Pronounced solvent effect on the absorption spectra of the photochemically produced 2,4-dinitrobenzyl carbanion

Abstract

The photochromism of 2,4-dinitrotoluene in aqueous solution has been attributed to formation of the 2,4-dinitrobenzyl carbanion (J. Chem. Phys. 39, 1218 (1963)). The spectrum of the colored transient is, however, significantly different from that of ground state carbanion when formed in aprotic solvents through thermal decarboxylation of potassium 2,4-dinitro-phenylacetate (J. Org. Chem. 49, 413 (1984)). In this paper absorption spectra obtained upon 248 nm laser photolysis are reported, for solutions in water, acetonitrile, and their mixtures. The spectrum in acetonitrile does closely resemble that of the carbanion formed by decarboxylation. It is suggested that the same ion is formed in water, and a solvent effect is responsible for the significantly different spectrum. Spectra in the mixed solvents suggest that there is an equilibrium between differently solvated forms of the anion, the extremes being an unspecifically solvated form with λmax 400 and 640 nm as in acetonitrile and a form in water that is hydrogen bonded through the nitro group with λmax 350 and 500 nm. Experiments with conductivity detection provide corroborative evidence for carbanion formation. In acetonitrile a non-conducting transient is produced as a precursor to the anion. This is suggested to be the conjugate acid of the aci-nitro anion, formed by intramolecular hydrogen abstraction in the excited nitrotoluene.

Laser photolysis of caged calcium: rates of calcium release by nitrophenyl-EGTA and DM-nitrophen

“…It is well known that flash photolysis of o-nitrobenzyl compounds at 350 nm produces transient intermediates (or aci-nitro species; cf. structures 1 and 2 in Scheme 1) that absorb at 450 nm (Wettermark, 1962;McCray et al, 1980McCray et al, , 1992Schupp et al, 1987;McClelland and Steenken, 1987). A single pulse from a frequency-doubled ruby laser (150 mJ at 347 nm) generated such aci-nitro species, the rate of disappearance of which was probed with the 458-nm line of an argon laser.…”

“…This species then decays to products via the bridged intermediate of type 3 with a half-time of 10 ms (the solvent was 60% ethanol, 40% H20). McClelland and Steenken (1987) have found that the aci-nitro conjugate acid has a very brief half-time in aqueous solution. In their experiments the conjugate base was formed in the lifetime of the laser pump pulse (35 ns).…”

A caged substrate peptide for matrix metalloproteinases

“…Flash photolysis experiments indicate that the photogenerated nitronic acid intermediates 6 of 2-nitrobenzyl compounds undergo deprotonation in water at neutral pH with rates in the 10 7 -10 8 s −1 range. [34][35][36] The deprotonation rate is essentially determined by the pK a of the particular nitronic acid intermediate, with the more acidic compounds having Table 1 Second-order rate constant k cat /K M , Michaelis constant K M for the hydrolysis of Fluo-LK and Fluo-KK in comparison with commercially available fluorogenic peptides (FS-1 and FS-6, see Scheme 1) for MT1-MMP at 25 °C…”

Deprotonation yields, pKa, and aci-nitro decay rates in some substituted o-nitrobenzaldehydes

“…ation with rates in the 10 8 s Ϫ1 range. [6][7][8][9] The high deprotonation rate is mainly due to the low pK a of the aci-nitro intermediate. 10 This property also accounts for the fact that the aci-nitro intermediate is completely deprotonated at pH above 3-4, depending on the compound.…”