Donor ability

Many phenomena ask for local, site-specific properties of a molecule such as the partial charge on a specific atom in a molecule or the hydrogen bond donor ability of a certain OH group. It would be highly desirable to have methods as simple as an additivity model to estimate such site-specific molecular properties. 

The finding that highly deactivated aromatics do not react with N02 salts is in accord with the finding that their greatly diminished TT-donor ability no longer snffices to polarize NOi. Similarly, (j-donor hydrocarbons such as methane (CH4) are not able to affect such polarization. Instead, the linear nitronium ion is activated by the superacid. Despite the fact that is a small, triatomic cation, the 11011-... 

Estimation of the influence of end groups by using their topological indexes, Oq and E, has been proposed. The first parameter, Oq, characterizes the shift of the MO modes and the level positions of a PMD containing end groups relative to unsubstituted polymethines. Thus it corresponds to the end-group basicity concept (7). The parameter Oq was found to be related directiy to the electron donor ability 1 = lim ( q. The other index, E,... 

Hydrogen bonding to the nitrogen lone pair leads to an upheld shift, the extent of which depends on the proton-donor ability of the solvent, and the acceptor ability of the base shifts of some 20 p.p.m. are commonly found. 

Figure 8-2. Plot of etx (from Table 8-2) against a (from Reference 46). a measure of H-bond donor ability. The numbers identify solvents in Table 8-2.

Some of these model-dependent quantities were formulated as measures of a particular phenomenon, such as electron-pair donor ability but many of them have been proposed as empirical measures of solvent polarity, with the goal, or hope, that they may embody a useful blend of solvent properties that quantitatively accounts for the solvent effect on reactivity. This section describes many, although not all, of these empirical measures. Reichardt has reviewed this subject. 

Note, Added in Proof-. In their study of the autoxidation of 2-butyl-isoindoline, Kochi and Singleton showed that 2-butylisoindole is formed and is converted by further oxidation to 2-butylphthalimide and 2-butylphthalimidine. The rate of oxidation of 2-butylisoindoline to the isoindole was found to be markedly dependent on hydrogen donor ability of the solvent and was shoivn to involve a free radical chain process. Autoxidation of 2-butylisoindole also appears to be a radical process since it can initiate autoxidation of 2-butylisoindoline. 

CDCI3, DMSO-d, pyridine-ds), the equilibrium content of the ring-opened form 273 increases with increase in solvent polarity and electron-donor ability of the aryl substituent R. In the sohd, the cyclic tautomers are preferred as proven by IR spectroscopy (97ZOR418). 

RuCl2(PPh3)2 reacts with 4-R2P-dibenzothiophene (R = Ph, p-Tol) and forms 303, in which the dibenzothiophene ligand is coordinated to ruthenium via the phosphorus and sulfur atoms [84JA5379, 87JOM(318)409]. The donor ability of the sulfur atom is relatively weak. Complex 303 (R = Ph) is able to add carbon monoxide and yield the monocarbonyl adduct. 

TABLE 5. Hydrogen bond donor ability (AvOH) of hydroxylic compounds with DMSO in CC1,... 

Later, Taft and his coworkers examined the H-bonding donor ability of the amino protons in 4-nitroaniline and 4-nitrophenol as the hydrogen bond donors. They also tried... 

The donor ability of the monomer is decreased by electron-attracting substituents. 

The donor ability of the monomers could be quantitatively characterized, by their interaction with a constant acceptor, e.g. 

The electronic influence on the donor ability of the monomers as well as the acceptor behavior of the corresponding cation has been characterized by variation of the para-substituents of the styrenes. More detailed assertions can be expected when one varies the structure directly on the double bond by substitution at a- and P-position. A suprisingly good correlation appears between the energetic level of the HOMO and energy of theCT-absorption of the complexes with TCNE (ECT = 20.8 5.55x(HOMO) r = 0.80 n = 12) for styrenes, both with and without a methyl substituent at a- and p-position ... 

The relative graduation of ECT and x(HOMO) is not influenced during variation of X by introduction of a methyl group at a- and p-position. However, ECT as well as x(HOMO) are shifted in the direction of increased donor ability of the monomer by the methyl group. This shift is much more visible at the p-position than the a-position, for instance, the CT energy is decreased by an a-methyl group by 9 kJ mol-1 and by P-methyl group by about 24 kJ mol-1 (see Table 12). 

Apparently the complex formation with a 71-acceptor is suitable for characterization of the donor ability of the entire -system of the monomers. Simultaneously, it can be derived that the EDA-complex formation is only insignificantly influenced by steric effects. Because the above named variation in structure does not disturb the planarity of the center of the monomer double bond, the interaction of the 71-systems from both donor and coplanar acceptor cannot be limited by steric effects. 

It can be determined from the higher effect of the p-substitution compared with the 7-substitution and the high donor ability of the stilbene (ECT = 200 kJ mol-1 x(HOMO) = 0.504 qa = qp = 1.000), that an even electron distribution in the n-system of the donor causing a high electron density in the vicinity of the monomer double bond is important for the strength of the EDA interaction between 71-donor and 7t-acceptor. 

A special case of the internal stabilization of a cationic chain end is the intramolecular solvation of the cationic centre. This can proceed with the assistance of suitable substituents at the polymeric backbone which possess donor ability (for instance methoxy groups 109)). This stabilization can lead to an increase in molecular weight and to a decrease in non-uniformity of the products. The two effects named above were obtained during the transition from vinyl ethers U0) to the cis-l,2-dimethoxy ethylene (DME)1U). An intramolecular stabilization is discussed for the case of vinyl ether polymerization by assuming a six-membered cyclic oxonium ion 2) as well as for the case of cationic polymerization of oxygen heterocycles112). Contrary to normal vinyl ethers, DME can form 5- and 7-membe red cyclic intermediates beside 6-membered ringsIl2). 

In addition to the ratio of concentrations olefine/HA, the donor ability, or the nucleophilicity of the anion A- is a deciding factor for the manner in which the reaction continues. This anion is formed simultaneously with the carbenium ion. When the nucleophilicity of the anion is sufficiently high, as in the case of CP, Br-, I-, for instance, the reaction proceeds as an addition by the formation of a covalent bond between A- and the carbenium ion72). 

Enhanced o donor ability of the CO towards the enhanced r accepting metal. 

In the N-Bz derivatives of Gly and Gly-Gly, the planar [R3Sn(IV)] moieties are bridged by -COO groups. Because of the negative inductive effect of the Ph group, which reduces the donor ability of oxygen atoms, coordination of the amide -C = O in the latter compounds could be ruled out. 

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