Strength Of Intramolecular Hydrogen Bond Research Articles

Raman study of hydrogen bonding in the 2-haloethanols

The high frequency (3000–3700 cm −1) Raman spectra of the 2-haloethanols (XCH 2CH 2OH 2 X  F, Cl, Br and I) were studied as a function of temperature in the neat liquid phase. The two bands in this region, at 3300 and 3600 cm −1, were assigned to the valence mode of inter- and intramolecularly hydrogen bonded OH groups respectively. The intensity ratio, I intra/I inter, determined from the resolved band parameters, increases with temperature; calculated values of ΔH exhibit the trend, IBr>Cl>F. A simple picture is introduced which shows these results to be consistent with the order of intramolecular hydrogen bond strengths deduced from an earlier investigation of these alcohols in dilute solution.

Rates of proton transfer, acid dissociation constants, and the strengths of intramolecular hydrogen bonds in 8-methoxy-NN-dimethyl-1-naphthylammonium ion and 8-hydroxy-NN-dimethyl-1-naphthylamine

The rate coefficients for proton transfer from 8-hydroxy-NN-dimethyl-1-naphthylamine in 80%(v/v) dimethyl sulphoxide–water and from 8-methoxy-NN-dimethyl-1-naphthylammonium ion in aqueous solution are respectively 104 fold and 20–50 fold lower than expected for normal proton transfers from a hydroxy- and protonated amino-group. The low rates are due to intramolecular hydrogen bonds in the acids and by comparison with results for protonated 1,8-bis(dimethylamino)naphthalene, the rates show that in this series of peri-substituted naphthalenes the order of hydrogen bond strengths is [graphic omitted]–H ⋯ O < O–H ⋯ N < [graphic omitted]–H ⋯ N. The intramolecular hydrogen bonds also account for the unusual acidities: 8-methoxy-NN-dimethyl-1-naphthylammonium ion (pK 7.75) is a weaker acid by three pK units than NN-dimethyl-1-naphthylammonium ion and 8-hydroxy-NN-dimethyl-1-naphthylamine (pK ca. 14.9) is six pK units weaker than 1-naphthol.

Proton Magnetic Resonance Study of Intramolecular Hydrogen Bonding in Halophenols

The long-range spin–spin coupling constants over five bonds between the hydroxyl proton and the ring protons in a series of trihalophenols imply that the intramolecular hydrogen bond strength (negative enthalpy) to fluorine is greater than that to iodine by 75 ± 20 cal/mol, whereas the strengths to chlorine and bromine are 460 ± 60 cal/mol greater than to iodine. If a distinction can be made between chlorine and bromine, then chlorine–hydrogen bonds more strongly by only a few tens of calories per mol. The measurements were made mainly on dilute solutions in carbon tetrachloride at 32 °C.

Proton-ligand stability constants of some methyl-substituted ortho-hydroxy acetophenones and their oximes

The proton-ligand stability constants of 3-methyl, 4-methyl and 5-methyl derivatives of 2-hydroxyacetophenone and its oxime have been determined by Bjerrum-Calvin pH-titration technique in a water-dioxane (25–75% by volume) medium of ionic strength of 0·1 M (KNO 3) and at 40°C. The influence of methyl substitution is discussed in terms of its inductive effects and the strength of intramolecular hydrogen-bonding in these compounds.