Ultraviolet absorption and vibrational spectra of 2-fluoro-5-bromopyridine

The ultraviolet absorption spectrum in the range 340-185 nm in the vapour and solution phase has been measured for 2-fluoro-5-bromopyridine. Three fairly intense band systems identified as the pi* <-- pi transitions II, III and IV have been observed. A detailed vibronic analysis of the vapor and solution spectra is presented. The first system of bands is resolved into about sixty-two distinct vibronic bands in the vapour-phase spectrum. The 0,0 band is located at 35944 cm(-1). Two well-developed progressions, in which the excited state frequencies nu'25 (283 cm(-1)) and nu'19 (550 cm(-1)) are excited by several quanta, have been observed. The corresponding excited state vibrational and anharmonicity constants are found to be omega'i = 292 cm(-1), x'ii = 4.5 cm(-1) (i = 25) and omega'i = 563.8 cm(-1), x'ii = 6.9 cm(-1) (i = 19). The other two band systems show no vibronic structure, the band maxima being located at 48346 and 52701 cm(-1), respectively. The oscillator strength of the band systems in different solutions and the excited state dipole moments associated with the first two transitions have been determined by the solvent-shift method. The infrared spectrum in the region 4000-130 cm(-1) and the laser Raman spectrum of the molecule in the liquid state have been measured and a complete vibrational assignment of the observed frequencies is given. A correlation of the ground and excited state fundamental frequencies observed in the UV absorption spectrum with the Raman or infrared frequencies is presented.