Collection of Czechoslovak Chemical Communications - digital archive 

Abstract

Strong downward frequency shifts of ν(C=O) of benzoyl or carboxyl group have been found in IR spectra of some pyrazolecarboxylic acid derivatives, and bathochromic shifts of maximum of the first band of π-π^* transition have been found in their UV spectra, which can be explained by the presence of strong hydrogen bonds in seven-membered rings. Considerable frequency shift of ν(C=O) indicates lowering of order of this bond which is comparable with the bond order lowering of C=O group in six-membered conjugated chelates of the type of enolized β-diketones. In our opinion, formation of strong hydrogen bonds in seven-membered rings in pyrazolecarboxylic acids is due to the presence of the adjoining five-membered aromatic ring, which enables resonance stabilization of the whole π-electronic system in a pseudo-azulene structure.