Structure and vibrational spectroscopy of cyclic diketones and their hydrogen bonded complexes in cold inert gas matrices Amit K. Samanta, Prasenjit Pandey, Anamika Mukhopadhyay and Tapas Chakraborty Department of Physical Chemistry, Indian Association for the Cultivation of Science, Calcutta, India IACS

Conformers and tautomers of cyclohexanediones IACS

Diketo 4.2 kcal/Mol Trans monoenol (hydrogen bonded) 0.0 Cis monoenol 6.8 kcal/mol Relative energies DFT/ G(d,p) Means of identification: Infrared spectroscopy in cold inert gas matrices 1,2 cyclohexanedione IACS

Wavenumber (cm -1 ) Wavenumber (cm -1 ) 1740 # Wavenumber (cm -1 ) FTIR spectrum of 1,2-cyclohexanedione in Ar matrix at 8 K # B3LYP/ G(d,p) IACS

Chair (C) Twist boat (T) Relative energies DFT/ G(d,p) kcal/mol 1,4 cyclohexanedione CC 3.16 kcal/mol CT 3.37kcal/mol TT 3.62 kcal/mol Relative energies DFT/ G(d,p) C–H···O hydrogen bonded dimers IACS

Wavenumber (cm -1 ) 1740 FTIR spectrum of 1,4-cyclohexanedione in N 2 matrix at 8 K IACS

8K 20K 30K Effects of annealing on spectral features Effects of higher sample concentrations in the gas mixture IACS

Diketo Monoenol Chair 0.0 Boat 1.3 kcal/mol Twist boat 4.03 kcal/mol Cis monoenol (1.28 kcal/mol) Trans monoenol (3.2 kcal/mol) Mixed Dimer (O-H---O) Dimers Keto Dimer (C-H---O) 1,3-cyclohexanedione IACS

Wavenumber (cm -1 ) FTIR spectrum of 1,3-cyclohexanedione in N 2 matrix at 8 K Chair diketo Boat diketo IACS

FTIR spectra of 1,3-cyclohexanedione in CCl 4 solution 0.01M 0.02M 0.03M 0.04M IACS

Dimer binding # energy Proton affinity # C-H∙∙∙O hydrogen-bonded mixed dimers in room-temperature solutions CBN∙∙∙CHCl 3 CPN∙∙∙CHCl 3 CHN∙∙∙CHCl kcal/mol3.95 kcal/mol4.39 kcal/mol C=O str. frequency 198 kcal/mol204 kcal/mol 208 kcal/mol 1786 cm cm cm -1 System studied: Chloroform (donor) and small cyclic ketones (acceptor) # DFT/B3LYP/ G(d,P) IACS

Blue shifting of chloroform C-H stretching fundamental on complexation # DFT/B3LYP/ G(d,p) CHN  CHCl CPN  CHCl CBN  CHCl Experiment Theory #  C-H  C-H cm -1 cm -1 Complex Green: CBN Red: CPN Blue: CHN Blue shifts occurs with enhancement in transition intensity IACS

monomer Ketone:chloroform 1:1 1:5 1:10 1:15 1:20 Anomalous red-shifts of C=O stretching fundamentals cyclohexanone cyclopentanone cyclobutanone  Red shift of C=O stretching is uncorrelated with blue shifts of C–H stretching  Red shifts of C=O stretching has no correlation with the dimer binding energy IACS

Acknowledgement Amit K Samanta Prasenjit Pandey Biman Bandyopadhyay Anamika Mukhopadhyay Moitrayee Mukherjee Financial Support: Department of Science and Technology, Govt. of India Council of Scientific and Industrial Research, Govt. of India IACS