Polycyclic Aromatic Hydrocarbons with armchair edges and the 12.7 μm band
aa r X i v : . [ a s t r o - ph . GA ] O c t Polycyclic Aromatic Hydrocarbons with armchair edges and the12.7 µ m band. A. CandianLeiden Observatory, Niels Bohrweg 2, 2333-CA, Leiden, The Netherlands [email protected] andP. J. SarreSchool of Chemistry, The University of Nottingham, Nottingham, UKandA. G. G. M. TielensLeiden Observatory, Niels Bohrweg 2, 2333-CA, Leiden, The NetherlandsReceived ; acceptedTo appear in ApJL 2 –
ABSTRACT
In this Letter, we report the results of density functional theory calculationson medium-sized neutral Polycyclic Aromatic Hydrocarbon (PAH) moleculeswith armchair edges. These PAH molecules possess strong C-H stretching andbending modes around 3 µ m and in the fingerprint region (10-15 µ m), and alsostrong ring deformation modes around 12.7 µ m. Perusal of the entries in theNASA Ames PAHs Database shows that ring deformation modes of PAHs arecommon, although generally weak. Therefore, we then propose that armchairPAHs with N C >
65 are responsible for the 12.7 µ m aromatic infrared band inH ii regions and discuss astrophysical implications in the context of the PAHlife-cycle. Subject headings: astrochemistry - infrared: ISM - ISM: lines and bands - ISM:molecules - line: identification - molecular data
1. Introduction
Polycyclic Aromatic Hydrocarbon (PAH) molecules are commonly accepted tobe responsible for a family of emission bands, the so-called Aromatic Infrared Bands(AIBs), which are seen in a wealth of astronomical environments: H ii regions,photodissociation regions (PDRs), planetary and reflection nebulae, star formingregions, young stellar objects, galactic nuclei and (Ultra) Luminous Infrared Galaxies(Hony et al. 2001; Van Diedenhoven et al. 2004; Tielens 2008). However, despite mucheffort not a single PAH molecule has been identified unequivocally. Understanding thedifferent components of the PAH population in terms of their molecular structure, chargeand (de)hydrogenation would allow us to use them as a powerful tool to investigate thephysical and chemical properties of astronomical objects (Tielens 2008).The 11-15 µ m region shows a host of features which have been attributed to C-Hout-of-plane (OOP) bending modes (Hony et al. 2001). The strongest of these bandspeak at 11.2 µ m and 12.7 µ m and they come together with weaker ones at 11.0, 12.0,13.5 and 14.2 µ m (Hony et al. 2001); all of them are perched upon a featureless andbroad plateau (Tielens 2008). The 12.7 µ m band, due to the blending with atomic lines(Hu α at 12.37 µ m, [NeII] at 12.8 µ m) and molecular lines (H − µ m)was not studied in much detail before the advent of the Infrared Space Observatory(Kessler et al. 1996). In contrast to the red shaded asymmetric profiles of the 6.2 and11.2 µ m bands, the 12.7 µ m band is characterised by a slow blue rise and a red steepside (Hony et al. 2001); its intensity with respect to the nearby 11.2 µ m band varies byalmost an order of magnitude according to object, reaching its relative maximum in H ii regions (Hony et al. 2001). Regardless of their charge, PAHs with duo or trio H showa band near 12.7 µ m due to C-H OOP bending modes. As OOP modes are generallyweak relative to the CC modes in cations (Hudgins et al. 1994), the 12.7 µ m band is 4 –attributed to the OOP modes in neutral molecules containing duo or trio hydrogens(Hony et al. 2001; Bauschlicher et al. 2008; Bauschlicher et al. 2009; Ricca et al. 2012).Here we reexamine the assignment of the 12.7 µ m feature based on the results of newquantum chemical calculations.This Letter is organised as follows: Section 2 contains details of the specific PAHstructures studied and the computational methods used; Section 3 discusses the results ofthe calculations, and in Sections 4 and 5 the astrophysical implications and the conclusionsare presented.
2. Computational details
There are several ways to classify PAHs, mostly based on the structure of their carbonskeleton and on the number of adjacent peripheral hydrogen atoms. If we consider PAHs assmall sized graphene flakes, they can be divided according to the shape of their edges. Thuswe have PAHs with armchair edges like chrysene and PAHs with zigzag edges like tetracene(Figure 1(a)). PAHs with zigzag edges possess mostly solo (non-adjacent) hydrogen atoms,while PAHs with armchair edges possess mostly duo hydrogens which suffer from sterichindrance in the so called bay regions (Figure (1a))Density functional theory has proven to be a powerful tool to study the vibrationalspectra of large molecules such as PAHs (Langhoff 1996; Pathak & Rastogi 2005;Pathak & Rastogi 2007; Bauschlicher et al. 2008; Bauschlicher et al. 2009). In this study,the popular Becke three-parameter (Becke 1993), Lee-Yang-Parr (Lee et al. 1988) B3LYPfunctional was used in conjunction with the 4-31G basis set (Frisch et al. 1984) onGaussian 03 (Frisch et al. 2003) to optimize molecular geometries and to compute harmonicvibrational spectra. Langhoff (1996) showed that a scaling factor, 0.958, is needed to 5 –bring theoretical harmonic frequencies into agreement with experimental frequencies; thisholds also for PAHs with armchair edges (Langhoff 1996). The spectra are then scaledand convolved with Lorentzian profiles of 30, 20 and 10 cm − full-width half-maximum(FWHM) in the 3.1-3.4, 6-9, and 9-15 µ m regions, respectively. Theoretical IR spectra arein absorption while astronomical IR spectra are in emission; commonly a redshift of 15 cm − is invoked to account for anharmonicity in the emission process (Bauschlicher et al. 2008).Given the lack of robust constraints on the value of the shift, in this study we chose tobe conservative and we do not apply a redshift to the theoretical spectra. The softwareGABEDIT (Allouche 2010) was used to visualize the results of the calculations, and inparticular to view the vibrational modes. The molecular structures studied are summarisedin Figure 1(b): they were constructed to have increasing number of bay regions whileretaining D h symmetry.
3. Results & Discussion
The theoretical IR spectra of the studied PAHs between 3 and 15 µ m are shownin Fig. 2 (upper and middle rows). They all show strong bands around 3.25 µ m and inthe fingerprint (11-15 µ m) region caused by C-H stretches and bends, as is typical forneutral PAHs (Langhoff 1996). The pyrene-like PAHs (Fig. 2, upper row, first panel)possess several C-H stretching modes resulting in two strong peaks at 3.23 and 3.26 µ m,in agreement with previous studies (Bauschlicher et al. 2009; Candian et al. 2012). Themajor contribution to the first peak is due to the symmetric stretching of C-H bondsinvolving duo hydrogens, while the second is the result of asymmetric stretching of C-Hbonds involving duo hydrogens and trio hydrogens. Increasing the size of the moleculeresults in more pronounced peaks, with the relative intensity I . /I . varying from 0.6 to0.8 and a blue shift of the 3.23 µ m peak. Perylene-like PAHs (Figure 2, middle row, first 6 –panel) also possess a double-peaked band in the 3 µ m region that behaves similarly.In pyrene-like PAHs, the 6-9 µ m region appears devoid of strong features (Figure2, upper row, second panel). However, for perylene-like structures (Fig. 2, second row,middle panel) two moderately strong bands occur at ≈ µ m, the former dueto C-C stretching and the latter to a concerted in-plane bending motion of C-H bonds andstretching of C-C bonds. The positions of the bands appear independent of size, whilethe intensity increases with PAH size as can be expected for the larger number of bondsinvolved in the mode.In the fingerprint region (10-15 µ m) of pyrene-like PAHs (Figure 2, upper row, thirdpanel) two peaks are noticeable around 12.0 and 12.7 µ m. The peak at shorter wavelengthis due to C-H OOP bending mode in duo Hs – in agreement with accepted assignments(Allamandola et al. 1989). Visualisation of the atomic displacements proves that the peakat 12.7 µ m is the results of two equally strong, almost overlapping transitions due to a duoC-H OOP bending mode and a ring deformation mode (Table 1 and Figure 3). This lastvibrational mode is shown as a movie in Figure 3. It involves the rings belonging to theupper part of the armchair structure; the central “lone” rings keep the structure flexible,thus also preserving the intensity of the band for longer molecules. The intensity of thedeformation mode increases steadily with number of carbon atoms and/or size as for theC-H OOP mode (Table 1).In perylene-like molecules (Figure 2, middle row, third panel), the fingerprint region ismore complex; the peak around 12.7 µ m originates again from the C-C ring deformationmode and the two peaks at ≈ . µ m and ≈ . µ m are due to duo and trio C-H OOPbending modes, respectively. Weak modes present at shorter wavelengths are due to a mixof C-H in plane and C-C bending modes.The infrared spectra of positively charged C H and C H were computed to verify 7 –the effect of ionisation on the ring deformation modes around 12.7 µ m. The band suffers apronounced decrease in the intensity (Figure 2, lower row, third panel and Table 1), whileits position shifts to shorter wavelengths; this is an effect which is generally true for allcationic PAHs compared to their neutral counterparts (Hudgins et al. 1994). Vibrationalmodes in other regions of the spectrum behave as expected (Figure 2, lower row).We then inspected the NASA AMES PAH Theoretical Database (Bauschlicher et al. 2010;Boersma et al. 2014) to check whether the ring deformation mode around 12.7 µ m waspresent in other neutral and charged PAHs. We found that all PAHs in the database witharmchair structure possess a ring deformation mode in the range 12.3-12.8 µ m. Also PAHswith a non-rigid central ring, such as dicoronylene (C H , uid=100), show a deformationmode around 12.55 µ m. The intensity of the ring deformation mode strongly dependson the flexibility of the carbon structure and on the degree of symmetry. It reaches amaximum in the molecules analysed in this Letter (Figure 1(b)), where the (series) of lonecentral rings facilitates the ring deformation mode without disrupting the symmetry; alower symmetry results in decoupling of the vibrational modes. As the carbon structurebecomes more rigid, e.g. in the case of C H (uid=555), the intensity of the deformationmode around 12.59 µ m rapidly decreases and it becomes very weak in pericondensed PAHswith armchair edges like C H (uid=128).
4. Astrophysical implications
While the 12.7 µ m band is seen in a wide range of astronomical environments, itappears to be particurlarly prominent in the IR spectra of H ii regions, where its strengthcan be comparable to that of the 11.2 µ m band (Hony et al. 2001). Likewise, the 12.7 µ mband is very strong in the surface layers of PDRs where the 6.2 and the 7.7 µ m bands arealso (relatively) strong. Our calculations show that neutral PAHs with armchair edges 8 –and/or central flexible ring(s) possess a ring deformation mode in the 12.3-12.7 µ m range,where the intensity depends on the flexibility of the molecule and its degree of symmetry.These PAH molecules can thus make an important contribution to the 12.7 µ m bandstrength, especially in H ii regions. However, to reproduce the relatively low 11.2/12.7 µ mobservational intensity ratio, these molecules should not possess solo (non-adjacent)hydrogen atoms. Indeed, the intrinsic intensity of the solo OOP bending mode is higherthan the intensity of a ring deformation mode (Candian 2012). Thus, the interstellar 11.2and 12.7 µ m bands must be carried by separate populations of PAHs. In contrast, withinthis framework, the 12.0 and 12.7 um bands would result from these armchair PAHs. InFigure 4 we compare the 12.0/12.7 µ m ratio as function of size (N c ) in pyrene-like molecules(squares) with the values measured in H ii regions (dotted lines) by Hony et al. (2001).This comparison reveals that PAHs in excess of 65 C atoms are needed to reproduce theobserved low ratio. This is in agreement with the size estimate for interstellar PAHs derivedfrom the observed 3.3/11.2 µ m ratio (Tielens 2008; Ricca et al. 2012).The observed good correlation between the 12.7 µ m band and the 6.2 µ m band hasalways presented an enigma for the PAH model as the former is invariably attributedto neutral PAHs while the latter is carried by ionized PAHs. Generally, this correlationis taken to imply that the conditions that are leading to ionization of interstellar PAHsalso promote a high abundance of PAHs with corners ( i.e. , PAHs with duo and trio Hatoms; (Hony et al. 2001; Ricca et al. 2012)). This may then imply that PAHs are rapidlybroken down in the ionization zones and the fragmentation process produces small PAHsthat carry the 12.7 µ m band. However, as the lifetime of small PAHs is expected to be Visualisation of the vibrational modes of dicoronylene with the NASA AMES PAHDatabase online tool shows a strong (126.1 km/mol) solo OOP bending mode at 11.19 µ m,compared to the deformation mode at 12.55 µ m (26.3 km/mol). 9 –much shorter than for large PAHs, it is unlikely that this interpretation can withstand aquantitative analysis. Moreover, the main destruction channel for interstellar PAHs is likelyinitiated by complete dehydrogenation and the formation of pure carbon graphene flakesand/or cages (Ekern et al. 1998; Joblin 2003; Zhen et al 2014; Bern´e & Tielens 2012).Therefore, breakdown of interstellar PAHs will not favour the formation of the 12.7 µ mcarriers. The identification of the 12.7 µ m band with armchair PAHs proposed here shinesnew light on this problem. In particular, PAHs with armchair edges are known to bemuch more stable than PAHs with zigzag edges (Poater et al. 2007). Thus, in the surfacelayers of PDRs, UV photons lead to rapid ionization while also reducing the interstellarPAH family to its most stable form, the armchair PAHs. Theoretical studies have shownthat carbon loss (C H n ) from large compact PAHs leads to the formation of armchairstructures (Bauschlicher & Ricca 2014). While this class of PAH molecules can describethe qualitative behaviour of the observed 12.7/6.2 µ m bands, experimental studies on thephotochemical evolution of large PAHs are needed to establish their survival rate and toquantify the abundance of elongated armchair PAH molecules in the ISM.Candian et al (2012) proposed that the 3.3 µ m C-H band of class A – as observed inH ii regions – can be explained in terms of a two-component population of PAHs: compactmolecules and molecules with bay regions. The PAHs investigated here possess bay regionsand show two strong peaks in the C-H stretch region (Figure 3). They can thus contributeto the bay component responsible for the short-wavelength part of the 3.3 µ m band. Aninteresting way to explore this hypothesis would be follow the spatial variation of the twocomponents of the 3.3 µ m band and of the 12.7 µ m band in a sample of PDRs. 10 –
5. Conclusions
In this Letter we studied the vibrational spectra of PAHs with armchair edges bymeans of Density Functional Theory. The results presented here demonstrate that couplingbetween the C-H OOP mode and the C-C ring deformation mode in this sub-class ofspecies lead to a strong 12.7 µ m band. Hence we propose that armchair PAHs with N c > µ m band, previously attributed to duo andtrio C-H out-of-plane bending modes. It is inferred that PAHs with armchair edges arefavoured in regions exposed to strong UV processing. In the future, the full coverage ofthe AIB spectrum attained by the James Webb Space Telescope will allow us to studythe spatial variation of single AIB features with unprecedented sensitivity, helping us tofurther understand the contribution of PAHs with armchair edges to the astronomical PAHpopulation.AC acknowledges STFC and The University of Nottingham for scholarships andCameron Mackie for helping with the animation. Studies of interstellar PAHs at LeidenObservatory are supported through advanced European Research Council Grant 246976and a Spinoza award. The calculations were performed at the High Performance Computer(HPC) facility at The University of Nottingham (UK) and at the SARA supercomputercenter in Amsterdam, The Netherlands (project MP-270-13). 11 –
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Chrysene Tetracene
Bay b)a)
Fig. 1.— (a) Examples of isomeric PAH molecules (C H ) with different edges. Chrysene(left) possesses armchair edges, while tetracene (right) has zigzag edges. Note that PAHswith armchair edges have bay regions. (b) Molecular carbon structures of studied PAHs witharmchair edges in order of size. These moelcules can be described as pyrene-like (ending witha group of three adjacent hydrogens, e.g. C H ) and perylene-like (ending with two groupsof three adjacent hydrogens, e.g. C H ). All of them belong to the D h point group. 14 – pyrene-likeperylene-like Fig. 2.— Theoretical absorption IR spectra of the molecular structures of Figure 1(b). ALorentzian broadening and a scaling factor are applied (see text for details). For ease ofcomparison, dotted lines are drawn at 3.22, 3.26, 12.0 and 12.7 µ m. The upper row showsthe spectra of pyrene-like structures, the middle row the spectra of perylene-like structuresand the lower row a comparison between the IR spectrum of the first member of each series(dashed line), and the spectrum of its positively charged counterpart (solid line). 15 –Fig. 3.— Visualisation of the vibrational mode occurring around 12.7 µ m in C H . Thecarbon skeleton is shown in grey, the hydrogen atoms in white, and the arrows represent themotion direction and intensity. The ring deformation mode, belonging to the B u symmetry,involves rings forming the upper part of the armchair edge. (An animation of this figure isavailable in the online journal. 16 –Fig. 4.— Ratio of the theoretical intrinsic intensity of the 12.0 µ m band (C-H OOP bends)and the 12.7 µ m band (duo C-H OOP bends+ C-C deformation mode) as function of thenumber of carbon atoms for the pyrene-like series. The solid line shows the result of a linearfit. Calculation of the intensity through an emission model would only slightly increasethe ratio given the proximity of the 12.0 and 12.7 µ m bands. The horizontal dotted linesindicates the observed astronomical range in H ii regions as measured by Hony et al (2001). 17 –Table 1: Vibrational modes in the 12-14 µ m Region for the molecules studiedPyrene-like Perylene-likeMolecule µ m (cm − ) Int Type Molecule µ m (cm − ) Int TypeC H H H H H H H H H +14 H +16 Frequencies are scaled. Infrared intensities are in km mol −1