Covalent versus Electrostatic Nature of the Strong Hydrogen Bond:  Discrimination among Single, Double, and Asymmetric Single-Well Hydrogen Bonds by Variable-Temperature X-ray Crystallographic Methods in β-Diketone Enol RAHB Systems

Abstract

Beta-diketone enols are known to form intramolecular...O=C-C=C-OH... resonance-assisted hydrogen bonds (RAHBs) with O...O distances as short as 2.39-2.44 A. However, even the most accurate diffraction studies have not been able to assess with certainty whether these very strong hydrogen bonds (H-bonds) are to be described as proton-centered O...H...O bonds in a single-well (SW) potential or as the dynamic or static mixing of two O-H...O <= => O...H-O tautomers in a double-well (DW) one. This contribution reexamines the problem and shows that diffraction methods are fairly able to assess the SW or DW nature of the H-bond formed and, in the second case, its dynamic or static nature, provided a Bayesian approach is used which associates a number of experimental techniques (X-ray crystallography at variable temperature, difference Fourier maps, least-squares refinement of proton populations, Hirshfeld's rigid-bond test) with a reasonable prior, that is the full set of possible proton-transfer (PT) pathways for the O-H...O system derived from theoretical calculations. The method is first applied to three beta-diketone enols, whose crystal structures were determined in the interval of temperatures 100-295 K and then generalized to the interpretation of a much wider set of beta-diketone enol structures derived from the literature, making it possible to establish a general relationship between chemical structure (symmetric or dissymmetric substitution, steric compression or stretching, increased pi-bond delocalizability), H-bond strength, and the shape of the PT-barrier. Final results are interpreted in terms of simplified VB theory and state-correlation (or avoided-crossing) diagrams.

How resonance assists hydrogen bonding interactions: An energy decomposition analysis

“…6,8 This view has been challenged recently as more and more strong and unconventional hydrogen bonds have been recognized. [9][10][11][12] As a matter of fact, several types of strong hydrogen bonds have been investigated, for example, charge assisted hydrogen bonds (CAHBs), 12,13 low barrier hydrogen bonds (LBHBs), 14,15 dihydrogen bonds (DHBs), 8,16 and resonance-assisted hydrogen bonds (RAHBs). [10][11][12] In a CAHB, a positive or negative charge on the proton donating or accepting group remarkably increases the strength of the hydrogen bond.…”

“…8 RAHB highlights the co-operativity between the -electron delocalization and hydrogen bonds and the term was coined by Gilli and coworkers in the late 1980s, who have continued to refine their theory through a series of papers. [10][11][12] In RAHB, the hydrogen bond donor and acceptor atoms are connected through -conjugated double bonds. A typical example of RAHB is -diketone enols which form intramolecular ÁÁÁO¼ ¼CÀ ÀC¼ ¼CÀ ÀOHÁÁÁ hydrogen bonds enhanced by the resonance with OÁÁÁO distances as short as 2.39-2.44 Å .…”

“…A typical example of RAHB is -diketone enols which form intramolecular ÁÁÁO¼ ¼CÀ ÀC¼ ¼CÀ ÀOHÁÁÁ hydrogen bonds enhanced by the resonance with OÁÁÁO distances as short as 2.39-2.44 Å . 12 The shortening of the hydrogen bonds is associated with a decrease of OÀ ÀH vibrational frequencies and abnormal downfield 1 H NMR chemical shifts. 11 Heteronuclear NÀ ÀHÁÁÁO RAHBs have a crucial role in protein folding and DNA pairing.…”

Nanomechanical strength mechanisms of hierarchical biological materials and tissues

“…Several theories exist that describe competing processes due to mechanically induced instabilities of protein structures. These concepts stem primarily from a theory originally postulated by Bell (1978), Evans and Ritchie (1997), Gilli et al (2004), Dudko et al (2006) and Wiita et al (2006).…”

“…Bell's theory is one of the most widely used models to describe the statistical nature of bond breaking. In Bell's theory (Bell 1978;Evans and Ritchie 1997;Gilli et al 2004;Dudko et al 2006;Wiita et al 2006), the off rate x is the product of the natural bond vibration frequency, v 0 , and the quasi-equilibrium likelihood of reaching the transition state with an energy barrier E b that is reduced by mechanical energy F·x b ·cos(u), where F is the applied force, x b is the distance between the equilibrated state (minimum of the well) and the transition state, and u is the angle between the direction of the reaction pathway of bond breaking (x-direction) and the direction of applied load F (see also the schematic shown in Figure 4). The angle can be determined by analysing the molecular geometry.…”