Donor acceptor capacities

Dithiolane oxides, synthesis of 241 Dithiolanes, asymmetric oxidation of 291 Divinyl sulphones, reactions of 642, 1021 Divinyl sulphoxides reactions of 349, 352, 353, 359 synthesis of 240 Donor-acceptor capacities 387... 

The above relationships between the thiiranes (20) and their dioxides (17) are reminiscent of those between cyclopropane and cyclopropanone. The entire phenomena of the C—C bond lengthening and the concomitant C—S bond shortening in the three-membered ring sulfones and sulfoxides can be accounted for in terms of the sulfur 3d-orbital participation and the variation in the donor-acceptor capacities of the S, SO and S02 . The variations of the calculated valence-state orbital energies, together with the corresponding variations of the C—C overlap populations, can be used to understand the discontinuous variations of the C—C and the C—S bond lengths in the series thiiranes -... 

A second major mode of photocydoaddition involves 1.2-addition to the aromatic ring, and this predominates if there is a large difference in electron-donor/acceptor capacity between the aromatic compound and the alkene. It is therefore the major reaction pathway when benzene reacts with an electron-rich alkene such as 1,1-dimethoxyethylene (3.43) or with an electron-deficient alkene such as acrylonitrile (3.441. When substituted benzenes are involved, such as anisole with acrylonitrile (3.45), or benzonitrile with vinyl acetate (3.46), reaction can be quite efficient and regioselective to give products in which the two substituents are on adjacent carbon atoms. 

Membrane analogous polarity H-bond donor/acceptor capacity Because of water content in 1-octanol phase no desolvatization of polar moieties... 

The sensitivity of IR bands to changes in physical state led to numerous studies concerned with the thermodynamics of hydrogen bonding in solvents and the influence of solvent effects (donor/acceptor capacities). Phase transitions (polymorphism) in solids could easily be detected by changes in band intensity and position. 

The second important influence of the solvent on Lewis acid - Lewis base equilibria concerns the interactions with the Lewis base. Consequently the Lewis addity and, for hard Lewis bases, especially the hydrogen bond donor capacity of tire solvent are important parameters. The electron pair acceptor capacities, quantified by the acceptor number AN, together with the hydrogen bond donor addities. O, of some selected solvents are listed in Table 1.5. Water is among the solvents with the highest AN and, accordingly, interacts strongly witli Lewis bases. This seriously hampers die efficiency of Lewis-acid catalysis in water. 

In this equation, the substituent parameters and reflect the incremental resonance interaction with electron-demanding and electron-releasing reaction centers, respectively. The variables and r are established for a reaction series by regression analysis and are measures of the extent of the extra resonance contribution. The larger the value of r, the greater is the extra resonance contribution. Because both donor and acceptor capacity will not contribute in a single reaction process, either or r would be expected to be zero. 

The great electron-pair acceptor capacity (Lewis acidity) of SbFs has been utilized in the production of extremely strong proton donors (Brpnsted acids, p. 48). Thus the acidity of anhydrous HF is substantially increased in the presence of SbF ... 

However, an evaluation of the observed (overall) rate constants as a function of the water concentration (5 to 25 % in acetonitrile) does not yield constant values for ki and k2/k i. This result can be tentatively explained as due to changes in the water structure. Arnett et al. (1977) have found that bulk water has an H-bond acceptor capacity towards pyridinium ions about twice that of monomeric water and twice as strong an H-bond donor property towards pyridines. In the present case this should lead to an increase in the N — H stretching frequency in the o-complex (H-acceptor effect) and possibly to increased stabilization of the incipient triazene compound (H-donor effect). Water reduces the ion pairing of the diazonium salt and therefore increases its reactivity (Penton and Zollinger, 1971 Hashida et al., 1974 Juri and Bartsch, 1980), resulting in an increase in the rate of formation of the o-complex (ik ). 

The nonlinear character of log has not often been discussed previously. Nevertheless, Jorgensen and Duffy [26] argued the need for a nonlinear contribution to their log S regression, which is a product of H-bond donor capacity and the square root of H-bond acceptor capacity divided by the surface area. Indeed, for the example above their QikProp method partially reflects for this nonlinearity by predichng a much smaller solubility increase for the indole to benzimidazole mutation (0.45 versus 1.82 [39, 40]). Abraham and Le [41] introduced a similar nonlinearity in the form of a product of H -bond donor and H -bond acceptor capacity while all logarithmic partition coefficients are linear regressions with respect to their solvation parameters. Nevertheless, Abraham s model fails to reflect the test case described above. It yields changes of 1.8(1.5) and 1.7(1.7) [42] for the mutations described above. 

The solvent triangle classification method of Snyder Is the most cosDBon approach to solvent characterization used by chromatographers (510,517). The solvent polarity index, P, and solvent selectivity factors, X), which characterize the relative importemce of orientation and proton donor/acceptor interactions to the total polarity, were based on Rohrscbneider s compilation of experimental gas-liquid distribution constants for a number of test solutes in 75 common, volatile solvents. Snyder chose the solutes nitromethane, ethanol and dloxane as probes for a solvent s capacity for orientation, proton acceptor and proton donor capacity, respectively. The influence of solute molecular size, solute/solvent dispersion interactions, and solute/solvent induction interactions as a result of solvent polarizability were subtracted from the experimental distribution constants first multiplying the experimental distribution constant by the solvent molar volume and thm referencing this quantity to the value calculated for a hypothetical n-alkane with a molar volume identical to the test solute. Each value was then corrected empirically to give a value of zero for the polar distribution constant of the test solutes for saturated hydrocarbon solvents. These residual, values were supposed to arise from inductive and... 

The retention depends on the nature of both the stationary phase and the organic modifier in the mobile phase. Therefore CHI values obtained using different systems show different sensitivities towards solute characteristics. This has been studied systematically and used for the quantitative calculation of solute molecular descriptors (H-bond donor capacity, H-bond acceptor capacity and dipolarity/polarizability) for application in a general solvation equation [21]. 

Figure 3.63 illustrates the gauche effect for vicinal lone pairs and polar C—F bonds with the examples of (a) hydrazine and (b) 1,2-difluoroethane, respectively. As seen in Fig. 3.63(a), the

lone pairs are anti to one another (thus squandering their powerful donor strength on vicinal moieties with no acceptor capacity) is disfavored by 3.2 kcal mol-1 relative to the preferred = 93.9° conformer in which each nN hyperconjugates effectively with... 

In the latter case, complementary donor-acceptor interactions of lc—>2c type (e.g., nM— Oah ) can augment the 2c—s-lc (e.g., Cah iim ) interactions described in the previous section. Such complementary pairs of oppositely directed interactions are intrinsically favored compared with single (uni-directional) donor-acceptor interactions, which lead to unfavorable charge separation. Hence, stronger agostic interactions leading to full metal insertion (4.98b) will tend to be associated with transition-metal atoms having favorable donor (nM) and acceptor (nM ) capacity. 

As the logarithm of 1-octanol-water partition coefficient (log P) describes the hydrophobicity of molecules and the retention of solutes in RP-HPLC depends on the hydrophobicity, a strong correlation can be expected between the log V value and the retention of solutes in RP-HPLC. Besides log P, a considerable number of physicochemical parameters have been tested for their capacity to predict retention in RP-HPLC. Thus, Snyder s polarity index, fraction of positively and negatively charged surface area, molecular bulkiness, nonpolar surface area, electron donor and acceptor capacity, various ster-ical parameters, and the energy of highest occupied molecular orbit have all been included in QSRR calculations. 

Also, due to the stronger d -donor ability of Ru11 and Os11, equilibrium (2) can be set up for M = Ru, Os and L = EtOH. The dicarbonyls, e.g. Ru(CO)2(OEP) (97) and Os(CO)2(OEP) (78, 79), can be isolated, but one of the CO molecules is easily displaced, e.g. by H20, and the dynamic tram effect is still operative. Qualitatively, the stability of these dicarbonyls increases with the decrease of the 7r-acceptor capacity (or with the increase of the nitrogen basicity) of the attached porphyrin ligand, as expected from the porphyrin cis effect which is documented in an appropriate variation of the CO-stretching frequencies (see Sect. 7.2, Table 15). 

However, this order cannot be directly assigned to differences in porphyrin a-donor and rr-acceptor capacities. The observed differences from the expected series cited above are probably due to steric effects. The abnormal positions of (Meso-DME) or (Deut-DME) could be caused by a larger difference in the Fe-O-Fe angle being rather close to 180° in the former and rather far from 180° in the latter case 152). The extreme position of (OEPMe2), showing the weakest Fe—0—Fe interaction, is rationalized by either a lengthening of the Fe-O-Fe system by steric strain or the weak ff-acceptor capacity of the porphodimethene skeleton or both 153). 

Assisted by the two water solvate molecules, anions and cations aggregate in slabs stacked along c. Dpg+ exhausts its capacity as a donor and the anion is very much involved in a donor/acceptor role, only 04 is left rmperturbed (Figme. 4). 

---