theoretical and experimental investigations on the structure and vibrational spectra of 6-amino-3-methyl-1-phenyl-1h-pyrazolo-[3,4-b]pyridine-5-carboxylic acid and 6,7-dihydro-3-methyl-6-oxo-1-phenyl-1h-pyrazolo[3,4-b]pyridine-5-carbonitrile

The solid phase FT-IR and FT-Raman spectra of 6-amino-3-methyl-1-phenyl-1H-pyrazolo[3,4-b]pyridine-5-carboxylic acid (PYRPCA) and 6,7-dihydro-3-methyl-6-oxo-1-phenyl-1H-pyrazolo[3,4-b]pyridine-5-carbonitrile (PYRPCN) were recorded in the region 4000–400 cm-1. The spectra were interpreted with the aid of normal coordinate analysis following full structure optimization and force field calculations based on the density functional theory (DFT) using standard B3LYP, BLYP and ab initio RHF methods with 6-31G* basis set and were scaled using a recommended set of scaling factors yielding fairly good agreement between the observed and calculated frequencies. Based on the present good quality, the scaled quantum mechanical (SQM) force field, a reliable description of the fundamentals of PYRPCA and PYRPCN, was provided. The calculations predicated a predominance of different tautomers in PYRPCA and keto-enol tautomers in PYRPCN. For PYRPCA, the most stable conformer is stabilized by intramolecular hydrogen bonding. The characteristic of the hydrogen bonding is its strengthening effect on the conjugation of the NH2 and COOH groups with the pyridine ring.