Ab Initio Density Functional Theory Study of the Structure and Vibrational Spectra of Cyclohexanone and its Isotopomers

Abstract

We report ab initio density functional theory (DFT) calculations of the structure of cyclohexanone (1) in its chair conformation and of the unpolarized vibrational absorption (IR) spectra of four isotopomers of 1 (d0, d4, d6, and d10). DFT calculations use hybrid functionals, B3PW91 and B3LYP, and the TZ2P basis set. The results are compared to the electron diffraction (ED) structure of Dillen and Geise and liquid-phase IR spectra. Calculated and experimental structures are in reasonable agreement; however, the constraints imposed on the ED structure determination limit its accuracy. In the mid-IR (<2000 cm-1), predicted vibrational spectra are in excellent agreement with experiment, permitting an essentially unambiguous assignment of all fundamentals; in the C−H and C−D stretching regions, agreement is worse due to anharmonicity/Fermi resonance and the spectra cannot be unambiguously assigned. Our assignments differ substantially from earlier assignments of Fuhrer et al., based on a constrained valence force field. Vibrational circular dichroism (VCD) spectra of d1 and d2 isotopomers have been reported for C−H and C−D stretching regions. Predicted spectra are in poor agreement with experiment, a result again attributable to anharmonicity.