Donor-acceptor fragments

The second rule is directly relevant to conformational isomerism. In Part IV of this work we have provided the theoretical justification of this rule and also identified the situation where this rule may break down. A typical exception is shown below and involves a molecule where two vicinal polar bonds constitute the best donor-acceptor fragment combination and, thus, lead to a conformational preference placing them in an anti-relationship. 

Fig. 4.7 Correlation between electronic coupling matrix element from sFODFT and overlap between SOMO orbitals of donor-acceptor fragments. Taken from Ref. [135]...

Each of the pharmacophore queries consisted of one donor, one acceptor and one of the two hydrophobic points indicated in Figure 1.16. The directionality of hydrogen bonds was inferred from the X-ray structure and reasonably loose tolerances of 1.5 A were used for donor-acceptor distances and 2-2.5 A to the hydrophobe were chosen to allow for the flexibility seen across kinase structures and to maximise the diversity amongst the identified fragments. Four pharmacophore queries were completed and the results were combined. The chosen fragments were further filtered by molecular weight (between 150 and 250), 51og P (between 2.5 and —2.5) and the presence of... 

Coordination is further promoted by secondary donor-acceptor interactions between orbitals of the metal fragment and the ligand. Representative examples of such interactions are shown in Fig. 4.35 for the special case of H2W(NH3)2 (cf. Fig. 4.33(b)). 

Purely dative coordination of H2 apparently requires a hypovalent metal complex, such as HfH4 or TaHs. Figure 4.59 displays the geometrical structures and leading donor-acceptor interactions resulting from coordination of H2 to these fragments. 

Before departing the topic of structural isomerism, we wish to point out that the key ideas presented here fit within a broad scheme of chemical reasoning. Thus, the combination of two different types of fragments, namely, two A and two B fragments to yield A2 plus B2 is less stable than the combination of the same fragments to yield 2 AB, the latter being superior because it involves a donor-acceptor pairing. 

Similarly, the combination of three different types of fragments, namely, A, B and two C fragments to yield AB plus C2 may be more or less stable than the combination of the same fragments to yield AC plus BC depending upon the donor-acceptor interrelationships. In all cases, however, the success of the approach is guaranteed only if matrix elements remain relatively constant. 

Fig. 3 Typical ICT probes (left) and representative spectroscopic responses toward selected metal ions (right). Color code (left) coordinating atoms in blue, bridgehead atom of the fluorophore that takes part in complexation in orange, formal donor fragment in red, formal acceptor fragment in green (right) hypsochromic shifts in red, bathochromic shifts in green, fluorescence enhancement in violet, fluorescence quenching in blue. Symbols in table Aabs, 7em,

In principle, the trends should be exactly opposite for ICT probes carrying the receptor in the acceptor fragment. However, as can be seen for 9 in Fig. 3 and some other probes of this type (e.g., in [50-52]), the relationship is not straightforward. Moreover, since the literature on acceptor-type ICT probes is much less abundant than on their donor counterparts, the database for a comprehensive analysis and discussion is still rather weak and further conclusions will not be drawn here. 

Tetranitromethane produces strongly coloured electron donor-acceptor (EDA) complexes with derivatives of the anthracene213, in dichloromethane. Specific irradiation of the charge transfer absorption band at X > 500 nm produces a rapid fading of the colour of the solutions. From these solutions, adduct 91 is obtained (reaction 24) its structure is ascertained by X-ray crystallographic diffraction. 91 is derived from an anti-addition of fragments of tetranitromethane by a multistep pathway214. 

In this case, the initial donor-acceptor interaction yields radicals of identical activity. The presence of styrene gives rise not to a polymer but to a low-molecular individual compound containing fragments of the probe and both radicals formed. 

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