Interpretation of Spectroscopic Markers of Hydrogen Bonds

Abstract

Quantum calculations are used to examine whether an AH⋅⋅⋅D H-bond is unambiguously verified by a downfield shift of the bridging proton's NMR signal or a red (or blue) shift of the AH stretching frequency in the IR spectrum. It is found that such IR band shifts will occur even if the two groups experience weak or no attractive force, or if they are drawn in so close together that their interaction is heavily repulsive. The mere presence of a proton-acceptor molecule can affect the chemical shielding of a position occupied by a protondonor by virtue of its electron density, even if there is no H-bond present. This density-induced shielding is heavily dependent on position around the proton-acceptor atom, and varies from one group to another. Evidence of a hydrogen bond rests on the measurement of a proton deshielding in excess of what is caused purely by the presence of the proton acceptor species.

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

“…The method is based [49] on the notion that when placed between two atoms that are potentially H-bonded to one another, the chemical shielding of the bridging proton derives from two separate phenomena. In the first place, there is what might be called a positional effect σ p .…”

“…An attempt was made in this direction recently [49] in a study which considered the effects of HBs on IR and NMR spectral data. It was shown that the red shift of the A-H··B HB is not necessarily indicative of an attractive interaction.…”

“…Likewise, red shifts were observed even when the two molecules were rotated so as to void the normal alignment of a HB. The NMR chemical shielding of the bridging proton was shown to have greater potential in this regard [49], following earlier attempts [45] to utilize induced ring currents. However, before this quantity can be used to assess HB strength, it must first be cleansed of a spurious artifact which can obscure its relation to the H-bonding phenomenon itself.…”

Hydrogen Atomic Positions of O–H···O Hydrogen Bonds in Solution and in the Solid State: The Synergy of Quantum Chemical Calculations with 1H-NMR Chemical Shifts and X-ray Diffraction Methods

“…The concept, therefore, of a three center – four electron sharing due to resonance type n B → interaction: O–H: O ↔ O: H–O may be the basis for a revised IUPAC definition of the hydrogen bond [ 89 ]. This linear correlation between Wiberg bond order and δ( 1 H) may resolve the ambiguity that a deshielded bridging proton signal is a necessary but not sufficient demonstration of a hydrogen bond [ 90 ]. As pointed out by Scheiner [ 90 , 91 ], deshielding effects can be observed in cases of the proximity of an electron cloud of a second molecule even if no hydrogen bond is present.…”

“…This linear correlation between Wiberg bond order and δ( 1 H) may resolve the ambiguity that a deshielded bridging proton signal is a necessary but not sufficient demonstration of a hydrogen bond [ 90 ]. As pointed out by Scheiner [ 90 , 91 ], deshielding effects can be observed in cases of the proximity of an electron cloud of a second molecule even if no hydrogen bond is present.…”