The imino-amino tautomeric equilibrium in cyanoguanidine dissolved in several aprotic solvents; an FT-Raman spectroscopic study

The FT-Raman spectra of cyanoguanidine solutions in NN-dimethylformamide (D MF), dimethylsulfoxide (DMSO) and hexamethylphosphoric triamide (HMPA), in a range of concentrations between R = 3 and R = 10 (R = solvent/solute mole ratio), are studied. The intense doublet that appears in the spectral regi on of the nitrile stretching fundamental is attributed to the presence of t he imino (2147 cm(-1)) and amino (2186 cm(-1)) tautomeric forms. Furthermor e, a shoulder located in the higher frequency wing that could arise from cy anoguanidine molecules associated through hydrogen bonds affecting the nitr ogen atom of the nitrite group. When the solute concentration is increased, the relative intensity of that band arising from the imino tautomer increa ses and a further increase of the associated species can also be observed. These observations are common for the three solvents studied. The concentra tion of the imino form strongly depends on the solvent, increasing in the o rder DMF < DMSO < HMPA, which qualitatively corresponds with the donor numb er ranking. The presence of associated species, however, seems to be contro lled by the dielectric constant of the solvent, being scarce in DMSO, moder ate in DMF and more important in HMPA. (C) 2001 Elsevier Science B.V. All r ights reserved.