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LOCALLY AROMATIC POLYCYCLIC HYDROCARBONS AS POTENTIAL CARRIERS OF INFRARED. EMISSION FEATURES. Simon Petrie and Robert Stranger.
The Astrophysical Journal, 594:869–873, 2003 September 10 # 2003. The American Astronomical Society. All rights reserved. Printed in U.S.A.

LOCALLY AROMATIC POLYCYCLIC HYDROCARBONS AS POTENTIAL CARRIERS OF INFRARED EMISSION FEATURES Simon Petrie and Robert Stranger Department of Chemistry, the Faculties, Australian National University, Canberra, ACT 0200, Australia

and Walter W. Duley Department of Physics, University of Waterloo, Waterloo, ON N2L 3G1, Canada; [email protected] Received 2003 February 13; accepted 2003 May 22

ABSTRACT We report B-LYP/6-31G* and B3-LYP/6-31G* density functional theory calculations on a set of polycyclic hydrocarbons, ranging in size from C19H22 to C36H32, combining aromatic (unsaturated) and aliphatic (saturated, sp3-hybridized carbon) ring systems. These locally aromatic polycyclic hydrocarbons (LAPHs), generally exhibiting large deviations from planarity, may be considered as intermediate structures between polycyclic aromatic hydrocarbons (PAHs) and nanodiamonds. Calculated infrared vibrational frequencies are found to be similar to those observed experimentally in spectra of hydrogenated amorphous carbon (HAC) and other carbonaceous solids. In the C–H stretching region (3.1–3.6 lm) these species are characterized by strong absorption/emission within both the aliphatic and aromatic C–H bands. They also show spectral features associated with tertiary C–H. Similar features are evident in calculated spectra of the corresponding ions, which we have characterized in some cases. Ionization results in the particular enhancement of a spectral feature typically seen at 6.4 lm, in the aromatic C–C stretching region. In keeping with previous experimental and theoretical studies on the spectra of neutral and cationic PAHs, we find that the influence of ionization on the relative intensities of C–C and C–H stretching features is much greater than the influence of molecular structure. We suggest that LAPHs may be significant contributors to emission in Type B unidentified infrared emission sources. Subject headings: astrochemistry — circumstellar matter — dust, extinction — infrared: ISM — ISM: molecules — molecular data

(2000), in the context of an experimental study on plasma reactor processing of naphthalene, are effectively sheetlike, containing a regular array of hexagonal carbon rings. This ordered structure contrasts with the untidy threedimensional networks present in hydrogenated amorphous carbon (HAC; Duley & Williams 1983; Duley 1994; Grishko & Duley 2000) or carbon onions (Tomita, Fujii, & Hayashi 2002). A distinction between planar aromatic molecules and nonplanar hydrocarbons may, however, be more a matter of chemical classification than of astrophysical consequence since many hydrocarbon species apparently coexist in typical UIR sources (Allamandola, Tielens, & Barker 1989). A limited number of objects such as the post– asymptotic giant branch (AGB) star HR 4049 (Geballe et al. 1989; Guillois, Ledoux, & Reynaud 1999), the protoplanetary nebula CRL 2688 (Geballe et al. 1992), and the Herbig Ae/Be stars HD 97048 and Elias 1 (Aitken & Roche 1981; Guillois et al. 1999; Van Kerckhoven, Tielens, & Waelkens 2002) also show the presence of highly ordered nanodiamond particulates in addition to these hydrocarbons. A broad absorption feature, attributable to tertiary C–H groups in hydrocarbons, has also been detected in absorption spectra of dense protostellar clouds (Allamandola et al. 1992). In this study, we investigate some aspects of the structure and vibrational spectroscopy of a class of hydrocarbons having several of the gross structural features of sp3-hybridized carbon networks, such as nanodiamond and HAC, while still containing recognizable benzenoid (aromatic) rings which are, in principle, capable of mutual

1. INTRODUCTION

Detailed identification of the carriers of the unidentified infrared emission (UIR) bands is an important problem in molecular astrophysics. While the UIR features are generally attributed to emission from neutral or cationic polycyclic aromatic hydrocarbons (PAHs; Leger & Puget 1984; Beintema et al. 1996; Allamandola, Hudgins, & Sandford 1999; Bakes, Tielens, & Bauschlicher 2001; Kim, Wagner, & Saykally 2001; Sellgren 2001), variations in UIR line profiles in different astrophysical environments (Joblin et al. 1996; Sloan et al. 1997) suggest that no single molecular species is responsible for the full range of observed spectral features. Several studies have recently explored the possible implications of hydrogenation (Bernstein, Sandford, & Allamandola 1996; Arnoult, Wdowiak, & Beegle 2000; Wagner, Kim, & Saykally 2000; Beegle, Wdowiak, & Harrison 2001; Pauzat & Ellinger 2001) and of aliphatic substitution (Geballe et al. 1992; Joblin et al. 1996; Sloan et al. 1997; Langhoff et al. 1998; Pauzat, Talbi, & Ellinger 1999) on emission by prototypical PAH molecules. Such studies have, so far, considered modifications to PAH skeletons consisting of six-membered rings, although exploration of irregular PAHs featuring central four-, five-, and seven-membered rings has also recently been undertaken (Pauzat & Ellinger 2002). There remains a sense, throughout all of these studies, that interstellar hydrocarbons with some degree of aromaticity can be satisfactorily viewed as planar or near-planar structures. Even the highly hydrogenated networks depicted by Arnoult, Wdowiak, & Beegle 869

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through-space, but not through-bond, electronic interaction. Such structures, with large deviations from planarity within or between C6 rings, may be considered a missing link between PAHs and nanodiamonds. They can also be viewed as molecular analogues of particulate HAC and represent molecular carriers of both aromatic and sp3-derived C–H spectral features. We show that these structures, termed locally aromatic polycyclic hydrocarbons (LAPHs) have vibrational spectra consistent with the general features of the UIR bands. 2. THEORETICAL METHODS

Hydrocarbon structures investigated in the present work have been subjected to geometry optimization and vibrational frequency calculation at the B-LYP/6-31G* level of theory. This combination of the Becke exchange (Becke 1988) and Lee-Yang-Parr correlation (Lee, Yang, & Parr 1988) functionals is one of the most widely used and reliable density functional theory (DFT) approaches and performs very well in the context of predicting the frequencies of vibrational modes. A detailed study by Scott & Radom (1996) has established that, of eight widely used theoretical approaches using the 6-31G* basis set, B-LYP offers the closest agreement (before scaling) with a test set of 1066 frequencies for 122 small molecules. For many of the smaller LAPH structures (i.e., those containing 28 or fewer C atoms), optimization and frequency calculations on both neutral and ionized species were also pursued via the B3-LYP/6-31G* level of theory. This hybrid DFT approach features the same correlation functional and basis set as noted above but utilizes a threeparameter exchange functional developed by Becke (1993). While unscaled B3-LYP/6-31G* frequencies are found to deliver somewhat poorer agreement with experiment than

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do the more economical B-LYP/6-31G* values (Scott & Radom 1996), the B3-LYP approach yields very good scaled harmonic frequencies (Scott & Radom 1996) and has superior performance in the context of calculating vibrational mode IR intensities (Halls & Schlegel 1998). All calculations reported in this work were performed using the GAUSSIAN98 suite of programs (Frisch et al. 1998). 3. RESULTS AND DISCUSSION

Geometry optimizations and frequency calculations have been carried out for representative LAPHs ranging in size from C19H22 to C36H32. The gross structural features of these molecules are summarized in Table 1. The size range adopted is a constraint resulting from the limitations of the computational platforms used in execution of the frequency calculations and does not necessarily reflect the size range of possible significance in interstellar sources. Furthermore, many of our LAPH structures have been engineered with inherent Cs or C2v symmetry to minimize the time required for geometry optimization. In all cases, we find that molecular size or the imposition of comparatively high symmetry has a negligible effect on our calculated solutions for the C–H stretching features. The LAPHs considered are of two basic forms, which we term anchored and bracketed. Anchored LAPHs feature aromatic ring systems attached at one side to an alicyclic backbone, while bracketed LAPHs feature attachment of two alicyclic brackets to the two opposite sides of each aromatic subunit. We may also describe adjacent aromatic layers as either single-spaced or double-spaced. The distinctions between these structural motifs are illustrated in Figure 1. The single-spaced structures exhibit significant internal strain, arising from a mismatch between sp2 and sp3 spacing

TABLE 1 Overview of Structural Details for LAPHs and Other Polycyclic Hydrocarbons Studied in the Present Work Formula

Typea

B1b

B2b

ALc

Spectrum Referenced

C19H22 ........................ C27H26 ........................ C29H26 ........................ C28H26 ........................ C34H36 ........................ C36H32 ........................ C11H18 ........................ C26H32 ........................ C14H10 ........................ C20H12 ........................

DA DAe,f SAe,g SB DBe,h SBe,i aliph aliph arom arom

meCH meCH meCH CH meCH meCH methyladamantane superadamantane ... ...

... ... ... CH meCH meCH ... ... ... ...

bz//bz naph//naph bz/naph/bz bz/naph bz//anth bz/naph/bz ... ... phenanthrene benz[e]pyrene

2a 2b 2c 2d 2e 2f 2g 2h 2i 2j

a SA, SB, DA, and DB indicate LAPH type (S = single-spaced, D = double-spaced, A = anchored, B = bracketed). Aliph and arom indicate fully aliphatic and fully aromatic hydrocarbons, respectively. b Aliphatic component(s) of the molecular structure (B1, B2 = brackets). LAPH brackets correspond to the following parent hydrocarbons: CH = cyclohexane, meCH = methylcyclohexane, dmeCH = dimethylcyclohexane, AD = adamantane, and meAD = methyladamantane. c Aromatic component(s) of the molecular structure (AL = aromatic layers). LAPH aromatic layers correspond to the following parent hydrocarbons: bz = benzene, naph = naphthalene, and anth = anthracene. d The calculated vibrational spectrum of this species is shown in Fig. 2. e Other LAPHs of this type were studied but not depicted in Fig. 2. f Other DA LAPHs studied: C H (B1 = meCH, AL = naph//bz) and C H (B1 = meAD, AL = bz//bz). 23 24 23 26 g Other SA LAPHs studied: C H (B1 = AD, AL = naph/bz) and C H (B1 = meCH, AL = bz/naph/bz). 26 24 33 30 h Other DB LAPHs studied: C H (B1 = B2 = meCH, AL = bz//bz) and C H (B1 = meCH, B2 = dmeCH, 26 32 27 34 AL = bz//bz). i Other SB LAPHs studied: C H (B1 = CH, B2 = meCH, AL = bz/naph), C H (B1 = B2 = AD, AL = bz/ 29 28 36 34 naph), and C40H30 (B1 = B2 = dmeCH, AL = naph/anth).

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Fig. 1.—Optimized geometries of four LAPHs typical of the range of structures studied in the present work: C29H26 (top left), C19H22 (bottom left), C40H30 (top right), and C26H32 (bottom right). Shading distinguishes between aromatic (dark) and alicyclic (light) carbon atoms. The structures shown can be described as anchored (left) and bracketed (right), or as single-spaced (top) and double-spaced (bottom).

requirements: the spacing between rows of available ˚ ) is anchorage points on an sp3-hybridized surface (2.6 A much less than the optimum spacing between sp2-hybridized ˚ for graphite). Strain is manifested via carbon layers (3.4 A a tendency for single-spaced aromatic substructures to splay apart. As a result, single-spaced anchored LAPHs feature markedly nonplane-parallel aromatic layers, while similarly distorted anchorage points in the single-spaced bracketed LAPHs give these molecules a distinctly bowed appearance. These symptoms of internal strain suggest that single spacing of aromatic layers is a destabilizing effect, particularly in bracketed LAPHs. This might inhibit formation of singlespaced LAPHs in astrophysical sources. However, it should be noted that other carbonaceous molecules with high strain, notably fullerenes, are formed quite efficiently under conditions that have been held to mimic certain astrophysical environments (Kra¨tschmer et al. 1990; Kroto & Jura 1992). A tendency toward structural disorder might also be considered to favor LAPH formation in high-temperature circumstellar environments. Table 1 lists the LAPH species considered in the present study. Calculated IR spectra for several LAPHs versus those for representative fully aromatic or fully aliphatic hydrocarbons are shown in Figure 2. Note that throughout this work, we have restricted our scope to analysis of the stretching modes ( > 1200 cm1) of our LAPHs. Lower frequency bending modes are generally less well served by density functional methods, and such bending modes are likely to be more characteristic of structural subtleties of individual LAPHs, thereby limiting their utility in the context of understanding the general features of LAPH IR spectra. Even for the vibrational modes discussed here, the level of precision in our DFT calculations is not considered sufficient to yield detailed agreement with experimental frequen-

Fig. 2.—Calculated spectra, at the B-LYP/6-31G* level of theory, for several polycyclic hydrocarbons. Peak heights for all species are scaled relative to the mode of largest calculated IR intensity over the wavelength range encompassed, while the wavelength for each signal is obtained by scaling the calculated frequency by the recommended factor (0.9945; Scott & Radom 1996). Spectra covering the 6–8.4 lm wavelength range are shown, in each instance, on an intensity scale one-third that of the 3.1–3.5 lm spectrum for the same species. The structures represented in (a)–(c) are anchored LAPHs, (d )–( f ) are bracketed LAPHs, while (g) and (h) are alicyclic (saturated) hydrocarbons, and (i) and ( j) are PAHs (see also Table 1).

cies. It remains clearly apparent that aliphatic C–H, aromatic C–H, and aromatic C–C stretching frequencies in LAPHs are all similar to those of aromatic or aliphatic hydrocarbons, with any systematic shift in the frequencies of these modes being much smaller than the uncertainty intrinsic to vibrational frequency determination via B-LYP/6-31G*. An important characteristic of LAPH spectra is that features are observed in both the aromatic and aliphatic C–H spectral regions. In the B-LYP/6-31G* formalism, calculated wavelengths occur in the range 3.10– 3.27 lm for the aromatic band and 3.33–3.47 lm for aliphatic vibrations. In all LAPH spectra, features in the aliphatic C–H region are generally more intense than those attributed to aromatic C–H vibrations. This arises because of the intrinsic weakness of the aromatic C–H stretching mode relative to aliphatic C–H bands. It should be noted that IR intensities calculated at the B-LYP/6-31G* level are less accurate than vibrational frequencies. The spectral properties of ionized LAPHs are also of interest, and we have performed calculations, employing

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Fig. 3.—Calculated spectra and inset optimized structure for the singlespaced, anchored LAPH C26H24 and its corresponding molecular positive ion. On the C–H stretching region graph (3.2–3.5 lm), all spectra are shown at the same relative intensity scale so as to aid comparison of the influence of ionization upon intensity, but note that for the C–C stretching region (6.1–8.7 lm), the C26Hþ 24 intensities have been cut by a factor of 10 compared to the C26H24 intensity scale.

B-LYP/6-31G*, or B3-LYP/6-31G* on the molecular ions of several of the smaller LAPHs. The calculations on ions, with results typified by Figure 3, show that C–H stretching features are generally blueshifted on ionization, but the shift in C–C stretching frequencies is much less systematic. Furthermore, ionization generally results in a substantial increase in the intensity of C–C stretching band emission (i.e., in the region 6.2–8.7 lm) at the expense of C–H band emission, with aromatic C–C stretching frequencies around 6.4 lm showing particular enhancement in intensity. There is an indication (Fig. 3) that the intensity ratio for aromatic C–H versus aliphatic C–H emission is also sensitive to the LAPH’s charge state, with aromatic emission in the wavelength range 3.2–3.3 lm showing much heavier dampening on ionization than is seen for the aliphatic C–H emission (3.35–3.5 lm). The effect of ionization upon the IR spectral properties of LAPHs appears broadly consistent with the findings that have been reported in many previous experimental (Hudgins, Sandford, & Allamandola 1994; Hudgins & Allamandola 1995a, 1995b; Piest, von Helden, & Meijer 1999; Oomens et al. 2001; Kim & Saykally 2002) and theoretical (Langhoff 1996; Pauzat, Talbi, & Ellinger 1997; Bauschlicher & Langhoff 1997; Kjaergaard, Robinson, & Brooking 2000; Bakes, Tielens, & Bauschlicher 2001; Bakes et al. 2001) studies for ionization of PAHs. For example, experimental measurements of cationic PAH IR emission, whether for gas-phase (Piest, von Helden, &

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Meijer 1999; Oomens et al. 2001; Kim & Saykally 2002) or matrix-isolated (Hudgins & Allamandola 1995a, 1995b) ions, show very strong emission due to C–C stretching, generally without detectable accompanying C–H stretching features. Nevertheless, calculations on large condensed PAH cations such as C96Hþ 24 (Bauschlicher 2002) suggest that aromatic C–H stretching emission in such ions is not completely quenched. An experimental study on C–H stretching modes in jet-cooled alkylbenzene cations, supported by density functional theory calculations, has also found that the intensity of some aliphatic C–H stretching modes in alkylated PAHs such as toluene and ethylbenzene is actually increased on ionization (Fujii et al. 2000). Overall, however, it appears that aliphatic C–H stretching modes are generally reduced in intensity, albeit to a lesser extent than is seen for aromatic C–H stretches, on ionization of alkylated PAHs (Fujii et al. 2000; Bauschlicher & Langhoff 1997). Another inference drawn from an indepth survey of the laboratory and theoretical data is that the influence of charge state on relative intensities of the broad bands of IR emission from PAHs dominates over any effect of molecular structure (Bakes et al. 2001). This tallies with the indications from our calculations on LAPHs that the overall intensities of the C–C and C–H stretching modes are broadly similar for a wide range of LAPH structural motifs (Fig. 2) but are changed very substantially upon ionization (Fig. 3). The dramatic spectral changes resulting from LAPH ionization, evident in the relative intensity of features due to C–C, aliphatic C–H, and aromatic C–H stretching might also be related to the observed variations in intensity seen for these bands in different astrophysical environments (Peeters et al. 2002; Van Diedenhoven et al. 2003). While it is premature to conclude that LAPHs can account for observed astrophysical spectra, the general features of LAPH spectra (Fig. 2) are seen to be similar to those observed in emission spectra of type B UIR sources (Tokunaga 1996; Geballe 1996). Specifically, our simulated spectra (with the caveat concerning intensity) yield comparable band strengths at 3.3 and 3.4 lm in agreement with observation. These molecules contain tertiary C–H groups and would absorb/emit near 3.5 lm, but our calculated spectra for the various small LAPHs do not show the sharp features near 3.53 lm characteristic of tertiary C–H on the surface of larger crystalline diamond particles (Guillois et al. 1999; Chen et al. 2002). These sharp features are also not seen in calculated spectra of extended adamantane, C26H32, which with an effective diameter 0.6 nm might be termed a picodiamond. Note, however, that spectral features attributable to tertiary C–H groups in LAPHs do appear in the region of the broad diamond band detected in absorption in dense protostellar clouds (Allamandola et al. 1992). A surprising result of these calculations is that the IR spectra calculated for LAPHs having only 19–36 C atoms reproduce the general features of the spectra of HAC and other carbonaceous solids (Duley 1993). This suggests that LAPH structures, containing both aromatic and diamondlike carbon bonds, replicate the dominant structural elements of extended amorphous carbon networks. If this result is true in detail, then this would explain the excellent agreement between IR spectra of laboratory samples of HAC and emission spectra of type B UIR sources (Scott & Duley 1996; Scott, Duley, & Jahani 1997). A significant con-

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clusion would then be that HAC particles in the shells of post-AGB stars are effectively nanoparticles and would therefore be subject to single-photon heating. 4. CONCLUSIONS

Density functional theory calculations using the B-LYP/ 6-31G* and B3-LYP/6-31G* levels of theory have allowed us to characterize the structural and spectroscopic features of a set of locally aromatic polycyclic hydrocarbons (LAPHs), which are of interest as possible common carriers of both sp2-derived (aromatic) and sp3-derived (aliphatic) IR emission features seen in various dusty astrophysical environments. They also can incorporate tertiary C–H groups and absorb/emit near 3.5 lm, but the small LAPHs studied here do not exhibit the 3.53 lm feature attributed

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to tertiary C–H stretching in interstellar nanodiamonds. Calculated spectra display features in common with those observed in interstellar sources and with those seen in HAC. The influence of ionization on spectral intensities is found to be substantial and supports the results of previous studies on the spectral properties of ionized PAHs. Further study is needed to establish the optimum structural characteristics of larger LAPHs and the possible involvement of LAPHs as precursors to the formation of nanodiamonds, PAH conglomerates, and particulate HAC. This research was supported by a grant from the NSERC of Canada (W. W. D.) and by the allocation of supercomputing resources, housed at the ANU Supercomputing Facility, from the Australian Partnership of Advanced Computing (S. P. and R. S.).

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