₁- and 1,3,5-d₃-benzene. The measurements gave for J(CC, ortho) and J(CC, para) the values of 55.3 ± 0.5 Hz and 10.08 ± 0.10 Hz, respectively. The ratios of the direct dipolar C? C coupling constants were calculated using the previous results of 1-¹³C-benzene in an oriented phase. These ratios differ from the theoretical ones obtained assuming the benzene ring to be hexagonal, showing significant indirect contributions in the D values.     

The rα-structures of partially oriented 1,2,4,5-and 1,2,3,4-tetrachlorobenzene determined from their proton magnetic resonance spectra with natural abundance 13C-satellites

By recording two spectra of the same molecule in different nematic liquid crystals, and analysing the data simultaneously, information has been obtained on the molecular structures of 1,2,4,5- (1) and 1,2,3,4-tetrachlorobenzene (2). Carbon–hydrogen and carbon–carbon internuclear distance ratios, as well as bond lengths and bond angles, were computed and corrected for harmonic vibrations. Only part of the molecular structure of 2 could be derived with reasonable precision; the reasons for this limitation are discussed. The spectra of the two isomers were also analysed in isotropic solvents in order to determine the indirect coupling constants. One- and two-bond ¹³C isotope effects on the chemical shifts of the protons were observed. The solvent effects of the different nematic liquid crystals on the structure were found to be small compared with the experimental error.     

Generalized parton distributions from nucleon form factor data

We present a simple empirical parameterization of the x- and t-dependence of generalized parton distributions at zero skewness, using forward parton distributions as input. A fit to experimental data for the Dirac, Pauli and axial form factors of the nucleon allows us to discuss quantitatively the interplay between longitudinal and transverse partonic degrees of freedom in the nucleon (nucleon tomography). In particular we obtain the transverse distribution of valence quarks at given momentum fraction x. We calculate various moments of the distributions, including the form factors that appear in the handbag approximation to wide-angle Compton scattering. This allows us to estimate the minimal momentum transfer required for reliable predictions in that approach to be around . We also evaluate the valence contributions to the energy-momentum form factors entering Jis sum rule.Received: 16 September 2004, Revised: 15 October 2004, Published online: 17 December 2004     

Pseudomorphic growth of a single element quasiperiodic ultrathin film on a quasicrystal substrate

An ultrathin film with a periodic interlayer spacing was grown by the deposition of Cu atoms on the fivefold surface of the icosahedral Al70Pd21Mn9 quasicrystal. For coverages from 5 to 25 monolayers, a distinctive quasiperiodic low-energy electron diffraction pattern is observed. Scanning tunneling microscopy images show that the in-plane structure comprises rows having separations of S=4.5+/-0.2 A and L=7.3+/-0.3 A, whose ratio equals tau=1.618... within experimental error. The sequences of such row separations form segments of terms of the Fibonacci sequence, indicative of the formation of a pseudomorphic Cu film.     

Two-photon annihilation into baryon-antibaryon pairs

We study the handbag contribution to two-photon annihilation into baryon-antibaryon pairs at large energy and momentum transfer. We derive factorization of the process amplitude into a hard subprocess and form factors describing the soft transition, assuming that the process is dominated by configurations where the (anti)quark approximately carries the full momentum of the (anti)baryon. The form factors represent moments of time-like generalized parton distributions, so-called distribution amplitudes. A characteristic feature of the handbag mechanism is the absence of isospin-two components in the final state, which in combination with flavor symmetry provides relations among the form factors for the members of the lowest-lying baryon octet. Assuming dominance of the handbag contribution, we can describe current experimental data with form factors of plausible size, and predict the cross sections of presently unmeasured channels. Received: 3 July 2002 / Revised version: 7 October 2002 / Published online: 20 November 2002 RID="a" ID="a" e-mail: mdiehl@physik.rwth-aachen.de RID="b" ID="b" e-mail: kroll@physik.uni-wuppertal.de RID="c" ID="c" e-mail: cvogt@nordita.dk     

Search for direct CP violation in nonleptonic decays of charged Xi and lambda hyperons

A search for direct CP violation in the nonleptonic decays of hyperons has been performed. In comparing the product of the decay parameters, alpha(Xi)alpha(Lambda), in terms of an asymmetry parameter, A(XiLambda), between hyperons and antihyperons in the charged Xi-->Lambdapi and Lambda-->ppi decay sequence, we found no evidence of direct CP violation. The parameter A(XiLambda) was measured to be 0.012+/-0.014.     

Exclusive electroproduction of pion pairs

We investigate electroproduction of pion pairs on the nucleon in the framework of QCD factorization for hard exclusive processes. We extend previous analyses by taking the hard-scattering coefficients at next-to-leading order in α_s . The dynamics of the produced pion pair is described by two-pion distribution amplitudes, for which we perform a detailed theoretical and phenomenological analysis. In particular, we obtain constraints on these quantities by comparing our results with measurements of angular observables that are sensitive to the interference between two-pion production in the isoscalar and isovector channels.     

Nonlinear robust optimization via sequential convex bilevel programming

In this paper, we present a novel sequential convex bilevel programming algorithm for the numerical solution of structured nonlinear min–max problems which arise in the context of semi-infinite programming. Here, our main motivation are nonlinear inequality constrained robust optimization problems. In the first part of the paper, we propose a conservative approximation strategy for such nonlinear and non-convex robust optimization problems: under the assumption that an upper bound for the curvature of the inequality constraints with respect to the uncertainty is given, we show how to formulate a lower-level concave min–max problem which approximates the robust counterpart in a conservative way. This approximation turns out to be exact in some relevant special cases and can be proven to be less conservative than existing approximation techniques that are based on linearization with respect to the uncertainties. In the second part of the paper, we review existing theory on optimality conditions for nonlinear lower-level concave min–max problems which arise in the context of semi-infinite programming. Regarding the optimality conditions for the concave lower level maximization problems as a constraint of the upper level minimization problem, we end up with a structured mathematical program with complementarity constraints (MPCC). The special hierarchical structure of this MPCC can be exploited in a novel sequential convex bilevel programming algorithm. We discuss the surprisingly strong global and locally quadratic convergence properties of this method, which can in this form neither be obtained with existing SQP methods nor with interior point relaxation techniques for general MPCCs. Finally, we discuss the application fields and implementation details of the new method and demonstrate the performance with a numerical example.