Section 22.4.5.7. The answer `no'

Previous sections have illustrated the location and characterization of some important non-covalent interactions. Equally important is a knowledge of when such interactions do not occur although chemical sensibility might indicate that they should. We provide four examples from the CSD: (a) only 4.8% of more than 1000 thioether S atoms form hydrogen-atom contacts that are within van der Waals limits, despite the obvious analogy with the potent acceptor C—O—C (Allen, Bird et al., 1997b); (b) of 118 instances in which a furan ring coexists with N—H or O—H donors, the O atom forms hydrogen bonds on only three occasions (Nobeli et al., 1997); (c) the ester oxygen (R₁)(O=)C—O—R₂ almost never forms strong hydrogen bonds, although the adjunct carbonyl oxygen atom is well known as a highly potent acceptor (Lommerse et al., 1997); and (d) covalently bound fluorine atoms rarely form hydrogen bonds (Dunitz & Taylor, 1997).