Assignment of the perfluoropropionic acid-formic acid complex and the difficulties of including high Ka transitions.

Abstract

We recently began an investigation into the perfluoropropionic acid$\cdots$formic acid complex using broadband microwave spectroscopy. This study aims to examine the possible double proton transfer between the two interacting carboxcyclic acid groups. The spectrum presented as a doubled set of lines, with spacing between transitions of $<\ 1 $\ MHz. Transitions appeared to be $a$-type, R branch transitions for an asymmetric top. Assignment of all $K_a=1,0$ transitions yields decent fits to a standard rotational Hamiltonian. Treatment of the doubling as either a two state system (presumably with a double proton transfer) or as two distinct, but nearly identical conformations of the complex produce fits of similar quality. Including higher $K_a$ transitions for the $a$-type, R-branch lines greatly increases the error of these fits. A previous study involving the trifluoroacetic acid$\cdots$formic acid complex published observed similar high $K_a$ transitions, but did not include them in the published fit.\footnote{Martinache, L.

Kresa, W.

Wegener, M.

, Vonmont, U.

and Bauder, A. \textit{Chem. Phys.} \textbf{148} (1990) 129-140.} We hope to shed more light on this conundrum. Similarities to other double-well potential minimum systems will be discussed.