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

Abstract

Polycyclic aromatic hydrocarbons (PAHs) have been shown to be ubiquitous in a large variety of distinct astrophysical environments and are therefore of great interest to astronomers. The majority of these findings are based on theoretically predicted spectra, which make use of scaled DFT harmonic frequencies for band positions and the double harmonic approximation for intensities. However, these approximations have been shown to fail at predicting high-resolution gas-phase infrared spectra accurately, especially in the CH-stretching region (2950-3150 cm, 3 μm). This is particularly worrying for the subset of hydrogenated or methylated PAHs to which astronomers attribute the observed non-aromatic features that appear in the CH-stretching region of spectral observations of the interstellar medium (ISM). In our previous work, we presented the anharmonic theoretical spectra of three linear PAHs and five non-linear PAHs, demonstrating the importance of including anharmonicities into theoretical calculations. In this work we extend these techniques to two methylated PAHs (9-methylanthracene, and 9,10-dimethylanthracene) and four hydrogenated PAHs (9,10-dihydroanthracene, 9,10-dihydrophenanthrene, 1,2,3,4-tetrahydronaphthalene, and 1,2,3,6,7,8-hexahydropyrene) in order to better understand the aliphatic IR features of substituted PAHs. The theoretical spectra are compared with the spectra obtained under matrix isolation low-temperature conditions for the full vibrational fundamental range and under high-resolution, low-temperature gas-phase conditions for the CH-stretching region. Excellent agreement is observed between the theoretical and high-resolution experimental spectra with a deviation of 0.00% ± 0.17%, and changes to the spectra of PAHs upon methylation and hydrogenated are tracked accurately and explained.

Far-IR and UV spectral signatures of controlled complexation and microhydration of the polycyclic aromatic hydrocarbon acenaphthene

“…We believe that this transition is a combination mode similar to the transitions in PAHs above 1700 cm -1 as calculated in Ref. 63.…”

Anharmonic interstellar PAH molecules

“…Once the QFF is known, the energy of the anharmonic vibrational states can be written as where χ kl are the anharmonic constants (see Ref. for a detailed explanation). VPT2 is a powerful method but it is affected by resonances, that is, instances where the sum/difference/combination of some harmonic mode energies are nearly equal.…”

“…For each polyad a matrix is built, containing the relevant anharmonic constants, and is diagonalised. The resulting eigenvectors give the degree of mixing of the states, while the eigenvalues give the new energies of the states …”

The Carrier of 3.3 μm Aromatic Infrared Bands: Anharmonicity and Temperature Effects on Neutral PAHs

“…However, the application of VPT2 to PAHs shows appreciable discrepancy in comparison to experimental spectra, which could be due to the improper choice of functionals and basis sets. It has been reported that for hydrogenated and methylated PAHs, changing from B971/TZ2P to B3LYP/N07D, the improvement in agreement between theory and experiment was found to be from an error of 1% down to 0.1% (30-3 cm −1 in the CH-stretching region; Mackie et al 2018).…”