Bulk-molecule transition

Chander Mohan was born in 1975 at Dhariwal, Punjab, India. He received his B.Pharm. and M.Tech.(Pharm.) in bulk drugs from Guru Nanak Dev University and the National Institute of Pharmaceutical Education and Research (NIPER), Mohali. After that he worked as senior chemist at Dr. Reddy s Research Foundation, Hyderabad. He then joined Professor M. P. Mahajan s research group in 2002 for his doctoral studies. His research is focused on the synthesis and chemical transformation of C-5/C-6-substituted pyrimidinones. His research interests include synthesis of medicinally important molecules, transition metal-induced transformations in organic synthesis, reaction mechanism and total synthesis of natural products. 

In early work on molecular shape-selectivity [1-3], three kinds of mechanisms were envisaged. Reactant selectivity occurs when some molecules of the feed are too bulky to diffuse through the zeolite pores and are prevented from reacting. Product selectivity occurs when among all the product molecules formed within the pores, only those with the proper dimensions can diffuse out and appear as products in the bulk. Restricted transition-state selectivity occurs when certain reactions are... 

Aray et a/. analysed the topology V p of bulk 3d transition metals and zoomed in on the atomic graphs. The authors were able to correlate the value of at the local minima with the experimental heats of adsorption for the O2 and CO molecules on the 3d transition metals. 

The microscopic understanding of tire chemical reactivity of surfaces is of fundamental interest in chemical physics and important for heterogeneous catalysis. Cluster science provides a new approach for tire study of tire microscopic mechanisms of surface chemical reactivity [48]. Surfaces of small clusters possess a very rich variation of chemisoriDtion sites and are ideal models for bulk surfaces. Chemical reactivity of many transition-metal clusters has been investigated [49]. Transition-metal clusters are produced using laser vaporization, and tire chemical reactivity studies are carried out typically in a flow tube reactor in which tire clusters interact witli a reactant gas at a given temperature and pressure for a fixed period of time. Reaction products are measured at various pressures or temperatures and reaction rates are derived. It has been found tliat tire reactivity of small transition-metal clusters witli simple molecules such as H2 and NH can vary dramatically witli cluster size and stmcture [48, 49, M and 52]. 

Metals and alloys, the principal industrial metalhc catalysts, are found in periodic group TII, which are transition elements with almost-completed 3d, 4d, and 5d electronic orbits. According to theory, electrons from adsorbed molecules can fill the vacancies in the incomplete shells and thus make a chemical bond. What happens subsequently depends on the operating conditions. Platinum, palladium, and nickel form both hydrides and oxides they are effective in hydrogenation (vegetable oils) and oxidation (ammonia or sulfur dioxide). Alloys do not always have catalytic properties intermediate between those of the component metals, since the surface condition may be different from the bulk and catalysis is a function of the surface condition. Addition of some rhenium to Pt/AlgO permits the use of lower temperatures and slows the deactivation rate. The mechanism of catalysis by alloys is still controversial in many instances. 

RAIRS spectra contain absorption band structures related to electronic transitions and vibrations of the bulk, the surface, or adsorbed molecules. In reflectance spectroscopy the ahsorhance is usually determined hy calculating -log(Rs/Ro), where Rs represents the reflectance from the adsorhate-covered substrate and Rq is the reflectance from the bare substrate. For thin films with strong dipole oscillators, the Berre-man effect, which can lead to an additional feature in the reflectance spectrum, must also be considered (Sect. 4.9 Ellipsometry). The frequencies, intensities, full widths at half maximum, and band line-shapes in the absorption spectrum yield information about adsorption states, chemical environment, ordering effects, and vibrational coupling. 

At temperatures well above the glass transition of the polymers, the molecular segments are highly flexible and slip past each other almost without restriction. They behave like the molecules of a liquid except for the fact that their ends are linked with each other. Just the existence of crosslinks distinguishes rubberlike materials from ordinary liquids. The bulk moduli K of liquids and of rubberlike materials are of similar magnitude, e.g. K = 1 to 2 GPa [26]. 

The rapid rise in computer speed over recent years has led to atom-based simulations of liquid crystals becoming an important new area of research. Molecular mechanics and Monte Carlo studies of isolated liquid crystal molecules are now routine. However, care must be taken to model properly the influence of a nematic mean field if information about molecular structure in a mesophase is required. The current state-of-the-art consists of studies of (in the order of) 100 molecules in the bulk, in contact with a surface, or in a bilayer in contact with a solvent. Current simulation times can extend to around 10 ns and are sufficient to observe the growth of mesophases from an isotropic liquid. The results from a number of studies look very promising, and a wealth of structural and dynamic data now exists for bulk phases, monolayers and bilayers. Continued development of force fields for liquid crystals will be particularly important in the next few years, and particular emphasis must be placed on the development of all-atom force fields that are able to reproduce liquid phase densities for small molecules. Without these it will be difficult to obtain accurate phase transition temperatures. It will also be necessary to extend atomistic models to several thousand molecules to remove major system size effects which are present in all current work. This will be greatly facilitated by modern parallel simulation methods that allow molecular dynamics simulations to be carried out in parallel on multi-processor systems [115]. 

The relationship between film thickness of hexadecane with the addition of cholesteryl LCs and rolling speed under different pressures is shown in Fig. 25 [50], where the straight line is the theoretic film thickness calculated from the Hamrock-Dowson formula based on the bulk viscosity under the pressure of 0.174 GPa. It can be seen that for all lubricants, when speed is high, it is in the EHL regime and a speed index 4> about 0.67 is produced. When the rolling speed decreases and the film thickness falls to about 30 nm, the static adsorption film and ordered fluid film cannot be negligible, and the gradient reduces to less than 0.67 and the transition from EHL to TFL occurs. For pure hexadecane, due to the weak interaction between hexadecane molecules and metal surfaces, the static and ordered films are very thin. EHL... 

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