Biman Bandyopadhyay, Prasenjit Pandey, Amit K. Samanta, Anamika Mukhopadhyay, Tapas Chakraborty Association Complexes of 1,3-Cyclohexanedione: Probing of Keto-Enol Tautomeric Equilibriain Cold Inert Gas Matrix, Solution and Vapor Phase by Infrared SpectroscopyandQuantum Chemistry Study Intermolecular 66 th OSU International Symposium on Molecular Spectroscopy June 20, 2011 Indian Association for the Cultivation of Science Kolkata, India

Why 1,3-Cyclohexanedione? β-diketones have been subject to considerable interest centering their keto-enol tautomeric equilibria Linear and cyclic variants show markedly different behavior owing to relative orientation of carbonyl groups 1,3-cyclohexanedione is a good prototype for the later variant ↕ ↕ Detailed information on the tautomeric equilibria in cyclic systems is still lacking

Tools we used: Infrared spectroscopy At cryogenic temperature: Ar matrix, neat sample At room temperature: Vapor, solution Electronic structure calculation Geometry optimization Normal mode analyses Transition state search

Who is where? *Energies are in kcal/mole Calculations done at DFT/B3LYP/ G**

Vapor Phase Spectrum: Ketoenol:Diketo at room temperature ≈ 0.05

Matrix Isolation Spectrum: Observed Diketo Calculated Ketoenol Calculated

Solution Phase Spectrum: Concentration VariationTemperature Variation Wavenumber (cm -1 ) M. Takasuka, T. Saito and H. Nakai, Vib. Spectrosc. 1996, 13, 65

Effect of annealing:

Neat Sample: 30 K ≈ 20 cm K ≈ 120 cm -1

Effect of dimer formation on tautomeric barrier: Ketoenol Dimer Diketo Dimer Diketo Ketoenol Monomeric TS 61 kcal/mol 36 kcal/mol Dimeric TS G. Alagona and C. Ghio, Int. J. Quantum Chem. 2008, 108, 1840 Calculations done at DFT/B3LYP/ G**

Summary: First experimental approach to estimate the effect of dimer formation on keto-enol tautomeric barrier for 1,3-cyclohexanedione The diketo form is in the global minimum of PES, but upon dimer formation ketoenol homodimers become more stable over diketo homodimers Dimer formation is found to be a necessary condition for presence of appreciable amount of ketoenol tautomer at ambient temperature Calculation predicts that dimer formation effectively reduces the tautomeric barrier by almost 50% compared to that in monomer The weak C―H‐ ‐ ‐O hydrogen bond present in diketo dimer plays an important role in lowering the barrier

Acknowledgement: Mrs. Moitrayee Mukherjee Mr. Arup Kumar Ghosh Ms. Shreetama Karmakar Ms. Pujarini Banerjee Financial Support: Department of Science and Technology, Government of India

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