Infrared Study of the Effect of Solvent upon ν(NH) in Pyrrole

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Abstract
New data are presented on the NH group stretching frequency of pyrrole in dilute and concentrated solutions in fourteen different solvents and in several solvent mixtures. It is shown that in nonpolar solvents ν(NH) of pyrrole obeys the Kirkwood-Bauer relationship, δν/ν=C(D—1)/(2D+1), where C is a constant, and D is the solvent dielectric constant. Furthermore, it is shown that C is only slightly affected as ν(HCl) of hydrochloric acid, ν(OD) of methanol-d, or ν(OH) of methanol, ethanol, propanol, or phenol is substituted for the ν(NH) of pyrrole. The deviations from the Kirkwood-Bauer relationship produced by polar solvents and aromatic hydrocarbon solvents are analyzed in terms of hydrogen bonding and the formation of molecular complexes. The effect of solvent upon band intensity is also briefly examined. A classification of types of OH and NH solution frequencies is suggested.