Intramolecular hydrogen bonding and molecular conformation. Part 4.—Corrections of thermodynamic properties obtained by IR measurements at different temperatures and pressures

Abstract

Corrections have been examined for infrared measurements of the enthalpy change, AH, and the partial molar volume change, AV, for the rupture of an intramolecular hydrogen bond of some hydroxyketones and ohalophenols in dilute solutions. Expressions of AH and AV were given by allowing for the temperature and pressure dependences of these quantities, and the molar integrated intensities. The temperature and pressure coefficients of the molar integrated intensities were assumed to be given by experimental values of model compounds which are similar to the molecules concerned, but which do not exhibit conformational equilibrium. It has been found that the corrections are not small enough to be negligible, compared with the values obtained by the usual procedures.Solvation is an important factor in determining the conformation of a wide variety of molecules in solution, from simple to complex biological m~lecules.'-~ Molecules with weak to moderate intramolecular hydrogen bonds are particularly interesting. Usually, the functional groups (OH, NH, , C=O etc.) which take part in hydrogen bonding interact with the solvent fairly strongly and, therefore, conformations involving intramolecular hydrogen bonds will be easily influenced by solvation. The effects of solvation on these intramolecular hydrogen bonds are characterized by changes in the enthalpy and in the partial molar volume, A H and AV, respectively, for their rupture. These properties can be conveniently obtained from analysis of the effects of temperature and pressure on the IR spectra. The observed values are usually given by4-6where R is the gas constant, and I , and are the integrated intensities of the bands due to the hydrogen-bond-free (HBfree) and the intramolecularly hydrogen-bonded (HB) conformers, respectively. Usually, observed values of ln(I,/I,) are approximately linear with both 1/T and p, and the slopes of these plots provide reasonable values of AHob, and A%,,, which are useful for examining the effect of s~lvation.'-~ However, it should be remembered that the above relationships are derived on the basis of the following assumptions:(1) the ratio of the molar integrated intensities, af/c(b, is independent of both temperature and pressure; (2) A H is independent of temperature; (3) A V is independent of pressure. These assumptions are not always valid and this may affect the values of A H and A V to some extent. The molar integrated intensities will, more or less, vary with temperature and pressure. This is partly due to changes in the effect of the internal dielectric field,8 which depends on the solvent density. Both A H and AV, which are good measures of the state of solvation, should also be dependent on both temperature and -f Part 3: Ref. 7.

Intramolecular Hydrogen Bonding and Conformation of Small Peptides: Variable-Temperature FTIR Study on N-Acetyl-l-Pro-l-Leu-Gly-NH2 and Related Compounds

“…Thermodynamic properties of the HBd conformers are obtained from analysis of the band intensities as a function of temperature. , Figure shows van't Hoff plots for Ac-PLG-NH 2 in CDCl 3 . Intensity ratios of the 3412-cm -1 band, which is assigned to the HB-free NH of the C-terminal amide, to each of the two hydrogen-bonded bands, I f / I b , were plotted against inverse absolute temperature.…”

“…Entropy difference between two conformers is given by 12 if the ratio of the molar integrated intensities, α 1 /α 2 , is given. This ratio could be given by a slope of a I 1 vs I 2 plot, but unfortunately it is usually difficult to estimate owing to temperature dependence of the molar integrated intensities . It is well-known that the molar integrated intensities of the N−H or O−H stretching bands increase remarkably when the NH or OH groups take part in hydrogen bonds .…”

“…It is well-known that the molar integrated intensities of the N−H or O−H stretching bands increase remarkably when the NH or OH groups take part in hydrogen bonds . In addition, the local environment of the NH or OH bonds changes with the hydrogen bonding and so does the temperature coefficients of the intensities . Therefore, it is difficult to estimate Δ S between HB-free and HBd conformers.…”

Temperature and pressure effects on conformational equilibria of alanine dipeptide in aqueous solution

“…The ⌬H values are listed in Table I. Using the skeletal stretching mode of AAlaMA in chloroform, ⌬H In previous study, Yasuda et al 34 reported the temperature dependence of the IR absorbance of the OH stretching for 4-hydroxy-4-methylpentan-2-one in chloroform solution. They estimated the enthalpy change accompanying the rupture of their intramolecular hydrogen bond, ⌬H (free3 HBd) , at Ϫ8.4 Ϯ 0.2 kJ/mol.…”

Conformational Analysis of Quaternary Ammonium-Type Ionic Liquid Cation, N,N-Diethyl-N-methyl-N-(2-methoxyethyl) Ammonium Cation

“…Finally, we mention the relation of the present calculation of [DEME] + monomer in the gas phase and in the liquid state such as [DEME][X](X = BF 4 and I). The average ΔE between the folded and extended forms of [DEME] + is about 10 kJ/mol, and this energy corresponds to the energy of the hydrogen-bonded formation of molecules in the liquid state [30]. Therefore, the energy balance between the conformational equilibrium of [DEME] + in the liquid state may be different with that in the gas phase.…”