Abstract
The IR spectrum and p K a values of 1-methyl-4,5-dicarboxyimidazole (H 2MDCI) support a zwitterionic structure with one acidic hydrogen on the non-methylated nitrogen (N3) and the second acidic hydrogen involved in a covalent three-center hydrogen bond between carboxylate groups; the N3 hydrogen is involved in strong intermolecular hydrogen bonding. On half-neutralization the carboxylate hydrogen is removed, and the remaining N3 hydrogen acts as a weak hydrogen bond donor. The solubility of the diacid in H 2O is increased by the addition of fluoride ion. The stoichiometry of the dissolution process is H 2MDCI + 4F − = MDCI 2− + 2HF − 2. The unexpected removal of both acidic hydrogens in the presence of undissociated diacid is driven by strong chelation of sodium ion by MDCI 2−. Evaporation of a 3:1 NaF to H 2MDCI solution yields Na 2MDCI and NaH 2F 3; with excess NaF, Na 2MDCI and NaHF 2 are obtained. With quantities of NaF below a 3:1 stoichiometry, various mixtures of H 2MDCI, NaHMDCI, and Na 2MDCI are obtained on evaporation.
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