Empirical polarity effect

Abstract Current methodologies for modelling electronic polarization effects in empirical force... 

Finally, the effect of reaction conditions and solvents on chemoselectivity should also be considered.18 Empirically, polar, more basic solvents, e.g. HMPA, DMF, serve to minimize counterion effects by formation of solvent-separated ion pairs and promote electron-transfer processes which are conducive for... 

Reid DL, Armstrong DA, Rauk A, Nese C, Schuchmann MN, Westhoff U, von Sonntag C (2003) H-atom abstraction by C-centered radicals from cyclic and acyclic dipeptides. A theoretical and experimental study of reaction rates. Phys Chem Chem Phys 5 3278-3288 Roberts BP (1996) Understanding the rates of hydrogen abstraction reactions empirical, semi-em-pirical and ab initio approaches. J Chem Soc Perkin Trans 2 2719-2725 Russell GA (1973) Reactivity, selectivity, and polar effects in hydrogen atom transfer reactions. In Kochi JK (ed) Free radicals. Wiley, New York, pp 275-331 Russo-Caia C, Steenken S (2002) Photo- and radiation-chemical production of radical cations of methylbenzenes and benzyl alcohols and their reactivity in aqueous solution. Phys Chem Chem Phys 4 1478-1485... 

S. Mengali, R. Moccia, Non-empirical core polarization effects on the optical properties of Mg(I) I. discrete and continuum energy spectrum up to the third ionization threshold, J. Phys. B At. Mol. Opt. Phys. 29 (1996) 1597. 

One of the difficulties is that there is no convenient theory for selecting the appropriate surfactants a priori. The simplest procedure is to try to match the HLB (hydrophile-lipophile balance) character of the surfactants to the polarity of the particular polymers. Tabulated HLB values are available [23, 24] (the higher the HLB number the more hydrophilic is the soap), but the rating is essentially empirical and effective use requires some experience and intuition. 

Lorentz and polarization effects [14,15], No correction for crystal decomposition was necessary. Inspection of the azimuthal scan data [16] showed a variation which was not symmetric around 180° in phi, probably due to a mis-centering of the crystal. For this reason an empirical correction was made to the data based on the combined differences of Fobs and Fcic following refinement of all atoms with isotropic thermal parameters (Tmax = 1.16, Tmin = 0.87, no theta dependence) [17]. Inspection of the systematic absences indicated uniquely space group Ia3. Removal of systematically absent data and averaging of redundant data yielded the unique data in the final sets, which were 149 for 20°C and 241 for -127°C For the former, R(I) = 5.1% for observed data and 5.4% for all data, and for the latter R(I) = 3.9% for the observed data and 4.3% for all data. 

All X-ray intensity data for compound 1 were collected with use of a Broker SMART diffractometer equipped with a CCD area detector using Mo radiation. Intensity data were corrected for Lorenz and polarization effects and were empirically corrected for absorption using... 

Several solvent polarity scales vere proposed to quantify the polar effects of solvents on physical properties and reactivity parameters in solution, such as rate of solvolyses, energy of electronic transitions, solvent induced shifts in IR, or NMR spectroscopy. Most of the polarity scales vere derived by an empirical approach based on the principles of the linear free energies relationships applied to a chosen reference property and system vhere hydrogen bonding effects are assumed negligible [Reichardt,1965, 1990 Kamlet, Abboud et al., 1981, 1983]. 

In practice, (f) can be calculated by inserting experimental copolymerization rates into Eq. (7.64). The values of (j> thus obtained are frequently greater than unity, and these deviations are ascribed to polar effects that favor cross-termination over homotermination. However, this is not always unambiguous, since the apparent cross-termination factor may vary with monomer feed composition in a given system [25,26]. It is clear also that termination reactions are at least partially diffusion controlled [27,28]. A dependence of segmental diffusivity on the structure of macroradicals is to be expected and dependence of diffusion controlled termination on copolymer composition seems reasonable. It is therefore plausible that the value of the overall termination rate constant ku in copolymerizations should be functions of fractions F and Fi) of the comonomers incorporated in the copolymer. An empirical expression for ku has thus been proposed [27] ... 

The above discussion neglects polarization effects. In part, these are taken into account by the empirical scaling procedures (e.g., the dipole moment of the water molecule in condensed-phase models is chosen to be larger than the gas-phase value).12 Internal charge rearrangements due to conformational... 

Several other treatments of solvent effects on solvolysis rates have been developed. The equations typically include several terms related to (a) macroscopic nonspecific solvent properties, such as the dipole moment and dielectric constant (b) empirical polarity criteria, such as Ej.(30) (c) solvent electrophilicity and nucle-ophilicity parameters and (d) terms related to solvent cohesivily. The last term accounts for the difference in work required to disrupt structure within the solvent, when, for example, there is expansion in volume between reactants and the TS. 

The term solvatochromism was introduced in 1922 by Hantzshlater [45] to explain the influence of the solvent on electronic absorption spectra. In 1951 Brooker et al. [46] suggested that the solvatochromic effect induced by a solvent on certain probes that have a strong absorption in the UV/Vis could be used as a visual indicator of the solvent polarity. One of the main reasons that spectroscopic measurements are such a popular method to obtain empirical polarity parameters is the simplicity of the experiment Electronic absorption spectroscopy is a readily available tool that requires little speciahst skill to operate. Hence, several of these scales have been used with ILs. 

Several approaches for calculating excited states in protein environments were proposed to improve the ordinary QM/MM description. The effect of polarization was included as a classical force field [27], and the excitation energy calculated for bacteriorhodopsin (bR) was 0.34 eV less than that from a fixed-charge non-polarizable QM/MM method [27]. Later, a triple-layer QM1/QM2/MM approach was proposed, and DFT(PBEO) calculations were performed for the QM2 layer, which consisted of the amino acids 4 A from the retinal PSB [28]. The calculated excitation energy of bR was only 0.08 eV smaller than that obtained using the ordinary QM/MM method [28]. In another study, an empirical polarization model combined with the QM/MM calculation produced a red shift of 0.14-0.17 eV [29]. However, these pioneering studies neglected the CT effects between the retinal and the protein environments. 

Figure 5 Illustrates the effect of solvent changes on the rJ of the ionophore free acid and its anion. Kosower s Z values proved empirically an effective function for ranking solvents according to their Integrated polar and protic properties (18).

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