Polarisability-dipolarity

Kirkwood took a more rigorous statistical-mechanical approach in an attempt to incorporate the effect of local ordering. His theory is only valid for rigid dipoles, and it was left to Frohlich to extend the treatment properly to a system of polarisable dipolar molecules. The work is well described in Frohlich s classic text (1949). The final outcome was the following formula,... 

MEASUREMENTS OF H-BOND ACIDITY, BASICITY AND POLARISABILITY-DIPOLARITY BY HPLC... 

The importance of H-bond acidity, basicity and polarisability-dipolarity in describing various partition processes and solubility... 

The power of the solvation equation approach is that it can characterise various partition systems and lipophilicity scales by five molecular descriptors (dispersivity, size, polarisability-dipolarity, H-bond acidity and basicity). All of the five descriptors should be included into the linear regression equations by definition. When one or two coefficients are not significantly different from zero, it means that that particular property does not play a significant role in the partition. 

It has been shown above that basic molecular descriptors like size, H-bond acidity-basicity and polarisability-dipolarity can be used to describe various water-solvent partitions, biological partition processes, and can be used even for estimating the aqueous solubility of compounds. However, the measurement of these molecular descriptors is very time-consuming, and it is not easy to access existing data. Promising results revealed that the fast reversed-phase chromatographic retention data (CHI) could be correlated with such descriptors with a good overall statistical fit (low standard error. 

Using a solver technique (we have used the solver add-in in Microsoft Excel 6.0 ), one can calculate the 3 molecular descriptors (H-bond acidity, H-bond basicity and polarisability-dipolarity). Plass et al. 1122] published the molecular descriptors of tripeptide derivatives based on the above-described method. Although reasonably sensible data were obtained, the method has not yet been validated on a large number of... 

Using the measured CHI data of the model compounds we can set up a correlation equation to express the H-bond acidity (a), H-bond basicity (f) and the polarisability-dipolarity (n) parameters by the CHI values obtained on a particular stationary phase with a particular mobile phase. The plot of the database descriptors as a function of the estimated ones based on the measured CHI values can be seen in Figs. 12.20-12.22. The best equations for the estimation of the descriptors are also shown in the figures. It can be seen that the CHI values obtained on Luna C-18 columns with acetonitrile and trifluoroethanol gradients are used for the fi and the ir calculations. The CHI values... 

While this theory holds for ionic liquids in general when compared to organic solvents, it cannot be efficiently used to select the best ionic liquid from a pool of ionic liquids. This has been attributed to the fact that ionic liquids feature very similar Jt -values, which determine polarisability/dipolarity. 

These criteria can be met by the "design" and synthesis of polariseable dipolar molecules. As indicated above, incorporation of a chiral centre and separation and purification of the enantiomers can afford non-centrosymmetric crystals which may exhibit SHG. 

Recently the solvent effect on the [4+2] cycloaddition of singlet oxygen to cyclic dienes has been subjected to a multiparameter analysis. A pre-equilibrium with charge-transfer character is involved, which is affected by the solvent through dipolarity-polarisability (n ) and solvophobic interactions ( Sjf and Another multiparameter analysis has been published by Gajewski, demonstrating the... 

The regioselectivity benefits from the increased polarisation of the alkene moiety, reflected in the increased difference in the orbital coefficients on carbon 1 and 2. The increase in endo-exo selectivity is a result of an increased secondary orbital interaction that can be attributed to the increased orbital coefficient on the carbonyl carbon ". Also increased dipolar interactions, as a result of an increased polarisation, will contribute. Interestingly, Yamamoto has demonstrated that by usirg a very bulky catalyst the endo-pathway can be blocked and an excess of exo product can be obtained The increased di as tereo facial selectivity has been attributed to a more compact transition state for the catalysed reaction as a result of more efficient primary and secondary orbital interactions as well as conformational changes in the complexed dienophile" . Calculations show that, with the polarisation of the dienophile, the extent of asynchronicity in the activated complex increases . Some authors even report a zwitteriorric character of the activated complex of the Lewis-acid catalysed reaction " . Currently, Lewis-acid catalysis of Diels-Alder reactions is everyday practice in synthetic organic chemistry. 

MHz MAS CP DD (magic-angle spinning with cross-polarisation and H dipolar decoupling) 13 C NMR spectra performed at room temperature for the three poly(cycloalkyl methacrylates) (cyclopentyl, cyclohexyl, cyclohep-tyl) are shown in Fig. 8. 

Solvation of thiolates is similarly low in both protic and dipolar aprotic solvents because of the size and polarisability of the large weakly basic sulfur atom, so is unlikely to contribute appreciably to the observed solvent effect. The intermediate nitro radical anion is stabilised by H-bonding in a manner which retards its dissociation in the SrnI mechanism (upper equation in Scheme 10.35). In contrast, the electron flow in the direct substitution at X (lower equation in Scheme 10.35) is such that solvation by methanol promotes the departure of the nucleofuge. In summary, protic solvation lowers the rate of the radical/radical anion reactions, but increases the rate of the polar abstraction yielding disulfide. 

Induced dipolar interactions. The electron clouds in many (especially large) organic molecules are readily polarised resulting in the formation of induced dipoles that can interact, resulting in complex stabilisation. Both cations and anions can induce dipoles in aromatic molecules, for example. 

Cross-polarisation (CP) is a solid-state NMR experiment designed to achieve a higher sensitivity for the rare nucleus through the transfer of polarisation, driven by the heteronuclear dipolar interaction, from an abundant ( ll) to a dilute spin (13 C).118 Transfer of magnetisation is possible... 

Figure 12.13 shows the Xe spectra of a iPP/EP blend obtained with single pulse excitation and with => 129Xe cross-polarisation at -33 °C [13]. In the cross-polarisation spectrum only the Xe line from Xe in iPP is observed, because the dipolar interaction between the polymer spins and the 129Xe spins due to the Xe mobility is too weak in EP. Also the cross-polarisation signal of iPP disappears at temperatures higher than the Tg of iPP. 

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