Vibrational Stark fields in carboxylic acid dimers

Abstract

Carboxylic acids form exceptionally stable dimers and have been used to model proton and double proton transfer processes. The stabilization energies of the carboxylic acid dimers are very weakly dependent on the nature of substitution. However, the electric field experienced by the OH group of a particular carboxylic acid is dependent more on the nature of the substitution on the dimer partner. In general, the electric field was higher when the partner was substituted with an electron-donating group and lower with an electron-withdrawing substituent on the partner. The Stark tuning rate (Δ) of the O–H stretching vibrations calculated at the MP2/aug-cc-pVDZ level was found to be weakly dependent on the nature of substitution on the carboxylic acid. The average Stark tuning rate of O–H stretching vibrations of a particular carboxylic acid when paired with other acids was 5.7 cm⁻¹ (MV cm⁻¹)⁻¹, while the corresponding average Stark tuning rate of the partner acids due to a particular carboxylic acid was 21.9 cm⁻¹ (MV cm⁻¹)⁻¹. The difference in the Stark tuning rate is attributed to the primary and secondary effects of substitution on the carboxylic acid. The average Stark tuning rate for the anharmonic O–D frequency shifts is about 40–50% higher than the corresponding harmonic O–D frequency shifts calculated at the B3LYP/aug-cc-pVDZ level, much greater than the typical scaling factors used, indicating the strong anharmonicity of O–H/O–D oscillators in carboxylic acid dimers. Finally, the linear correlation observed between pK_a and the electric field was used to estimate the pK_a of fluoroformic acid to be around 0.9.