FTIR STUDIES OF HYDROGEN-BONDING BETWEEN ALPHA,BETA-UNSATURATED ESTERS AND ALCOHOLS

The enthalpy (and entropy) of hydrogen bond formation has been measure d between the ester carbonyl groups of the two alpha,beta-unsaturated esters thienylacryloyl (TAOMe) and 5-methylthienylacryloyl (5MeTAOMe) methyl ester and the hydrogen bond donors ethanol, phenol and 3,5-dich lorophenol in CCl4. For the esters, the hydrogen bonding strengths wer e measured by quantitating the amount of bound and unbound donor, usin g the O-H stretching region, as a function of temperature and applying the van't Hoff equation. The decrease in nu(C=O) of the ester carbony l group upon hydrogen bond formation (Delta nu(C=O)) has also been mea sured and correlated with the enthalpy of hydrogen bond formation. A l inear correlation is observed between the enthalpy of hydrogen bond fo rmation -Delta H and Delta nu(C=O), with -Delta H = 1.36 Delta nu(C=O) - 16.1, where Delta H is measured in kJ mol(-1) and Delta nu in cm(-1 ) Comparison with data for other carbonyl acceptor compounds indicates that the carbonyl group of the above alpha,beta-unsaturated esters is more readily polarized than the carbonyl group of saturated esters or ketones, The quantitative relationship between -Delta H and Delta nu( C=O) derived here has been used to determine the change in the enthalp y of hydrogen bond formation between substrate and enzyme groups in a series of acylserine proteases.