Ab initio density functional theory study of the structure and vibrationalspectra of cyclohexanone and its isotopomers

We report ab initio density functional theory (DFT) calculations of the str ucture of cyclohexanone (1) in its chair conformation and of the unpolarize d vibrational absorption (IR) spectra of four isotopomers of 1 (d(0), d(4), d(6), and d(10)). DFT calculations use hybrid functionals, B3PW91 and B3LY P, and the TZ2P basis set. The results are compared to the electron diffrac tion (ED) structure of Dillen and Geise and liquid-phase IR spectra. Calcul ated and experimental structures are in reasonable agreement; however, the constraints imposed on the ED structure determination limit its accuracy. I n the mid-IR (<2000 cm(-1)), predicted vibrational spectra are in excellent agreement with experiment, permitting an essentially unambiguous assignmen t of all fundamentals in the C-H and C-D stretching regions, agreement is w orse due to anharmonicity/Fermi resonance and the spectra cannot be unambig uously assigned. Our assignments differ substantially from earlier assignme nts of Fuhrer et al., based on a constrained valence force field. Vibration al circular dichroism (VCD) spectra of di and dl isotopomers have been repo rted for C-H and C-D stretching regions. Predicted spectra are in poor agre ement with experiment, a result again attributable to anharmonicity.