NMR and IR Investigations of Strong Intramolecular Hydrogen Bonds

For the purpose of this review, strong hydrogen bonds have been defined on the basis of experimental data, such as OH stretching wavenumbers, nu(OH), and OH chemical shifts, delta(OH) (in the latter case, after correction for ring current effects). Limits for O-H center dot center dot center dot Y systems are taken as 2800 > nu(OH) > 1800 cm(-1), and 19 ppm > delta(OH) > 15 ppm. Recent results as well as an account of theoretical advances are presented for a series of important classes of compounds such as beta-diketone enols, beta-thioxoketone enols, Mannich bases, proton sponges, quinoline N-oxides and diacid anions. The O center dot center dot center dot O distance has long been used as a parameter for hydrogen bond strength in O-H center dot center dot center dot O systems. On a broad scale, a correlation between OH stretching wavenumbers and O center dot center dot center dot O distances is observed, as demonstrated experimentally as well as theoretically, but for substituted beta-diketone enols this correlation is relatively weak.