Unsymmetric hydrogen bonding

An unsymmetrical hydrogen bonded system may be represented diagramaticaUy by... 

Unsold s theorem. 19 Unstable complexes, 547-548 Unsymmetric hydrogen bonding. 302... 

It becomes less probable with unsymmetrical hydrogen bonds, and may be negligible in the moderate to weak hydrogen bonds. 

For unsymmetrical hydrogen bonding, such as that of B 3- HA BHA shown in Figure 6-7, the pattern is similar. The two electron pairs in the lower orbitals have a lower total energy than the total for the electrons in the two reactants. 

FIGURE 6-7 Molecular Orbitals for Unsymmetrical Hydrogen Bonding. 

A noteworthy example is potassium hydrogen mesotartrate (Sect. XVII C), both because of the high accuracy of the analysis and because symmetrical and unsymmetrical hydrogen bonds occur in the same crystal and in chemically similar situations. The unsymmetrical bond has 0 0 = 2.543(1) A. The two symmetrical ones are very significantly shorter at 2.452(2) and 2.483(2) A. 

Infra-red and n.m.r, spectra show that the molecules have an almost symmetrical structure, due to a time-averaging effect [209,247], and investigation of its microwave spectrum indicates that the barrier to interconversion cannot extend much above the ground vibrational level [249]. X-Ray and neutron diffraction studies show, however, a slightly unsymmetric hydrogen bond [272]. 

At ordinary temperatures, KH2PO4 has a symmetrical tetragonal structure (Figure 13.4), but at -151°C this changes (reversibly) to a slightly less symmetrical orthorhombic structure which is ferroelectric. This change involves a shift of the H positions in the crystal to produce unsymmetrical hydrogen bonds (12.405). 

For unsymmetrical hydrogen bonds, the bond force constants are unequal and the hydrogen is in motion in the symmetric stretch, which now shows some sensitivity to isotopic substitution. At the same time the isotopic sensitivity of the asymmetric stretch is diminished. As is shown below through application of the Product... 

In the limit of a weak and highly unsymmetrical hydrogen bond, isotopic sensitivity is again confined to a single vibration, and characteristically one vibration approximates that of the monovalent hydrogen while the other is of low frequency and isotopically insensitive [17]. This behaviour has been discussed in detail for triatomic... 

---