Formaldehyde interstellar

The validity of quantum-chemical kinetics of low-temperature reactions, the existence of the limit of chemical reaction rate,6 and the applicability of the previously mentioned general principle of cold formation of low-entropy products can be illustrated by recent finding of formaldehyde polymers in interstellar space.10... 

Polymers of formaldehyde were found recently in interstellar space by N. Wickramasinghe [Nature, 252, 462 (1974)]. It is well known that polyformaldehyde is thermodynamically unstable already at not very high temperatures (close to room temperature), but it should be stable versus depolymerization near absolute zero. Therefore the formation of poly-oxymethylene near absolute zero is not a thermodynamic but a kinetic problem. 

Cosmic ray particles can work as a trigger of the chain of polymerization of formaldehyde adsorbed (e.g., at the surface of interstellar silicate dust). However, the triggering of the polymerization chain is a necessary but still not sufficient condition for obtaining the interstellar polymers. If the addition of any new link of the chain would require a hit of adsorbed monomer layer by another cosmic ray particle (i.e., if there is no spontaneous growth of chains after they are started by some external factor), the formation of polymers in interstellar space would be highly improbable. Therefore the spontaneous growth of polymer chains near absolute... 

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... 

In the past 10 years a large number of organic molecules have been found in interstellar dust clouds mostly by emission lines in the microwave region of the spectrum (for a summary see Ref. 38). The concentration of these molecules is very low (a few molecules per cm3 at the most) but the total amount in a dust cloud is large. The molecules found include formaldehyde, hydrogen cyanide, acetaldehyde, and cyanoacetylene. These are important prebiotic molecules, and this immediately raises the question of whether the interstellar molecules played a role in the origin of life on the earth. In order for this to have taken place it would have been necessary for the molecules to have been greatly concentrated in... 

Carbohydrates are especially unstable, even at temperatures well below the boiling point of water. This instability arises from the fact that they contain a C=0 unit. Because of this instability, some have proposed that carbohydrates could not have been part of genetic molecules in early life, as they are in modem life, at least until advanced metabolic repair, and sequestration became available to manage their reactivity. Indeed, until recently, no nonbiological process was well established that would yield carbohydrates under plausibly prebiotic conditions and in sufficient concentrations before the carbohydrates were then destroyed under the same conditions in which they were formed.1 The simplest carbohydrate that has been observed in the interstellar medium is formaldehyde the most complex, glycolaldehyde (Figure 2.7). 

If the effective temperature of our defined system is less than the universal radiation background temperature of 2.7 K, transitions between the two levels can be observed in absorption. This is the case with interstellar formaldehyde. Alternatively absorption can be observed against the continuum radiation from a nearby bright source. Spontaneous emission will always occur provided the upper of the two levels is populated, and can be observed if the populations are different. There are, in addition, examples of the exceptional situation in which N2 > N the result of this population inversion is that stimulated emission dominates, and maser emission is observed. Interstellar OH and SiO provide diatomic examples of this unusual situation, as also does interstellar H2O we shall describe the results for OH later in this chapter. Departures from local thermodynamic equilibrium are very common, and the concept of temperature in interstellar gas clouds is not simple this is a major part of astrophysics which is, however, beyond the scope of this book. 

Fig. 20 shows the observed interstellar molecular lines of various isotopic species of formaldehyde, H2CO, as detected by Gardner et al., 1971. This particular line, the lowest asymmetry-doublet transition 110 — lu, is seen in absorption in the continuum radiation of the strong radio source Sgr B2, which is located behind the molecular gas cloud. Frequency is plotted along the abscissa and the ordinate is intensity, expressed in the ratio of line-to-continuum antenna temperatures. For all three formaldehyde isotopes the continuum temperature is Tc T >b Tex- This is the case because the formaldehyde molecules are in approximate equilibrium with the microwave background... 

From the preceding discussions it is evident that at least four different temperatures have to be considered which under laboratory conditions are all equal the excitation temperature Tex of the molecule, defined by the relative populations of the levels, the kinetic temperature Tk, corresponding to the Maxwellian velocity distribution of the gas particles, the radiation temperature Traa, assuming a (in some cases diluted) black body radiation distribution, and the grain temperature 7, . With no thermodynamic equilibrium established, as is common in interstellar space, none of these temperatures are equal. These non-equilibium conditions are likely to be caused in part by the delicate balance between the various mechanisms of excitation and de-excitation of molecular energy levels by particle collisions and radiative transitions, and in part by the molecule formation process itself. Table 7 summarizes some of the known distribution anomalies. The non-equilibrium between para- and ortho-ammonia, the very low temperature of formaldehyde, and the interstellar OH and H20 masers are some of the more spectacular examples. 

Formaldehyde, H CO, is a widely distributed interstellar molecule which occurs in diffuse and in dense molecular clouds. Specific gas phase reactions have been proposed. But formaldehyde has remained a test case molecule where gas-phase reactions cannot completely explain the observed interstellar abundances and where surface reactions might play a role. However, the recently observed HDCO shows that its abundance (Langer et al., (1979)) is not in disagreement with the proposed reaction mechanisms for H CO. Three mechanisms have been proposed (Watson, (1976), Herbst and Klemperer (1973)) ... 

Formaldehyde, a strong irritant, is considered one of these aggressors. Since it is a well defined chemical, it has become an easy target for elimination. However, we should remember that formaldehyde is not only an industrial chemical, but is omnipresent in nature Formaldehyde is present in traces in the living organism where it plays an important part in the metabolic cycles (biosynthesis of the puric nucleus). We can find it in apples, onions, etc. It was also one of the first organic compounds discovered in interstellar space. In fact, in the direction of... 

Dust particles of a size of several micrometers probably contained metals and condensates of water, ammonia, methane and admixtures of formaldehyde, acetonitrile, methylacetylene and other substances occurring in interstellar space. 

The recent discoveries of complex organic molecules associated with interstellar dust clouds have, of course, attracted great interest among students of chemical evolution. Inspection of Table IV will reveal several compounds which have been referred to with regard to the syntheses of biologically significant compounds HCN, formaldehyde and cyanoacetylene. It is perhaps unlikely that the processes which have produced these com-... 

Wicke, E., M. Eigen and T. Ackermann (1954) Uber den Znstand des Protons (Hydroniu-mions) in waBriger Losung. Zeitschrift fur Physikalische Chemie (Neue Folge) 1, 343-364 Wickramasinghe, N. C. (1974) Formaldehyde polymers in interstellar space. Nature 252, 462-463... 

Whether ammonia can react with simple carbonyls (formaldehyde, acetaldehyde, and acetone) at ultracold temperature in water clusters (i.e. interstellar ice analogues) has been examined theoretically. Almost barrier-free C-N bond formation was found for 4 X HjO clusters, with proton transfer becoming spontaneous at 9 x H2O. Consideration of what the teU-tale IR frequencies might be for such species in such clusters is also discussed. ... 

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