Theoretical studies on the pK~a~ values of perfluoroalkyl carboxylic acids: Chain helicity

Abstract

AbstractSemiempirical, ab initio, and density functional theory (DFT) studies were conducted on a homologous series of straight chain perfluoroalkyl carboxylic acids (PFCAs) ranging in chain length from C~1~ (trifluoroacetic acid; TFA) to C~9~ (perfluorodecanoic acid). Regardless of perfluoroalkyl chain length and computational method and level employed, no significant change in the electrostatic or steric characteristics of the carboxylate head group was found between C~2~ and C~9~. Application of a thermodynamic cycle to calculations at the B3LYP/6-311++G(3df,2p) level provided an accurate prediction of the experimental monomeric pK~a~ for TFA. At this – and similar – levels of theory, monomeric pK~a~ values of longer straight chain PFCAs are expected to display no significant increase with increasing chain length, having a relatively constant pK~a~ value of about 0 that is in excellent agreement with earlier predictions and with recent experimental evidence. Contrary to previous claims, perfluoroalkyl chain helicity does not appear to result in increased monomeric PFCA pK~a~ values at chain lengths >5 perfluorocarbons.