Anharmonic vibrational spectroscopy of polycyclic aromatic hydrocarbons (PAHs)

Abstract

While powerful techniques exist to accurately account for anharmonicity in vibrational molecular spectroscopy, they are computationally very expensive and cannot be routinely employed for large species and/or at non-zero vibrational temperatures. Motivated by the study of Polycyclic Aromatic Hydrocarbon (PAH) emission in space, we developed a new code, which takes into account all modes and can describe all IR transitions including bands becoming active due to resonances as well as overtones, combination and difference bands. In this article, we describe the methodology that was implemented and discuss how the main difficulties were overcome, so as to keep the problem tractable. Benchmarking with high-level calculations was performed on a small molecule. We carried out specific convergence tests on two prototypical PAHs, pyrene (C16H10) and coronene (C24H12), aiming at optimising tunable parameters to achieve both acceptable accuracy and computational costs for this class of molecules. We then report the results obtained at 0 K for pyrene and coronene, comparing the calculated spectra with available experimental data. The theoretical band positions were found to be significantly improved compared to harmonic Density Functional Theory (DFT) calculations. The band intensities are in reasonable agreement with experiments, the main limitation being the accuracy of the underlying calculations of the quartic force field. This is a first step towards calculating moderately high-temperature spectra of PAHs and other similarly rigid molecules using Monte Carlo sampling.

Molecular mass growth through ring expansion in polycyclic aromatic hydrocarbons via radical–radical reactions

“…10 and 11); these two-dimensional nanostructures resemble graphene-type layers and define molecular building blocks of graphite as detected in the L3 chondrite Khohar 36 and in Murchison 37 . The mechanism outlined here will be crucial in interpreting chemical models of combustion processes 5 and of carbon-rich circumstellar environments 4 , help explain observation of graphitized carbon grains as detected in carbonaceous chondrites 36–40 , and enrich our knowledge of the formation and evolution of carbonaceous matter in the galaxy.…”

“…However, the elucidation of the fundamental reaction pathways to polycyclic aromatic hydrocarbons (PAHs)—organic molecules composed of fused benzene rings with naphthalene (C 10 H 8 ) denoting the simplest representative—in combustion environments 1 and in the interstellar medium (ISM) 2 has posed a long-standing challenge. Sophisticated combustion 3 and astrochemical reaction networks 4 along with flame sampling studies 5 and astronomical surveys 2 reveal that PAHs represent the critical link between resonantly stabilized free radicals (RSFRs) like propargyl (C 3 H 3 • ), allyl (C 3 H 5 • ), and cyclopentadienyl (C 5 H 5 • ) 6,7 and carbonaceous nanoparticles universally referred to as soot and interstellar grains on Earth and in extraterrestrial environments, respectively 2,8,9 . Whereas on Earth, PAHs represent undesirable toxic, often carcinogenic byproducts released in incomplete combustion 10 , in the interstellar medium, PAHs embody up to 20% of the galactic carbon budget 11 and are—along with carbonaceous grains—suggested to play a central role in the formation of vital precursors to molecular building blocks of life such as amino acids 8 .…”

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.…”

Density-functional tight-binding: basic concepts and applications to molecules and clusters

“…However, MD allows to go beyond the harmonic approximation, integrating the IR absorption spectra at finite temperature on-the-fly via the Fourier transform of the autocorrelation function of the electric dipole μ along the trajectories [182] IðωÞ / ω 2 ð þ1 À1 dt hμð0Þ:μðtÞi e iωt (59) where < > indicates a statistical average to minimize spurious correlations. Let us mention that anharmonic effect can also be obtained from a posteriori treatment of cubic and quartic derivatives of the PES [183,184]. However, the quartic constant can only be obtained at the DFT level for small systems, whereas their computation at the DFTB level could allow for the application of such approaches to larger molecules [185,186].…”