This research deals with a very special kind of a chemistry - that of the interstellar medium. About 130 chemical compounds were hitherto identified in the interstellar space. Among those, the profusion of chain molecules, held together by carbon backbones with multiple bonds, is striking. Cyanopolyacetylenes, H-(CC)n-CN (with n=1-5), constitute a representative example.
The interpretation of astronomical IR data cannot be accomplished without the detailed knowledge of characteristic vibrational frequencies for individual compounds. Paradoxically, the molecules of interest - most abundant in the Galaxy, and possibly also in the entire Universe - are often unstable and/or hard to synthesize in laboratories. Our current research concentrates on 1) studying the molecules already discovered in space, 2) predicting the properties of unknown, yet potentially interstellar species (when possible, we try to synthesize the latter), and 3) looking for possible interstellar synthesis pathways.
Several cyanoacetylene-related molecules of already known or potential astrophysical significance. Hydrogen, carbon, and nitrogen atoms are represented by red, green, and blue balls, respectively.
Cyanoacetylene, H-C≡C-C≡N (molecule 1 in the Figure above), found in the dense clouds of interstellar matter, and in some evolved circumstellar shells, is the archetype for a family of cyanopolyacetylenes, and for a multitude of related isomers, radicals, and ions. The interstellar microwave (pure rotational) emissions from two isomers of this compound, namely an isonitrile, HCCNC (2), and imine, HNCCC (3) - were found in early 1990s. It seemed important to extend the studies of cyanoacetylene isomers into complementary spectral regions, especially IR, to look for additional data on molecular structures, and to broaden the possibilities of astronomical observations.
Rare gas solids offer a useful laboratory environment for the photochemical synthesis and isolation of unstable chemical species. The majority of our experiments is based on the ultraviolet photolysis of an appropriate precursor compound (e.g. cyanoacetylene), dispersed in solid argon. A typical argon-to-molecule ratio of such low temperature matrices is close to 1000. Alternatively, the gaseous argon/precursor mixture can be subjected to electrical discharges, prior to the solidification on a cold surface.
The identification of photolysis (or discharge) products is accomplished mostly via IR spectroscopy. For example, we found that irradiations with UV quanta, energetic enough to cleave single bonds in the cyanoacetylene molecule, invariably lead to the isonitrile (2) formation, that is to the 180 degree rotation of the cyano group -CN [1]. Moreover, when the excess energy left in the system after the dissociation is low (due to the proper choice of the photolysis wavelength), another isomer of cyanoacetylene is formed along with 2, namely an imine (3) - i.e. the product of H atom migration to the opposite terminal of the molecule. The identification of the photolysis product usually involves isotopic substitution studies, as well as quantum-chemical calculations.
The impressive development of astronomical IR techniques resulted in the discovery of vibrational absorptions of molecules like cyanoacetylene, known to radioastronomers, but also in the finding of several centrosymmetric species: C3, C5, benzene, acetylene, di- and triacetylene [2], which remained out of reach at microwave wavelengths. In connection to these (mostly very recent) results, it was of interest to launch the laboratory IR spectroscopic studies of dicyanopolyacetylenes, NC-(CC)n-CN, and related isomers [3]. We considered the n=2 compound (6) as a representative member of this homologous series, and studied its photolysis in argon matrices. Two isonitriles (7 and 8) were identified, since either a single or both cyano groups can experience a turnaround for this symmetric molecule. These results complemented previous similar studies [4] on a simpler (n=1) compound (9) (carried out in the research group of V. Bondybey, in Munich), which led to molecules 10 and 11.
Another approach to astrochemistry is accomplished by quantum chemical calculations, namely by theoretical searches for simple, yet thus far unknown arrangements of basic elements, which can reasonably be proposed as candidates for interstellar molecules. These studies resulted e.g. in the discovery of CCCNCN molecule (12), which was first theoretically predicted [5], and subsequently found in argon matrices (in the research group of J.-P. Aycard [6]). Other molecules of interest include HCNCC (4), cyanovinylidyne CC(H)CN (5), and dicyanovinylidene CC(CN)CN (13). (See Chapter IV.2 for details.)
An interesting regularity can be noticed in the series of dicyanoacetylenes: calculations predict their isomerizations to be usually accompanied by gigantic amplifications (factor of 100 or more) of IR absorption abilities. On the contrary, small (or zero) dipole moments of these compounds make the microwave recognition difficult or impossible. Generally, unsaturated carbon-nitrogen chains may constitute a new important class of interstellar molecules, potentially detectable via IR spectra [7].
Plans for the nearest future include experimental and theoretical studies of the electronic spectroscopy of "exotic" carbon-nitrogen unsaturated compounds. Some of these may turn out to be the carriers of unidentified spectral features known as diffuse interstellar bands. We also hope for the detection of new interstellar molecules within the framework of the Polish participation in the Herschel Space Observatory mission (dedicated to IR measurements), to be started in 2007.
1. R. Kołos, J.Waluk; J. Mol. Structure 408/409 (1997) 473-476; R. Kołos, A.L. Sobolewski; Chem. Phys. Lett. 344 (2001) 625-630
2. J. Cernicharo et al.; Astrophys. J. 546 (2001) L123-L126; J.R. Heath et al. Science 244 (1989) 564-565
3. R. Kołos; Chem. Phys. Lett. 299/2 (1999) 247-251; R. Kołos; Polish J. Chem. 74 (2000) 1723-1730
4. A.M. Smith et al., J. Chem. Phys. 98 (1993) 1776-1785
5. R. Kołos; Chem. Phys. 117 (2002) 2063-2067
6. Z. Guennoun, I. Couturier-Tamburelli, N. Piétri, J.-P. Aycard; Chem. Phys. Lett. 368 (2003) 574-583
7. R. Kołos, Z.R. Grabowski; Astrophysics & Space Science 271 (2000) 65-72
