Transient species decay constant

The formation of diphenylphosphino radicals on photolysis of triphenyl-phosphine, diphenylphosphine, and tetraphenylbiphosphine has been verified. In the case of the reactions of the phosphines, the radicals were trapped with t-nitrosobutane and the resultant nitroxyl radical [Ph2PN(0)Bu ] was identified by e.s.r. The nitroxyl radical has a small P splitting constant, demonstrating that there is no extensive delocalization onto the phosphorus atom. The e.s.r. spectrum of diphenylphosphino radicals, generated by photolysis of tetraphenylbiphosphine in benzene at 77 K, has been observed. When methanolic solutions of the biphosphine or triphenylphosphine are flash-photolysed, a transient species having Amax = 330 nm and which decays by first-order kinetics (A 4 x 10 s )... 

Most importantly, the careful kinetic analysis of the rise and decay of the transient species in equation (69) shows that the decarboxylation of Ph2C(OH)CO occurs within a few picoseconds (kc c = (2-8) x 1011 s-1). The observation of such ultrafast (decarboxylation) rate constants, which nearly approach those of barrier-free unimolecular reactions, suggests that the advances in time-resolved spectroscopy can be exploited to probe the transition state for C—C bond cleavages via charge-transfer photolysis. 

In the case of NOS 12, different kinetics were observed at 436 nm. In cyclohexane, there was a rapid rise, with a lifetime of 6.6 psec followed by a decay with a 100-psec lifetime. In 1-butanol, there was a rapid rise (lifetime=4.3 psec), a decay (43-psec lifetime), and a second longer decay within a 1.4-nsec lifetime. These findings were confirmed by picosecond time-resolved resonance Raman spectroscopy. In these Raman studies in cyclohexane, a single rate constant was observed, whereas in 1-butanol, three spectral components grew with different time constants. The data were said to be consistent with the photo-formation of two or three isomers trans about the central methine bond however, other transient species could be responsible for the observed kinetics because the absorption envelope obviously shifts and this would affect the resonance Raman bands. 

It is common for many second-order reactions to exhibit pseudo-first-order behavior under conditions of nLFP. This is due to the fact that, while reactive intermediates are present in micromolar concentrations, (typically 10-50 pA/), the molecules with which they react are present in concentrations several orders of magnitude larger. As a result, the concentration of these reagents remains essentially constant during the decay of the transient species. An example is shown in Figure 18.4, where triplet benzophenone is quenched by melatonin. ... 

The decay constant of the transient species can be measured directly by using nonstationary state methods. In these methods the system is irradiated intermittently so that transients are created in the light period and decay in the dark period. The simplest form of the arrangement is the use of sector wheel in which a number of sectors are cut (Figure 10.9). The cut and uncut portions can be of equal size or may vary in a definite... 

Similar IPCT photochemistry is observed for salts involving other organic acceptors, with results indicating that the cage escape efficiency of the CT pair is sensitive to the charge of the acceptor, being 1.0 for A+ but only 0.1 for A2+. The related tetrahedral complex MV[Zn(mnt)2] displays similar CT photochemistry, although the decay of the transient species occurs with a second-order rate constant of k = 3 x 109 M 1 s 1 (69). 

The transient Q-band EPR experiments provide direct evidence for sequential electron transfer from the primary to the secondary radical pair of the triplet channel in a triad consisting of a zinc-9-desoxo-meso-methylpyrochlorophyllide donor (ZC), a pyromellitimide primary acceptor (PI), and a naphthalene-1,8 4,5-diimide secondary acceptor oriented in a liquid crystal (Heinen et al., 2002). At room temperature this process occurs with an exponential time constant of tT = 50 + 1 ns. In the singlet-initiated channel, the intramolecular electron-transfer rates are too fast for direct EPR detection. The species decay with a time constant of tS = 36 1 ns by charge recombination to the singlet ground state. 

Transient bivalent gold ions and two unidentified species, possibly Au°, were prepared and stiuded in different aqueous matrices by pulse radiolysis. Their absorption spectra and rate constants of formation and decay are reported. Bivalent gold was prepared by the oxidation of Au(CN)2 in neutral chloride solutions of N20, and also by the reduction of AuClf in the presence of CH3OH. The kinetics of its decay indicates disproportionation. The reduced species of Au(CN)2 from H atoms is different from that obtained with e aq, the latter being attributed to an electron adduct. Both of these species decay, forming colloidal gold. 

Formation, decay, and absorption spectra of transient species produced by reaction of OH free radicals with DNA and nucleic acid constituents have been investigated by pulse radiolysis. Dilute aqueous N20 saturated solutions were exposed to 2-500 nsec, pulses (750-1700 rads/pulse) of 10 Mev. electrons, and changes in optical density were measured. The rate constant for formation of the DNA transient (nucleotide M.W. 350) is 6 X 108 M I seer1. The transient persists for more than 1 msec, with no significant change in spectrum. Thymine, uracil, deoxyribose, and thymidine transients have complex decay patterns which vary with pH. Comparisons of transient spectra confirm that the sites of attack by OH on thymine, 5-methylcytosine, and thymidine depend on pH. Abstraction from 5,6-dihydro-thymine is slower than addition to thymine. 

Table I. First Order Rate Constants for the Decay of the Longest Lived Transient Species Formed in Radiolysis of N2O Saturated Aqueous Thymine Solutions (10 4 k)...

In fact, it is well known that Cr(lll) polypyridyl complexes are reduced to the labile Cr(ll) species upon irradiation with 351 nm laser, in agreement with their strong oxidant character in the excited state. Flash-photolysis experiments showed transient decay constants in the ps and sub-ps timescale that could be quenched by both, oxidants ([Ru (NH3)6]) and reductants (F, triethanolamine and alcohols) [39]. A very complex photophysic and photochemical behavior were described. 

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