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

Abstract

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 O─H⋯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 (K_d) 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 K_d 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.