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, pyri dine, and a 1:1 adduct. The formation of solid N-trifluoroacetylpyridi nium trifluoroacetate on cooling to -78 degrees C has been claimed pre viously. This product is now shown to be pyridinium trifluoroacetate f ormed by partial hydrolysis. The 1:1 TFAA-pyridine mixture is characte rized by H-1, C-13, F-19, and N-14 NMR spectroscopy and by Raman spect roscopy. The latter definitely excludes the N-trifluoroacetylpyridiniu m trifluoroacetate structure for the 1:1 complex. Instead, this is a s igma-complex with a partial dipolar structure arising from attack of t he pyridine nitrogen on the C = O group in TFAA. The enhanced reactivi ty of TFAA in pyridine is attributed to the presence of this complex. Furthermore, strong pi-pi interactions are present between TFAA and fr ee pyridine.