Delocalised electron interactions

While the Overhauser shift is due to delocalised electrons, the ENDOR experiments require more localised electrons. Both groups were able to observe ENDOR on the effective mass donor resonance but not on the deep donor signal. An illustrative ENDOR spectrum is shown in FIGURE 4, in which the 69 71Ga ENDOR lines are observed. The 69Ga line is clearly split by quadrupole interactions just as in the Oveihauser shift measurements, while the quadrupolar interaction is much less resolved in the 71 Ga line, due to its smaller quadrupole moment. In the three samples investigated by the two groups, the linewidths were all too broad to resolve any hyperfine interaction and so no definitive identification of the residual donor was possible. 

One idea was that conduction should be looked for in materials based on organic unsaturated molecules, where electrons and holes are more easily excited into delocalised states. Little proposed that, in these materials, carriers created by such excitonic excitations could couple into electrons and holes plasmons, as later indeed observed in classical semiconductors under light, and thus produce a conductive state hopefully a superconductor state could develop under the same electron-electron interactions, leading possibly to high T<. s. The search for such excitonic conduction had been initiated shortly before as a porposal by W. Kohn for normal covalent semiconductors it had been carried out notably by D. Jerome, when he came to Orsay after on year s stay with Kohn in La Jolla various mineral semiconductors were studied, with small gaps and often low dimensionally. 

The amide bond that links different amino acids together in peptides is no different from any other amide bond (Section 24.4). Amide nitrogens are nonbasic because their unshared electron pair is delocalised by interaction with the carbonyl group. This overlap of the nitrogen p orbital with the p orbitals of the carbonyl group imparts a certain amount of double-bond character to the C-N bond and restricts rotation around it. As indicated by the stereo views of alanylserine and serylalanine shown in the previous section, the amide bond is planar and the N-H is oriented 180° to the C=0. 

Alkene hydrogenation is significantly exothermic, and it is not always easy to keep the catalyst isothermal except at low rates. Heats of hydrogenation for a number of alkenes were measured many years ago (Table 7.1), the use of a catalyst ensuring that calorimetry could be conducted at ambient temperature. The values are similar to but perhaps more accurate than those derived from heats of combustion, where subtraction of two large numbers is involved they reflect the extents to which the jt electrons interact with the electrons in the C—H bonds by hyperconjugation. This electron delocalisation is also reflected in the relative stabilities of alkene complexes with Ag+ cations. ... 

Two or more molecules close together can interact, leading to splitting of their electronic states to produce delocalised electron systems. If these are in solid crystals, the electronically excited states can be considered as chargeless quasi-... 

When the electronic interaction between the chromophores along a polymer chain is so strong that the electrons may be delocalised over several cbromophores, the exciton may also be similarly delocalised. Such a delocalised exciton is called a Frenkel exciton. Frenkel excitons have been observed in conjugated polymers such as polyphenylacetylene (Figure 13.10). 

The properties of the a-Per2M(mnt)2 compounds, especially those derived from the coexistence in the same solid of delocalised electron chains (perylene molecules) and chains of localised spins (M(mnt)2 units for some M), were recognised early on as a unique and interesting feature of this family of compounds [57], and are the main topic of two short reviews [70,71]. In fact, this type of compound produced the first examples of one-dimensional molecular metals where delocalised electrons were found to clearly interact with localised spins. In this respect they resemble some of the properties of... 

Finally, the solvent also interacts with sites of the Lewis acid and the Lewis base that are not directly involved in mutual coordination, thereby altering the electronic properties of the complex. For example, delocalisation of charges into the surrounding solvent molecules causes ions in solution to be softer than in the gas phase . Again, water is particularly effective since it can act as an efficient electron pair acceptor as well as a donor. 

The exact reverse of the above is seen with aniline (13), which is a very weak base (pKa = 4-62) compared with ammonia (pKa = 9-25) or cyclohexylamine (pKa = 10-68). In aniline the nitrogen atom is again bonded to an sp2 hybridised carbon atom but, more significantly, the unshared electron pair on nitrogen can interact with the delocalised 7r orbitals of the nucleus ... 

A group with a more powerful (electron-withdrawing) inductive effect, e.g. NOa, is found to have rather more influence. Electron-withdrawal is intensified when the nitro group is in the o- or p-position, for the interaction of the unshared pair of the amino nitrogen with the delocalised it orbital system of the benzene nucleus is then enhanced. The neutral molecule is thus stabilised even further with respect to the cation, resulting in further weakening as a base. Thus the nitro-anilines are found to have related p a values ... 

Perhaps the most important conclusion to be drawn from results for metal atoms in groups such as 7SiL3 or -PL3+ is undetectably small (70,71). Indeed, the R2C- moiety displays hyperfine interaction with H and 13C that suggest normal planarity at carbon with essentially unit spin-density thereon, and coupling to the metal atom (specifically, 31P) is small and probably negative. This implies that spin-density is acquired by spin-polarisation of the C-M o-electrons and not by p -d delocalisation, as is so often... 

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