Link between the kinetic‐ and exchange‐energy functionals in the generalized gradient approximation

An approximate kinetic‐energy functional of the generalized gradient approximation form was derived following the “conjointness conjecture” of Lee, Lee, and Parr. The functional shares the analytical form of its gradient dependency with the exchange‐energy functionals of Becke and Perdew, Burke, and Ernzerhof. The two free parameters of this functional were determined using the exact values of the kinetic energy of He and Xe atoms. A set of 12 closed‐shell atoms was used to test the accuracy of the proposed functional and more than 30 others taken from the literature. It is shown that the conjointness conjecture leads to a very good class of kinetic‐energy functionals. Moreover, the functional developed in this work is shown to be one of the most accurate despite its simple analytical form. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002     

Oxygen-17 nuclear magnetic resonance: The effects of remote unsaturation on 17o-chemical shifts in polycyclic ethers

Natural-abundance ¹⁷O-NMR spectra of 7-oxanorbornane exo-3-oxatricyclo [3.2.1.0^2.4]octane and their unsaturated derivatives (endo cyclic and exocyclic double bonds) have been measured. Linear correlation laws were observed for $\text{δ}{}_{\mathrm{o}}\text{δ}{}_{\mathrm{c}}$ of these ethers/corresponding hydrocarbons. The “cyclization shifts” for δ_o in ethers were not correlated by the “cyclization shifts” for δ_c of the corresponding hydrocarbons.     

An application of the phosphine-catalyzed [4 + 2] annulation in indole alkaloid synthesis: formal syntheses of (+/-)-alstonerine and (+/-)-macroline

[reaction: see text] An application of the phosphine-catalyzed [4 + 2] annulation in the formal synthesis of alstonerine and macroline is reported. A phosphine-catalyzed [4 + 2] reaction between imine 7a and allene 8 formed the D ring of the target indole alkaloids. A subsequent intramolecular Friedel-Crafts acylation provided the C ring of the bridged tetracycle. Deprotection, followed by methylation of the bridged nitrogen, deoxygenation of the C6 ketone, and reduction of the C16 carbethoxy group provided the previously known intermediate 3.     

Potentiometric titration of sorbed hydrogen on palladium carrier catalysts

A method of determination of adsorbed and absorbed hydrogen on palladium catalysts with carbon carriers is suggested and verified. The method allows H_ad and H_ab to be determined with an accuracy of about 10% using catalysts containing at least 5% Pd.

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- . . . 10% , , - , 5% Pd.

     

Femtosecond time resolved coherent anti-Stokes Raman spectroscopy: experiment and modelization of speed memory effects on H2-N2 mixtures in the collision regime

With the aim of temperature diagnostic, femtosecond time-resolved CARS (coherent anti-Stokes Raman spectroscopy) is applied to probe H2 in H2-N2 mixtures. In a first part, a Lorentzian profile is used to model the femtosecond CARS response. A difference between the experimental broadening and the expected one is observed in the collision regime. The observed broadening increases strongly in an inhomogeneous way with respect to the perturber concentration. This is of considerable importance for temperature measurements. In a second part, we show that in the collision regime, this inhomogeneous broadening is due to the speed dependence of the collisional parameters and the memory effects of the radiator speed. A new modelization of the time-resolved CARS response taking into account the speed memory effects is presented and applied to the temperature diagnostic in H2-N2 mixtures. The numerical results are in good agreement with experiments.     

High-pressure adsorption capacity and structure of CO2 in carbon slit pores: theory and simulation

We present new simulation results for the packing of single-center and three-center models of carbon dioxide at high pressure in carbon slit pores. The former shows a series of packing transitions that are well described by our density functional theory model developed earlier. In contrast, these transitions are absent for the three-center model. Analysis of the simulation results shows that alternations of flat-lying molecules and rotated molecules can occur as the pore width is increased. The presence or absence of quadrupoles has negligible effect on these high-density structures.     

Effect of carbon nanotubes on the crystallization and properties of polypropylene

The effect of different concentrations of single‐walled carbon nanotubes (SWNTs) on the nonisothermal crystallization kinetics, morphology, and mechanical properties of polypropylene (PP) matrix composites obtained by melt compounding was investigated by means of X‐ray diffraction, differential scanning calorimetry, optical and scanning electron microscopy, and dynamic mechanical thermal analysis. Microscopy showed well‐dispersed nanotube ropes together with small and large aggregates. The modulus was found to increase by about 75% at a level of 0.5 wt % nanotubes. The SWNTs displayed a clear nucleating effect on the PP crystallization, favoring the α crystalline form rather than the β form. The crystallization kinetics analysis showed a significant increase in activation energy on incorporating nanotubes. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2445–2453, 2005     

Crystal Structures of Twisted Di(2-Pyridyl)ketone and Planar Azo-di(2-Pyridine) and Their Discussion Based on PM3 Enthalpy Hypersurfaces for the Isoelectronic Molecules

Single crystal structure determinations prove the two pyridine substituents in di(2-pyridyl)ketone (H₄C₄NC)₂C=O to be twisted out of the carbonyl skeleton plane by torsion angles (OCCN) of 41° and –163°, in contrast to their planar arrangement in azo-di(2-pyridine) H₄C₄NC)-N=N-(CNC₄H₄). In order to rationalize the surprising difference between the two isoelectronic molecules, approximate PM3 enthalpy of formation hypersurfaces have been calculated for each of the two ring torsions, which are assumed to be the dominant ones among the 3N – 6 = 60 degrees of freedom. For both the ketone and the azo derivative, global minima are calculated, the torsion angles of which deviate from the crystal structure results, and, therefore, support the assumption that both the experimentally determined twisting of di(2-pyridyl)ketone as well as the flattening of azo-di(2-pyridine) might be affected by the crystal packing.     

Natural Abundance 17O-NMR. Study of Some Macrocyclic Complexes with Alkali Metal Cations

Complexation between crown ethers 12C4, 15C5, 18C6 and cryptand 222, and alkali cations Li⁺, Na⁺, K⁺ in various solvents were studied by ¹⁷O-NMR. spectroscopy. Small diamagnetic shifts arising from the cation electric field are observed. They increase according to the sequence K⁺ < Na⁺ < Li⁺. ¹⁷O-linewidth are discussed and compared to the ¹³C relaxation times. Linewidth modification results mainly from modifications of the effective correlation time. In general, for crown ethers, considerable line broadening occurs when the cation fits well into the cavity but line narrowing occurs when the cation is much smaller than the cavity.