An NMR and Raman Study of Trifluoroacetic Anhydride in Pyridine.

Equimolar amounts of pyridine and trifluoroacetic acid anhydride (TFAA) form an equilibrium mixture containing similar amounts of TFAA, pyridine, and a 1:1 adduct. The formation of solid N-trifluoroacetylpyridinium trifluoroacetate on cooling to -78 degrees C has been claimed previously. This product is now shown to be pyridinium trifluoroacetate formed by partial hydrolysis. The 1:1 TFAA-pyridine mixture is characterized by H-1, C-13, F-19, and N-14 NMR spectroscopy and by Raman spectroscopy. The latter definitely excludes the N-trifluoroacetylpyridinium trifluoroacetate structure for the 1:1 complex. Instead, this is a sigma-complex with a partial dipolar structure arising from attack of the pyridine nitrogen on the C = O group in TFAA. The enhanced reactivity of TFAA in pyridine is attributed to the presence of this complex. Furthermore, strong pi-pi interactions are present between TFAA and free pyridine.