Absorption Spectrum of pyrazine was examined in mixed solvents of trifluoroacetic acid-hexane (I), trifluoroacetic acid-water (II), isobutyric acid-hexane (III), and isopropyl alcohol-hexane (IV). Basicity constant pK_ of pyrazine was determined to be 0.6 in the mixture II, whereas the determination of pK_> was unsuccessful, Spectral data showed no discernible amount of protonated species in the mixtures III and IV. The concentration dependence of the absorption coefficient of a n-π^* transition in the mixture III can be interpreted by assuming that pyrazine forms two hydrogen bonds with two isobutyric acid molecules
