AQUEOUS INFRARED CARBOXYLATE ABSORBANCES - ALIPHATIC DI-ACIDS

Aqueous attenuated total reflectance Fourier transform infrared (ATR-F TIR) spectra of 18 aliphatic di-carboxylic acids are reported as a fun ction of pH. The spectra show isosbestic points and intensity changes which indicate that Beer's law is obeyed, and peak frequencies lie wit hin previously reported ranges for aqueous carboxylates and pure carbo xylic acids. Intensity sharing from the symmetric carboxylate stretch is evident in many cases, so that bands which are nominally due to alk yl groups show increased intensity at higher pH. The asymmetric stretc h of the HA(-) species is linearly related to the microscopic acidity constant of the H(2)A species, with sigma(pK) < 0.25 log units; this r elationship falls on the same line as previously observed for mono-car boxylic acids. The linear relationship applies to the acidity constant of the HA(-) species only when the two acid groups are well separated (> 2 intervening atoms). The results suggest that aqueous ATR-FTIR ma y be able to estimate 'intrinsic' pK(a) values of carboxylic acids, in addition to providing quantitative estimates of ionization. (C) 1998 Elsevier Science B.V. All rights reserved.