A-H—B bond

Hydroboration, discovered by Herbert C. Brown of Purdue University (co-winner of the Nobel Prize for Chemistry in 1979), involves an addition of a H-B bond (a boron hydride) to an alkene. 

Figure 2.2 Potential of the A-H stretching mode, Ulnstrating the enhanced anharmonicity cansed by formation of an A-H- -B bond.

Similar curves are obtained for other types of A-H B bonds but symmetrical bonds are automatically ruled out when A and B are different. 

Merrifield (1258) found a linear correlation between Af, and R. Furthermore, their data include one F— H-- F system and one N—H 0 system, implying that the linear relationship applies to any A—H---B bond. Pimentel and Sederholm (1637) examined the data for N—H- -O systems and N—H- -N systems, and concluded that a separate straight line is required for each type of H bond. They list the following equations as the best relations ... 

A—H Distances in Hydrogen Bonded Crystals. There are a few scattered data for A—H B bonds where either A or B is not oxygen. Most of these cases were studied by proton magnetic resonance and, in some, proton-proton distances were determined rather than A—H distances. Table 9-IV shows the data available. 

One of the major corollaries that we deduce from the list of the nonconven-tional H-bonded systems collected in Tables 3 and 6 is that fluoromethanes act as the proton donors in a majority of the blue-shifting and blue-shifted hydro-gen-bonded systems. Notice that the quintessential distinction between the blue-shifting and blue-shifted hydrogen bonds lies in the following. The former has only nonconventional A-H- - -B bond(s), that is, the bond whose donor A-H sub-bond contracts under the A-H- - -B bond formation and whose corresponding stretching mode u(A H) undergoes a blue shift with respect to the isolated donor molecule. In contrast, the latter, which comprises of the... 

For an acid to transfer its proton to a base, the two entities must collide. Moreover, they must collide with a specific geometry. The trajectory of the base (B ) must be toward the H of H—A, not toward the A group. Further, it makes sense that there would be an optimal trajectory for the approach of H—A and of B that will lead to the lowest-energy pathway for passing the proton from the acid to the base. In this case, a linear approach is optimal. The transition state has a linear A—H—B bond angle, and the proton is partially shared between both A and B (look back at Figure 4.1). 

Boranes are typical species with electron-deficient bonds, where a chemical bond has more centers than electrons. The smallest molecule showing this property is diborane. Each of the two B-H-B bonds (shown in Figure 2-60a) contains only two electrons, while the molecular orbital extends over three atoms. A correct representation has to represent the delocalization of the two electrons over three atom centers as shown in Figure 2-60b. Figure 2-60c shows another type of electron-deficient bond. In boron cage compounds, boron-boron bonds share their electron pair with the unoccupied atom orbital of a third boron atom [86]. These types of bonds cannot be accommodated in a single VB model of two-electron/ two-centered bonds. 

Figure 2-60. Soine examples of electron-deficient bonds a) diborane featuring B-H-B bonds b) diborane in a tentative RAMSES representation c) the orbital in a B-B-B bond (which occurs in boron cage compounds),...

Fig. 5. Modes of M—H—B bonding where M—H—B represents a three-center hydrogen bridge bond for (a), (b), (c) tetrahydroborates and for (d), (e), (f)...

Intimate information about the nature of the H bond has come from vibrational spectro.scopy (infrared and Raman), proton nmr spectroscopy, and diffraction techniques (X-ray and neutron). In vibrational spectroscopy the presence of a hydrogen bond A-H B is manifest by the following effects ... 

It follows from the preceding discussion that the unbranched H bond can be regarded as a 3-centre 4-electron bond A-H B in which the 2 pairs of electrons involved are the bond pair in A-H and the lone pair on B. The degree of charge separation on bond formation will depend on the nature of the proton-donor group AH and the Lewis base B. The relation between this 3-centre bond formalism and the 3-centre bond descriptions frequently used for boranes, polyhalides and compounds of xenon is particularly instructive and is elaborated in... 

Figure 3.11 Schematic representation of the energy levels in various types of 3-centre bond. The B-H-B ( electron deficient ) bond is non-linear, the ( electron excess ) F-Xe-F bond is linear, and the A-H B hydrogen bond can be either linear or non-linear depending on the compound.

Some force fields make special provision for hydrogen-bonded atoms A-H B, and modify the Lennard-Jones 12-6 potential to a 12-10 model ... 

In general the A—H B hydrogen bond can be taken to be approximately linear for example, in diaspore the angle between the inter-... 

The conjugate acid of [MntCO) ]" HMnfCO) , protonates BjH, to give 2,2,2-(CO)3-2-MnB5H,o that contains two Mn — H—B bonds as well as a direct Mn—B bond between the apical boron atom and manganese. ... 

---