Anion polarisation

Fig. 1.2 The Faber-Ziman partial structure factors Sap(k) and partial pair-distribution functions gafi (r) (a, /S = M, X) as calculated for models using two different values for the anion polarisability ax [61]. The curves in (a) and (b) correspond to a rigid ion model (RIM) with ax = 0, while the curves in (c) and (d) correspond to a polarisable ion model (PIM) with ax = 20 au. The introduction of anion polarisability leads to the appearance of an FSDP in S mm (k) at kpsop 1.2 A and to an alignment of the principal peaks in aU three Safi(k) functions atkpp 2 A. The alignment of the principal peaks in (c) arises from in-phase large-r oscillations in the ga r) functions shown in (d)...

Injax. The are complex numbers with amplitudes related by IMsNll-e ccI = W-Hc and the phases are related by 0nn + 6cc = 26>nc- Equations (1.13)-(1.15) also hold for binary mixtures of hard-spheres having different diameters, i.e. when both the Coulomb and dispersion terms are absent from (1.12), where the common wavelength of oscillation is set by one or other of the hard sphere sizes depending on the thermodynamic conditions [71], The effect on (1.13)-(1.15) of introducing anion polarisability has yet to be fully explored. 

This theory, when applied to the SPC water geometry but with a fixed dipole equal to that of an isolated H2O molecule, p = 1.85D, leads to the result = 2.62D. This is in good agreement with the well-known extended simple point charged model SPC/E which allows for the polarisation correction. This validates the self-consistent mean-field procedure. As an illustration of the anion polarisation effect. Fig. 7... 

Similar considerations apply to oxidation. An anion which is considerably more stable than water will be unaffected in the neighbourhood of the anode. With a soluble anode, in principle, an anion only needs be more stable than the dissolution potential of the anode metal, but with an insoluble anode it must be stable at the potential for water oxidation (equation 12.4 or 12.5) plus any margin of polarisation. The metal salts, other than those of the metal being deposited, used for electroplating are chosen to combine solubility, cheapness and stability to anode oxidation and cathode reduction. The anions most widely used are SOj", Cl", F and complex fluorides BF4, SiFj , Br , CN and complex cyanides. The nitrate ion is usually avoided because it is too easily reduced at the cathode. Sulphite,... 

Many baths in which metal is reduced from complex anions (e.g. cyanide baths, stannate baths) give high throwing indices because both polarisation and cathode efficiency variation favour a low value of M. The cathode efficiency for a typical copper cyanide bath (40°C) was ... 

There have been ab initio studies of the bonding for a series of imino and amino phosphorus molecules,15 and for the phosphonium ylides (8).14 The anion character of the ylidic carbon is intermediate between that of olefinic and substituted carbanions. Polarisation of the H3P group has a stabilising effect. The influence of the substituent X on the inversion of the carbanion and also the tendency of the molecule to dissociate to carbene and phosphine is also discussed.14 CNDO/S MO calculations on the... 

Protonation, if forced upon pyrrole, is found to take place not on nitrogen but on the a-carbon atom (19). This occurs because incorporation of the nitrogen atom s lone pair of electrons into the aromatic 6jre system leaves the N atom positively polarised protons tend to be repelled by it, and are thus taken up by the adjacent a-carbon atom. The basicity situation rather resembles that already encountered with aniline (p. 70) in that the cation (19) is destabilised with respect to the neutral molecule (18a). The effect is much more pronounced with pyrrole, however, for to function as a base it has to lose all aromatic character, and consequent stabilisation this is reflected in its related pKa (-0-27) compared with aniline s of 4-62, i.e. pyrrole is a very weak base indeed. It can in fact function as an acid, albeit a very weak one, in that the H atom of the NH group may be removed by strong bases, e.g. eNH2 the resultant anion (20) then retains the aromatic character of pyrrole, unlike the cation (19) ... 

In this discussion of nucleophilic displacement at a saturated carbon atom, interest has tended to centre on attack by nucleophilic anions Nu e, especially eOH, on polarised neutral species, especially alkyl halides, +R—Hala . In fact this general type of displacement is extremely common involving, in addition to the above, attack by non-charged nucleophiles Nu- on polarised neutral species,... 

If the cation is small and has a high charge, i.e. it has a high charge density, it is very polarising. Such a cation will distort the electron cloud of the anion, pulling the electron density towards itself. 

This introduces covalent character into the ionic compound. Anions with a large radius and high negative charge are very polarisable. 

Thermal decomposition occurs more readily in compounds where the metal cation polarises the anion. Therefore ... 

Emission spectra have been recorded for electron injection into Au and Ag spherical electrodes and hole injection into Au(lll) planar electrodes. These processes were brought about in solutions of acetonitrile containing tetrabutylammonium hexafluoro-phosphate (TBAHP), using the trans-stilbene radical anion as the electron injector and the thianthrene radical cation as hole injector. The spectrum for the hole injection process into planar Au(lll) electrodes has been resolved into the P S-polarised components of the emitted light. A comparison of the spectral distribution of emitted light for the above electron injection process, occurring at both Au and Ag... 

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