Correlation of observed and model vibrational frequencies for aqueous organic acids: UV resonance Raman spectra and molecular orbital calculations of benzoic, salicylic, and phthalic acids

The aromatic carboxylic acids benzoic, salicylic and phthalic acid were used to study the interaction of soluble organics compounds with metal cations. To accomplish this, we have developed methods for studying the carboxylic acids using UV resonance Raman (UVRR) combined with molecular orbital density functional theory calculations. The pH values of the acid solutions were based on the pKa's for the different acids to examine the neutral and charged species. Deprotonation of the organic acids was detectable down to 10−4 M using UVRR (two orders of magnitude lower than previous vibrational spectroscopy studies). Limitations to decreasing the concentration lower using the current UVRR facilities are discussed. Two methods were used to calculate the optimized geometry and frequencies of the acids: explicit and continuum solvation. The frequencies from the experimental spectra were then compared to the theoretical results obtained from the two methods.