Flexibility, framework selection

These frameworks therefore demonstrate breathing behaviour that results from rotations and twists of structural units in order to achieve better coordination to bound species. A second type of flexible framework appears to be able to break bonds to permit the uptake of adsorbate molecules. In Ni2(4,4 -bipy)3(N03)4, for example, it is found that the windows observed in the X-ray crystal structure do not limit the size of the molecules that can be adsorbed into internal cavities they must be able to open to permit guest passage. This type of behaviour, which must require selective bond breaking and reforming, begins to resemble that exhibited by some biological systems. These effects of structural response to adsorption are discussed further in Chapter 7. 

Calculations were performed with both rigid and flexible lattices. Because of increased computational demands of allowing the framework to relax, flexible lattice simulations were done with a smaller system, together with a shorter time step relative to the rigid framework simulations. The adsorbate atoms were distributed evenly throughout the channels at the start of each simulation, and a nominal temperature of 300 K was selected. The size... 

In most cases, evaluations of ecological impacts are site-specific and, as a consequence, are not considered when establishing a generally applicable waste classification system. These impacts normally are addressed in disposal site selection, design, and operation, and they may be used in establishing waste acceptance criteria for the site. To the extent that ecological impacts can be evaluated generi-cally, NCRP believes that the principles and framework for risk-based waste classification presented in this Report are sufficiently flexible to take them into account. 

POMA A complete mathematica implementation of the NMR product-operator formalism—Guntert et al. JMR 101A, 103-105 (1993). POMA is a flexible implementation of the product operator formalism for spin-1/2 nuclei written for Mathematica. It provides analytical results for the time evolution of weakly coupled spin systems under the influence of free precession, selective and non-selective pulses, and phase cycling. As part of Mathematica, it requires a license, but the source code is free. Mathematica also provides a framework for visualizing and storing results. 

It has already been mentioned that zeolites are shape selective with respect to molecular adsorption. This property relates to their micropores stmcture. The zeolite framework shows a limited flexibility, which is essential. For instance, Yashonath et al. have shown in their classical dynamic simulations study of molecular diffusion within zeolite micropore that the zeolite framework flexibility affects significantly diffusion when the molecules have a size comparable with the micropore size. To get an idea of the order of magnitude of this flexibility, one can consider the hybrid semi-empirical DFT periodic study of chabazite zeolite of Ugliengo et al. V They introduced in the unit cell of chabazite Br0nsted acidic sites which are known to induce an increase of the volume of around 10 This increase of the volume relates with the difference of volume between a Si04 tetraheron and a... 

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