Collect. Czech. Chem. Commun. 1979, 44, 781-787

Spectral manifestations of strong hydrogen bonds in seven-membered rings of pyrazolecarboxylic acid derivatives

Emil Svátek, Viktor Zikán and Miroslav Semonský

Research Institute for Pharmacy and Biochemistry, 130 00 Prague 3


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.