Abstract
The crystal structure of squaric acid (dihydroxycyclobutene-dione), and the structures of the neutral and the acid potassium salts, both of which are hydrated, are reviewed, and additional experimental material (x-ray and optical) is contributed. Crystals of these three compounds are grown and examined by laser-Raman techniques. These Raman spectra, and the corresponding infrared absorption spectra, are interpreted in terms of the structures, shown to be essentially monomeric in the case of squaric acid and the neutral salt, and pseudo-dimeric for the acid salt. Crystals of KHCO[3] are grown, and their Raman and infrared spectra used to support the interpretation of the acid squarate spectra. Vibrational interaction is proposed, to account for the repulsion of observed C-C stretch frequencies, as the skeletal symmetry of the squarate ion is lowered from D[4h] to C[2v]. Hydrogen bonding into a pseudo dimer is proposed to account for the prolixity and frequency separation of the C-O stretch bands in the acid squarate spectra. The two windows that are observed in the infrared spectra of the acid squarate are shown not to arise from the water of hydration, and are in consequence assigned to fundamental modes of the anion. Some anion modes are explained in terms of single mechanical models, and an approximate frequency calculated for the torsional mode nu[3], inactive in the 'free' squarate ion. Explanations are found both for the weakness of certain totally symmetric fundamentals, and for the appearance of overtones of librational fundamentals in particular elements only of the polarisibility tensor.