Lone pair donor strength

Hydrogen bonds (H-bonds) are ideal noncovalent interactions to construct supramolecular nanoporous architectures since they are highly selective and directional [16]. H-bonds are formed when a donor with an available acidic hydrogen atom interacts with an acceptor that carries available nonbonding electron lone pairs. The strength of the H-bond depends mainly on the solvent and number and sequence of the H-bond donors and acceptors. Various supramolecular polymer materials have been developed which use H-bonds as structural element to position molecules. After removal of these molecular templates, a porous material is obtained to fabricate molecule specific systems. 

Figure 3.63 illustrates the gauche effect for vicinal lone pairs and polar C—F bonds with the examples of (a) hydrazine and (b) 1,2-difluoroethane, respectively. As seen in Fig. 3.63(a), the

lone pairs are anti to one another (thus squandering their powerful donor strength on vicinal moieties with no acceptor capacity) is disfavored by 3.2 kcal mol-1 relative to the preferred = 93.9° conformer in which each nN hyperconjugates effectively with... 

The basic features of the model are the following a) Lone pair and bonds are classified according to their intrinsic donor and acceptor abilities. Tables 36 and 37 summarize the relative intrinsic donor and acceptor strengths of various lone pairs and bonds. 

Since phenol has an appreciable dipole moment, and no low energy acceptor orbitals, it should interact best with the donors that have the largest lone pair dipole moment — the oxygen compounds. Iodine has no dipole moment and the interaction with iodine is expected to be essentially covalent. Iodine should interact best with the donors that have the lowest ionization potential, i.e., the ones whose charge clouds are most easily polarized. Similar considerations have been employed to explain the donor strengths of primary, secondary and tertiary amines 35a) and the acid strengths of (35b) ICl, Bt2, I2. CeHsOH and SO2. 

The strength of the hydrogen bond is at a maximum when the proton donor group and the axis of the lone pair orbital are collinear. The strength of the bond decreases as the distance between X and Y increases. 

A large number of molecules can react in this way and typically HX contains an H—S or H—O bond or else is a hydrohalic acid. There are both kinetic and thermodynamic considerations as to whether this type of reaction can take place. Firstly, the mechanism of the reaction rarely involves direct protonation of the M—OR bond. Instead, initial coordination of HX through lone pairs of electrons on X is necessary prior to transfer of die proton. Hence, the rate of the reaction will be dependent on the steric constraints of both HX and the metal coordination sphere as well as the electronic donor-acceptor properties of the two substrates. Thermodynamically the position of the equilibrium will depend on a number of variables, the relative strengths of the M—O and M—X bonds being important ones. 

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