Trapping rate

Also, the distance of the injection system from the first oxidized guanine influences the product ratio Pggg/Pgu because the injection system itself can influence the water trapping rate of the nearby Gf+. Therefore, the experimentally detected efficiencies of the charge transfer (measured by the ratio of the chemical products) depend upon the special injection system and the reaction conditions. This has to be taken into account if the experimental results obtained under different reaction conditions are to be compared with each other. 

Interference filters with 7-17 nm band width Tungsten lamp [G] [LDi] [L-ns]12 Phototaxigraph (light trap). Rate photoaccumulation. Spectrum is relative response to that at 499 nm corrected for incident quanta Diehn and Tollin29)... 

Two possible approaches are indicated in Schemes 4 and 5. In the first, a reactive radical R> is spin-trapped in competition with its pseudo-first order reaction with a substrate SH, which occurs at a known rate to give RH and S. The growth of both spin-adducts (ST—R ) and (ST—S ) is monitored, and simple analysis leads to the trapping rate constant kT. In the second approach, R-does not react with a substrate, but undergoes unimolecular rearrangement or fragmentation at a known rate to give a new species R. This latter procedure... 

Rate constants for spin trapping of alkyl radicals measured by the procedures outlined here, are collected with other spin-trapping rate data in Table 5. It will be seen that most nitrone and nitroso traps scavenge reactive radicals of diverse types with rate constants generally in the range 10s-10 1 mol-1 s l. Of the nitroso-compounds, the nitroso-aromatics (except for the very crowded TBN) are particularly reactive, whilst MBN and DMPO are the most reactive nitrones. Much of the data for spin trapping by nitrones has been accumulated by Janzen and his colleagues, who have discussed in a short review how steric and electronic factors influence these reactions (Janzen et ai, 1978). 

The trapping rate dPT/dt is given by the sum of energy-transfer rates from any site donor i to any trap site J. 

On the assumptions that the triplet TMB biradical 37 is the reactive intermediate and that its reaction with O2 occurs at the encounter-controlled rate, the authors estimated that the triplet is more stable than the singlet by at least 4-5 kcal/mol, or more if the diffusion-limited trapping rate assumed is actually lower. 

Laser flash photolysis methods have also been applied to the study of nitrenium ion trapping rates and hfetimes. This method relies on short laser pulses to create a high transient concentration of the nitrenium ion, and fast detection technology to characterize its spectrum and lifetime The most frequently used detection method is fast UV-vis spectroscopy. This method has the advantage of high sensitivity, but provides very little specific information about the structure of the species being detected. More recently, time-resolved infrared (TRIR) and Raman spectroscopies have been used in conjunction with flash photolysis methods. These provide very detailed structural information, but suffer from lower detection sensitivity. 

TABLE 13.7. Absorption Maxima (nm), Lifetimes, and Trapping Rate Constants (M s ) of Nitreninm Ions Characterized by Laser Flash Photolysis of A-Aminopyridininm Salts"... 

The aminopyridinium route has been employed in flash photolysis studies of aryl as well as diarylnitrenium ions. Several examples of nitrenium ion species, along with their absorption maxima and some trapping rate constants are given in Table 13.7. To the extent the data are comparable, there is good agreement with the behavior of nitrenium ions generated by the azide route. For example, the 4-biphenylyl systems from the azide protonation and /-aminopyridinium routes both give absorption maxima at 460 nm and live for several microseconds in water. Likewise, the 4-methoxyphenyl systems show maxima at 300 nm (from azide) and 320 nm (from aminopyridinium ion). The discrepancy in this case can be attributed to the A -methyl substituent, present in the aminopyridinium route, but absent in the azide experiment. 

Two different explanations have been advanced for the difference in trapping rates of GG and GGG. Berlin, Burin, and Ratner [54] base their explanation on the assumptions that (1) both traps are deep, i.e., -0.5 eV for GG and -0.7 eV for GGG, and (2) the ionization potential of guanine is lower than that of the other nucleobases by at least 0.4 eV. The different reactivities of the two traps were ascribed in [54] to different relaxation times of a hole in the trap. The GG units were taken to have a long relaxation time, so that a hole is likely to make a further hop before the trap closes on it, while the relaxation time of the GGG units was supposed to be relatively short, faster than the hopping time. 

Thermolysis of l,l-difluoro-2,3-diphenylcyclopropane in supercritical CO2 has allowed the rate of geometrical isomerization [i.e. cis-( 109) to /ra/M-(109)] and racemization [i.e. (/< )-( 109) to (S)-( 109)] to be determined from O2 dependence of the trapping rate of the postulated intermediate 1,3-biradical.246 Above 150 °C, the formation of 2,2-difluoroindane and its decomposition products is reported. A similar thermally induced equilibrating series of stereomutations has been observed with the analogous non-fluorinated cyclopropane in which rate constants and deuterium exchange isotope effects are reported.247 Theoretical studies of this isomerization have focused on classical248 and quasi-classical trajectories.249... 

---