Experimental and theoretical anharmonicity for benzene using density functional theory

The anharmonic force field of benzene has been calculated using a finite di fference 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 [J. Chem. Phys. 97, 4233 (1992)]. The vapor phase infr ared (IR) spectra of benzene (natural isotopic mixture) and of C-12-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 b ands nu(11), nu(4)+nu(12), nu(5)+nu(12), nu(2)+nu(11), and nu(7)+nu(16), us ing the Wilson numbering, with their accompanying hot bands, have been anal yzed and their origins determined to test our computed anharmonic force fie ld. The Raman spectra of gas-phase benzene have been also recorded at mediu m resolution (similar to 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 e xperimental and the theoretical frequencies and band profiles of the parall el nu(1), nu(2), 2 nu(16), 2 nu(4), and 2 nu(14) and of the corresponding h ot bands, taking into account the l-vibrational doubling and all Fermi reso nances within 100 cm(-1). By comparison with experiment, the DFT B3LYP is s hown to be more accurate than the self-consistent field (SCF): the fundamen tals are calculated with a mean absolute error of 10.7 cm(-1) and most of t he spectroscopic constants are in better agreement with the experimental va lues. (C) 2000 American Institute of Physics. [S0021-9606(00)31201-6].