A monopole mass filter with a hyperbolic V-shaped electrode

In this article we compare the classical monopole mass filter of von Zahn and the monopole mass filter with a hyperbolic V-shaped electrode. The experimental results and those of computer simulation for both mass spectrometers are presented. We show that the replacement of a conventional 90 degrees V-shaped electrode by an electrode with a hyperbolic profile substantially improves the peak shape of any given mass, and increases the mass resolution by a factor of 3-4 and the abundance sensitivity by a factor of 100. The potential of high analytical performance combined with electroforming techniques for electrode manufacture indicate future practical uses of such instruments. Copyright 1999 John Wiley & Sons, Ltd.     

Surfactant-Assisted Acid Pretreatment of Sugarcane Tops for Bioethanol Production

Sugarcane tops is one of the largest biomass resources in India and in tropical countries such as Brazil in terms of surplus availability. Conversion of this feedstock to ethanol requires pretreatment to make it more accessible for the enzymes used in saccharification. Though several pretreatment regimens have been developed for addressing biomass recalcitrance, very few seem to be promising as an industrial process. A novel hybrid method involving use of mild acid and surfactant was developed which could effectively remove lignin and improve the sugar yield from sugar cane tops. Operational parameters that affect the pretreatment efficiency (measured as yield of sugars) were studied and optimized. Changes in structural properties of the biomass were studied in relation to the pretreatment process using scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier Transform Infrared (FTIR) analysis, and the changes in chemical composition was also monitored. The biomass pretreated with the optimized novel method could yield 0.798?g of reducing sugars per gram of pretreated biomass upon enzymatic hydrolysis.     

Hierarchical Modeling of Entangled Polymers

Summary : We demonstrate that it is possible to link multi-chain molecular dynamics simulations with the tube model using a single chain slip-links model as a bridge. This hierarchical approach allows significant speed up of simulations, permitting us to span the time scales relevant for a comparison with the tube theory. Fitting the mean-square displacement of individual monomers in molecular dynamics simulations with the slip-spring model, we show that it is possible to predict the stress relaxation. Then, we analyze the stress relaxation from slip-spring simulations in the framework of the tube theory. In the absence of constraint release, we establish that the relaxation modulus can be decomposed as the sum of contributions from fast and longitudinal Rouse modes, and tube survival. Finally, we discuss some open questions regarding possible future directions that could be profitable in rendering the tube model quantitative, even for mildly entangled polymers.     

A three body approach to study the structural properties of 2-n halo nuclei and the search for Efimov states

The discovery of neutron rich isotopes of the lightest elements on the neutron drip line exhibiting a halo structure has opened up new vistas in research activities. The novel structural features associated with the halo phenomena have been the subject for extensive theoretical and experimental investigations in recent times. In this talk, I propose to present a broad overview of the recent developments in this field, bringing out the striking features which show that a large number oflight nuclei near the neutron drip line are characterized by a clear separation between a ‘normal’ core nucleus and a loosely bound low density veil of neutrons. Specifically, the two neutron halos offer a natural premises, from a theoretical standpoint, to employ three body techniques for studying their detailed structural properties. A considerable part of the talk will be devoted to report and highlight the results on a number of light halo nuclei such as ¹¹Li, ¹¹Be, ¹⁹B and ²²C on which we have been carrying out investigations employing a simple but realistic three body model. These three body systems which have been termed as ‘Borromean’ (i.e while three body systems are bound, the corresponding binary subsystems on the other hand are unbound) are characterized by large spacial extension and very low separation energy of the neutron. They are, therefore, ideally suited for exploring the possibility of the existence of Efimov states in two neutron halo nuclei. We have recently carried out the three body analyses to predict the possibility of the occurrence of such states on which experimental work at various laboratories is underway.     

Spin state and exchange in the quasi-one-dimensional antiferromagnet KFeS2

We report the optical spectra and single crystal magnetic susceptibility of the one-dimensional antiferromagnet KFeS₂. Measurements have been carried out to ascertain the spin state of Fe³⁺ and the nature of the magnetic interactions in this compound. The optical spectra and magnetic susceptibility could be consistently interpreted using aS=1/2 spin ground state for the Fe³⁺ ion. The features in the optical spectra have been assigned to transitions within thed-electron manifold of the Fe³⁺ ion, and analysed in the strong field limit of the ligand field theory. The high temperature isotropic magnetic susceptibility is typical of a low-dimensional system and exhibits a broad maximum at ∼565K. The susceptibility shows a well defined transition to a three dimensionally ordered antiferromagnetic state atT _N=250 K. The intra and interchain exchange constants,J andJ′, have been evaluated from the experimental susceptibilities using the relationship between these quantities, andχ _max,T _max, andT _N for a spin 1/2 one-dimensional chain. The values areJ=−440.71 K, andJ′=53.94 K. Using these values ofJ andJ′, the susceptibility of a spin 1/2 Heisenberg chain was calculated. A non-interacting spin wave model was used belowT _N. The susceptibility in the paramagnetic region was calculated from the theoretical curves for an infiniteS=1/2 chain. The calculated susceptibility compares well with the experimental data of KFeS₂. Further support for a one-dimensional spin 1/2 model comes from the fact that the calculated perpendicular susceptibility at 0K (2.75×10⁻⁴ emu/mol) evaluated considering the zero point reduction in magnetization from spin wave theory is close to the projected value (2.7×10⁻⁴ emu/mol) obtained from the experimental data.     

Helical aromatic oligoamides: reliable, readily predictable folding from the combination of rigidified structural motifs

Factors responsible for the folding of aromatic oligoamides with backbones rigidified by local three-center H-bonds were investigated. The stability of the three-center H-bonds was quantified by the half-lives of amide proton-deuterium exchange reactions, which show that the three-center H-bonds were largely intact at room temperature in the oligomer examined. This result is consistent with our current and previous 2D NMR studies. The overall helical conformation of nonamer 1 was found by variable-temperature NOESY studies to be dynamic. As temperature rose, the end-to-end NOEs rapidly disappeared, while the amide side chain NOEs were still readily detectable, corresponding to the "breath" and stretching of the helix by slightly twisting the local H-bonded rings. Based on the simple repetition of the same structural motif and local conformational preference, undecamer 2 was found to fold into well-defined helical conformation. The predictability of the folding of these backbone-rigidified aromatic oligoamides was demonstrated by a simple modeling method using structural parameters from oligomers with known crystal structures. The reliability and generality of the modeling methods were shown by the excellent agreement between the modeled structures corresponding to 1 and 2 and data from NOESY studies.     

Accommodating unwelcome guests in inorganic layered hosts: inclusion of chloranil in a layered double hydroxide

The host-guest chemistry of most inorganic layered solids is limited to ion-exchange reactions. The guest species are either cations or anions to compensate for the charge deficit, either positive or negative, of the inorganic layers. Here, we outline a strategy to include neutral molecules like ortho- and para-chloranil, that are known to be good acceptors in donor-acceptor or charge-transfer complexes, within the galleries of a layered solid. We have succeeded in including neutral ortho- and para-chloranil molecules within the galleries of an Mg-Al layered double hydroxide (LDH) by using charge-transfer interactions with preintercalated p-aminobenzoate ions as the driving force. The p-aminobenzoate ions are introduced in the Mg-Al LDH via ion exchange. The intercalated LDH can adsorb ortho- and para-chloranil from chloroform solutions by forming charge-transfer complexes with the p-aminobenzoate anions present in the galleries. We use X-ray diffraction, spectroscopy, and molecular dynamics simulations to establish the nature of interactions and arrangement of the charge-transfer complex within the galleries of the layered double hydroxide.     

TICT formation in para- and meta-derivatives of N-phenylpyrrole

The photophysical properties of m- and p-cyano N-phenylpyrrole (m- and p-PBN) are compared. Both compounds show highly red-shifted and strongly forbidden emission in polar solvents, assigned to a charge transfer state. The forbidden nature is indicative of very weak coupling between the two pi-systems, and a twisted emissive structure is suggested (TICT state). Comparison to quantum chemical calculations indicates that the twisted structure possesses an antiquinoid distortion of the benzonitrile group, i.e., the central bonds in the ring are lengthened instead of shortened. m-PBN is the first meta compound which shows a CT emission assignable to a TICT state. It differs from p-PBN by a less exergonic formation of the CT state from the LE/ICT quinoid state. Consequently, it shows only single LE/ICT fluorescence in nonpolar alkane solvents, whereas p-PBN shows dual fluorescence in this solvent (LE/ICT and TICT).