RESUMO
The crystal structures, synthesis procedure, thermal behavior, and spectroscopic properties of a new squaraine SqINH·H2O and its anhydrous arrangement are described. This squaraine is obtained through an acid-base reaction using squaric acid (H2Sq) and isoniazid (INH) as precursors. Both squaraines crystallize in the monoclinic system, but in different space groups: the hydrated and anhydrous arrangement crystallizes in the P21 and P21/c space group, respectively. The crystallographic data strongly suggest that the structures present an expressive increase in their electronic delocalization all over the molecular structure of both compounds, when compared with the reagents. The bond distances for both structures present an average value intermediate between a single and double character (1.463(3) Å for SqINH·H2O and 1.4959(3) Å for SqINH). The vibrational and electronic data also corroborate with this proposal, since the band shifts indicate that the conjugation over the system is increased, as indicated by the blue shift observed for the carbonyl stretching bands for both compounds. The presence of the water molecule is responsible for a decrease in fluorescence emission, as determined by the emission spectra recorded for both compounds.
RESUMO
This work presents a comparative study of dihydrated and anhydrous forms of potassium croconate crystals by vibrational spectroscopy, X-ray powder diffraction, and thermogravimetry. These compounds have different colors (dihydrated is orange, and dehydrated is yellow) due to the presence of coordinated water molecules. X-ray diffraction patterns show that the unit cell of the yellow compound is smaller than that of the orange analogue, suggesting that the croconate ion layers are more closely bonded in this salt. The loss of water is reversible due to the potassium cation size which is intermediate between small (Li+ and Na+) and large (Rb+ and Cs+) alkaline metal ions. However, the hydrated compound (orange) is more stable, and with a small quantity of water the yellow compound is quickly converted to the orange compound. A diagnostic feature of the Raman spectrum for the orange (hydrated) and yellow (anhydrous) analogues is the singlet at 1240 cm(-1) in the former, assigned to a nu(CC) + delta(CCC) + nu(CO) + beta(CO) mode of E'2 symmetry, which splits in the yellow form to a doublet at 1256 and 1232 cm(-1).