The anharmonic quartic force field infrared spectra of five non-linear polycyclic aromatic hydrocarbons: Benz[a]anthracene, chrysene, phenanthrene, pyrene, and triphenylene

Abstract

The anharmonic quartic force field infrared spectra of three polycyclic aromatic hydrocarbons: Naphthalene, anthracene, and tetracene J. Chem. Phys. 143, 224314224314 (2015); 10.1063/1.4936779 THE JOURNAL OF CHEMICAL PHYSICS 145, 084313 (2016) The anharmonic quartic force field infrared spectra of five non The study of interstellar polycyclic aromatic hydrocarbons (PAHs) relies heavily on theoretically predicted infrared spectra. Most earlier studies use scaled harmonic frequencies for band positions and the double harmonic approximation for intensities. However, recent high-resolution gas-phase experimental spectroscopic studies have shown that the harmonic approximation is not sufficient to reproduce experimental results. In our previous work, we presented the anharmonic theoretical spectra of three linear PAHs, showing the importance of including anharmonicities into the theoretical calculations. In this paper, we continue this work by extending the study to include five non-linear PAHs (benz[a]anthracene, chrysene, phenanthrene, pyrene, and triphenylene), thereby allowing us to make a full assessment of how edge structure, symmetry, and size influence the effects of anharmonicities. The theoretical anharmonic spectra are compared to spectra obtained under matrix isolation low-temperature conditions, low-resolution, high-temperature gas-phase conditions, and high-resolution, low-temperature gas-phase conditions. Overall, excellent agreement is observed between the theoretical and experimental spectra although the experimental spectra show subtle but significant differences. Published by AIP Publishing.[http://dx

The anharmonic quartic force field infrared spectra of hydrogenated and methylated PAHs

“…The theoretical methods of this work follow similar techniques to our previous work. 21,23,24 The software package Gaussian09 25 is used to optimize the geometry of the molecules, as well as to calculate the quartic force fields (QFFs) and IR intensities. In order to handle the large number of mutually resonating modes (polyads 26 ), we use a locally modified version of the SPECTRO 27 software package to perform second order vibrational perturbation theory treatment (VPT2) after transforming the QFF constants from normal to Cartesian coordinates.…”

“…The major source of error in calculating the intensities of the CH-stretching region is the reliance on the distribution of harmonic intensities through the polyads rather than anharmonic intensities. 21,23 The calculation and redistribution of ''true'' anharmonic intensities should improve the fits. The discrepancies between the theoretical and experimental intensities did not hamper reliable band identifications.…”

“…In our previous work, [20][21][22][23] we showed that good agreement between theoretical and experimental spectra can be obtained when an anharmonic theoretical approach is used; specifically, a second-order vibrational perturbation treatment that accounts for large numbers of mutually resonating modes and combination bands. This work extends these studies in order to address the high-resolution spectra of hydrogenated and methylated PAHs.…”

Experimental Approach to the Study of Anharmonicity in the Infrared Spectrum of Pyrene from 14 to 723 K

“…Type Absorbance Fit Range Empirical anharmonicity factors (300 K) (300 K) can be valuable data to test the reliability and accuracy of codes that calculate anharmonic spectra. 24,25,33 The biggest advancement of this work is to report uniformly measured data over the full 14 to 723 K temperature range, whereas previous studies were either limited to low temperature (∼4 K) [30][31][32] or to high temperatures [573-873] K. 7 The main drawback of the used experimental methodology, though, is that the data are recorded in condensed phase, consisting of solid grains embedded in KBr pellets. The stable configuration of solid pyrene in normal conditions is referred to as Phase I (cf.…”

“…[19][20][21] Other type of codes aim at modeling in details the band structures by describing the connection between states with explicit consideration of the resonances. [22][23][24][25] For these theoretical studies, the best way to test and validate the method is by comparison between calculated IR anharmonic spectra and experimental spectra for a range of temperatures. Unfortunately, experimental data remain scarce.…”