Structure of the NH Stretching Band and (NH).. N Hydrogen Bond Frequencies in Purine

Abstract

THE AH stretching band of a strongly hydrogen bonded AH..B system is usually very broad and may present a complicated structure consisting of numerous sub-bands. In order to explain these sub-bands, the frequency modulation theory—that is, that the sub-band frequencies can be represented, by the expression ν = νAH±nν(AH)..B, where n = 0,1,2… and ν(AH)..B is any hydrogen bond frequency—has been advanced in the literature^1–3. In the case of imidazole, which contains strong NH..N hydrogen bonds, we have suggested that the hydrogen bond-frequencies do not seem to be a decisive factor in determining the sub-band structure of the νNH absorption⁴. In this communication, we wish to show that the same is true for purine, which contains a similar hydrogen bonded system, by comparing the behaviour of the νNH sub-bands and the far infrared hydrogen bond frequencies of different isotopic species at different temperatures.