Donor pairs

Fig. 20.4. H-bond acceptor patterns observed two electron donor groups with a spatial in P-gp substrates. Type I units patterns separation of 4.6 + 0.6 A. A denotes a informed by electron donor pairs with a spatial bonding acceptor group (electron donor...

Fig. 4. Likely sites with axial symmetry for the hydrogen impurity in hydrogen-acceptor or -donor pairs. The sites BC, Si—AB, and X—AB refer to the bond-centered, siliconantibonding, and X (donor or acceptor)-antibonding sites, respectively. 

The most commonly accepted model of the hydrogen-donor pair has H... 

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 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. 

Because trans dispositions commonly result from co-bonding (the near-linear alignment of the hyperbonding 3c/4e X—M- L triad), it is not surprising that the origin of the trans influence can be traced to the resonance nature of co-bonding. When H is placed trans to a halide or PH3, the dominant resonance structure will be that with a 2c/2e M—H bond and a donor pair of electrons on the halide or phosphine ligand, as depicted on the left in (4.93) ... 

The direct photocyclization of another interesting acceptor-donor pair, the amine-enone system, has been reported by Mariano [224-226]. Direct irradiation of -(aminoethyl)cyclohexenones 281 leads to the excitation of the conjugated cyclohexenone chromophore. Intramolecular single-electron transfer from the amine donor to the cyclohexenone excited state results in the formation... 

Figure 2-26 Some of the possibilities for hydrogen bonding of side chain groups in proteins. Oxygen atoms can and frequently do form up to three hydrogen bonds at once. Open arrows point from H-atoms and toward electron donor pairs.

Evidence over the past 15 years has indicated that this assay for lymphocytes with bound antibody is indeed more sensitive in detecting antibodies directed against donor cells than is the traditional (microscope) assay for lymphocyte death. The flow assay also defines a group of recipient-donor pairs who are at risk for severe rejection crises and possibly, but not necessarily, for loss of the grafted organ. At the present time, donated organs are in short supply. One transplant... 

An enantioselective intermolecular Michael addition of aldehydes (138) to enones (139), catalysed by imidazolidinones (140), has been reported. Chemoselectivity (Michael addition versus aldol) can be controlled through judicious choice of hydrogen bond-donating co-catalysts. The optimal imidazolidinone-hydrogen bond donor pair affords Michael addition products in <90% ee. Furthermore, the enamine intermediate was isolated and characterized and its efficacy as a nucleophile in the observed Michael addition reactions was demonstrated.172... 

PET reactions where products arise by coupling of acceptor-donor pairs are summarised in this section. Usually these reactions do not involve direct coupling of radical ions, instead, recombination of radical pairs, formed by the... 

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