H-acceptor

Thiophene reacts with ReH7(PPh3)2 aided by the H-acceptor (3,3-dimethyl-l-butene) to give the thioallyl species 113 (92JA10767). Further reaction of 113 with trimethylphosphine yields organometallic products with a cleaved C—S bond. 

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 ). 

Upon dimerization, electron charge is transferred from the base (the H-acceptor molecule) to the acid (the H-donor molecule), in agreement with Lewis generalized definition of an acid and a base as an electron acceptor and donor, respectively. The amount of such a charge transfer (CT) is reported in Table 4, for the two SCF models considered in this paper and as a function of the basis set size. The CTs are small and, for the SCF-SM method, are found to decrease as the basis set size increases. 

Additionally, it has been noted that Tetralin operates via hydride transfer, at least in its reduction of quinones. Thus it has been shown that Tetralin readily donates hydrogen to electron-poor systems, such as quinones at 50°-160°C. The reaction is accelerated by electron-withdrawing substituents on the H-acceptor and polar solvents, and is unaffected by free radical initiators (6). These observations are consistent with hydride transfer, as is the more recent finding of a tritium isotope effect for the reaction (7). 

Unifying Summary of H-Acceptor and H-Donor Pairs in Silicon... 

Hopefully, just on the horizon is a thorough understanding of possible defect metastability. The results of a number of computational studies now support the notion that there may be one metastable configuration for H-acceptor pair (Si—AB) and perhaps two for H-donor pairs (X—AB and BC). This would be consistent with the experimental evidence for metastability from channeling and PAC studies for the acceptor complexes however, quantitative agreement may be lacking. 

In the calculations of Briddon and Jones (1988), the BC site was found to be the stable site of hydrogen for the H—Be (H-acceptor) pair. The H—As and H—Ga separations were found to be 1.54 A and 1.77 A, respectively. The Aj frequency was computed to be 2083 cm-1, very close to the experimentally determined value of 2037 cm-1. An E-mode was also calculated to have a frequency of (383, 346) cm-1. (This pair of computed results should be degenerate differences are artifacts of method approximations.)... 

In summary, the H-acceptor pairs appear to be very similar to their silicon counterparts, which we have discussed in depth. The H-donor pairs are similar in that the H occupies a silicon-antibonding site however, this is an antibonding site to the defect and not to the host as is found in silicon. It is also interesting to note that the computed hydrogen frequencies appropriate to the latter pairs are better described by theory than the silicon counterparts discussed earlier. It is not clear whether this is a consequence of the electronic-structure method used here, a natural consequence of the differences between the silicon and compound-semiconductor hosts, or simply an accident. 

An acid in the Br0nsted-Lowery theory is an H+ donor and a base is an H+ acceptor. In the Brpnsted-Lowery acid-base theory, there is a competition for an H+. Consider the acid-base reaction between acetic acid, a weak acid, and ammonia, a weak base ... 

Br0nsted-Lowery acids are H+ donors and bases are H+ acceptors. Strong acids dissociate completely in water. Weak acids only partially dissociate, establishing an equilibrium system. Weak acid and base dissociation constants (Ka and Kb) describe these equilibrium systems. Water is amphoteric, acting as both an acid or a base. We describe water s equilibrium by the Kw expression. A pH value is a way of representing a solution s acidity. Some salts and oxides have acid-base properties. A Lewis acid is an electron pair acceptor while a Lewis base is an electron pair donor. 

At the microscopic level, acids are defined as proton (H ) donors (Bronsted-Lowry theory) or electron-pair acceptors (Lewis theory). Bases are defined as proton (H+) acceptors (Bronsted-Lowry theory) or electron-pair donors (Lewis theory). Consider the gas-phase reaction between hydrogen chloride and ammonia ... 

Two additional systems in which hydrogen bonds are expected to play a dominant role, ammonia-water complex and 2-aminoethanol, were calculated ab initio and by the new MM3 force field. Two ammonia-water complexes were considered, one with an N... H—O bridge (32) and the other with an O... H—N bridge (33). As expected from the relative H-donor/H-acceptor properties of nitrogen and oxygen, 32 was calculated... 

With these improved techniques P-hydroxybutyrate, which penetrates mitochondria easily and is oxidized to acetoacetate using NAD+ as H acceptor, gave a P/O ratio of 3, the value equivalent to that from the reoxidation of NADH found by Lehninger. Succinate, which bypassed the NAD+/NADH step, gave a ratio of 2. When cytochrome c-Fe2+ was... 

Diols were converted to diones by RuQ3(PPh3)3/benzalacetone/THF/195°C in a process involving H-transfer to an olelinic H-acceptor Ph(CH)3COMe (Table 2.4) [899],... 

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