CH···O interaction lowers hydrogen transfer barrier to keto-enol tautomerization of β-cyclohexanedione: combined infrared spectroscopic and electronic structure calculation study.

Molecular association and keto-enol tautomerization of β-cyclohexanedione (β-CHD) have been investigated in argon matrix and also in a thin solid film prepared by depositing pure β-CHD vapor on a cold (8 K) KBr window. Infrared spectra reveal that, in low-pressure vapor and argon matrix, the molecules are exclusively in diketo tautomeric form. The CH···O hydrogen bonded dimers of the diketo tautomer are produced by annealing the matrix at 28 K. No indication is found for keto-enol tautomerization of β-CHD in dimeric complexes in argon matrix within the temperature range of 8-28 K. On the other hand, in thin film of pure diketo tautomer, the conversion initiates only when the film is heated at temperatures above 165 K. The observed threshold appears to be associated with excitation of the intermolecular modes, and the IR spectra recorded at high temperatures display narrowing of vibrational bandwidths, which has been associated with reorientations of the molecules in the film. The nonoccurrence of tautomerization of the matrix isolated dimer is consistent with the barrier predicted by electronic structure calculations at B3LYP/6-311++G** and MP2/6-311++G** levels of theory. The transition state calculation predicts that CH···O interaction has a dramatic effect on lowering of the tautomerization barrier, from more than 60 kcal/mol for the bare molecule to ~35-45 kcal/mol for dimers.