The anharmonic quartic force field infrared spectra of three polycyclic aromatic hydrocarbons: naphthalene, anthracene, and tetracene

Current efforts to characterize and study interstellar polycyclic arom. hydrocarbons (PAHs) rely heavily on theor. predicted IR (IR) spectra. Generally, such studies use the scaled harmonic frequencies for band positions and double harmonic approxn. for intensities of species, and then compare these calcd. spectra with exptl. spectra obtained under matrix isolation conditions. High-​resoln. gas-​phase exptl. spectroscopic studies have recently revealed that the double harmonic approxn. is not sufficient for reliable spectra prediction. In this paper, we present the anharmonic theor. spectra of three PAHs: naphthalene, anthracene, and tetracene, computed with a locally modified version of the SPECTRO program using Cartesian derivs. transformed from Gaussian 09 normal coordinate force consts. Proper treatments of Fermi resonances lead to an impressive improvement on the agreement between the obsd. and theor. spectra, esp. in the C-​H stretching region. All major IR absorption features in the full-​scale matrix-​isolated spectra, the high-​temp. gas-​phase spectra, and the most recent high-​resoln. gas-​phase spectra obtained under supersonically cooled mol. beam conditions in the CH-​stretching region are assigned. (c) 2015 American Institute of Physics.