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 Abstract 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. Ó 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 Ó 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.