No abstract

An understanding of the mechanism of a photochemical reaction may involve a knowledge of protolytic reactions, not only of the ground-state molecule, but also of excited states, free radicals and other intermediates involved. In molecules, such as ketones and quinones, having hydroxy and amino substituents, protonation raises the quantum yield of reaction from near zero to near unity and our interpretation of this observation involves a consideration of protolytic reactions in the ground and upper singlet states and in two types of triplet state. Protolytic equilibrium is maintained only between states of similar electron configuration. The relationship between proton, electron and hydrogen atom transfer is briefly discussed.

The absorption spectrum of triplet benzene has been observed by flash photolysis studies of benzene in rigid hydrocarbon glasses at 77°K.

Evidence is presented of similarities of transient spectra and kinetics in flash photolyzed and pulse radiolyzed aqueous solutions of p-nitrosodimethylaniline (RNO). This is unexpected since the energy in radiolysis is absorbed predominantly by H2O whereas the bulk of the photons in flash photolysis experiments are absorbed by RNO molecules. Detectable transient absorption after the flash photolysis is found only when irradiating with wavelengths of light shorter than 230 nm. Transients with maximum absorbance at 350, 540, and 580 nm have been studied as a function of pH, oxygen and RNO concentration, flash intensity, and wavelength of irradiating light. The 540-nm transient has been characterized as being quite insensitive to experimental conditions with a first-order decay constant of 5.7 X 102 sec-1 and is apparently the same transient as that found in the RNO pulse radiolysis system. A study of RNO bleaching under continuous irradiation demonstrated that the wavelength of light used has to be below a threshold value of 230 nm for RNO bleaching (at 440 nm) to occur. This bleaching is essentially zero order in RNO concentration and is enhanced by oxygen in a reaction which is first order in oxygen concentration. This, together with other evidence, strongly implicates water, rather than direct RNO, photolysis as the primary photolysis event. This primary process is probably direct water photolysis or photolysis of a water-oxygen charge transfer complex. It is also necessary to postulate that in flash photolysis, as well as in pulse radiolysis of oxygenated RNO solutions, there are two reactive intermediates, in addition to the hydrated electron (or O2-in the case of photolysis and radiolysis of oxygenated solutions), which react with RNO at relatively early times, giving rise independently to the 350-and 540-nm transients. One of these is probably the OH radical. The identity of the other is unknown, but appears to be related to the OH radical rather than the reducing species eaq~o r H.

The gaseous self-decomposition of nitric acid, 10-48 mm., made up with nitrogen to 680 mm., has been followed a t 303.5 and 349.5" by means of the pressure change. The same reaction has been studied by means of infrared analysis a t 349.5", more definitively, and as the reference-reaction for a corresponding study of the reaction between nitric acid, 5-17 mm., and methane, 90-700 mm., with nitrogen added as needed to make up a minimum total pressure of 700 mm.Confirming the conclusions of Johnston and his co-workers, we find the self-decomposition of nitric acid to have the kinetics of a unimolecular reaction, in which the rate-controlling first step of homolysis is reversible. We find the reaction with methane to have the kinetics, apart from a small rate-dependence on methane, of a unimolecular reaction rate-controlled by the same first step of homolysis, which is here irreversible, because it is followed by fast reactions, as shown in eqns. (12) (p. 1077). These reactions lead in parallel to nitration and oxidation, a t absolute rates, and with product compositions, which are plausibly consistent with observation. The small dependence of the rate on methane concentration may be the superposed effect of a concurrent, relatively unimportant, bimolecular heterolytic mechanism, as illustrated in eqn. (13) (p. 1077).