Symmetries of hydrogen-bonded enol forms of diketones as determined by x-ray photoelectron spectroscopy

"Strong" hydrogen bonds in chemistry and biology

Hydrogen bonds are a key feature of chemical structure and reactivity. Recently there has been much interest in a special class of hydrogen bonds called "strong" or "low-barrier" and characterized by great strength, short distances, a low or vanishing barrier to hydrogen transfer, and distinctive features in the NMR spectrum. Although the energy of an ordinary hydrogen bond is ca 5 kcal mol-1, the strength of these hydrogen bonds may be > or = 10 kcal mol-1. The properties of these hydrogen bonds have been investigated by many experimental techniques, as well as by calculation and by correlations among those properties. Although it has been proposed that strong, short, low-barrier hydrogen bonds are important in enzymatic reactions, it is concluded that the evidence for them in small molecules and in biomolecules is inconclusive.

Role of Chelation and Resonance on the Intrinsic Acidity and Basicity of Tropolone

The gas-phase basicity and acidity of tropolone have been determined by Fourier transform ion cyclotron resonance mass spectrometry (FT ICR) techniques. Ab initio calculations at the MP2/-6311+G(d,p) level were carried out to describe the effects that protonation and deprotonation have on the aromaticity and hence on the stability of the system. Experimental and calculated energetics of protonation and deprotonation are in excellent agreement. Our analysis shows that both the protonated and the deprotonated species are stabilized by resonance. As a consequence the acidity of tropolone (341.3 kcal/mol) is enhanced, and this compound is found to be surprisingly as acidic as benzoic acid. This is in good agreement with the results reported in the literature and obtained in DMSO solutions. The intramolecular hydrogen bond becomes significantly weaker upon protonation, and this effect tends to counterbalance the resonance stabilization of the cation. As a consequence, tropolone is found to be slightly less basic than tropone.