Donors characteristics

The solvent effect on the diastereofacial selectivity in the reactions between cyclopentadiene and (lR,2S,5R)-mentyl acrylate is dominated by the hydrogen bond donor characteristics of the solvent... 

The reason for the greater stability of M(PR3)n over M(CO)4 must lie in the difference in donor characteristics of the two kinds of ligand. CO is a poor (7-donor but a strong 7r-acceptor, while tertiary phosphines are much better [Pg.196]

Equation (6.20) and the semiquantitative trends it conveys, can be rationalized not only on the basis of lateral coadsorbate interactions (section 4.5.9.2) and rigorous quantum mechanical calculations on clusters89 (which have shown that 80% of the repulsive O2 - O interaction is indeed an electrostatic (Stark) through-the-vacuum interaction) but also by considering the band structure of a transition metal (Fig. 6.14) and the changes induced by varying O (or EF) on the chemisorption of a molecule such as CO which exhibits both electron acceptor and electron donor characteristics. This example has been adapted from some rigorous recent quantum mechanical calculations of Koper and van Santen.98... 

As noted for heteroatom attachment in 8-oxo-dG and C8-arylamine adducts, attachment of the Ph moiety to the C-8 site of dG enhances the one-electron donor characteristics of the purine nucleoside. The redox properties of 8-/>-X-Ph-dG (X = OH, OCH3, CH3, H, CN, CHO) adducts have been studied by cyclic voltammetry in anhydrous DMF. The C8-aryl adducts exhibited irreversible one-electron oxidation peaks with half-peak potentials ( p/2) ranging from 0.85 V versus saturated calomel electrode (SCE) for 8-/>-PhOH-dG (X = OH) up to 1.11 V/SCE for 8- -CHO-Ph-dG. All adducts were oxidized more readily than dG, which gave p/2= 1.14 V/SCE in DMF (Table 2). 

Excitation of increases its electron acceptor and donor characteristics, respectively,... 

Similar triplet energies (18) and ground state donor characteristics of MPT and PTH in solution imply that triplet redox potentials of the two substances must also be close. 

Another challenge is now to tune the emissive properties of these compounds, which implies the study of all the factors that modify them, such as the intemetallic distance, the coordination number and geometry around the metal centers—which in the case of T1(I) is variable49—or the donor characteristics of the ligands. 

In aqeous systems, containing uranyl ions, there are either mono- or polynuclear [U02(L)n] complexes with compositions determined by the chemical nature and concentration of acidic or/and electroneutral ligands L, their electron donor characteristics and their possible types of coordination to the U atoms. 

Table 3. Electron-donor characteristics of some ligands in the structures of uranyl compounds...

Electron donor characteristics determined on the basis of crystal structure data (Table 3) correspond to ligands with a certain composition and coordination type in both crystals and aqueous solutions. 

The U02 - XOs - H2O (X= C or N) systems have been chosen for analysis for two reasons. First, they are well studied experimentally, since carbonate and nitrate uranyl complexes are important from technological standpoints. Second, the isoelectronic anions XOs have the same planar structure (symmetry Dsh) and form bidentate coordination around uranyl ions (the type). However, their electron-donor characteristics are different El = 3.1 and 3.4 for the NOs and COs ions, respectively. From this viewpoint, it would be of interest to understand how the difference in the electron-donor properties influences complex formation in the U02 - XOs " - H2O systems. As in the case of aqua-complexes, we shall use the 18-electron rule to obtain answers to the following questions (a) what is the composition of stable complexes in aqueous solutions containing carbonate or nitrate uranyl complexes (b) what is the coordination number of U(VI) in these complexes. 

The fact that the same model can be applied equally well to crystals as well as to solutions is not something superficial. In our opinion, this provides additional evidence that the short-range order in atomic arrangements conforms to the same laws in both crystals and complexes existing in solutions. The successful application of crystal-structure-based values of electron-donor characteristics of ligands to the problems of complex formation (choice of coordination number, sequence and direction of replacement reactions, etc.) is most probably owing to the adequate character of the model and its description of the chemical interactions. 

In the 1960s, two direct coal liquefaction processes were under development in the U.S. the Exxon Donor Solvent (EDS) process and the H-Coal process. The distinguishing feature of the EDS process was a separate solvent hydrogenation step to carefully control the hydrogen donor characteristics of the solvent. The most important feature of the H-Coal process was the emulated bed reactor in the process. 

This work has been extended by a study of the donor characteristics of the decationated surface properly dehydrated with trinitrobenzene as the acceptor. This we shall discuss later in this chapter. 

Remarkable differences in behavior are found when the analogous permethyl derivatives are studied. The increased steric bulk, solubility, and electron-donor characteristics produce different spectroscopic and reactivity properties. Reduction of Cp 2TiCl2 with Mg in the presence of disubstituted 1,3-butadiynes leads to different final... 

Table HI. Acid/Base (Acceptor/Donor) Characteristics (in Arbitrary Units)...

Inverse gas chromatography at infinite dilution appears to be a powerful tool for studying the surface properties of carbon fibres and polymer matrices. The use of alkane probes and acid/base probes allows the characterization of the surfaces in terms of their London dispersive component of surface energy and their acid/base or acceptor/donor characteristics. A strong correlation was obtained between fibre-matrix adhesion, measured by a destructive fragmentation technique, and the level of acid base interactions calculated from the chromatographic analysis. 

In conclusion, the special influence of water on the endo/exo selectivity stems from the fact that all effects favouring the endo-adduct are combined in this solvent (1) strong hydrogen bonding, (2) polarity and (3) hydrophobicity. Water also increases the diastereofacial- and the regioselectivity of Diels-Alder reactions. Mechanistic investigations on the reaction between cyclopentadiene and methyl acrylate emphasized the importance of hydrogen-bond donor characteristics and polarity . 

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