Atom-specific vibrational analysis reveals labile bonds in linear and branched PFOA molecules

We have performed atom-specific vibrational analyses of a large number of perfluorooctanoic acid isomers as well as their anions and show that differences in the vibrational features of corresponding anion and neutral molecules clearly identify the C-F bonds that are most strongly activated in the anions. We discuss two analysis tools for associating vibrational modes with individual atoms, both based on density functional theory calculations. The first involves computing the Einstein frequencies for a given atom and the second projects the full vibrational spectrum onto a given atom using participation factors derived from the normal mode eigenvectors. We show that the two methods give results that are qualitatively the same and that either can be used to identify the most labile C-F bonds in the anions. We also show that the results of the vibrational analyses are consistent with systematically computed F atom removal energies. The vibrational analysis tools are shown to be related to the local mode analysis of Cremer and co-workers. Finally, we compare and contrast branched and linear PFAS molecules and analogous hydrogenated carbon chains.