The vibrational spectra and stability of dipyrrolylmethene hydrobromides and their oxa and thia derivatives

This paper presents the IR spectra of solid alkyl substituted α,α-, α,β-, and β,β-dipyrrolylmethene hydrobromides and their oxa and thia derivatives recorded in KBr pellets over the frequency range 400–4000 cm−1. The conclusion was drawn that the NH bonds in the pyrrole fragments of α,α-dipyrrolylmethene were identical, as distinct from the α,β and β,β isomers. The influence of functional substitution in the dipyrrolylmethene molecule on the frequencies of NH stretching vibrations was discussed. A satisfactory correlation between NH bond vibrational frequencies and the enthalpies of vaporization of gaseous hydrogen bromide from crystalline samples under heating was observed. This allows the IR data to be used as a criterion of the stability of dipyrrolylmethene salts and their oxa and thia derivatives.