Unshared electron pairs, interaction

As shown in Figure 4.11, the crncial electronic interaction is between an unshared electron pair- of Cl and the vacant 2p orbital of the positively charged car bon of (CFl3)3C. ... 

The amide bond that links different amino acids together in peptides is no different from any other amide bond (Section 24.3). Amide nitrogens are non-basic because their unshared electron pair is delocalized 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... 

Fig. 1. Electronic states [or iron-group atoms, showing number of states as qualitative [unction of electronic energy. Electrons in band A are paired with similar electrons of neighboring atoms to form bonds. Electrons in band B are d electrons with small interatomic interaction they remain unpaired until the band is half-filled. The shaded area represents occupancy of the states by electrons in nickel, with 0.6 electron lacking from a completely filled B band. (States corresponding to occupancy of bond orbitals by unshared electron pairs are not shown in the diagram.)...

The methylene protons that are very close to the oxirane oxygen atom are deshielded and appear as a broader signal near 8 = 4.2. This deshielding appears to be due to both van der Waals interactions between protons and the oxygen atom 331 and the effect of the unshared electron pair at the heteroatom 34). 

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 ... 

With substituents such as OH and OMe that have unshared electron pairs, an electron-donating, i.e. base-strengthening, mesomeric effect can be exerted from the o- and p-, but not from the m-position, with the result that the p-substituted aniline is a stronger base than the corresponding w-compound. The m-compound is a weaker base than aniline itself, due to the electron-withdrawing inductive effect exerted by the oxygen atom in each case. As so often, the effect of the o-substituent remains somewhat anomalous, due to direct interaction with the NH2 group by both steric and polar effects. The substituted anilines are found to have related pAa values as follows ... 

The role of excess antimony pentafluoride can best be explained by assuming that it is capable of solvating the alkylcarbonium ion salt through interaction of the unshared electron pairs of fluorine with the vacant p-orbital of the sp -hybridized planar central carbon atom of the carbonium ion. In the system of low dielectric constant the alkylcarbonium ion is present not in the free form, but as a tightly bound ion-pair solvation affects this species rather than the free ion. 

The peculiarity of this interaction is that the carbon atom participating in the appearance of intramolecular interaction possesses no unshared electron pairs which results in the impossibility of acceptor-donor interaction [13]. 

The quantum-mechanical theory of valence leads to the result that the interaction energy of two unshared electron pairs on adjacent... 

---