These studies characterize the primary process and products in the photodecarboxylation of nitrophenylacetate ions in aqueous solutions. 3-Nitrophenylacetate, 4-nitrophenylacetate, and 4-nitrohomophthalate ions decarboxylate with a quantum yield of about 0.6 at 367 nm, while that of o-nitro structures (2-nitrophenylacetate and 2,4-dinitrophenylacetate ions) is only 0.04. acz'-Nitro intermediates are identified spectrally by flash photolyses from the o-and p-nitro structures, but not from the m-nitro structure. Results are consistent with a mechanism of photodecomposition into carbon dioxide and a nitrobenzyl anion which is stabilized as an ««-nitro anion in o-and p-nitro structures. The nitrophenylacetic acids are more stable photochemically than their ions; neither C02 nor a«'-nitro intermediates are observed from photolysis of the undissociated acids. The ««-nitro yield varies with pH according to the pA-of the acid being photolyzed; thus a pK of 4.8 is calculated for the acarboxyl group of 4-nitrohomophthalic acid. Three of the ««'-nitro anions occur via photochemical hydrogentransfer reactions from other compounds so that by comparison with the photodecarboxylations the quantum yields of H-transfer photochromism in 2-nitrotoluene, 2,4-dinitrotoluene, and 5-nitro-otoluate ions are estimated to be about 0.01, 0.003, and 0.07, respectively.In the course of studies on the photochromism of compounds with nitrobenzyl structures,1 we noted that an irreversible photochemical decarboxylation occurs in aqueous solutions of compounds with nitrophenylacetate ion structures.2 In both types of reaction '-nitro intermediates are postulated to occur from photochemical acid-base processes. While the photochromic process appears to be a reversible proton-transfer reaction, the photodecarboxylation mechanism is based upon the formation of carbon dioxide and a carbanion residue, as in eq 1. The carboxyl forms of these nitrophenylacetic acids photodecompose less readily and do not yield the same intermediates or products as their ions.

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