Equilibria hydrogen bonding interactions

The main interaction of the complex of equilibrium 31 is probably a hydrogen bonding interaction between nitrofluorobenzenes and some amines. These complexes are more stable when the nitro groups are in position 2 of fluorobenzene than when they are in position 4. A reasonable explanation of this trend is the interaction of the ortho nitro group with the amine shown in 110, in which a second hydrogen bond (between the amino and the nitro groups) enhances the interaction. 

Equilibrium 29 regarding the tautomerism of pentane-2,4-dione (143) was found to have been completely shifted towards the enol form by a tertiary amine. The predominance of the enol form can be attributed to strong hydrogen-bonding interactions with the amine as shown in 145 and 146. Low polarity of solvents also favours the enol form 144. 

When the solvent cage contained more than 15% water, hydrogen bonding interactions shifted the tautomeric equilibrium to the thiol tautomers. The supermolecule solute-water seemed to be of 1 n type. Standing at ambient laboratory conditions in the dark favored the dithione tautomeric form, while indirect sunlight favored the monothiol... 

The SM2/AM1 model was used to examine anomeric and reverse anomeric effects and allowed to state that aqueous solvation tends to reduce anomeric stabilization [58]. Moreover, SM2/AM1 and SM3/PM3 models were accounted for in calculations of the aqueous solvation effects on the anomeric and conformational equilibria of D-glucopy-ranose. The solvation models put the relative ordering of the hydroxymethyl conformers in line with the experimentally determined ordering of populations. The calculations indicated that the anomeric equilibrium is controlled primarily by effects that the gauche/trans 0-C6-C5-0 hydroxymethyl conformational equilibrium is dominated by favorable solute-solvent hydrogen bonding interactions, and that the rotameric equilibria were controlled mainly by dielectric polarization of the solvent [59]. On the other hand, Monte Carlo results for the effects of solvation on the anomeric equilibrium for 2-methoxy-tetrahydropyran indicated that the AM1/SM2 method tends to underestimate the hydration effects for this compound [60]. 

There are two enol forms (center and right) that are in equilibrium with each other. Both enol forms benefit from intramolecular hydrogen-bonding interactions. 

The ratio of yHF to yDF was assumed to be 21/4 in deriving Eqn. (10). For yHF — 20-30°,Eqn.(10)givesadipolemomentdifferenceof0.03-0.08 D, as observed. Thus, the dipole moment data and the hyperfine interactions support a nearly linear equilibrium hydrogen bond, with large amplitude vibrational displacements from equilibrium. It must be emphasized that none of the experimental evidence discussed so far, including this, precludes the hydrogen bond from having a small departure, on the order of 5°, from linearity at equilibrium. 

Kisiel, Z., Legon, A.C., and Millen, D.J. (1982) Spectroscopic investigations of hydrogen bonding interactions in the gas phse. VII. The equilibrium conformation and out-of-plane bending potential energy function of the hydrogen-bonded heterodimer H20-HF determined from its microwave rotational spectrum. Proc. R. Soc. London Ser. A, 381, 419-442. 

The N—H 7i hydrogen bonding interaction was studied by uv spectrophotometry of the system 1-naphthylamine and the donors reported in Table 2126. The equilibrium constant increases by increasing the number of electron donor substituents on the n donor partner. This behaviour parallels that observed extensively for O—H n interaction between phenols, or alcohols with alkenes or aromatic hydrocarbons127. 

A. The chair conformation is shown in 74. One consequence of a chair conformation is that a pronounced l,3-diaxial interaction exists between a /-butyl group and a hydroxyl group. This can be avoided if the ring adopts a twist-boat conformation 75. The number of molecules that exist to any extent in the twist-boat conformation is small, and 75 has the benefit of a hydrogen-bonded interaction between the two hydroxyl groups. The twist-boat conformation is ca, 6 kJ mol 1 more stable than the boat conformation. In this work, Stolow8 claims an equilibrium between 74 and 75. Another molecule known to exist in a twist-boat conformation is cyclohexane-1,4-dione (76) (see Hoffmann and Hursthouse9). 

O Shea K, Kirmse K, Fox MA, Johnston KP. Polar and hydrogen-bonding interactions in supercritical fluids effects on the tautomeric equilibrium of 4-(phenylazo-l-naphthol). J Phys Chem 1991 95 7863. 

---