Infrared, Raman, and Ultraviolet Absorption Spectra and Theoretical Calculations and Structure of 2,3,5,6-Tetrafluoropyridine in its Ground and Excited Electronic States.

Infrared and Raman spectra of 2,3,5,6-tetrafluoropyridine (TFPy) were recorded and vibrational frequencies were assigned for its S0 electronic ground states. Ab initio and density functional theory (DFT) calculations were used to complement the experimental work. The lowest electronic excited state of this molecule was investigated with ultraviolet absorption spectroscopy and theoretical CASSCF calculations. The band origin was found to be at 35,704.6 cm-1 in the ultraviolet absorption spectrum. A slightly puckered structure with a barrier to planarity of 30 cm-1 was predicted by CASSCF calculations for the S1(π,π*) state. Lower frequencies for the out-of-plane ring bending vibrations for the electronic excited state result from the weaker bonding within the pyridine ring.