From: Bernstein, M. P.,, Sandford, S. A.,.and Allamandola , L. J. (1997). The Infrared Spectra of Nitriles and Related Compounds Frozen in Ar and H2O. Astrophys. J. 476, 932-942.
ABSTRACT
We present the 2320-2050 wavenumber (4.31-4.88 micron) infrared spectra of 16 solid state nitriles, isonitriles, and related compounds in order to becoming accessible to astronomers for the first time through the Infrared Space Observatory (ISO). This frequency range spans the positions of the strong CN stretching vibration of these compounds and is inaccessible from the ground due to absorption by CO2 in the terrestrial atmosphere. Band positions, profiles, and intrinsic strengths (A values) were measured for compounds frozen in Ar and H2O matrices at 12 K. The molecular species examined included acetonitrile, benzonitrile (phenylcyanide), 9-anthracenecarbonitrile, dimethylcyanamide, isopropylnitrile (isobutyronitrile), methylacrylonitrile, crotononitrile, acrylonitrile (vinyl cyanide), 3-aminocrotononitrile, pyruvonitrile, dicyandiamide, cyanamide, n-butylisocyanide, methylisocyanoacetate, diisopropylcarbodiimide, and hydrogen cyanide. The CN stretching bands of the majority of nitriles fall in the 2300-2200 wavenumber (4.35-4.55 micron) range and have similar positions in both Ar and H2O matrices, although the bands are generally considerably broader in the H2O matrices. In contrast, the isonitriles and a few exceptional nitriles and related species produce bands at lower frequencies spanning the 2200-2080 wavenumber (4.55-4.81 micron) range. These features also have similar positions in both Ar and H2O matrices and the bands are broader in the H2O matrices. Two of the compounds (dicyandiamide, and cyanamide) show unusually large shifts of their CN stretching frequencies when changing from Ar to H2O matrices. We attribute these shifts to the formation of H2O:nitrile complexes with these compounds. The implications of these results for the identification of the 2165 wavenumber (4.62 wavenumber) "XCN" interstellar feature and the 4550 wavenumber (2.2 micron) feature of various objects in the solar system are discussed.
1. INTRODUCTION
Astronomers are unable to make ground-based measurements in certain portions of the infrared spectrum because of strong absorption produced by telluric CO2. This is particularly true in the 2400-2200 wavenumber (4.17-4.55 micron) spectral region in which the asymmetric O=C=O stretching fundamental produces a strong feature. Since the large intrinsic strength of this absorption and the large scale height of CO2 preclude a solution to this problem using airborne platforms like the Kuiper Airborne Observatory, astronomical observations in this spectral region require observations be made with space-borne telescopes.With the recent successful launch of the European Infrared Space Observatory (ISO) (see Encrenaz & Kessler 1992), the 2400-2200 wavenumber portion of the infrared spectrum has become accessible for the first time. There are a wide variety of compounds that produce infrared absorption features in this range and, despite the rather limited extent of this spectral region, it is anticipated that ISO will detect a number of new features here (Allamandola 1993). Examples of molecular fundamentals that produce bands at these frequencies include the stretching vibrations of oxides of nitrogen and carbon, the CD stretch, and various CN stretches. Here we focus on the absorption bands produced by the CN stretching vibrations of nitriles, isonitriles, and related compounds. Nitriles are compounds that contain one or more -C triple bond N functional groups that typically produce a strong absorption feature or features in the 2400-2200 wavenumber range due to stretching vibrations of the CN group.
There are a number of reasons to expect that ISO will detect bands due to nitriles in the interstellar medium (ISM), especially in the spectra of ices in dense molecular clouds. First, a number of nitriles and related compounds like HCN, HNC, NH2CN, CH2CN, CH3CN, CH2CHCN, and CH3CH2CN have been detected in the gas phase in dense clouds using radio spectral techniques (see Mann & Williams 1980; Irvine et al. 1987; van Dishoeck et al. 1993 for reviews). At the low temperatures typical of dense interstellar clouds (T < 40 K) these molecules will be frozen out efficiently onto dust grains in the clouds (see Sandford & Allamandola 1993). Infrared spectroscopic studies have shown already that mixed molecular ices in these clouds contain H2O, CO, CH3OH, H2, and a host of other molecular species that were either condensed out of the gas phase or formed in situ (see Sandford 1996 for a recent review). The nitriles seen in the gas phase in these clouds should be present in the ices as well.
Indeed, there is some direct observational evidence that interstellar solids do contain nitriles or related species. Spectra obtained from a limited number of lines of sight through dense clouds contain a broad, often weak, feature centered at about 2165 wavenumber (4.62 micron) near the CO ice feature (Lacy et al. 1984; Tegler et al. 1993). The specific molecule or molecules responsible for this absorption have yet to be identified conclusively. While a variety of assignments for this band have been proposed, including the Si-H stretching vibration (Nuth & Moore 1988; Moore et al. 1991) and the CN stretch in the OCN- ion (Grim & Greenberg 1987), most work has focused on the possibility of nitrile or isonitrile carriers. Early laboratory experiments by Moore et al. (1983) and Lacy et al. (1984) demonstrated that a feature similar to the observed interstellar band was produced when laboratory ice mixtures were exposed to ionizing radiation. Lacy et al. further noted that the feature was produced only by ices containing C and N. This, in conjunction with the feature's position, lead them to suggest that the absorption is due to the stretching vibration of a CN functional group in a larger molecule, i.e. a nitrile or isonitrile, and that some sort of energetic processing is required to produce it. Subsequent laboratory irradiation experiments have confirmed the importance of C and N for the formation of the 2165 wavenumber band and have demonstrated that excellent spectral fits to the astronomical data can be obtained using more astrophysically relevant ice mixtures (Tegler et al. 1993; Bernstein et al. 1995). In addition, some of these laboratory experiments showed that energetic processing simultaneously produces a host of other related CN-bearing species (Bernstein et al. 1995). Thus, there are many reasons to expect that the infrared spectra of dense molecular clouds will contain a feature or features due to nitriles or related compounds in the 2300-2200 wavenumber spectral region.
Finally, there are indications that nitriles may also be present in a variety of solar system objects. Low resolution infrared spectra of the comets Panther (1981 II) and Bowell (1982 I) contained absorption features centered near 4420 wavenumber (2.26 micron) (Jewitt et al. 1982). Cruikshank et al. (1991) noted that absorption features in this region could be due to overtones of the fundamental CN stretch in nitriles or isonitriles. Recently, Cruikshank et al. (1996) have discovered a well defined feature near 4400 wavenumber (2.27 micron) in the spectrum of the icy planetesimal 5145 Pholus. The possibility that nitriles and related compounds could be present in the solar system has motivated some recent laboratory work to characterize the spectral properties of these materials (Khare et al. 1995; Russo & Khanna 1996). Interpretation of any features found in this spectral region will require comparison to appropriate laboratory data. Although there is a large chemical literature on the infrared spectral properties of nitriles, nitrile complexes, and related compounds (see Freedman & Nixon 1972; Hunt & Andrews 1987; Bohn & Andrews 1989; Jacobs et al. 1992; Kubelkov et al. 1995; and references therein) these studies cover only a small range of nitrile compounds and there is a dearth of spectral information on nitriles frozen into astrophysically-relevant, H2O-rich ices at low temperature. In this paper we present the infrared spectra of 16 nitriles and related compounds (see Figure 1) frozen in both argon and H2O matrices to facilitate the search for and assignment of any features observed by ISO in the 2400-2200 wavenumber (4.17-4.55 micron) region of the spectrum.
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