NMR and IR Investigations of Strong Intramolecular Hydrogen Bonds

Abstract

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

Quantitative decomposition of resonance‐assisted hydrogen bond energy in β‐diketones into resonance and hydrogen bonding (π‐ and σ‐) components using molecular tailoring and function‐based approaches

“…For comparison, the O⋯O distance is longer up to 1 Å for the other intramolecular O─H⋯O H‐bonding motifs . From spectroscopic viewpoint, the abnormal lowering in the v O─H frequency (2566–2675 cm −1 ) and downfield shift of the bridge hydrogen resonance (15.3–17.0 ppm) are observed in the enol β‐diketones compared, for instance, with the same spectral parameters in the phenol (4605 cm −1 and 4.67 ppm, respectively). Based on the experimental data combined with theoretical calculations, Gilli et al proposed the RAHB concept to describe strong H‐bond in the molecules where the donor and acceptor atoms are separated by the conjugated single and double bonds …”

“…[7] For comparison, the OÁ Á ÁO distance is longer up to 1 Å for the other intramolecular O─HÁ Á ÁO H-bonding motifs. [8] From spectroscopic viewpoint, the abnormal lowering in the v O─H frequency (2566-2675 cm −1 ) and downfield shift of the bridge hydrogen resonance (15.3-17.0 ppm) are observed in the enol β-diketones [9,10] compared, for instance, with the same spectral parameters in the phenol (4605 cm −1 and 4.67 ppm, respectively [11] ).…”

Hydrogen‐Bond Structure and Dynamics of TADDOL Asymmetric Organocatalysts Correlate with Catalytic Activity

“…Consistent with this assignment, we find the integrals of these ranges to have an approximate 28:2:2:6 ratio. The chemical shift of OH protons is ap articularly sensitive reporter for the bonding state [17][18][19] and allows assessing the intramolecular hydrogen-bond strength of diols. [15,16] To obtain information on the hydrogen-bonding characteristics of both TADDOL catalysts we investigate the chemical-shift changes of the hydroxyl protons of TADDOL in the NMR spectra in more detail.…”

“…[17,21,[24][25][26][27][28] For similar diols that form an intramolecular hydrogen-bond, ab road lower fre-quencyO Hs tretching band, whichi sr ed-shifted due to the intermolecular hydrogen-bond ("bound OH"), and ab lue-shifted OH stretching mode, with the OH group exposed to the solvent ("free OH") could be discriminated. In contrast to NMR (using frequencieso fh undreds of MHz), IR spectroscopy (at frequencies of tens to hundreds of THz) probes the bonding state on shorter timescales.…”

“…[15,16] To obtain information on the hydrogen-bonding characteristics of both TADDOL catalysts we investigate the chemical-shift changes of the hydroxyl protons of TADDOL in the NMR spectra in more detail. The chemical shift of OH protons is ap articularly sensitive reporter for the bonding state [17][18][19] and allows assessing the intramolecular hydrogen-bond strength of diols. [20,21] For 2nTADDOL the OH protons lead to as ingle peak at % 4.8 ppm, consistent with the disappearance of this NMR signalu pon exchanging this proton for deuterium by adding D 2 O( see inset of Figure 2b).…”