Characterization of covalent linkages in organically functionalized MCM-41 mesoporous materials by solid-state NMR and theoretical calculations

The covalent linkages formed during functionalization of MCM-41 mesoporous molecular sieves with five chloroalkylsilanes ((EtO)3Si(CH2Cl), (MeO)3Si(CH2CH2CH2Cl), Cl3Si(CH2CH2CH3), Cl2Si(CH3)(CH2Cl) and Cl2Si(CH3)2) have been investigated using high-resolution solid-state NMR spectroscopy and DFT calculations. Structural information was obtained from 1H-13C and 1H-29Si heteronuclear (HETCOR) NMR spectra, in which high resolution in the 1H dimension was obtained by using fast MAS. The 1H-13C HETCOR results provided the assignments of 1H and 13C resonances associated with the surface functional groups. Sensitivity-enhanced 1H-29Si HETCOR spectra, acquired using Carr-Purcell-Meiboom-Gill refocusing during data acquisition, revealed the identity of 29Si sites (Qn, Tn, and Dn) and the location of functional groups relative to these sites. Optimal geometries of local environments representing the Qn, Tn and Dn resonances were calculated using molecular mechanics and ab initio methods. Subsequently, DFT calculations of 29Si, 13C, and 1H chemical shifts were performed using Gaussian 03 at the B3LYP/6-311++G(2d,2p) level. The theoretical calculations are in excellent accord with the experimental chemical shifts. This work illustrates that state-of-the-art spectroscopic and theoretical tools can be used jointly to refine the complex structures of inorganic-organic hybrid materials.     

D-Glucosamine as a green ligand for copper catalyzed synthesis of primary aryl amines from aryl halides and ammonia

Replacing conventional reagents with environment friendly reagents is one of the primary goals of modern synthetic methodology and in this very primitive study about utilizing green, naturally available carbohydrate molecules as ligands in transition metal catalyzed reactions, we report Cu/D-glucosamine as an efficient catalyst for aniline synthesis.     

Identification of distinct Brønsted acidic sites in zeolite mordenite by proton localization and [27Al]-1H REAPDOR NMR

[27Al]-1H REAPDOR (Rotational Echo Adiabatic Passage DOuble Resonance) NMR experiments distinguish structurally dissimilar Br?nsted acid sites located in main 8 and 12 membered-ring channels of the zeolite mordenite.     

Tuning rheochaos by temperature in wormlike micelles

We investigate the critical role played by the mean micellar length during the route to rheochaos for wormlike micellar gels of surfactant cetyltrimethylammonium tosylate in the presence of salt sodium chloride that show coupling of flow to concentration fluctuations. To this end, we have carried out stress/shear rate relaxation experiments at a fixed shear rate/stress but at different temperatures to take the sample through the route to rheochaos. We see the type-II intermittency route to rheochaos in stress relaxation measurements and the type-III intermittency route to rheochaos in shear rate relaxation measurements. We have also carried out linear rheology measurements at different temperatures to estimate the mean micellar length (-)L, the reptation time tau(rep), and the breaking time tau(break). It is shown that (-)L changes by approximately 58%, as the sample goes through the route to rheochaos.     

B(C6F5)3 catalyzed synthesis of dihydropyrano[3,2-b]chromenediones under solvent-free conditions

Abstract

A simple, efficient, solventless, and one step, B(C₆F₅)₃ catalyzed, synthesis of dihydropyrano[3,2-b]chromenediones from dimedone, aldehyde and kojic acid is described. This protocol proceeds smoothly, accommodates aromatic as well as heteroaromatic aldehydes and gives dihydropyrano[3,2-b]chromenediones in excellent yield.     

Dispersion and nonlinear management for femtosecond optical solitons

We consider the concept of femtosecond propagation for optical solitons in a dispersion management fiber and study the optimal amplification of optical solitons through dispersion wells and barriers and also for the dispersion tailored profile case. For the former, we observed periodic soliton trapping for the in-phase injection case when their respective velocities were equal and opposite with their amplitudes being unequal and no soliton trapping for the off-phase injection case when the two pulses are having a phase difference of π. For the latter, we observed an enormous amplification of the soliton pulses which is one of our main results in this Letter.     

Thermal convection in an infinite porous medium due to a source in sphere

A simple mathematical theory is proposed using the Brinkman model, for the analysis of the free convection motion in an unbounded porous medium, inducedby an instantaneous point heat source which is enveloped by a solid sphere. The theory consists of retaining only the leading terms of the series expansions of the dependent variables in terms of the Rayleigh number and is valid in the limit of small Rayleigh number only. The heat generating rate is assumed to be not excessive so that the induced flow is slow. The evolution of the flow field is demonstrated by drawing the streamlines at various times and the results are delineated by comparing them with those of the Darcy flow model. The significance of the presence of the enveloping solid sphere is highlighted.     

Hydrodynamic response of tension-leg platforms—A model

The tension-leg platform (TLP) is a hybrid type of offshore platform, fixed for the vertical degrees of freedom and compliant for the horizontal degrees of freedom. The platform structure is connected to the sea-bed foundation by vertical moorings (tension legs) which are kept in tension under all conditions by the excess buoyancy over weight of the platform. Hydrodynamic analysis is an important stage of the design of these structures; model tests are usually done for this purpose. This paper gives the details of an experimental study to analyze the behavior of a 1:50 TLP model subjected to the action of regular and irregular waves. Platform motions in surge, sway and heave directions were measured using accelerometers. Tether tensions were measured using specially developed load cells. Details of instrumentation, testing and analysis to obtain response amplitude operators (RAOs) are explained. The connection of tether to the column, arrangement for adjusting pretension from top, connection of load cell to the tether and provision for changing the angle of incidence of waves are some special features of this experimental study. The work reported will serve as a useful practical guide for similar types of experimental study. In addition, the conclusions on the comparison of RAOs of the model for regular and irregular waves will be useful in planning hydrodynamic model tests.