Quinone Derivatives as Main Organic Component

In course of the mission so far a lot of interstellar dust impact negative ion mass spectra have been taken. As already inferred from laboratory test measurements by Kissel and Krueger (2001) and comparison with related ion generation methods, negative ion mass spectra of organic compounds, especially polymers, contain much more specific information than positive ion ones do under these ion generation conditions (i.e., impact speed and grain size). Thus it was possible to elucidate better these ion generation rules and chemically assign the principal component of those interstellar grains. This has been treated already widely by Krueger et al. (2004). Thus it seems sufficient here to summarize the results, omitting the mass spectral arguments given in detail there.

The hydrocarbon polymers are very much dominated by oxygen. The mean atomic ratio O/C can be estimated to be 20%. On the other hand, nitrogen seems to play a minor role only. The mean atomic ratio N/C may be estimated to be 3 ± 1%. The mean degree of saturation can roughly be estimated only as medium: H/C ~ 1 seems to be a fair guess. Sulfur does not play a role (except for possible traces).

However, it seems very interesting that oxygen apparently is much more often bound to carbon only rather than to hydrogen. Thus quinone, furane, and ether moieties are more probable than hydroxyl moieties as present in alcohols, phenoles, or hydroquinones. The nitrogen seems to be fully integrated into (e.g., aromatic) carbon structures, such as pyrroles, pyridines, or quino-lines. Also nitriles are possible. There is no hint whatsoever for the substantial presence of amino- or imino-groups, i.e., nitrogen is not bound to hydrogen; a remarkable relation to many of the gas phase molecules found in interstellar clouds (see, e.g., Ehrenfreund and Charnley, 2000).

In negative ion mass spectrometry from stable solids, preferably electronegative, subunits of larger molecules and polymers play the major role. This is in sharp contrast to positive ion mass spectrometry from solids with which generally a large number of reaction and decay products result in, mainly nonradical, daughter ions. Thus, negative ions are much more sensitive to the molecular structure in question than positives are, especially within the impact speed domain we deal with here. Another interesting negative ion species is due to a complete stripping of all (but one, sometimes, a few only) hydrogen atoms from a backbone structure, and cleavage after a charge carrier. This gives rise to ion types, the sum formulae being such as C-. , C„H-, C„N-, C„O-. , C„OH-. (n = 1, 2, 3,4). These are found here, too, with the strong prevalence of oxygen-bearing species.

The smallest structurally more or less intact and intense ions from apparently molecular substructures are due to CeOH- (x = 0,1, 2,...). An aromatic (like phenole) or a quinoic source is very probable. Moreover, the whole redox-system benzoquinone/ hydroquinone shows up in the spectra. The anions of benzopyrane and benzopyranone (chromone) are also present. Here we have apparently the basic structures for a lot of quinones. However, although there is a lot of evidence for higher quinones (see next section), for flavones indications are poor. A further basic structure may be C14H6O3, benzo-naphthalin-(ortho)-quinone with furan ring closure (Fig. 12.1).

Fig. 12.1. (a) An example of phenanthrene. Due to its low polarity, it cannot be directly measured by CIDA. However, pure PAH's are very probable in interstellar grains by indirect conclusions. (b) The first oxygen, containing species of a series of larger homologues, directly found. (c) A very probable (sub)-structure found in interstellar dust.

Fig. 12.1. (a) An example of phenanthrene. Due to its low polarity, it cannot be directly measured by CIDA. However, pure PAH's are very probable in interstellar grains by indirect conclusions. (b) The first oxygen, containing species of a series of larger homologues, directly found. (c) A very probable (sub)-structure found in interstellar dust.

As mentioned above, a probability (although small) is given that N-heterocyclic compounds may play an additional role. Just removing some CH-moieties in rings, and with some reorder, we end up with the basic structure of pyrrolo-quinoline-quinone. There is some evidence for carboxylic groups, too. Thus, the famous "PQQ" (see Sect. 12.3.5, and Duine (1999)) and relatives thereof could easily have been built up in diffuse interstellar dust as such, or by further hydrolysis in liquid water.

As pure PAHs have not been found, but, in contrary, strong evidence for quinoic (nonaromatic, nonaliphatic) ring structures and some hetero-aromates was found, we like to call this stuff "dirty PAHs" (O and N being the "dirt" in the overwhelming majority of C backbone atoms in the molecules).

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