THE PHOTOCHEMISTRY OF AQUEOUS NITRATE ION REVISITED

Aqueous nitrate solutions were photolysed at 254 nm in the absence of oxidizable additives, in the presence of methanol or propan-2-ol and o xygen and in the presence of cyclopentane under anaerobic conditions. The main nitrogen-containing products are nitrite and peroxynitrite. T he quantum yields depend on the pH, nitrate concentration, nature of t he additive and the light intensity. The intrinsic nitrite yield in al kaline solutions could not be determined directly because, under the c onditions of the nitrite assay, the accompanying peroxynitrite decompo ses to form nitrite and nitrate; it is smaller than the apparent nitri te yield. In the acidic (pH 4-7) range, the intrinsic nitrite quantum yield is equal to the apparent nitrite yield because there is no build up of peroxynitrite under these conditions. The apparent nitrite quant um yield increases from 0.01 (no oxidizable additive) to approximately 0.03 (cyclopentane (millimolar range), oxygen free) to 0.06 (methanol (millimolar range), air saturated). At pH 13 and in the absence of ox idizable additives, the apparent nitrite quantum yield increases to ab out 0.1, whereas from material balance considerations the intrinsic ni trite quantum yield is estimated to be 0.06, twice the oxygen quantum yield of 0.03. Spectrophotometrically, peroxynitrite is detected in th e alkaline range only, because its protonated form is unstable. In the absence of oxidizable additives, the quantum yield of peroxynitrite i s about 0.1, i.e. only about two-thirds of the quantum yield in the pr esence of oxidizable additives. Mechanistic considerations on the basi s of the pH dependence of the quantum yields of the products nitrite, peroxynitrite and oxygen, as well as their dependence on the kind of a dditive, indicate that the decisive factor of photolysis in the absenc e of additives is the formation of the nitric oxide peroxyl radical, O NOO., formed by reaction of peroxynitrite with the primarily generated (OH)-O-. radical. The decay of ONOO. is the source of O-2 in this sys tem. Nitric oxide, NO., the other fragment of this decay reaction, rea cts with nitrogen dioxide, which is one of the primarily formed interm ediates. The latter reaction is one of the pathways to the product nit rite, particularly in the alkaline range. The formation of NO. during photolysis has been verified by electron spin resonance (ESR) spectros copic detection of the nitroxide 1,1,3,3-tetramethyl-isoindolin-2-oxyl , the NO adduct to 7,7,8,8-tetramethyl-o-quinodimethane. Of the three primary processes discussed in the literature, we conclude that reacti ons (1) and (2) occur with quantum yields of approximately 0.09 and 0. 1 respectively NO3- + h nu --> NO2. + O.- (O.- + H2O --> (OH)-O-. + OH -) (1) NO3- + h nu --> ONOO- (2) It appears that none of the peroxynit rite anion is formed in a cage reaction through the recombination of t he primary fragments from reaction (1). The primary process shown in r eaction (3) is of relatively minor importance, with a quantum yield of no more than 0.001 (3) NO3- + h nu --> NO2- + O (3) In the presence o f methanol (or propan-2-ol) and oxygen under acidic conditions, formal dehyde (or acetone) is formed in an amount equivalent to nitrite via p eroxyl radical reactions (quantum yield of approximately 0.06 for both alcohols). In the alkaline range, the apparent formaldehyde quantum y ield decreases with increasing pH, while formic acid is produced in in creasing amounts. The formation of formic acid is ascribed to the reac tion of peroxynitrite anion with photolytically generated formaldehyde . The acetone quantum yield does not decrease with increasing pH over the whole alkaline pH range. In the presence of cyclopentane under oxy gen-free conditions, apart from nitrite (and peroxynitrite when alkali ne), the compounds nitrocyclopentane, cyclopentyl nitrate, cyclopenten e, cyclopentanol and cyclopentanone are produced. The formation of the organic nitrogen compounds leads to an increase in the pH as photolys is proceeds. This pH shift is particularly pronounced in the neutral r ange.