Variations in the Peak Position of the 6.2 μm Interstellar Emission Feature: A Tracer of N in the Interstellar Polycyclic Aromatic Hydrocarbon Population

Abstract

This paper presents the results of an investigation of the molecular characteristics that underlie the observed peak position and profile of the nominal 6.2 m interstellar emission band generally attributed to the CC stretching vibrations of polycyclic aromatic hydrocarbons (PAHs). It begins with a summary of recent experimental and theoretical studies of the spectroscopic properties of large (>30 carbon atoms) PAH cations as they relate to this aspect of the astrophysical problem. It then continues with an examination of the spectroscopic properties of a number of PAH variants within the context of the interstellar 6.2 m emission, beginning with a class of compounds known as polycyclic aromatic nitrogen heterocycles (PANHs; PAHs with one or more nitrogen atoms substituted into their carbon skeleton). In this regard, we summarize the results of recent relevant experimental studies involving a limited set of small PANHs and their cations and then report the results of a comprehensive computational study that extends that work to larger PANH cations including many nitrogen-substituted variants of coronene + (C 24 H þ 12 ), ovalene + (C 32 H þ 14 ), circumcoronene + (C 54 H þ 18 ), and circum-circumcoronene + (C 96 H þ 24 ). Finally, we report the results of more focused computational studies of selected representatives from a number of other classes of PAH variants that share one or more of the key attributes of the PANH species studied. These alternative classes of PAH variants include (1) oxygen-and silicon-substituted PAH cations; (2) PAH-metal ion complexes (metallocenes) involving the cosmically abundant elements magnesium and iron; and (3) large, asymmetric PAH cations.Overall, the studies reported here demonstrate that increasing PAH size alone is insufficient to account for the position of the shortest wavelength interstellar 6.2 m emission bands, as had been suggested by earlier studies. On the other hand, this work reveals that substitution of one or more nitrogen atoms within the interior of the carbon skeleton of a PAH cation induces a significant blueshift in the position of the dominant CC stretching feature of these compounds that is sufficient to account for the position of the interstellar bands. Subsequent studies of the effects of substitution by other heteroatoms (O and Si), metal ion complexation ( Fe + , Mg + , and Mg 2+ ), and molecular symmetry variation-all of which fail to reproduce the blueshift observed in the PANH cations-indicate that N appears to be unique in its ability to accommodate the position of the interstellar 6.2 m bands while simultaneously satisfying the other constraints of the astrophysical problem. This result implies that the peak position of the interstellar feature near 6.2 m traces the degree of nitrogen substitution in the population, that most of the PAHs responsible for the interstellar IR emission features incorporate nitrogen within their aromatic networks, and that a lower limit of 1%-2% of the cosmic nitrogen is sequestered within the interstellar PAH populati...

PRESSURE EFFECTS IN POLYCYCLIC AROMATIC NITROGENATED HETEROCYCLES (PANHs): DIAGNOSTIC QUALITIES AND COSMOBAROMETRY POTENTIAL

“…Acridine and anthracene have previously been observed to be practically indistinguishable at ambient conditions using infrared spectroscopy (Fialkovskaya & Nefedov 1968;Langhoff et al 1998;Hudgins et al 2005). Vibrational assignments have been made for both molecules and are summarized in Table 2 (Brigodiot & Lebas 1972).…”

“…The shift of the 6.2 μm peak to lower wavelengths is hypothesized to reflect N-substitution inside the PAH structure as opposed to on the outside edge. Multiple N-substitutions are predicted to have a similar peak-broadening effect (Langhoff et al 1998;Hudgins et al 2005). Although the consquences of varying levels of N-substitution have been studied, the effects of pressure on PANHs have not previously been investigated.…”

“…It has been observed previously that the infrared spectra of the PAH anthracene and the PANH acridine, despite the obvious differences in atomic structure, molecular, and crystal symmetry, are functionally indistinguishable (Fialkovskaya & Nefedov 1968;Langhoff et al 1998;Hudgins et al 2005). Although additional small peaks are observed in the spectra of acridine owing to symmetry splitting in the C-C stretching and C-H bending regions, these are small and difficult to observe without a high signal-to-noise source such as synchrotron infrared light (Langhoff et al 1998;Charnley et al 2005).…”

Laboratory infrared spectroscopy of PAHs

“…Recent applications include the study of nitrogen containing PAHs (Mattioda et al 2003), which have been suggested to provide a better match for the interstellar 6.2 µm emission band (Hudgins et al 2005;Peeters et al 2002), and of complexes of PAHs and atomic Fe, which have been suggested to be stable in interstellar clouds (Klotz et al 1996;Wang et al 2007). The method has also been applied to record PAH spectra in the far-infrared range of the spectrum (Mattioda et al 2009), where very molecule-specific vibrational modes -the socalled "drumhead" modes -are located.…”

State-Resolved Dynamics of the CN(B2Σ+) and CH(A2Δ) Excited Products Resulting from the VUV Photodissociation of CH3CN

“…18,22 Furthermore, CN provides a mechanism for incorporating nitrogen into the ring structure of polycyclic aromatic hydrocarbons to form polycyclic aromatic nitrogen heterocycles (PANHs). 23 Much improved fits to a 6.2 µm interstellar emission feature can be achieved by considering PANHs, 24 thus these species may provide a route to understanding the origins of the diffuse interstellar bands. 25 The need for additional spectroscopic studies of excited species relevant to astrochemistry has also recently been noted.…”