CH⋯O H-bond mediated tautomerization of 2-methyl-1,3-cyclohexanedione: A combined IR spectroscopic and theoretical study

Abstract

Molecular association and its impact on the keto-enol tautomerization of 2-methyl-1,3-cyclohexanedione (MCHD) have been investigated in low temperature argon matrix and thin solid film. The system exists exclusively in diketo tautomeric form in argon matrix. The CH⋯O H-bonded homodimers of the diketo tautomer are produced by annealing the matrix at 28 K. No trace of the keto-enol tautomer is observed in matrix isolated homodimers in the temperature range of 8–28 K. However, tautomeric conversion initiates in a thin film of pure diketo tautomer when the temperature of the film is raised to ~170 K. Transition state calculations on the monomeric and dimeric MCHD demonstrate that CH⋯O H-bond formations between diketo tautomers play a vital role in lowering the tautomerization barrier. However, the extent of CH⋯O H-bonded dimer formation in matrix isolation, as well as extent of tautomerization in the neat sample are found to be smaller than that for the previously reported 1,3-cyclohexanedione (CHD) under similar experimental conditions (J. Phys. Chem. A 2012, 116, 3836–3845). Electronic structure calculations suggest that formation of the CH⋯O H-bonded dimer is less feasible in presence of the bulky 2-methyl groups of MCHD, as compared to CHD. Additionally, the transition state geometry of the dimeric keto-enol product of MCHD, as compared to the same for CHD, is more strained and offers a weaker CH---O H-bond that contributes to lesser tautomeric conversion in the former.