Polar effects combination

In the next section we shall consider an attempt to combine both resonance and polarity effects. 

That these expressions do combine resonance and polarity effects can be seen as follows ... 

One of the most efficient ways to treat this problem is to combine the ab initio MO method and the RISM theory, and this has been achieved by a slight modification of the original RISM-SCF method. Effective atomic charges in liquid water are determined such that the electronic structure and the liquid properties become self-consistent, and along the route of convergence the polarization effect can be naturally incorporated. 

In summary, all the experiments expressly selected to check the theoretical description provided fairly clear evidence in favour of both the basic electronic model proposed for the BMPC photoisomerization (involving a TICT-like state) and the essential characteristics of the intramolecular S and S, potential surfaces as derived from CS INDO Cl calculations. Now, combining the results of the present investigation with those of previous studies [24,25] we are in a position to fix the following points about the mechanism and dynamics of BMPC excited-state relaxation l)photoexcitation (So-Si)of the stable (trans) form results in the formation of the 3-4 cis planar isomer, as well as recovery of the trans one, through a perpendicular CT-like S] minimum of intramolecular origin, 2) a small intramolecular barrier (1.-1.2 kcal mol ) is interposed between the secondary trans and the absolute perp minima, 3) the thermal back 3-4 cis trans isomerization requires travelling over a substantial intramolecular barrier (=18 kcal moM) at the perp conformation, 4) solvent polarity effects come into play primarily around the perp conformation, due to localization of the... 

In this case polarization effects must also be taken into account. By several arguments it was possible to show that the Fe(II) + Ti(IV) combination has the lower energy [12]. 

The mere exposure of diphenyl-polyenes (DPP) to medium pore acidic ZSM-5 was found to induce spontaneous ionization with radical cation formation and subsequent charge transfer to stabilize electron-hole pair. Diffuse reflectance UV-visible absorption and EPR spectroscopies provide evidence of the sorption process and point out charge separation with ultra stable electron hole pair formation. The tight fit between DPP and zeolite pore size combined with efficient polarizing effect of proton and aluminium electron trapping sites appear to be the most important factors responsible for the stabilization of charge separated state that hinder efficiently the charge recombination. 

Although we have illustrated the effects of polarizability of ions by considering a few cases where the effects are large, there must be some polarization effect for any combination of ions. However, there is an even more important consideration. It is known that the apparent radius of a given ion depends somewhat on the environment of the ion. For example, an ion surrounded by four nearest neighbors will appear to be slightly different in size from one that is surrounded by six ions of opposite charge. We have treated the ionic radius as if it were a fixed number that is the same in any type of... 

The comparisons made by Parchment et al. [271] illustrate the importance of combining electronic polarization effects with corrections for specific solvation effects. The latter are accounted for parametrically by the explicit simulation, but that procedure cannot explicitly account for the greater polarizability of tautomer 8. The various SCRF models do indicate 8 to be more polarizable than any of the other tautomers, but polarization alone is not sufficient to shift the equilibrium to that experimentally observed. Were these two effects to be combined in a single theoretical model, a more accurate prediction of the experimental equilibrium would be expected. 

Through the years from his laboratory studies under Thorpe until his return to London in 1944, and still afterward, the heart of Ingold s theoretical chemistry was a recognition of the complementary roles in chemical processes of three-dimensional structure and physicochemical forces. His early career is marked by an effort to sort out the directing role in reacting molecules played by electrical and polar effects, in combination with stereochemical effects studied by chemists imbued with nineteenth-century concepts of structure and valence. 

Reactivity ratios for all the combinations of butadiene, styrene, Tetralin, and cumene give consistent sets of reactivities for these hydrocarbons in the approximate ratios 30 14 5.5 1 at 50°C. These ratios are nearly independent of the alkyl-peroxy radical involved. Co-oxidations of Tetralin-Decalin mixtures show that steric effects can affect relative reactivities of hydrocarbons by a factor up to 2. Polar effects of similar magnitude may arise when hydrocarbons are cooxidized with other organic compounds. Many of the previously published reactivity ratios appear to be subject to considerable experimental errors. Large abnormalities in oxidation rates of hydrocarbon mixtures are expected with only a few hydrocarbons in which reaction is confined to tertiary carbon-hydrogen bonds. Several measures of relative reactivities of hydrocarbons in oxidations are compared. 

Figure 7 shows an aberration-free intensity distribution at the focus of a typical objective lens similar to that used for DLW lithography. Calculations were carried out using a vectorial Debye theory, which accounts for the polarization effects. For the linearly polarized wave it can be seen that the spot is elongated along the polarization vector. To reduce this asymmetry, a X/4-plate can be used to convert the polarization of the incident beam to circular, which can be interpreted as a combination of two mutually perpendicular linearly polarized components. Thus, width of the photomodified line becomes independent of the beam scanning direction in the sample. 

At room temperature H2 molecules striking those crystallites of a platinum surface which have the highest work function (I) decompose into atoms. If a pure platinum surface is contacted by hydrogen, crystallites I will be covered by H atoms, those with a low work function, II, by H2 molecules, because the decomposition of H2 molecules on crystallites II requires a higher energy of activation than on crystallites I. The work function of crystallites I is lowered by the polarized H atoms that of crystallites II will not be changed essentially. All effects combined, the photoelectric emission therefore increases if hydrogen is adsorbed on a pure platinum surface. 

SAMPLE SOLUTION (a) Boron trifluoride is planar with 120° bond angles. Although each boron-fluorine bond is polar, their combined effects cancel and the molecule has no dipole moment. 

The rates and orientation of free radical additions to fluoroalkenes depend upon the nature of the attacking radical and the alkene, but polar effects again are important For instance, methyl radical adds 9 5 times faster to tetrafluoroethylene than to ethylene at 164 °C, but the tnfluoromethyl radical adds 10 times taster to ethylene [7551 The more favorable polar transition states combine the nucleophilic radical with the electron deficient olefin 17 and vice versa (18) These polar effects account for the tendency of perfluoroalkenes and alkenes to produce highly regular, alternating copolymers (see Chapter starting on page 1101)... 

It seems to the author that the incorporation of the equilibrium (24) into the mechanism of reaction does not help in the interpretation of the results of Tag-liavini and co-workers and that a direct reaction between the tetraalkyltin and the solvated Me2Sn2 + cation cannot be ruled out. The leaving group effects shown in Table 25 are very small and can be accounted for by combinations of minor steric effects with polar effects in some transition state of the SE2 type. 

Two effects seem to be in force here, a polar effect in the sense Me < Et < Pr < Bu and a steric effect in the sense Me > Et > Pr > Bu" > Pr1 > Bu. The total combination leads to the results shown in sequence (47) for the trialkyltin bromides. It may be noted that the polar effect is quite the reverse of the expected inhibition of nucleophilic coordination to tin by electron releasing groups attached to the tin atom. It appears, from sequence (47), that variations in AT(41) are likely to make comparatively small contributions to the overall variation in k°f. ... 

High-level ab initio calculations have shown that the AN2 reaction of the cyanide ion with ethyl chloride is catalysed by 1,4-benzenedimethanol in dipolar aprotic solvents through selective two hydrogen bonds.7 In non-polar solvents, combined with phase-transfer catalysis, the 1,4-benzenedimethanol could replace some water molecules hydrating the cyanide ion and induce a substantial rate acceleration effect. 

Meanwhile, cation - crown complexes were fully optimized by ab initio QM techniques in the gas phase, allowing to investigate the effect of electronic reorganization (mainly charge transfer + polarization effects) on the recognition properties/15,161 QM/MM hybrid methods which combine a QM representation of the ion coordination sphere with a MM representation of the more remote species, have been tested on 18C6/ cation complexes/17, 18] Typical applications of "computational approaches in supramolecular chemistry" up to 1994 can be found in ref. [191 There are also reviews on more specialized fields like calixarenes/20,21]... 

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