Upconverted 5d-4f luminescence from Er and Nd ions doped into fluoride hosts excited by ArF and KrF excimer lasers

We report on observation of upconverted VUV luminescence due to 5d-4f radiative transitions in Er³⁺ and Nd³⁺ ions doped into some fluoride crystals, under excitation by ArF and KrF excimer lasers, respectively. Only spin-forbidden 5d-4f luminescence of Er³⁺ (at 165 nm) was detected from the LiYF₄:Er³⁺ crystal whereas both spin-forbidden (at 169 nm) and spin-allowed (at 160.5 nm) components are observed from the BaY₂F₈:Er³⁺ crystal, the latter being much weaker than in the case of one-photon excitation. Nd³⁺ 5d-4f luminescence at 180 and 173 nm has been detected from the LiYF₄:Nd³⁺ and LaF₃:Nd³⁺ crystals, respectively. The shift of short-wavelength edge of 5d-4f emission spectra towards longer wavelengths is observed under temperature increase from 15 to 293 K. The observed effects in the spectra of Er³⁺ and Nd³⁺ doped crystals were interpreted as a result of reabsorption of 5d-4f luminescence escaping from the bulk of the crystals.     

Divergence en route to nonclassical annonaceous acetogenins. Synthesis of pyranicin and pyragonicin

Syntheses of the nonclassical annonaceous acetogenins, pyranicin, and pyragonicin from common late-stage intermediates are presented. The construction of key elements relies on asymmetric HWE reactions, including the desymmetrization of a meso-dialdehyde and a parallel kinetic resolution of a racemic aldehyde. A stereoconvergent Pd-catalyzed substitution serves to install the C4 stereocenter in protected form with different orthogonal protective groups. A divergent strategy to form 1,4- and 1,6-diols, employing stereoselective Zn-mediated alkynylations, is used for completion of the core structures. Notably, the stereoselective coupling reaction toward pyragonicin proceeds with highly functionalized fragments. The methodology is further expanded by a divergent synthesis of all stereoisomers of the 2,3,6-trisubstituted tetrahydropyran subunit.     

Forward flux sampling-type schemes for simulating rare events: efficiency analysis

We analyze the efficiency of several simulation methods which we have recently proposed for calculating rate constants for rare events in stochastic dynamical systems in or out of equilibrium. We derive analytical expressions for the computational cost of using these methods and for the statistical error in the final estimate of the rate constant for a given computational cost. These expressions can be used to determine which method to use for a given problem, to optimize the choice of parameters, and to evaluate the significance of the results obtained. We apply the expressions to the two-dimensional nonequilibrium rare event problem proposed by Maier and Stein [Phys. Rev. E 48, 931 (1993)]. For this problem, our analysis gives accurate quantitative predictions for the computational efficiency of the three methods.     

Exact results for noise power spectra in linear biochemical reaction networks

We present a simple method for determining the exact noise power spectra and related statistical properties for linear chemical reaction networks. The method is applied to reaction networks which are representative of biochemical processes such as gene expression. We find, for example, that a post-translational modification reaction can reduce the noise associated with gene expression. Our results also indicate how to coarse grain networks by the elimination of fast reactions. In this context we have discovered a breakdown of the sum rule which relates the noise power spectrum to the total noise. The breakdown can be quantified by a sum rule deficit, which is found to be universal, and can be attributed to the high-frequency noise in the fast reactions.     

The use of Mosher derivatives for the determination of the absolute configuration of substituted isoxazolidines

Isoxazolidines serve as intermediates in the synthesis of natural products. In addition, they can display significant biological activity, much of which derives from their ability to act as nucleoside analogues. As a result, considerable effort has been applied toward the asymmetric synthesis of isoxazolidines. However, a rapid and straightforward method for determination of the absolute configuration of isoxazolidines has not yet been reported. Herein we report the application of Mosher derivatives for the determination of the absolute configuration of substituted isoxazolidines. The Mosher derivatives exhibit conformational behavior similar to the Mosher derivatives of cyclic secondary amines. Interpretation of the chemical shift anisotropy, with these conformational biases in mind, can be used for the assignment of the absolute configuration of substituted isoxazolidines.     

Calculation of molecular quadrupole moments and a demonstration of the importance of overlap densities in the theory of polyatomic molecules

A computational method for calculating quadrupole moments from molecular wave functions in a Slater orbital basis set is described. Using both IEHT and CNDO wave functions quadrupole moments for a series of polyatomic molecules are calculated. They are compared with experimental results and the IEHT wave functions are found to give agreement with experiment while CNDO wave functions do not. The importance of bicentric densities (overlap densities) in the calculation of multipole moments is shown. This is followed by a discussion of the usefulness of these wave functions for a quantitative characterization of the electronic structure of large molecules.     

Perturbation theories and wave functions for calculation of electronic polarizabilities application to DNA bases

Three different forms of perturbation theories, variational perturbation, finite perturbation and second-order, are evaluated regarding their value for calculation of electronic polarizabilities of small and intermediate size molecules. It is concluded that with the practical constraint of a small basis set the variational perturbation method is the most promising alternative for calculation of polarizabilities. For several small molecules, our calculated polarizabilities indicate that both IEHT and ab initio wave functions give values in close agreement with each other. Variational perturbation calculations of polarizabilities with IEHT wave functions also include the DNA bases.     

Mathematical Description of Strongly Nonlinear Pressure Dependence of Phase Transition Temperatures

In order to describe mathematically the pressure dependence of phase transition temperatures we have derived an equation using the following assumption: i. The transition enthalpy Δ_tH = const. ii. The transition volume Δ_tV depends on the pressure p according to: The modified Simon-Glatzel-equation T = T₀(1 + p/b)^a exp (cp) allows to calculate the transition temperatures T also in cases of strongly non-linear relations.     

Recent developments in the field of star-shaped polymers

The interest for star-shaped polymers arises from their unique structure and enhanced segment density. This paper will review the different methods of preparation for star-shaped polymers. Two cases of special interest will be considered in greater detail: the core-first method which gives access to end-functionalized star-shaped polymers and the three-step method which leads to the so called hetero-star copolymers with rather specific properties.