Aqueous infrared carboxylate absorbances: Aliphatic monocarboxylates

Aqueous attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectra of 24 aliphatic monocarboxylates have asymmetric COO- stretch frequencies (nu(as)) of 1541 cm(-1) to 1678 cm(-1), while the symmetric stretch frequencies range from 1202 cm(-1) to 1417 cm(-1). The pK(a) values of these acids are linearly correlated with nu(as) according to the equation (pK(a) = 66.70-0.04018 nu(as) sigma = 0.266 pK(a) units, r(2) = 0.968). This correlation should be useful in determining the pK(a) values of acids in complex mixtures and the site pK(a) values of carboxyl groups in polyacids. Semi-empirical molecular orbital calculations using the AMI parameter set give nu(as) values which correlate well with experimental nu(as) (sigma = 11.3 cm(-1)), although model nu(as) values are approximate to 500 cm(-1) too high; the PM3 parameter set predicts nu(as) values that are only approximate to 300 cm(-1) too high, but which do not correlate as well with experiment (sigma = 17.6 cm(-1)).