Hydrogen laser from with

Cyclohexyl xanthate has been used as a model compound for mechanistic studies [43]. From laser flash photolysis experiments the absolute rate constant of the reaction with (TMS)3Si has been measured (see Table 4.3). From a competition experiment between cyclohexyl xanthate and -octyl bromide, xanthate was ca 2 times more reactive than the primary alkyl bromide instead of ca 50 as expected from the rate constants reported in Tables 4.1 and 4.3. This result suggests that the addition of silyl radical to thiocarbonyl moiety is reversible. The mechanism of xanthate reduction is depicted in Scheme 4.3 (TMS)3Si radicals, initially generated by small amounts of AIBN, attack the thiocarbonyl moiety to form in a reversible manner a radical intermediate that undergoes (3-scission to form alkyl radicals. Hydrogen abstraction from the silane gives the alkane and (TMS)3Si radical, thus completing the cycle of this chain reaction. 

The dependence of the dissociation constant kj) and the recombination constants kjt in the reaction (a) on the optical polarization of the atoms A was predicted by Bernheim [65] and Kastler [225] and later demonstrated experimentally [7, 363]. The dependence can be understood from the obvious fact that only hydrogen-like atoms with opposite electronic spins may recombine and form a molecule A2(X1E+). Hence we have kR = kft(l — S2), where S is the degree of electron spin polarization of the atoms. A convenient indicator of dimer formation is provided by the kinetics of the laser-induced molecular fluorescence after switching on magnetic resonance which destroys the polarization of the atoms, as performed by Huber and Weber [201] for a Na — Na2 mixture. The se-... 

The relative yields of the respective explosion pathways can be derived from the number of coincidence events. For CH3CN, the relative yields for the n = 1 and 2 pathways with respect to the n = 0 pathway are 1.3(1) and 1.0(1), respectively. The comparable yields for the three explosion pathways show that the migration of hydrogen atoms competes with the two-body Coulomb explosion of acetonitrile in the intense laser field (0.15 PW/cm2). 

S -state. Experimentally, the IS — 2S transition can be induced Doppler-free by absorbing two photons from two identical counter-propagating laser beams being generated by either a sufficiently powerful pulsed laser (as with muonium [14]) or a continuous laser (as with hydrogen [5,6]). In the latter case, for example, where 1.3 Hz, the quality factor of 8v/v 10-15 has already been... 

In many synthetically useful radical chain reactions, hydrogen donors are used to trap adduct radicals. Absolute rate constants for the reaction of the resulting hydrogen donor radicals with alkenes have been measured by laser flash photolysis techniques and time-resolved optical absorption spectroscopy for detection of reactant and adduct radicals Addition rates to acrylonitrile and 1,3-pentadienes differ by no more than one order of magnitude, the difference being most sizable for the most nucleophilic radical (Table 8). The reaction is much slower, however, if substituents are present at the terminal diene carbon atoms. This is a general phenomenon known from addition reactions to alkenes, with rate reductions of ca lOO observed at ambient temperature for the introduction of methyl groups at the attacked alkene carbon atom . This steric retardation of the addition process either completely inhibits the chain reaction or leads to the formation of rmwanted products. 

BPH and BPDH were generated from the photoreduction of BP and BPD in cyclohexane. BP and BPD in the lowest triplet excited states (BP(T ) and BPDCTj)) decayed through the hydrogen abstraction from cyclohexane to produce BPH and BPDH after the first 266-nm nanosecond-laser irradiation. The generated BPH was excited at the visible absorption band using the second laser (532 nm, 30 ps FWHM) with the delay time of 1 ps after the first laser. Upon the excitation, BPH showed fluorescence with a peak at 564 nm. Similarly, fluorescence of BPDH was observed with the second laser irradiation. The measured xf of BPH is 2.0 ns, which is close to the reported value [112-114]. 4-Chloro-, 4,4 -dichloro-, 4-bromo-, 4,4 -dibromo-, and 4-trifluoromethyl-substituted... 

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