Assessment of the Presence and Strength of H-Bonds by Means of Corrected NMR

Abstract

Abstract:The downfield shift of the NMR signal of the bridging proton in a H-bond (HB) is composed of two elements. The formation of the HB causes charge transfer and polarization that lead to a deshielding. A second factor is the mere presence of the proton-accepting group, whose electron density and response to an external magnetic field induce effects at the position of the bridging proton, exclusive of any H-bonding phenomenon. This second positional shielding must be subtracted from the full observed shift in order to assess the deshielding of the proton caused purely by HB formation. This concept is applied to a number of H-bonded systems, both intramolecular and intermolecular. When the positional shielding is removed, the remaining chemical shift is in much better coincidence with other measures of HB strength.

NMR and IR Investigations of Strong Intramolecular Hydrogen Bonds

“…For applications of DFT theory in calculations of 1 H chemical shifts of intramolecular hydrogen bonds, see the work by Siskos et al [ 10 , 32 ]; these authors use OH chemical shifts to predict the OH proton positions [ 10 ]. Most recently, Scheiner [ 11 ] has published a computational analysis of the downfield shift of the NMR signal of the bridging proton in a hydrogen bond. There is an interplay between the shielding effects caused by charge transfer and polarization due to formation of the hydrogen bond, and those due to the mere presence of the proton-accepting group.…”

“…The nature of the hydrogen bond is an issue of great current interest [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 ]. The title of this paper includes the term “strong intramolecular hydrogen bonds”.…”

“…The definition of a strong intramolecular hydrogen bond can therefore be based on the two abovementioned criteria. However, in the case of XH chemical shifts, these should be corrected for possible ring current effects and anisotropy contributions (see later) [ 11 , 12 ]. Taking this into account, we have adopted an experimental approach for XH = OH: 2800 cm −1 > ν OH > 1800 cm −1 and 19 ppm > δ OH > 15 ppm (see below).…”

“…For large aromatic systems the effects may be larger [ 30 ]. Recently, ring current effects have been estimated on OH chemical shifts in a series of intramolecular hydrogen bonded cases [ 11 , 12 ]. Furthermore, for intramolecular hydrogen bonds as described in Scheme 2 an anisotropy effect may also be at play.…”

Special Issue: Intramolecular Hydrogen Bonding 2017

“…Bilonda and Mammino apply their calculations to quinine [ 35 ], and the naphthalene series is examined by Sanchez-Sanz et al who compare H-bonds to their chalcogen analogues [ 36 ]. Scheiner explains how quantum calculations of NMR data can provide unambiguous evidence of H-bonds, as well as their strength [ 37 ]. In addition to quantum calculations, methods of dynamics can also be useful, as explained by Huang et al [ 38 ].…”