Influence of Cation Size on the Structural Features and Interactions in Tertiary Alkylammonium Trifluoroacetates: A Density Functional Theory Investigation.

We present the results of electronic structure calculations based on density functional theory (DFT) in order to investigate the reactions of the interaction of tertiary alkylamines with alkyl groups of different sizes (triethyl, tributyl, dimethylethyl, and diisopropylethyl) with trifluoroacetic acid. We have obtained data on the affinity of the studied amines with a proton. It has been shown that amine interaction with the acid leads to proton transfer from the acid to the amine and formation of ions held together in the ion pair by electrostatic interaction and a very strong hydrogen bond. We have also investigated the energy profiles of the proton transfer from the tertiary alkylammonium cation to the trifluoroacetate anion within the ion pair and found different correlations between the geometric characteristics of the H-bond and the parameters obtained by the natural bond orbitals and quantum theory of atoms in molecules analyses. It has been established that the interionic interactions in these systems weaken as the length and the degree of cation alkyl chain branching increase. A good qualitative agreement between the theoretical results and the experimental data on physicochemical properties has been obtained.