Cooperative strengthening of an intramolecular O—H···O hydrogen bond by a weak C—H···O counterpart: matrix-isolation infrared spectroscopy and quantum chemical studies on 3-methyl-1,2-cyclohexanedione

Abstract

Matrix-isolation infrared spectra of 1,2-cyclohexanedione (CD) and 3-methyl-1,2-cyclohexanedione (3-MeCD) were measured in a nitrogen matrix at 8 K. The spectral features reveal that, in the matrix environment, both molecules exist exclusively in the monohydroxy tautomeric form, which is stabilized by an intramolecular O—H···O═C hydrogen bond (HB). The ν_O—H band of the enol tautomer of 3-MeCD appears at a relatively lower frequency and displays a somewhat broader bandwidth compared to that of CD, and these spectral differences between the two molecules are interpreted as being due to the formation of an interconnected C—H···O HB, where the enolic oxygen is the HB acceptor and one of the C—H covalent bonds of the methyl group is the HB donor. Electronic structure calculations at the B3LYP/6-311++G**, MP2/6-311++G**, and MP2/cc-pVTZ levels predict that this tautomer (enol-2) is ∼3.5 kcal/mol more stable than a second enolic form (enol-1) where such interconnected H-bonding is absent. Theoretical analysis with a series of molecules having similar functional groups reveals that part of the excess stability (∼1 kcal/mol) of enol-2 originates from a cooperative interaction between the interconnected C—H···O and O—H···O HBs. In the IR spectrum, a weak band at 3007 cm^-1 is assigned to ν_C—H of the methyl C—H bond involved in the H-bonded network. The spectra predicted by both harmonic and anharmonic calculations reveal that this transition is largely blue-shifted compared to the fundamentals of the other two methyl C—H stretching frequencies that are not involved in H-bonding. The conclusions are corroborated further by natural bond orbital (NBO) analysis.