Atom-Specific Vibrational Analysis Reveals Labile Bonds in Linear and Branched PFOA and its Hydrogenated Analogues

We have performed atom specific vibrational analyses of a large number of perflurooctanoic acid (PFOA) isomers and their anions and show that differences in the vibrational features of corresponding anion and neutral molecules can clearly identify the F atoms that are most weakly bound in the anions. We introduce two analysis tools for associating vibrational modes with individual atoms. The first involves computing the Einstein modes 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 weakly bound F atoms. We also show that the results of the vibrational analyses are consistent with systematically computed F atom removal energies. These vibrational analysis tools are shown to be closely related to the local mode analysis of Cremer and co-workers. Finally, we compare and contrast branched and linear PFAS molecules and analogous fluorinated and hydrogenated carbon chains.