2- S -Methyl-5,5-dimethylimidazolin-4-one: chemical behaviour and crystal structure of its Δ 1 tautomer

2-S-Methyl-5,5-dimethylimidazolin-4-one can be crystallized as Δ 1 or Δ 2 tautomers, depending on the solvent polarity. The structure, solved on crystals obtained from benzene, showed it to be the Δ 1 tautomer in the anti configuration. Δ 1 -2-S-Methyl-5,5-dimethylimidazolin-4-one is monoclinic, space group P2 1 /c, with a = 8.347(1), b = 10.121(1), c = 9.996(3) Å, β = 107.20(2)°, R = 0.037 for 1395 independent reflections with I > lσ(I). The quantum mechanical (MNDO, PM3, and AMI) calculations indicate that among the four conformers the anti-Δ 1 tautomer has both the lowest dipole moment and the lowest enthalpy of formation. The ultraviolet spectrum of the title compound has been recorded in solvents with different dielectric constants. The infrared spectra of the Δ 1 and Δ 2 solid samples are discussed relative to the v(NH) and v(CO) absorptions and in comparison with those recorded in carbon tetrachloride and in chloroform solution. Its acid–base behaviour has been studied by determining the pK a in aqueous solutions and the results are compared with those obtained for some related hydantoins. Key words: 2-S-methyl-5,5-dimethylimidazolin-4-one, tautomerism, hydantoin derivatives, crystal structure.