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-carbonitrile