Acidities and basicities of carboxylic acids. Correlations between core-ionization energies, proton affinities, and gas-phase acidities

Abstract

Core-ionization energies for a number of carboxylic acids and esters have been measured with the view of establishing the relative importance of initial-state charge distribution and final-state charge rearrangement in determining core-ionization energies, proton affinities, and gas-phase acidities. Results are reported for RCOOH (R = H, CH/sub 3/, CF/sub 3/, CF/sub 2/H), for RCOOCH/sub 3/ (R = H, CH/sub 3/, CF/sub 3/), for RCOOC/sub 2/H/sub 5/ (R = H, CH/sub 3/, CF/sub 3/, CF/sub 2/H, CFH/sub 2/, CClH/sub 2/, CBrH/sub 2/), and for acetone and methyl carbonate. A linear correlation with slope -1.6 is found between core-ionization enegies and proton affinities for double-bonded oxygen. Compounds in which initial-state effects are important and those in which final-state effects are important fit the correlation line equally well. Although there is no simple correlation between anion proton affinity (the negative of acidity) and core-ionization energies, the data are consistent with an expected negative correlation when inductive effects are important and a positive correlation when relaxation is important. A quantitative analysis of these results is in good agreement with theoretical results obtained by Davis and Shirley. Correlations among the various core-ionization energies indicate that positive charge on a carboxyl group, whether produced by coremore » ionization or by protonation, is extensively delocalized by valence-electron rearrangement.« less