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

Abstract

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 π*←π 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 ν′25 (283 cm−1) and ν′19 (550 cm−1) are excited by several quanta, have been observed. The corresponding excited state vibrational and anharmonicity constants are found to be ω′i=292 cm−1, x′ii=4.5 cm−1 (i=25) and ω′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.