Molecular framework-cation interactions

In a similar manner, p-bis(9-anthryl) phenylene gives a mono(anion-radical) or a mono(cation-radical) under reductive or oxidative conditions with spin delocalization around the whole molecular framework. In the case of m-bis(9-anthryl) phenylene, reduction or oxidation leads to the formation of dianion or dication-diradicals. Based on ESR experiments at cryogenic temperatures (6.5-85 K), these species contain two separated ion-radical moieties. They have parallel aligmnent of their spins (Tukada 1994). The work gives clear experimental evidence for the so-called ferromagnetic interaction between these ion-radical substituents. In some cases, release of the electron depends on temperature. See, for example, the anion-radical shown in Scheme 3.63. 

The surface character of the AlPO molecular sieves differs from that of the silica molecular sieves even though both framework types are neutral with no extra-framework cations. The molecular sieve silicalite is hydrophobic and the AlPO molecular sieves are moderately hydrophilic. Zeolites are hydrophilic due to the interaction of the dipole of the Hz0 molecule with the electrostatic fields of the anionic aluminosilicate framework and the balancing nonframework cations. The hydrophilicity of the AlPOi, materials is apparently due to the difference in electronegativity between Al(1.5) and P(2.1). Neither mechanism is possible with silica molecular sieves. The AlPOi, molecular sieves do exhibit less affinity for HzO than the hydrophilic zeolites such as Type A and Type X. 

Singlet cyclobutylidene (7) is found to have a C, symmetric nonclassical structure at the DFT, MP2, and CCSD(T) levels (Figure 9). The distances to pentacoordinated C4 are between 1.61 and 1.84 A. The structure of 7 is very similar to that of the well studied isoelectronic cyclobutyl cation. Both the carbene and the carbocation profit from hyperconjugative interaction of the Cl -C4 and C3-C4 bonds with the formally empty p orbital on C2 in spite of the associated distortion of the molecular framework. 

The separation of fmctose from glucose illustrates the interaction between the framework stmcture and the cation (Fig. 5) (50). Ca " is known to form complexes with sugar molecules such as fmctose. Thus, Ca—Y shows a high selectivity for fmctose over glucose. However, Ca—X does not exhibit high selectivity. On the other hand, K—X shows selectivity for glucose over fmctose. This polar nature of faujasites and their unique shape-selective properties, more than the molecular-sieving properties, make them most useful as practical adsorbents. 

If the full molecular symmetry is assumed, the ground states of the cation radical of fulvalene and the anion radical of heptafulvalene are both predicted to be of symmetry by using the semiempirical open-shell SCF MO method The lowest excited states of both radicals are of symmetry and are predicted to be very close to the ground states in the framework of the Hiickel approximation these states are degenerate in both cases (Fig. 4). Therefore, it is expected that in both these radicals the ground state interacts strongly with the lowest excited state through the nuclear deformation of symmetry ( — with the result that the initially assumed molecular... 

Monte Carlo simulations and energy minimization procedures of the non-bonding interactions between rigid molecules and fixed zeolite framework provide a reasonable structural picture of DPP occluded in acidic ZSM-5. Molecular simulations carried out for DPB provide evidence of DPB sorption into the void space of zeolites and the preferred locations lay in straight channels in the vicinity of the intersection with the zigzag channel in interaction with H+ cation (figure 1). 

Applications to microporous solids such as molecular sieves included examples of site blocking by cations [10] and a first example of intersite exchange in mordenites [2], In open framework molecular sieves, in order to avoid the contribution of Xe-Xe interactions to the... 

The adsorption of gas molecules on the interior surfaces of zeolite voids is an ionic interaction with a characteristic potential energy called the heat of adsorption. The molecular adsorption process results in an exothermic attachment of the gas molecules to the surface of the voids, and is characterized by a high order of specificity. Zeolites exhibit a high affinity for certain gases or vapors. Because of their "effective" anionic frameworks and mobile cations, the physical bonds for adsorbed molecules having permanent electric moments (N2, NH-j, H20) are much enhanced compared with nonpolar molecules such as argon or methane. 

For the Tar—Tar kissing loops, the P—B calculations are unable to discern their propensity to accumulate counterions accumulation at the loop—loop interface (data not shown). This is because the fully hydrated ions as defined by the Stem layer cannot penetrate into the central cation binding pocket (data not shown). Similarly, the axial spine of counterion density observed in the A-RNA helix (Fig. 20.5) is not captured by the P—B calculation (Fig. 20.7). No noticeable sequence specificity is observed in the counterion accumulation patterns in the P—B calculations, even though the sequence effects are explicitly represented in the P—B calculation through the appropriate geometry and assignment of point-charges. This is because the sequence specificity observed in the molecular dynamics simulations usually involves first shell interactions of base moieties with partially dehydrated ions, which cannot be accurately represented in the P—B framework. 

Subsequently, it is possible to consider that the adsorbate-adsorbent interaction field inside these structures is characterized by the presence of sites of minimum potential energy for the interaction of adsorbed molecules with the zeolite framework and charge-compensating cations. A simple model of the zeolite-adsorbate system is that of the periodic array of interconnected adsorption sites, where molecular migration at adsorbed molecules through the array is assumed to proceed by thermally activated jumps from one site to an adjacent site, and can be envisaged as a sort of lattice-gas. 

All chiral crown ethers incorporating one carbohydrate subunit possess two diastereo-topic faces of the macrocyclic ring. They are able to form diastereoisomeric complexes with primary alkylammonium cations. Since nonbonding interactions are responsible of the chiral recognition of optically active species, it would be desirable to form monofacial ligands in which the inclusion of a chiral molecule (or chiral ion) could proceed from the sterically hindered side only. This special molecular architecture may be followed by the fusion of the cryptand framework and the chiral unit. 

The complexity of xylene adsorption over zeolites is too high to predict the selectivity from the chemical properties of the zeolite only (electronegativity of the cations, charge of the framework oxygens). The interactions between xylenes and the zeolite must necessarily be considered, which explains the important development of molecular simulation methods. This is supported by the work of V. Lachet et al. (18) who succeeded in reproducing the inversion of selectivity between KY and NaY with Grand Canonical Monte Carlo Simulations. 

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