Local Acid Strength of Solutions and Its Quantitative Evaluation Using Excess Infrared Nitrile Probes

We propose the concept of local acidity in condensed-phase chemistry in this work. The feature is demonstrated in trifluoroethanol (TFE) by employing two Fourier-transform infrared spectroscopy (FTIR) nitrile probes, acetonitrile (CH₃CN) and benzonitrile (PhCN). Specifically, three positive excess peaks were found in the binary systems composed of TFE and a probe using excess spectroscopy. To characterize the local acidity quantitatively, we have tried to correlate the wavenumbers of the positive excess peaks of the probes and the pK_a values in water of a series of XH-containing compounds (X = O, N, and C). Good linear relationships were discovered. Accordingly, three different pK_a values of TFE were determined based on the three positive excess infrared peaks, which are attributed to the monomer, dimer, and trimer of TFE with the help of quantum-chemical calculations. The concept of local acidity and its quantitative evaluation enrich our knowledge of acid-base chemistry and will shed light on a better understanding of microstructures of solutions.