STABLE CARBOCATIONS .287. C-13 NMR AND AB-INITIO IGLO STUDY OF PROTONATED OXOCARBONS - POLY-O-PROTONATED SQUARIC, CROCONIC, AND RHODIZONIC ACIDS AND THEIR AROMATIC NATURE - PREPARATION AND STUDY OF TETRAHYDROXY-ETHENE AND DIAMINODIHYDROXYETHENE DICATIONS

The protonation of oxocarbons in superacidic solutions has been studied. Squaric acid (3,4-dihydroxy-3-cyclobutene-1,2-dione), 2), croconic acid (4,5-dihydroxy-4-cyclopentene-1,2,3-trione, 3), and rhodizonic acid (5,6-dihydroxy-5-cyclohexene-1,2,3,4-tetraone, 4) were poly-O-protonated with Magic Acid (1:1 SbF5 + FSO3H) or fluorosulfuric acid at low temperatures and characterized by C-13 NMR spectroscopy. The protonation of oxalic acid and oxamide has also been studied in HSO3F/SbF5/SO2 solution at -78-degrees-C and the resulting ethene dications were characterized by C-13 NMR spectroscopy. Dimethyl oxalate was also methylated under stable-ion conditions with CH3F/SbF5 in SO2 solution but gave only the corresponding monomethylated cation. Comparison of experimental with ab initio/IGLO calculated C-13 NMR chemical shifts gives insight into the nature of the protonated oxocarbons. Similar comparisons in the studied ethene dications show their preferred planar geometry.