Acidity in zeolites and their characterization by different spectroscopic methods

109-123An overview of the different acidic sites in zeolites is presented here. It is followed by a discussion of the methods for identification of the various acidic sites and their distribution in some widely used zeolites such as faujasite, ZSM-5 and mordenite. Emphasis is laid on the discussion of various techniques, their relative advantages and disadvantages and several example studies, where these techniques are applied. The techniques discussed include titration in aprotic solvents, temperature-programmed desorption, microcalorimetric measurement of the heats of adsorption of probe molecules, IR spectroscopy with and without probe molecules, MAS NMR, photoelectron spectroscopy and position annihilation spectroscopy. From this study it appears that a single technique is not sufficient for the characterization and interpretation of zeolite acid sites and for the prediction of catalytic activity. Combination of different techniques such as NMR with IR can provide more information about the structure of acid sites of zeolites.</span

Application of hard-soft acid-base principle to study bronsted acid sites in zeolite clusters: a quantum chemical study

The effect of isomorphous substitution of Si by Al, B, and Ga in the tetrahedral sites of a zeolite framework on the properties of Bronsted acid sites is investigated by computational techniques. We used the density functional theory (DFT) based acidity descriptor, "local softness”, and a new acidity descriptor, "relative electrophilicity"to predict the acidity sequence of the zeolite clusters. The finite difference approximation is used to calculate local softness. The relative electrophilicity is found to be a suitable descriptor of acidity of zeolite clusters. The trend of acidity derived from local softness values depends on several factors such as basis sets, geometry of zeolite clusters, and different population analyses used to calculate them. The acidity trend derived from the relative electrophilicity follows the experimental trend, and it is not affected by the choice of basis sets or cluster models

Dynamics of Fullerene-Mediated Heat-Driven Release of Drug Molecules from Carbon Nanotubes

We have performed molecular dynamics (MD) simulations as a function of temperature to investigate the controlled release of multiple pyrazinamide (PZA) drug molecules encapsulated within single-wall carbon nanotube (SWCNT) mediated by fullerene (C₆₀) fillers. The displacement of PZA by C₆₀ can be accounted to the comparatively higher π–π stacking between C₆₀–SWCNT and PZA–SWCNT. The root-mean-square deviation (RMSD) provides definitive insight into drug release and simultaneous C₆₀ entrapment within the nanotube. The diffusion coefficient, variation of center of mass (COM), and energetic profiles at different temperatures suggest that the rate of diffusion of PZA increases with temperature. These results can be quite instrumental in providing details about the role of temperature on drug release from confined SWCNTs and aims at achieving the drug delivery regime

Dynamics of Fullerene-Mediated Heat-Driven Release of Drug Molecules from Carbon Nanotubes

We have performed molecular dynamics (MD) simulations as a function of temperature to investigate the controlled release of multiple pyrazinamide (PZA) drug molecules encapsulated within single-wall carbon nanotube (SWCNT) mediated by fullerene (C₆₀) fillers. The displacement of PZA by C₆₀ can be accounted to the comparatively higher π–π stacking between C₆₀–SWCNT and PZA–SWCNT. The root-mean-square deviation (RMSD) provides definitive insight into drug release and simultaneous C₆₀ entrapment within the nanotube. The diffusion coefficient, variation of center of mass (COM), and energetic profiles at different temperatures suggest that the rate of diffusion of PZA increases with temperature. These results can be quite instrumental in providing details about the role of temperature on drug release from confined SWCNTs and aims at achieving the drug delivery regime

Inhibitory potential of furanocoumarins against cyclin dependent kinase 4 using integrated docking, molecular dynamics and ONIOM methods

Cyclin Dependent Kinase 4 (CDK4) is vital in the process of cell-cycle and serves as a G1 phase checkpoint in cell division. Selective antagonists of CDK4 which are in use as clinical chemotherapeutics cause various side-effects in patients. Furanocoumarins induce anti-cancerous effects in a range of human tumours. Therefore, targeting these compounds against CDK4 is anticipated to enhance therapeutic effectiveness. This work intended to explore the CDK4 inhibitory potential of 50 furanocoumarin molecules, using a comprehensive approach that integrates the processes of docking, drug-likeness, pharmacokinetic analysis, molecular dynamics simulations and ONIOM (Our own N-layered Integrated molecular Orbital and Molecular mechanics) methods. The top five best docked compounds obtained from docking studies were screened for subsequent analysis. The molecules displayed good pharmacokinetic properties and no toxicity. Epoxybergamottin, dihydroxybergamottin and notopterol were found to inhabit the ATP-binding zone of CDK4 with substantial stability and negative binding free energy forming hydrogen bonds with key catalytic residues of the protein. Notopterol exhibiting the highest binding energy was subjected to ONIOM calculations wherein the hydrogen bonding interactions were retained with significant negative interaction energy. Hence, through these series of computerised methods, notopterol was screened as a potent CDK4 inhibitor and can act as a starting point in successive processes of drug design. Communicated by Ramaswamy H. Sarma</p

Understanding the Atmospheric Oxidation of HFE-7500 (C₃F₇CF(OC₂H₅)CF(CF₃)₂) Initiated by Cl Atom and NO₃ Radical from Theory

Kinetics and mechanistic pathways for atmospheric oxidation of HFE-7500 (<i>n</i>-C₃F₇CF­(OCH₂CH₃)­CF­(CF₃)₂) initiated by Cl atom and NO₃ radical have been studied using density functional theory. Oxidative degradation pathways facilitated by H-abstraction from the −OCH₂ or −CH₃ groups in HFE-7500 have been considered. It has been shown that H-abstraction from the α-site (−OCH₂) is favored over other reaction pathways. The rate constants were computed employing transition-state theory and canonical variation transition-state theory incorporating small curvature tunnelling correction, over the temperature range of 250–450 K at atmospheric pressure. Calculated rate constants at 298 K and 1 atm compare well with earlier experiments. Temperature dependence of the rate constants and branching ratios for these pathways contributing to overall reaction are described. It has been shown that the rate constants over the studied temperature range was found to fit well to the modified Arrhenius equation (in cm³ molecule^–1 s^–1) <i>k</i>_Cl = 1.10 × 10^–14 <i>T</i>^0.04 exp­(−69.87 ± 1.41/T) and <i>k</i>_NO₃ = 7.66 × 10^–26 <i>T</i>^3.30 exp­(596.40 ± 1.22/T). Standard enthalpies of formation for the reactant (C₃F₇CF­(OCH₂CH₃)­CF­(CF₃)₂) and the products [C₃F₇CF­(OC^•HCH₃)­CF­(CF₃)₂ and C₃F₇CF­(OCH₂C^•H₂)­CF­(CF₃)₂] during H-abstraction are derived using the isodesmic approach. Atmospheric implications of the titled molecule are presented