FTIR studies of hydrogen bonding between α, β-usaturated esters and alcohols

The enthalpy (and entropy) of hydrogen bond formation has been measured between the ester carbonyl groups of the two α, β-unsaturated esters thienylacryloyl (TAOMe) and 5-methylthienylacryloyl (5MeTAOMe) methyl ester and the hydrogen bond donors ethanol, phenol and 3,5-dichlorophenol in CCl₄. For the esters, the hydrogen bonding strengths were measured by quantitating the amount of bound and unbound donor, using the O-H stretching region, as a function of temperature and applying the van't Hoff equation. The decrease in v_C=O of the ester carbonyl group upon hydrogen bond formation (Δv_C=O) has also been measured and correlated with the enthalpy of hydrogen bond formation. A linear correlation is observed between the enthalpy of hydrogen bond formation -ΔH and Δv_C=O, with -ΔH = 1.36Δv_C=O - 16.1, where ΔH is measured in kJ mol^-1 and Δv in cm^-1. Comparison with data for other carbonyl acceptor compounds indicates that the carbonyl group of the above α, β-unsaturated esters is more readily polarized than the carbonyl group of saturated esters or ketones. The quantitative relationship between -ΔH and Δv_C=O derived here has been used to determine the change in the enthalpy of hydrogen bond formation between substrate and enzyme groups in a series of acylserine proteases.