Intra- and intermolecular H-bond mediated tautomerization and dimerization of 3-methyl-1,2-cyclopentanedione: Infrared spectroscopy in argon matrix and CCl4 solution

Mid-infrared spectra of 3-methyl-1,2-cyclopentanedione (3-MeCPD) have been recorded by isolating the molecule in a cold argon matrix (8 K) and also in CCl4 solution at room temperature. The spectral features reveal that in both media, the molecule exists exclusively in an enol tautomeric form, which is stabilized by an intramolecular OH⋯O hydrogen bond. NBO analysis shows that the preferred conformer is further stabilized because of hyperconjugation interaction between the methyl and vinyl group of the enol tautomer. In CCl4 solution, the molecule undergoes extensive self association and generates a doubly hydrogen bonded centrosymmetric dimer. The dimerization constant (Kd) is estimated to have a value of ∼9 L mol−1 at room temperature (25 °C) and the thermodynamic parameters, ΔH°, ΔS° and ΔG°, of dimerization are estimated by measuring Kd at several temperatures within the range 22–60 °C. The same dimer is also produced when the matrix is annealed at a higher temperature. In addition, a non-centrosymmetric singly hydrogen bonded dimer is also identified in the argon matrix. A comparison between the spectral features of the two dimers indicates that the dimerization effect on doubly H-bonded case is influenced by cooperative interaction between the two H-bonds.