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The monoprotonated biguanides phenformin hydrochloride, buformin hydrochloride and metformin hydrochloride were investigated by 15N NMR spectroscopy. The structure of all hydrochlorides is best described by a 1,3-diazabutadienic skeleton with three amino groups and its corresponding mesomeric forms. There is no evidence for the presence of other tautomeric forms in solution.
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Clement et al. (1999) studied this question.
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