Peroxyacetyl Nitrate and Its Homologs

Shortly after the discovery of photochemical air pollution, Stephens and his co-workers (Stephens et al., 1956 Stephens, 1987) applied long-path infrared spectroscopy to identifying and measuring products in the photooxidation of organic-NO, mixtures. In the photooxidations of 3-methylheptane and, to a larger extent, 2,3-butanedione, a set of infrared bands that could not be assigned to known products was observed. These were assigned to a previously unobserved species, which was initially called compound X. It was ultimately shown to be peroxyacetyl nitrate (PAN)  

For a detailed discussion of the discovery of PAN in the context of early studies of photochemical air pollution, the reader is referred to an article by Stephens (1987). 

PAN is the simplest member of a series of compounds known as peroxyacyl nitrates, having the structure 

The nomenclature for this homologous series is somewhat confused. The term PANs has been used historically to denote peroxyacyl nitrates, and this terminology continues to be used extensively in the literature, despite the lack of adherence to traditional IUPAC rules of nomenclature. Because the PANs can be considered to be mixed anhydrides of carboxylic acids and nitric acid, another suggestion (Roberts, 1990) has been peroxyacetic nitric anhydride for CH,C(0)00NO2 and peroxy carboxylic nitric anhydrides for the whole class of compounds. Although it does not follow the IUPAC rules, it would be consistent with the widespread use of the name PAN but also reflect the structure more accurately. Table 6.20 shows the structures and commonly used names of some PANs that have been observed in the atmosphere and/or in laboratory studies. 

PAN is known to play an important role in tropospheric chemistry. As discussed in this section, its thermal decomposition releases both N02 and an organic free radical, so that it can act as an NOx reservoir and ultimately as a source of OH in the dark. In addition, PAN is a strong lachrymator (eye irritant), is mutagenic in certain bacterial assays, and is phytotoxic to plants. Because of these broad effects on a variety of systems, its formation and reactions have been studied in some detail. 

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