Radical interstellar

Fourier transform isotopic ( C and D) studies of potential interstellar species - C4H (butadiynyl radical) and QH (hexatriynyl radical) - have also been carried out. The radical C4H was produced (10) by trapping of products from the vacuum UV photolysis of diacetylene (C4H2) or 1,3-butadiene (C4H6) in solid argon at 10 K (Shen et al., 1990). Similarly the radical C6H was obtained (11) by vacuum UV photolysis of matrix-isolated... 

Free radicals are short-lived, highly-reactive transient species that have one or more unpaired electrons. Free radicals are common in a wide range of reactive chemical environments, such as combustion, plasmas, atmosphere, and interstellar environment, and they play important roles in these chemistries. For example, complex atmospheric and combustion chemistries are composed of, and governed by, many elementary processes involving free radicals. Studies of these elementary processes are pivotal to assessing reaction mechanisms in atmospheric and combustion chemistry, and to probing potential energy surfaces (PESs) and chemical reactivity. 

The above examples should suffice to show how ion-molecule, dissociative recombination, and neutral-neutral reactions combine to form a variety of small species. Once neutral species are produced, they are destroyed by ion-molecule and neutral-neutral reactions. Stable species such as water and ammonia are depleted only via ion-molecule reactions. The dominant reactive ions in model calculations are the species HCO+, H3, H30+, He+, C+, and H+ many of then-reactions have been studied in the laboratory.41 Radicals such as OH can also be depleted via neutral-neutral reactions with atoms (see reactions 13, 15, 16) and, according to recent measurements, by selected reactions with stable species as well.18 Another loss mechanism in interstellar clouds is adsorption onto dust particles. Still another is photodestruction caused by ultraviolet photons produced when secondary electrons from cosmic ray-induced ionization excite H2, which subsequently fluoresces.42... 

As discussed in Section II, neutral-neutral reactions involving atoms and/or small radicals play an uncertain role in the gas-phase chemistry of interstellar... 

Since the extent of neutral-neutral chemistry in dense interstellar clouds is currently unclear, we have constructed three different interstellar models according to the extent of neutral-neutral reactions incorporated in them.62 Our normal model, referred to as the new standard model, does not have a significant number of atom/radical-stable neutral reactions. Ironically, this model still shows the best... 

A second major class of ion-molecule reactions that is relatively poorly studied consists of systems involving very unsaturated hydrocarbon neutrals, especially radicals. The unsaturated nature of the organic chemistry in interstellar clouds leads to sizeable abundances of very unsaturated hydrocarbons such as the polyacetylenes HC H, the carbenes H2C , the radicals C H, and the clusters Cn. Although some work has been done on the chemistry of such species, much of the relevant ion-molecule chemistry involving ions such as C+, CH3, and even C2H2 must be guessed at from generalizations based on a small number of studied systems. 

Under certain conditions (such as in interstellar space), the OH radical has been observed. Construct the molecular orbital diagram for this species. Determine the bond order and determine what type of orbital contains the unpaired electron. 

Maser transitions have been observed in many important molecules and have been used to carry out surveys of the entire sky. The 22.235 GHz water maser transition is the strongest transition in the radio universe and represents an interesting candidate for an interstellar broadcast frequency. If extraterrestrial intelligence is trying to communicate with us, the choice of the broadcast frequency is an important one and would be known to all intelligent life. Of course it would have a different label, 22.235 GHz being a distinctly Earthly label, but it is a fundamental transition frequency and is observed everywhere. Other maser transitions include the 6.7 and 12.2 GHz methanol maser, the SiO maser v = 1, J = 7-6, 301.8 GHz, which occurs between levels in the first vibration state of the SiO molecule, and finally the OH maser first discovered in 1963 and buried deep in the 2n3/2 electronic state of the hydroxyl radical near 18 cm. This is actually four transitions at 1612, 1665, 1667 and 1720 MHz, all of which must be seen as a group but not necessarily of the same intensity. 

We are aware that our review is by no means complete, the topic of diene and polyene radical cations having ramifications into such diverse fields as biology or interstellar chemistry. In the following we shall first discuss the photoelectron spectra of dienes... 

Ethynyl radicals have recently been detected in interstellar medium by microwave spectroscopy [Tucker et al. (980a)]. 

Formaldehyde has been detected recently in the interstellar medium by microwave spectroscopy (593), It is a combustion product of hydrocarbons. The photolysis of H2CO by sunlight in the troposphere may produce H02 radicals by reactions such as... 

For these reasons, it is generally felt that the interstellar molecules played at most a minor role in the origin of life. However, the presence of so many molecules of prebiotic importance in interstellar space, combined with the fact that their synthesis must differ from that on the primitive earth where the conditions were very different, indicates that some molecules are particularly easily synthesized when radicals and ions recombine. Another way of saying this is that there appears to be a universal organic chemistry, which shows up in interstellar space, in the atmospheres of the major planets, and in the reducing atmosphere of the primitive earth. 

Schwarz and coworkers have used the technique of neutralization-reionization mass spectrometry (NRMS) to structurally characterize numerous elusive silicon-containing molecules of interstellar interest342. The identification of HNSi was supported by ab initio calculations. The radical ion [HNSi] + was produced from N2 and SiH3I in the chemical ionization source of the mass spectrometer. 

The higher acetylene radicals SiC4H and SiQ,II have received some experimental interest as these acetylenes are believed to play an important role in the interstellar genesis of silicon carbides (equation 27)168,169. 

Various forms of molecular carbon, from ions to radicals, have been detected in the diffuse interstellar medium (ISM) using electronic, rotational, and vibrational spectroscopies (Henning and Salama 1998 Snow and Witt 1995). Discrete absorption and emission bands seen toward diffuse interstellar clouds indicate the presence of numerous two-atom molecules such as CO, CN and C2. In addition to these interstellar features, a large family of spectral bands observed from the far-UV to the far-IR still defies explanation. Currently, it is the general consensus that many of the unidentified spectral features are formed by a complex, carbonaceous species that show rich chemistry in interstellar dust clouds (Ehrenfreund... 

In other papers (Cataldo and Keheyan 2002 Cataldo and Pontier-Johnson 2002) we have tried to extend the results and the conclusions we have drawn in the field of the interaction between carbon black surfaces, activity and morphology, with radical and macroradicals also in the field of interstellar carbon dust. 

The late 30 s brought a further important step in the investigation of the interstellar medium — the discovery of the first molecular species. In the optical region, the electronic spectra of the diatomic radicals CH, CH+, and CN, seen in absorption against the continuum spectra of bright background stars, furnished the first evidence that the interstellar medium was not devoid of molecules but contained at least some simple ones. However, the intensities of the molecular spectral peaks seen via optical absorption studies were quite weak compared with the spectra of atoms, indicating that the sources observed in these early studies were not rich in molecules. These sources, now labeled diffuse interstellar clouds, possess very low gas densities (n 102 cm-3) and are of limited interest chemically. 

The identification of all new radicals has been closely linked to the production of the appropriate species in the laboratory, mainly by glow discharges in acetylene and other molecules. Likewise, the detection of new interstellar ions has depended on laboratory work, typically involving glow discharges of a novel type designed by De Lucia et al. (1983). The newly discovered radicals are C3N, C3H, and C4H reported... 

The specific mechanisms that produce small interstellar molecules via ion-molecule reactions have been discussed by a large number of investigators since the original paper by Herbst and Klemperer (1973) and a large number of detailed model calculations have been undertaken. For a general discussion of processes, the reader is referred to earlier reviews by us (Winnewisser 1981 Herbst and Klemperer 1976). For the reader who is unfamiliar with gas phase interstellar chemistry, we will briefly consider some of the processes that form and destroy two significant molecules — H20 and the radical OH. 

Once molecules such as H20 and OH are formed, they are destroyed by either ion-molecule reactions or neutral-neutral reactions, or they are adsorbed onto the grains. The radical OH is a reactive neutral species and it is assumed by most investigators that it can react with neutral atoms such as O and N even at interstellar temperatures. The less reactive species H20 can only react with ions and, consequently, models predict that there is more H20 than there is OH. Examples of ion-molecule reactions involving H20 are... 

---