Oxygen ions, polarizabilities

Hope et al. (116) presented a combined volumetric sorption and theoretical study of the sorption of Kr in silicalite. The theoretical calculation was based on a potential model related to that of Sanders et al. (117), which includes electrostatic terms and a simple bond-bending formalism for the portion of the framework (120 atoms) that is allowed to relax during the simulations. In contrast to the potential developed by Sanders et al., these calculations employed hard, unpolarizable oxygen ions. Polarizability was, however, included in the description of the Kr atoms. Intermolecular potential terms accounting for the interaction of Kr atoms with the zeolite oxygen atoms were derived from fitting experimental results characterizing the interatomic potentials of rare gas mixtures. In contrast to the situation for hydrocarbons, there are few direct empirical data to aid parameterization, but the use of Ne-Kr potentials is reasonable, because Ne is isoelectronic with O2-. 

Water belongs to the group of compounds AB2 which, contrary to expectation, were proved by measurements of the dielectric constant not to possess a linear molecule. In a water molecule the oxygen ion, which is strongly polarizable, has two extremely small hydrogen... 

Zinc oxide may crystallize in either the wurtzite or, more rarely, the zinc blende structure, the former being more stable at lower temperatures. In both structures the zinc and oxygen ions are tetrahedrally co-ordinated to each other. The bonding is intermediate between the completely ionic and the completely covalent, both ions being more polarizable than in a perfect ionic crystal and carrying an effective charge of only 0.5e. As is well known, non-stoicheio-metric ZnO, with excess of Zn, is an n-type semiconductor with a band gap of 3.2 eV. 

Since a majority of the ions in any glass are usually anions, the contribution to the refractive index from the anions is very important. Replacement of fluorine by more polarizable oxygen ions, or by other halides, increases the refractive index. Conversely, partial replacement of oxygen in oxide glasses by fluorine to form fluoroborate glasses, for example, reduces the refractive index. Since non-bridging oxygens are... 

Owing to the high value of q, the polarizability of the B-ions, combined with the likewise high polarizability of the oxygen, ions contribute to a large extent to the effective (total) polarizability a. For the case of barium titanate. A = 0.20 leads to... 

The potential parameters of the hydroxide ions are the same as those used successfully by de Leeuw (1995), which were modified by Baram and Parker (1996) to include a polarizable oxygen ion and were applied in their woik on hydroxide formation at quartz and zeolite surfaces. Methanoic add and methylamine molecules were modeled using the Consistent Valence Force Held (CVFF) from the Insight 11 package (Molecular Simulations Inc, San Diego, CA). The interaction between methanoic acid and methylamine with a quartz surface was adopted according to the partial charges on the atoms. 

In their reactions with suitable nucleophiles, such as tt-aromatics or heteroatom donor nucleophiles, the readily polarizable linear acylium ions shift a Tt-electron pair to oxygen, bending the ions and developing an empty p-orbital at the carbocationic center. This enables the reaction with aromatics. The acetylation of benzene can be depicted as... 

Bases of low polarizabiUty such as fluoride and the oxygen donors are termed hard bases. The corresponding class a cations are called hard acids the class b acids and the polarizable bases are termed soft acids and soft bases, respectively. The general rule that hard prefers hard and soft prefers soft prevails. A classification is given in Table 3. Whereas the divisions are arbitrary, the trends are important. Attempts to provide quantitative gradations of "hardness and softness" have appeared (14). Another generaUty is the usual increase in stabiUty constants for divalent 3t5 ions that occurs across the row of the Periodic Table through copper and then decreases for zinc (15). 

The pA of 1,3-dithiane is 36.5 (Cs" ion pair in THF). The value for 2-phenyl-1,3-dithiane is 30.5. There are several factors which can contribute to the anion-stabilizing effect of sulfur substituents. Bond dipole effects contribute but carmot be the dominant factor because oxygen substituents do not have a comparable stabilizing effect. Polarizability of sulfur can also stabilize the carbanion. Delocalization can be described as involving 3d orbitals on sulfur or hyperconjugation with the a orbital of the C—S bond. MO calculations favor the latter interpretation. An experimental study of the rates of deprotonation of phenylthionitromethane indicates that sulfur polarizability is a major factor. Whatever the structural basis is, there is no question that thio substituents enhance... 

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