No abstract

No abstract

The interaction between triplet benzophenone-4-carboxylate (3BC) and some inorganic anions, X", has been studied by laser flash photolysis over a wide range of anion concentration (up to 6 M), at pH 11 and constant ionic strength. Spectra and extinction coefficients of 3BC and BC1 89" radical were determined by using the benzophenone actinometer. Triplet lifetime measurements gave primary interaction rate constants, kf-(M_1 s'1), close to diffusion controlled for iodide, thiocyanate, and azide (k ~2 X 12109), somewhat less for bromide (2 X 108) and very low for chloride (<106). Radical yields, 0.1 M, <¿-r increases with [X"] and equivalent amounts of BC'" and X2" radicals are formed, including N6~. At high anion concentrations, plots of versus [X"]"1 are linear. Limiting

No abstract

No abstract

Laser flash and steady irradiation studies have been carried out on the 4-carboxybenzophenone (CB)-4-carboxybenzhydrol (CBH2) and CB-hydrazine systems over the pH range 5.8-12. In reaction with CBH2, CB triplet is quenched in part by the carboxylate, Aq 4 x 106 M"1 s"1; total reaction, A4 = 3.0 X 107 M"1 s"1, leads to the ketyl radical CBH•, = 1.7. Absorption spectra were recorded for CB triplet, Xma, = 535 nm (e = 6600 M"1 cm"1), for CBH•, Xmal = 570 nm (e = 5500), and for the radical anion CB"•, X^, = 660 nm (e = 8100). The pAa of CBH• was determined, 8.2. Rate constants were determined: for combination of CBH• to pinacol, 2 k6 = 1.8 X 109 M"1 s'1; for reactions of CBH• with CB", k1 = 1.1X 109 M'1 s'1; for disproportionation of 2CB"•, 2Ag = 9.8 X 106 M"1 s"1. Factors to convert observed yields of photoreduction by hydrazine to yields of ketyl radical leading to pinacol and hydrol were determined from effects of pH on quantum yields of reduction of CB by CBH2. Reduction of CB triplet by neutral hydrazine occurs with kir = 6.8 X 108 M"1 s"1, forming ketyl radical with = 1.0, measured relative to the CBH2 reaction. The reaction with protonated hydrazine is slower, k'ir = 4.4 X 106 M"1 s"1. Computer simulation, using the measured rate constants, pKa and primary yields, and literature constants

Proton NMR spectroscopy has been used to study the protonation of meso-tetraphenylporphyrins with zero, one, or four para dimethylamino substituents. Changes in the spectra are similar for corresponding protons through this series of compounds. We give the following interpretations. The internal pyrrolenine nitrogen atoms always add two protons before any peripheral dimethylamino groups react. There is no evidence for a monoprotonated intermediate between the free base and diprotonated porphyrin in the chloroform-trifluoroacetic acid solvent system used. The chemical shifts of the NMR signals of the internal N-H protons are strongly acid-dependent and suggest a loss of aromatic ring current at the porphyrin core a t intermediate acid concentrations. Changes in chemical shifts of protons on the periphery of these aromatic systems are much smaller and in the opposite sense. The signals of the pyrrole Hp protons move discontinuously upfield upon diprotonation and then drift continuously downfield as acid concentration is increased beyond that required for diprotonation. These changes are also in the directions to be expected if aromatic ring currents of the porphyrin decrease to a minimum a t the acid concentration a t which the internal nitrogen atoms are fully protonated. W e suggest that this is due to the known distortion from planarity of the diprotonated tetraarylporphyrin aromatic system. Signals of protons attached to the meso-substituted aromatic rings move unidirectionally downfield as acid concentration increases.The changes in optical spectra of porphyrins on addition of acid have been used extensively to follow the evolution of solute species as basic sites are protonated.' Careful analysis of the shapes of these spectrophotometric titration curves can establish the sequence of distinguishable states and in favorable cases the number of protons added if successive sets of isosbestic points can be identified. However, there can be problems in determining the concentration of free acid in the nonaqueous solvents generally used for these studies. Furthermore, if several competing basic sites are available, the spectrophotometric data can at best tell only indirectly which sites are protonated. Although proton NMR spectra of porphyrins have been routinely recorded as a step in characterization, they often have not been used to count the number of protons at specific basic sites in these molecules, by integration of appropriate NMR signals. Also, with some exceptions, the spectra have been run only in a nonpolar neutral solvent or in trifluoroacetic acid but not at intermediate acid concentrations.2 Theexpectation has been that porphyrins without basic groups on the periphery of the ring system will add two protons on the two pyrrolenine nitrogen atoms which lack them in the free base. This expectation was confirmed by early acidbase titrations3 and NMR studies4 which have been interpreted as demonstrations of diprotonation.In this paper we report proton NMR studies on mesotetraphenylporphyrin with from zero...

No abstract