Acc. Chem. Res. 2001, 34, 699-706 The Formation of Nitriles in (CO2), germane (GeH4), hydrogen sulfide (H2S) as well as the hydrocarbons ethane (C2H6), acetylene (C2H2), ethyl- ene (C2H4), methylacetylene (CH3CCH), propane (C3H8), Hydrocarbon-Rich Atmospheres • and diacetylene (HCCCCH) and the methyl radical ( CH3) of Planets and Their Satellites: have also been found to be present in trace amounts with mole fractions between 10-9 and 10-5.3 Saturn's moon Laboratory Investigations by the Titan has attracted special attention, because Earth and Crossed Molecular Beam Titan are believed to have emerged with similar atmo- 4 spheres from the solar nebula. Molecular nitrogen (N2) Technique and methane (CH4) are the main constituents of Titan's present atmosphere, followed by molecular hydrogen (H2), RALF I. KAISER* nitrogen-bearing molecules, hydrogen cyanide (HCN), Department of Physics, Technical University Chemnitz, dicyan (C2N2), and cyanoacetylene (HCCCN), as well as 09107 Chemnitz, Germany, and Department of Chemistry, the hydrocarbons acetylene (C H ), ethylene (C H ), ethane University of York, York YO10 5DD, U.K. 2 2 2 4 (C2H6), methylacetylene (CH3CCH), propane (C3H8), and 3 NADIA BALUCANI diacetylene (C4H2). Even though the above-mentioned Dipartimento di Chimica, UniversitaÁ di Perugia, nitrilessmolecules containing a cyano (-CN) groups 06123 Perugia, Italy occur only in trace amounts of a few parts per billion, they Received March 13, 2001 are of particular importance because they are thought to be the key intermediates to form biologically relevant ABSTRACT molecules. Nitriles can be hydrolyzed and react via Crossed molecular beam experiments of cyano radicals, CN(X2Σ+), multistep synthesis ultimately to amino acids, thus pro- reacting with unsaturated hydrocarbons have been performed to viding one of the basic ªingredientsº for life.5 As opposed investigate synthetic routes to nitriles formation in hydrocarbon- to Earth, however, the surface temperature of Titan is rich atmospheres of planets and their moons. We have verified that s s all cyano radical reactions with acetylene, ethylene, methylacet- about 94 K too cold for liquid water to exist and the ylene, allene, benzene, and dimethylacetylene proceed without chemical evolution has remained frozen at an early stage. entrance barrier, have exit barriers well below the energy of the As a consequence, the study of the chemistry of Titan's reactant molecules, and are strongly exothermic. The identification atmosphere offers the unique opportunity to reconstruct of the CN versus H atom exchange channel makes these reactions compelling candidates to synthesize unsaturated nitriles in solar the scene of the primordial terrestrial atmosphere and to system environments. Some of these nitriles, hitherto unobserved unveil key concepts about how biologically active mol- in our solar system, now represent an ideal target to be detected ecules and their nitrile precursors were synthesized on - in the future Cassini Huygens mission to Titan. proto-Earth. One of the main basic questions is, how can the nitrile chemistry be initiated in those low-temperature environ- I. Introduction ments? The atmospheres of Titan and the giant gas planets During the past century, the atmospheres of Jupiter, are constantly bombarded with high-energy photons and Saturn, Uranus, and Neptune have been investigated with cosmic ray particles. In the upper layers of Titan's great effort because their atmospheric composition and atmosphere, for example, the energy deposition is mainly chemical evolution are expected to reveal key aspects of from strongly ionizing, high-energy electrons from Sat- 1 prebiotic chemistry of Earth. The Voyager I/II missions urn's magnetosphere and short-wavelength solar ultra- and ground-based telescopes revealed that the atmo- violet photons (λ < 155 nm). Hence, the ionospheric spheres of the giant gas planets are composed primarily chemistry is dominated by ion-molecule reactions.6 2 of the light elements hydrogen and helium; polyatomic However, longer wavelength photons penetrate down to molecules methane (CH4), ammonia (NH3), water (H2O), the stratosphere and photodissociate HCN to yield cyano phosphine (PH3), carbon monoxide (CO), carbon dioxide radicals, CN(X2Σ+). Since the cyano radical concentration profile overlaps with atmospheric regions containing Ralf I. Kasier was born on May 24, 1966, in Unna, Germany. He received his unsaturated hydrocarbons such as acetylene, it has been Ph.D. in chemistry from the University of Mu¨nster (Germany) and did postdoctoral speculated that these radicals react with hydrocarbons to work in Berkeley with Y. T. Lee and A. G. Suits from 1994 to 1997. From 1997 to produce unsaturated nitriles via neutral-neutral reac- 2000, he received a fellowship from the German Research Council (DFG) to 7 perform his Habilitation at the Department of Physics (University of Chemnitz, tions. The potential importance of the cyanopolyynes Germany; D. Gerlich) and Institute of Atomic and Molecular Sciences (Adademia (Hs(CtC)nsCtN) in that environment strongly motivates Sinica, Taiwan; Y. T. Lee). His research interests include chemical reaction laboratory studies, the aim of which is to elucidate their dynamics (gas phase and solid state), planetary chemistry, and laboratory studies routes of formation. relevant to astrochemistry. Laboratory data on the reactions of cyano radicals with Nadia Balucani was born on October 10, 1965, in Perugia, Italy. She received unsaturated hydrocarbons were lacking for a long time, her Ph.D. from the University of Perugia, did postdoctoral work in Berkeley with Prof. R. J. Saykally, and is currently Research Associate in Chemistry at the predominantly due to the experimental difficulty in University of Perugia. preparing a large concentration of CN(X2Σ+) radicals. Two 10.1021/ar000112v CCC: $20.00 2001 American Chemical Society VOL. 34, NO. 9, 2001 / ACCOUNTS OF CHEMICAL RESEARCH 699 Published on Web 08/18/2001 Nitriles Formation in Hydrocarbon-Rich Atmospheres Kaiser and Balucani kinds of experimental information are necessary to es- tablish whether these reactions might synthesize nitriles in low-temperature environments: (i) the values of the reaction rate constants and (ii) the reaction products themselves. Recent kinetic studies on CN reactions with acetylene, methylacetylene, and ethylene at temperatures as low as 13 K have proved that these reactions are indeed very fast and hold rate constants in the gas kinetics order (about 10-10 cm3 s-1).8 Therefore, these experiments have demonstrated that cyano radicals can react at the prevail- ing temperatures of Jupiter (165 K), Saturn (134 K), Uranus (76 K), and Neptune (57 K). However, in those investiga- tions, only the decay rate of the free radical concentration was monitored, and no information on the identity of the reaction products could be given. Because of this limita- tion, systematic laboratory studies to identify the reaction products of cyano radical reactions with unsaturated hydrocarbons are required in order to assess, once and for all, whether these routes of nitrile formation are open FIGURE 1. Schematic top view of the crossed molecular beam in the solar system environments. apparatus. The two pulsed beam source chambers are visible in The crossed molecular beam method is a powerful the top and left parts; in the case of the CN beam source (source experimental technique to provide this complementary I), the carbon rod holder and the incident laser beam are also piece of information. The complex networks of chemical sketched. The chopper wheel, the cold shield, and a sketch of the detector outer chamber are also shown. processes occurring in the planetary atmospheres consist of a series of elementary reactions, most of which are bimolecular collisions between a radical or an atom and II. The Crossed Molecular Beam Approach a closed-shell species. A detailed experimental knowledge The crossed molecular beam (CMB) method with mass- of the elementary processes involved at a fundamental spectrometric detection10 is a versatile technique to study microscopic level is therefore desirable, even if we want elementary reactions under single-collision conditions, simply to assess the nature of the primary reaction thus permitting the elucidation of the chemical dynamics products. Experiments under single-collision conditions andsin the case of polyatomic reactionssthe primary are essential to achieve this, because in a binary collision products. In contrast to bulk experiments, where reactants involving CN radicals and the molecules of a generic are mixed, the main advantage of a crossed beam experi- hydrocarbon, RH, where an intermediate is formed, i.e., ment is the capability to confine the radicals in a separate, CN(X2Σ+) + RH f [RHCN]* f RCN + H, a cyano radical supersonic beam; this implies that the radicals will collide will react only with one single hydrocarbon molecule, and only with the molecules of a second beam at a specific the stabilization of the [RHCN]* intermediate and/or collision energy and crossing angle, therefore ensuring the successive reaction of the primary products will be observation of the consequences of well-defined molec- avoided. Only this requirement can guarantee that the real ular encounters. All the experiments reported here were primary reaction products are identified. Three-body performed by using the 35º crossed molecular beam collisions may occur in dense planetary atmospheres, and machine (Figure 1).11 The apparatus