Absorption and emission spectra of molecules subject to hindered internal rotation with an application to α-dicarbonyls

A simple model of a flexible molecule with one hindered rotation and one totally symmetric vibration is presented and the resulting absorption and emission spectra are discussed. The results are applied to ¹S₀-¹S₁(¹nπ*) spectra of α-dicarbonyls.     

Quantization and Motion Law for Ginzburg–Landau Vortices

We study the vortex trajectories for the two-dimensional complex parabolic Ginzburg–Landau equation without a well-preparedness assumption. We prove that the trajectory set is rectifiable, and satisfies a weak motion law. In the case of degree  ±  1 vortices, the motion law is satisfied in the classical sense. Moreover, dissipation occurs only at a finite number of times. Away from these times, possible collisions and splittings of vortices are constrained by algebraic equations involving their topological degrees. Quantization properties of the energy and potential densities play a central role in the proofs.     

Guidelines for Modeling a 2D Rough Wall Channel Flow

The effects of 2-D roughness elements on the Reynolds stress anisotropy tensor, and of the energy dissipation rate anisotropy tensor of a turbulent channel flow are investigated using data obtained from direct numerical simulations (DNSs). The roughness elements consist of transverse square rods of size , placed on one wall of the channel only. While is kept constant (, is the half-width of the channel), the spacing between the rods is varied from to . The results show that the variation in can dramatically change the structure of the wall region flow. The modeling of the near-wall region needs to reflect the structural changes caused by the variation in . On the basis of the Reynolds stress budgets, attention needs to be given to the turbulent energy and pressure diffusion terms while local isotropy may be a reasonable approximation for the energy dissipation rate, especially over a range of for which the drag is near its maximum.     

Synthesis of some oligopyridine-galactose conjugates and their metal complexes: a simple entry to multivalent sugar ligands

Some galactose-oligopyridine conjugates were readily assembled by combining differently functionalized oligopyridines with peracetylated galactose derivatives. Variation in the structure of the components and of the linkers employed for their connection afforded adducts of different size, shape, and conformational mobility. Complexation of the bipyridine ligands with CuOTf and of the terpyridine ligand with Zn(OTf)₂ afforded the corresponding peracetylated 2:1 and 1:1 complexes, respectively, as single species. Their structures were determined to be tetrahedral (Cu complexes) and trigonal-bipyramidal (Zn complex), on the basis of spectroscopic evidence. Removal of the acetyl protecting groups from the ligands afforded the corresponding polyols. The terpyridine-Zn(II) complex, unlike the bipyridine-Cu(I) complexes maintained their structures upon removal of the acetyl protecting groups.     

Numerical and experimental study of the interaction between a vortex dipole and a circular cylinder

This paper describes a study of the centred collision between a dipolar vortex and a solid circular cylinder. The flow was analysed experimentally by using dye visualizations and streak photography. Flow characteristics such as vorticity fields and the transport of passive tracers were compared with numerical simulations. Observations revealed that thin layers of vorticity, created at the cylinder wall are advected by the primary dipole halves, which, while rolling up into compact patches, give rise to the formation of two new asymmetric dipoles that move away along curved trajectories. The structure of the vorticity distribution inside the dipole, before and after the collision, has been investigated. Both the numerical and the experimental results indicate that the vorticity patches originating from the original primary dipole approximately preserve their original functional relationship =f(), while the secondary vorticity patches show a tendency to organize into structures attaining a similar relationship.One of us (JBF) gratefully acknowledges financial support by the Foundation for Fundamental Research on Matter (FOM) of the Netherlands Organization for Pure Research (NWO).     

A method for determining the frictional velocity in a turbulent channel flow with roughness on the bottom wall

A method is proposed for determining the frictional velocity U on both walls of a fully developed turbulent channel flow, one smooth and the other rough. This should aid experimentalists in obtaining a reliable estimate of U with knowledge of only the pressure drop and location where the Reynolds shear stress is zero. The method is general and does not depend on the roughness geometry that is used. It has been validated against direct estimates of the wall stress using DNS databases for two types of two-dimensional roughness. Results for a surface composed of staggered cubes are also in accord with the method.     

Continuous interfaces with disorder: Even strong pinning is too weak in two dimensions

We consider statistical mechanics models of continuous height effective interfaces in the presence of a delta pinning of strength ε$\epsilon$ at height zero. There is a detailed mathematical understanding of the depinning transition in two dimensions without disorder. Then the variance of the interface height w.r.t. the Gibbs measure stays bounded uniformly in the volume for ε>0$\epsilon >0$ and diverges like |logε|$|log\epsilon |$ for ε↓0$\epsilon \downarrow 0$. How does the presence of a quenched disorder term in the Hamiltonian modify this transition? We show that an arbitrarily weak random field term is enough to beat an arbitrarily strong delta pinning in two dimensions and will cause delocalization. The proof is based on a rigorous lower bound for the overlap between local magnetizations and random fields in finite volume. In two dimensions it implies growth faster than that of the volume which is a contradiction to localization. We also derive a simple complementary inequality which shows that in higher dimensions the fraction of pinned sites converges to one with ε↑∞$\epsilon \uparrow \infty$.     

Vibronic coupling in pyrazine nπ* transitions

Vibronic coupling integrals governing the induced absorption of the first nπ* excited state have been evaluated theoretically. The calculations, based on the CNDO/S method, confirm the b_1g assignment of the 918 cm^?1 vibration, its prominent role in the intensity borrowing and its anharmonic character in the B_3u state. Furthermore they suggest that non-BO coupling and borrowing from higher electronic states cannot be neglected.     

Metal-Free Deoxygenation of Chiral Nitroalkanes: An Easy Entry to α-Substituted Enantiomerically Enriched Nitriles

A metal-free, mild and chemodivergent transformation involving nitroalkanes has been developed. Under optimized reaction conditions, in the presence of trichlorosilane and a tertiary amine, aliphatic nitroalkanes were selectively converted into amines or nitriles. Furthermore, when chiral β-substituted nitro compounds were reacted, the stereochemical integrity of the stereocenter was maintained and α-functionalized nitriles were obtained with no loss of enantiomeric excess. The methodology was successfully applied to the synthesis of chiral β-cyano esters, α-aryl alkylnitriles, and TBS-protected cyanohydrins, including direct precursors of four active pharmaceutical ingredients (ibuprofen, tembamide, aegeline and denopamine).