Advances in Space Research 36 (2005) 166–172 www.elsevier.com/locate/asr
Laboratory IR spectra of 4-azachrysene in solid H2O
M.P. Bernstein *, S.A. Sandford, R.L. Walker
NASA-Ames Research Center, MS 245-6, Moﬀett Field, CA 94035, USA
Received 8 October 2004; received in revised form 12 May 2005; accepted 12 May 2005
We present the mid-IR spectrum of 4-azachrysene (C17H11N) frozen in solid H2O at 14 K, data directly comparable to astronom- ical observations along dense cloud lines of sight. We tabulate the positions, proﬁles, and relative intensities of those 4-azachrysene peaks not obscured by strong H2O absorptions and note some signiﬁcant changes in position and/or intensity relative to the pre- viously published values for 4-azachrysene isolated in an argon matrix. In contrast to simple PAHs that do not interact strongly with solid H2O, PANHs, with their nitrogen atom(s), are potentially capable of hydrogen bonding with H2O, and this presumably gives rise to some of the spectral changes. This demonstrates that observers will not always be able to rely on peak positions of matrix isolated PANHs to correctly reﬂect the actual absorption band positions of PANHs along lines of sight where they will exist as pure solids or be frozen in H2O. In general these nitrogen heterocycles are of astrobiological interest since this class of molecules has been detected in meteorites, they could be pre-biotically important, and/or they could act as false biomarkers. 2005 COSPAR. Published by Elsevier Ltd. All rights reserved.
Keywords: IR spectroscopy; Dense molecular cloud; H2O; Aromatic; PAH; PANH
1. Introduction mately mixed with solid H2O and begged the question to what extent these conditions might change the posi- Polycyclic aromatic hydrocarbons (PAHs) are com- tions and proﬁles of absorptions from aromatic monly assigned to astronomical infrared (IR) emission compounds. features based on comparisons to laboratory data (All- In a previous paper, we measured IR spectra of the amandola et al., 1999). IR astronomy suggests that aro- smallest PAH, naphthalene (C10H8), in solid H2Oat matic molecules are ubiquitous and, as a class, the most various temperatures and concentrations and saw only common organic compounds in the universe (Cox and modest changes in the IR spectra (Sandford et al., Kessler, 1999, Snow and Witt, 1995). IR spectra of 2004). In this paper, we extend our lab spectroscopy of PAHs, and their cations, isolated in inert gas matrix aromatics under conditions germane to astronomy to a have been employed to ﬁt astronomical observations polycyclic aromatic nitrogen heterocycle (PANH) – a of IR emission from gas-phase interstellar molecules PAH-like molecule with a nitrogen in the ring structure. (Peeters et al., 2002). IR absorptions of aromatic mole- These compounds are present in meteorites (Stoks and cules along lines of sight that include cold dust and ice Schwartz, 1982; Basile et al., 1984; Pizzarello, 2001), (Smith et al., 1989; Sellgren et al., 1995; Brooke et al., are predicted to be a component of TitanÕs haze (Ricca 1999; Chiar et al., 2000; Bregman et al., 2000; Bregman et al., 2001), and should be present in the ISM (Mat- and Temi, 2001) suggest that aromatics might be inti- tioda et al., 2003; Hudgins and Allamandola, 2004), al- beit a lower abundance than their normal PAH * Corresponding author. Tel.: +1 650 604 0194; fax: +1 650 604 6779. counterparts (Kuan et al., 2003). Despite their potential E-mail address: [email protected] (M.P. Bernstein). importance there are to-date no spectra of any PANHs
0273-1177/$30 2005 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2005.05.050 M.P. Bernstein et al. / Advances in Space Research 36 (2005) 166–172 167
À16 À17 À17 at low temperature in solid H2O, as they should occur in tions (i.e., 1.7 · 10 ,1· 10 , and 2.8 · 10 cm/ dense molecular clouds or the outer Solar System. molecule at 3.0, 6.25 and 13.3 lm, respectively, Hudgins In contrast to normal PAHs composed solely of car- et al., 1993). Given these assumptions, we estimate the bon and hydrogen that have IR spectra insensitive to H2O/4-azachrysene ratio for the mixture depicted in matrix (Sandford et al., 2004; Bernstein et al., 2005), the ﬁgures is in the range 500–1400, so the 4-azachrysene PANHs with their nitrogen atom(s) are potentially molecules are largely isolated from one another within capable of hydrogen bonding with H2O, making their the H2O matrix. spectroscopy more interesting and complicated. In this Typical samples were deposited at a rate suﬃcient to paper, we present the spectrum of the PANH 4-azachry- produce samples 0.1 micron thick after a few minutes. sene in solid H2O at 14 K and compare the positions, Under these conditions the samples are composed of an proﬁles, and relative intensities of the absorptions to intimate mixture of the PANH in high density amor- those previously reported for this molecule in an argon phous H2O. This form of H2O is believed to be represen- matrix (Mattioda et al., 2003). We have chosen 4-aza- tative of H2O-rich ices in interstellar molecular clouds chrysene as a model compound to represent its class. (Jenniskens and Blake, 1994; Jenniskens et al., 1995). It is large enough that it is stable and easy to work with, The C–H stretching features (in the 3.2–3.4 lm re- and it has a non-symmetric structure that includes one, gion) of 4-azachrysene in H2O are far more uncertain two, and three adjacent C-Hs, so its spectrum contains than the other absorptions because of their proximity absorptions corresponding to many molecular motions. to the very strong 3 lmH2O band. The C–H stretches of 4-azachrysene in H2O seen in Fig. 2(b) came from spectra that had been modiﬁed to remove the strong 2. Experimental techniques H2O band that masked them (compare traces a and b in Fig. 2 for an example). This was done in the following The basic techniques and equipment employed for manner: an IR spectrum of pure H2O was measured un- this study have been described previously as part of der conditions identical to those of the H2O/4-azachry- our previous studies of aromatics matrix isolated in Ar- sene mixture, and it was scaled so that the 3 lmH2O gon and in H2O at low temperature (Mattioda et al., band in the spectra matched. Then, the spectrum of pure 2003). Details associated with the materials and meth- H2O was gradually subtracted from the spectrum of the ods used that are unique to this particular study are pro- H2O/4-azachrysene mixture until the C–H stretching vided below. features were revealed. Naturally, the spectrum of pure The H2O was puriﬁed via a Millipore Milli-Q water H2O is not identical to that of H2O in mixture, so system to 18.2 MX and freeze–pump–thawed three times attempting to subtract out all of the H2O absorptions to remove dissolved gases prior to use. 4-Azachrysene resulted in artifacts (such as negative peaks in some re- (C17H11N) was obtained from the National Cancer gions). We found that removing 75% of the H2O gave InstituteÕs Chemical Carcinogen Reference Standard a spectrum where the 4-azachrysene C–H absorptions Repository operated by the Midwest Research Institute. were apparent, while the spectrum seemed free of obvi- All samples are of unspeciﬁed purity; however, the ab- ous aberrations, and this is what appears in Fig. 2. This sence of any notable discrepant spectral features is the same technique we ﬁrst employed in our studies of between the theoretical calculations and IR spectra of H2O/naphthalene mixtures (Fig. 9 of Sandford et al., 4-azachrysene isolated in solid argon indicate impurity 2004). The table does not include values for the C–H levels are no more than a few percent. Samples were pre- stretches of 4-azachrysene because we do not regard pared by subliming the solid 4-azachrysene from a Pyrex their area as accurate after the spectral subtraction that tube at 104 C directly onto the CsI substrate while co- was used to reveal them. depositing H2O vapor from room temperature bulb. As a result we cannot know the concentration in our sam- ples with certainty. However, we can estimate the 3. Results and discussion H2O/4-azachrysene ratio given certain assumptions about absolute intensities of bands for 4-azachrysene The IR spectrum of the four ring PANH 4-azachry- À1 and H2O. The band strength for the 1410 cm peak sene (C17H11N) isolated in inert matrix has been pub- of 4-azachrysene in argon is 5 · 10À18 cm/molecule lished previously (Mattioda et al., 2003). However, this according to Mattioda et al. (2003). First, we assume is the ﬁrst IR spectrum of 4-azachrysene in solid H2O that this value of 4-azachrysene isolated in argon is valid at 14 K, conditions that are relevant to lines of sight for our H2O/C17H11N mixture, which may not be such a including dense cloud material. À1 bad approximation based on what we observed for H2O/ The 3800–550 cm (2.63–18.2 lm) mid-IR spectra of naphthalene mixtures (Sandford et al., 2004). Second, 4-azachrysene isolated in an argon matrix and in solid we assume that the absorptions of H2O in our mixtures H2O are presented in Fig. 1. The large broad absorp- are similar to those of pure H2O under similar condi- tions in the latter (lower spectrum) centered near 3300, 168 M.P. Bernstein et al. / Advances in Space Research 36 (2005) 166–172
À1 Fig. 1. The 3800–550 cmÀ1 (2.63–18.2 lm) IR spectra of 4-azachry- Fig. 2. The 3130–2950 cm (3.195–3.390 lm) IR spectra, from sene (C17H11N) isolated in an argon matrix (top trace, Mattioda et al., bottom to top, of (a) 4-azachrysene in H2O, (b) 4-azachrysene in 2003) and in solid H2O (bottom trace). The large broad absorptions in H2O minus 75% of a comparable spectrum of pure H2O, and (c) 4- the (lower) spectrum centered near 3300, 2200, 1600, and 750 cmÀ1 azachrysene isolated in an argon matrix (from Mattioda et al., 2003) at À1 (3.0, 4.6, 6.25, and 13.3 lm) are caused by amorphous solid H2Oat 14 K. Note how the strong 3300 cm (3.0 lm) O–H stretching band of 14 K. Because H2O absorptions dominate the spectrum many of the H2O reduces the spectral contrast of the aromatic C–H stretching 4-azachrysene features are obscured. An * indicates the presence of a bands. Subtracting out a spectrum of pure H2O reveals the C–H group of peaks in the matrix spectrum caused by, or with a large stretches of 4-azachrysene in H2O as an absorption centered near À1 À1 contribution from, matrix isolated H2O. 3050 cm (3.28 lm) and ranging from 3090 to 2990 cm (3.24– 3.34 lm), encompassing the frequencies of those of 4-azachrysene in an argon matrix. 2200, 1600, and 750 cmÀ1 (3.0, 4.6, 6.3, and 13.3 lm) are caused by the amorphous solid H2O at 14 K. Expan- sions of these spectra in the 3130–2950 cmÀ1 (3.17– 3.45 lm) region are presented in Fig. 2. The strong 3 lmH2O peak hides the C–H stretches of 4-azachry- sene in H2O that are plainly evident at 3090, 3060, and 3035 cmÀ1 (3.24, 3.27, and 3.29 lm) in the spectrum of 4-azachrysene in argon (compare traces a and c in Fig. 2). However, the central spectrum of 4-azachrysene in H2O from which some pure H2O has been subtracted (Fig. 2(b); see Section 2 for details) demonstrates that the C–H stretches merge into one broad feature centered near 3050 cmÀ1 (3.28 lm) and extending from 3090 to 2990 cmÀ1 (3.24–3.34 lm; FWHM 50 cmÀ1). This broad feature encompasses the individual C–H stretch- ing features of 4-azachrysene in argon reported by Mat- tioda et al. (2003) seen in trace c of Fig. 2, and is consistent with observations of aromatics in absorption, discussed further in Section 4.1. This broadening is sim- ilar to the C–H stretches of the two-ring PAH naphtha- Fig. 3. The 1625–990 cmÀ1 (6.15–10.1 lm) IR spectrum of 4-azachry- lene in H2O (Sandford et al., 2004). sene in H2O (below) compared to that of 4-azachrysene isolated in an Fig. 3 displays those 4-azachrysene peaks in the por- argon matrix (above, from Mattioda et al., 2003) at 14 K. To bring out tion of the spectrum where they are least obscured by the 4-azachrysene peaks in the lower spectrum we have subtracted 75% strong H2O absorptions, where we can most clearly of a comparable spectrum of pure H2O, i.e., this is a diﬀerent portion of the same spectrum seen in Fig. 2(b). The steep declines at the edges see the eﬀects of interactions with H2O. The lower trace of the lower spectrum at 1600 and 1000 cmÀ1 (6 and 10 lm) are caused in Fig. 3 is the spectrum 4-azachrysene in H2O from by strong H2O absorptions that can be better seen in Fig. 1. The which some pure H2O has been subtracted (i.e., this is vertical hatched lines help to compare the relative peak positions. a diﬀerent portion of the same spectrum seen in Fig. Arrows mark features in the H2O spectrum with unexpectedly large 2(b)) and above the previously published (Mattioda intensity relative to those in an argon matrix. M.P. Bernstein et al. / Advances in Space Research 36 (2005) 166–172 169 et al., 2003) argon matrix data. The peak positions and spectrum that merge to form one broader feature at relative intensities appear in Table 1. 1263 cmÀ1 (7.92 lm) in the spectrum of 4-azachry- Most of the sharp 4-azachrysene absorptions in the sene in solid H2O. In this case the relative intensity of matrix spectrum are broader as a result of interactions the new absorption is identical (within experimental with H2O, but appear at essentially the same position uncertainty) to the combination of those smaller fea- in the H2O diﬀerence spectrum (Fig. 3). For example, tures (see Table 1). all but one of the peaks in Fig. 3 have shifted less than In some cases peaks in the spectrum of 4-azachrysene 4cmÀ1, and roughly half less than 2 wave numbers. in argon are more intense than the corresponding Of those peaks that are observed in both spectra a shift absorptions in the spectrum of 4-azachrysene in H2O to higher frequency seems to be more common, consis- although the positions are still within a few cmÀ1 of tent with a previous study of pyridine and pyrimidine one another. For example, the feature near 1187 cmÀ1 complexes with H2O in Ar matrices (Destexhe et al., (8.425 lm) is more than four times more intense than 1994). The broadening of absorption bands results in the corresponding 1191 cmÀ1 (8.396 lm) feature in the overlap and combination in the H2O spectrum of the lower H2O/4-azachrysene spectrum. peaks that are close but baseline-separated in the argon In two cases the spectrum of 4-azachrysene in solid spectrum. This is illustrated by the pair of peaks near H2O displays peaks having both positions and intensi- 1257 and 1266 cmÀ1 (7.955 and 7.899 lm) in the argon ties that diﬀer suﬃciently from those observed from the matrix experiments that we suspect an intermolecu- lar interaction between the 4-azachrysene and H2O. The À1 Table 1 ﬁrst case is the broad lump near 1017 cm (9.833 lm) Positions and relative intensities of selected IR absorptions of 4- marked with an arrow at right in the lower spectrum azachryseneb of 4-azachrysene in H2O (Fig. 3). Although there are
Matrix isolated in Argon Frozen in solid H2O no strong absorptions in the 4-azachrysene matrix spec- À1 Peak position Relative areac Peak position Relative areac trum at 1017 cm (9.833 lm), there is a weak feature at À1 À1 cmÀ1 (lm) cmÀ1 (lm) 1029 cm (9.718 lm) that is 12 cm (0.1 lm) away. This latter peak was not reported in Mattioda et al. 1599.4 6.2523 0.71 1601 6.2461 0.16 1579.1 6.3327 0.13 1580 6.3291 0.11 (2003) because its intensity was below their limiting 1575 6.3492 0.13 threshold, but it can be seen in Fig. 3. Absorptions in 1510.0 6.6225 0.25 1513 6.6094 0.29 this region (1100–500 cmÀ1) correspond to CH out-of- 1506.5 6.6379 plane bending and skeletal deformation modes. Presum- 1487.6 6.7222 0.44 1488.2 6.7195 0.32 ably these diﬀerences can largely be attributed to the 1428.9 6.9984 0.22 1437.9 6.9546 0.20 1431.5 6.9857 presence of the nitrogen atom. While a normal PAH 1408.0 7.1023 1.0 1412.3 7.0806 1.0 would have hydrogen atoms all around the edges and 1394.3 7.1721 0.29 1394.0 7.1736 0.16 keeping out surrounding H2O molecules, there is a gap 1311.8 7.6231 0.18 1310.5 7.6307 0.17 adjacent to the 4-azachrysene nitrogen atom where an 1266.0 7.8989 0.09 1263.3 7.9158 0.22 H O can more closely approach the molecule (and pre- 1257.4 7.9529 0.13 2 1232.7 8.1123 0.10 1237 8.0841 0.12 sumably hydrogen bond). Perhaps the hydrogens on 1221.0 8.1900 0.10 carbon atoms adjacent to the nitrogen interact more 1186.9 8.4253 0.13 1191 8.3963 0.03 strongly with this closer H2O, causing changes in the 1167.7 8.5638 0.06 1171.1 8.5390 0.06 spectrum (Bauschlicher, personal communication). 1163.8 8.5925 0.09 1158 8.6356 0.04 The second case is the appearance of the weak broad 1153.9 8.6663 0.07 À1 1141.6 8.7596 0.02 1128 cm (8.865 lm) feature (also marked with an ar- 1130.3 8.8472 0.03 1128 8.8889 0.18 row) in the lower trace in of Fig. 3. The relative area un- 1081.4 9.2473 0.12 1083 9.2336 0.060 der this absorption in the lower H2O spectrum is six 1049.2 9.5311 0.05 times larger than that of the corresponding peak at 1038.9 9.6256 0.02 1130 cmÀ1 (8.850 lm) in the upper spectrum. The com- 1029.0 9.7182 0.09 1016.5 9.8377 0.33 bined areas of the three nearest peaks at in the upper a spectrum approach the intensity under that broad ar- Many absorptions of 4-azachrysene in H2O are given only to the À1 nearest wave number, because their positions could not be determined row-marked 1128 cm (8.865 lm) feature (0.12 vs. À1 more accurately. 0.18). The modes that fall in the 1600–1100 cm region b All of the argon data are a selection taken from Mattioda et al., correspond to the aromatic CC and CN stretching and 2003. CH in-plane bending vibrations. c À1 Areas are normalized to the peak near 1410 cm (7.1 lm). The This seems most consistent with the former coinci- band strength for the 1410 cmÀ1 peak of 4-azachrysene in argon is À18 dental superimposition hypothesis, but we do not claim 5 · 10 cm/molecule, but we do not know its band strength in H2O. Assuming it is the same then the 4-azachrysene is isolated in H2O (see to understand the cause of these changes beyond a pre- Section 2). sumably N-dependent intermolecular interaction with 170 M.P. Bernstein et al. / Advances in Space Research 36 (2005) 166–172
H2O. It is to be hoped that molecular modeling will shed between diﬀerent types of aromatics. However, it is some light on this mystery. Again, perhaps perturba- interesting to note that there is a diﬀerence in the posi- tions in the spectrum correspond to interactions between tion of the aromatic C–H stretches between YSOs where À1 adjacent hydrogen atoms and an H2O molecule that is it is consistently seen near 3075 cm (3.25 lm; Brooke near the 4-azachrysene nitrogen atom. et al., 1999) and a line-of-sight to the Galactic Center Whatever the reasons for the diﬀerences, it is clear (GCS 3) where it falls near 3050 cmÀ1 (3.28 lm; Chiar that intermolecular interactions between the PANH 4- et al., 2000). With the caveats regarding the uncertainty azachrysene and H2O can cause bigger changes in the engendered by spectral subtraction in mind (see Section IR spectrum than were observed for plain PAHs lacking 2) we note that the central position of the C–H stretch nitrogen atoms, like naphthalene (Sandford et al., 2004). absorption of 4-azachrysene in H2O exactly matches that measured by Chiar et al. (2000) towards GCS 3. This diﬀerence between the central position of the 4. Implications absorption towards YSOs and the Galactic Center could reﬂect a real diﬀerence in the composition of the aro- The laboratory results presented here have a number matic material(s), and suggests that features of PANHs of implications for the infrared spectral detection and could be present towards GCS 3 at longer-wavelength. identiﬁcation of PANHs in interstellar dense molecular Intermolecular interactions between 4-azachrysene clouds. and H2O produced two signiﬁcant diﬀerences between the IR spectra, at longer wavelength, of this PANH in 4.1. Detection of PANHs in astrophysical ices an argon matrix and in solid H2O at 14 K. Thus, in con- trast to normal PAHs, the peak positions and strengths In dense clouds where other ice components obscure of PANHs in inert gas matrix and/or measured or calcu- PAH absorptions, PANHs would be challenging to de- lated gas phase cannot be relied upon to correctly pre- tect, but just how hard might it be? In general PANHs dict the IR absorptions of PANHs in H2O. are like PAHs in the intensity of their absorption bands Our results suggest that for this class of aromatics (typically 10À18 cm/molecule), and thus will share simi- bearing nitrogen atoms we currently lack a reliable lar diﬃculties in their detection by IR techniques in method for predicting the positions of IR absorptions astrophysical environments. If, consistent with the aro- in interstellar dense clouds short of measuring the spec- matic-rich kerogen in primitive meteorites, we assume tra of each molecule in solid H2O. Uncertainties in the there is about one nitrogen atom for every hundred car- eﬀects of H2O on the intrinsic absorptivities (A values) bon atoms, then there would be the equivalent of at least mean similar problems exist with the estimation of col- one 4-azachrysene present for every twenty chrysenes. umn densities of any PANHs in dense clouds one might Our lab results suggest that 4-azachrysene would be eas- manage to identify. Only a better understanding of the ily discerned in such a 20-to-1 mixture. This is a very intermolecular interactions that govern the appearance simple-minded analysis, however; the lab is diﬀerent of the IR spectra will free the community from the slow from the telescope, and the interstellar molecules are and onerous task of measuring these molecules individ- probably larger. Nevertheless, given that most PANHs ually in H2O. seem to have a stronger absorption in the 6.6 lm region than normal PAHs it seems reasonable that they con- 4.2. Astrobiology of PANHs tribute a measurable part of the IR emission there (Mat- tioda et al., 2003). Thus, the detection of bands in There are only a few articles that present spectra of absorption in this same spectral region may well be pos- polycyclic aromatic nitrogen heterocycles under condi- sible provided data with suﬃciently good signal-to-noise tions germane to astrophysics (Mattioda et al., 2003), (i.e., a factor of 10 better than has been published) are and they have not been widely searched for in space obtained. (Kuan et al., 2003). However, these compounds have While we have seen that some IR longer wavelength been extracted from carbonaceous chondrites such as absorptions of 4-azachrysene are sensitive to the pres- the Murchison and Tagish Lake meteorites (Stoks and ence of H2O, the characteristic aromatic C–H stretches Schwartz, 1982; Basile et al., 1984; Pizzarello, 2001), between 3150 and 2950 cmÀ1 (3.18–3.39 lm) remain a and their aromatic structure is consistent with what is secure representative of the class, but not one that al- hypothesized to be present on Titan (Ricca et al., 2001). lows discrimination between diﬀerent types of aromat- Purines and pyrimidines (the informational elements ics. Their usefulness may also be mitigated by their of DNA and RNA, that are also present in carbona- very poor spectral contrast in H2O-rich ices because of ceous chondrites), three amino acids (proline, histidine, their proximity to the very strong 3 lmH2O band. Also, and tryptophan), and numerous other vital bio-organic the relative invariance in position aromatic C–H compounds (such as ﬂavins, porphyrins, nicotinamides) stretches would seem to limit their power to discriminate contain single and fused rings of carbon atoms contain- M.P. Bernstein et al. / Advances in Space Research 36 (2005) 166–172 171 ing nitrogen. Nitrogen heterocycles should be C05A and 344-50-92-02), Origins of Solar Systems considered potentially astrobiologically important com- (344-37-44-01), and Planetary Geology and Geophysics pounds for two reasons. First, they were exogenously (344-30-21-01) programs. We gratefully acknowledge delivered to the early earth and thus may have played the advice and guidance of Dr. L.J. Allamandola, extra a role in the origin or evolution of our biochemistry. argon matrix data provided by Dr. A.L. Mattioda, and Second, the presence of this class of molecules could discussions of intermolecular interactions with Dr. C. act as false biomarkers, confusing the search for species Bauschlicher. We acknowledge the helpful comments that indicate signs of life in the universe. Even simple of two anonymous reviewers. Thanks for samples of 4 methylated PAHs have been invoked as biomarkers in Azachrysene to the National Cancer InstituteÕs Chemi- the case of the Alan Hills 84001 Martian meteorite cal Carcinogen Reference Standards Repository oper- (McKay et al., 1996). Although 4-azachrysene is not a ated under contract by Midwest Research Institute, biological compound, we have chosen it as a model com- No. NO2-CB-07008. pound to represent the general class of aromatics con- taining nitrogen, some of which are of biological importance. References
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