Strength Of Intermolecular Hydrogen Bonds Research Articles

Infra-red studies and thermodynamics of hydrogen bonding in ethylene glycol monoalkyl ethers. Evidence for a ten membered ring dimer

An i.r. study in the hydroxyl stretching region of three monoalkyl (methyl, ethyl and butyl) ethers of ethylene glycol in CCl4 solutions (concentrations up to 0.1 mol dm–3) has been carried out over the temperature range 298–338 K. Apparent integrated intensities of the bands at ∼3635, ∼3600 and ∼3475 cm–1 of the free, intramolecularly and intermolecularly bonded OH groups have been measured and used to evaluate the equilibria between the monomeric and associated species. Various possible structures including open and cyclic dimers, trimers and tetramers for the associated species absorbing at ∼3475 cm–1 have been considered. A cyclic ten-membered ring dimer involving the ether oxygens in hydrogen bonding satisfactorily explains the observed results. Various thermodynamic quantities such as K, ΔH°ΔG° and ΔS° in the three way equilibrium involving free monomer, intramolecularly bonded monomer and the cyclic ten membered ring dimer have been evaluated. The values of intra- and inter-molecular hydrogen bond strengths as calculated from the respective ΔH° values are ∼14.1 kJ mol–1 and ∼28.2 kJ mol–1 respectively.

Molecular forces governing non-electrolyte permeation through cell membranes

The relations between the chemical structure of non-electrolytes and their ability to permeate cell membranes are analysed at the level of molecular forces, using the measurements of reflexion coefficients in gall-bladder epithelial cells tabulated in the preceding paper. Stronger solute:water forces and weaker solute:membrane forces are associated with lower permeating power. The portions of the membrane controlling non-electrolyte permeation behave as nearly pure hydrocarbons with very few hydrogen-bonding sites. Most substituents (hydroxyl, ether, carbonyl, ester, amino, amide, urea, nitrile) are shown to decrease permeation in proportion to the number and strength of intermolecular hydrogen bonds which they form with water, while intramolecular hydrogen bonding accelerates permeation. Carbon-carbon double bonds and triple bonds and aromatic residues decrease permeability due to hydrogen bonds involving π electrons. Inductive effects, in which a substituent indirectly modifies permeability by withdrawing or releasing electrons at an adjacent hydrogen bonding site, are most noticeable for halogens, the nitro group, double and triple bonds, and branched alkyl groups. Altered forces between membrane hydrocarbons and the solute retard the permeation (weaker forces) of fluorine compounds and branched compounds, and slightly accelerate the permeation (stronger forces) of other halogen derivatives and compounds with long carbon chains. The main factor in the increase of permeability with increasing hydrocarbon chain length is an entropy effect associated with a change in local water structure; and this effect is partly responsible for the decrease in permeability with chain branching, whose origin is particularly complex.

Molecular forces governing non-electrolyte permeation through cell membranes.

The relations between the chemical structure of non-electrolytes and their ability to permeate cell membranes are analysed at the level of molecular forces, using the measurements of reflexion coefficients in gall-bladder epithelial cells tabulated in the preceding paper. Stronger solute: water forces and weaker solute: membrane forces are associated with lower permeating power. The portions of the membrane controlling non-electrolyte permeation behave as nearly pure hydrocarbons with very few hydrogen-bonding sites. Most substituents (hydroxyl, ether, carbonyl, ester, amino, amide, urea,nitrile) are shown to decrease permeation in proportion to the number and strength of intermolecular hydrogen bonds which they form with water, while intramolecular hydrogen bonding accelerates permeation. Carbon-carbon double bonds and triple bonds and aromatic residues decrease permeability due to hydrogen bonds involving nelectrons. Inductive effects, in which a substituent indirectly modifies permeability by withdrawing or releasing electrons at an adjacent hydrogen bonding site, are most noticeable for halogens, the nitro group, double and triple bonds, and branched alkyl groups. Altered forces between membrane hydrocarbons and the solute retard the permeation (weaker forces) of fluorine compounds and branched compounds, and slightly accelerate the permeation (stronger forces) of other halogen derivatives and compounds with long carbon chains. The main factor in the increase of permeability with increasing hydrocarbon chain length is an entropy effect associated with a change in local water structure; and this effect is partly responsible for the decrease in permeability with chain branching, whose origin is particularly complex.

Studies on the Infrared Spectra of Heterocyclic Compounds. VI. Infrared Spectra of Substituted α-Pyridones and α-Quinolones. (1). The Region from 2000 to 4000cm<SUP>-1</SUP>.

The infrared spectra of a number of substituted α-pyridones and α-quinolones were investigated in the region of 2000 to 4000cm-1, and the characteristic behavior of NH absorption in α-pyridone structure was pointed out and discussed in terms of the mode of molecular association. Extremely complex and broad nature extending from 3300 down to 2400cm-1 of the bonded NH absorption was interpreted as being due to the extreme strength of intermolecular hydrogen bond associated through a cyclic dimer. Abnormal stability of the dimer, being approximately equal to that of carboxylic acid dimer, was pointed out and discussed in terms of resonance stabilization. Ratio of vNH/vND for strongest maxima was found to be 1.27 to 1.30 and 1.34 to 1.36, respectively, for dimeric and non-dimeric α-pyridone structure. Characteristic behavior in 8-substituted carbostyrils was also pointed out.