Tautomeric and Conformational Properties of Acetoacetamide:  Electron Diffraction and Quantum Chemical Study

The tautomeric properties of acetoacetamide, CH3C(O)CH2C(O)NH2, have been investigated by gas electron diffraction (GED) and quantum chemical calculations (B3LYP and MP2 approximations with 6-31G(d,p) and 6-311++G(3df,pd) basis sets). GED results in a mixture of 63(7)% enol tautomer and 37(7)% diketo form at 74(5) °C. Only one enol form with the O−H bond adjacent to the methyl group (CH3C(OH)CHC(O)NH2) and only one diketo conformer (with dihedral angles τ(OC(CH3)CC) = 31.7(7.5)° and τ(OC(NH2)C(H2)C(O)) = 130.9(4.5)°) are present. The calculated tautomeric composition varies in a wide range depending on the quantum chemical method and basis set. Only the B3LYP method with small basis sets reproduces the experimental compositon correctly.