Experimental and theoretical anharmonicity for benzene using density functional theory

The anharmonic force field of benzene has been calculated using a finite difference method by means of density functional theory (DFT) with the B3LYP functional and a TZ2P atomic orbitals basis set, and compared to the field calculated by Maslen et al. [J. Chem. Phys. 97, 4233 (1992)]. The vapor phase infrared (IR) spectra of benzene (natural isotopic mixture) and of 12C-benzene have been recorded from 450 to 6000 cm-1, at resolutions varying from 0.05 to 0.008 cm-1, and at various path lengths (0.18/42 m). The parallel bands v11, v4 + v12, v5 + v12, v2 + v11, and v7 + v16, using the Wilson numbering, with their accompanying hot bands, have been analyzed and their origins determined to test our computed anharmonic force field. The Raman spectra of gas-phase benzene have been also recorded at medium resolution (∼0.7 cm-1) using an argon laser (line at 514.5 nm) with a power of 0.8 W and a multipass cell. In this work we compare the experimental and the theoretical frequencies and band profiles of the parallel v1, v2, 2v16, 2v4, and 2v14 and of the corresponding hot bands, taking into account the l-vibrational doubling and all Fermi resonances within 100 cm-1. By comparison with experiment, the DFT B3LYP is shown to be more accurate than the self-consistent field (SCF): the fundamentals are calculated with a mean absolute error of 10.7 cm-1 and most of the spectroscopic constants are in better agreement with the experimental values. © 2000 American Institute of Physics.