Photolysis polarization effects

Dimers (73) and (74) were formed in approximately equal amounts in all cases, although, as in the cases of 2-cyclopentenone and 2-cyclohexenone, the relative amount of (72) (either cis-syn-cis or cis-anti-cis) was found to vary substantially with solvent polarity. As in 2-cyclopentenone, this increase in the rate of head-to-head dimerization was attributed to stabilization of the increase in dipole moment in going to the transition state leading to (72) in polar solvents. It is thought that the solvent effect in this case is not associated with the state of aggregation since a plot of Stem-Volmer plot and complete quenching with 0.2 M piperylene indicate that the reaction proceeds mainly from the triplet manifold. However, the rates of formation of head-to-head and head-to-tail dimers do not show the same relationship when sensitized by benzophenone as in the direct photolysis. This effect, when combined with different intercepts for head-to-head and head-to-tail dimerizations quenched by piperylene in the Stem-Volmer plot, indicates that two distinct excited triplet states are involved with differing efficiencies of population. The nature of these two triplets has not been disclosed. 

ItoO, Matsuda M (1979) Evaluation of addition rates of the thiyl radicals to vinyl monomers by flash photolysis. 2. Substituent effect on addition of substituted benzenethiyl radicals to methyl methacrylate or styrene. J Am Chem Soc 101 5732-5735 Ito O, Matsuda M (1981) Evaluation of addition rates of thiyl radicals to vinyl monomers by flash photolysis. 3. Polar effect in addition reactions of substituted benzenethiyl radicals. J Am Chem Soc 103 5871-5874... 

Ito O, Matsuda M (1982) Polar effects in addition reactions of benzenethiyl radicals to substituted styrenes and a-methylstyrenes determined by flash photolysis. J Am Chem Soc 104 1701-1703 Janata E, Veltwisch D, Asmus K-D (1980) Submicrosecond pulse radiolysis conductivity measurements in aqueous solutions. II. Fast processes in the oxidation of some organic sulphides. Ra-... 

Table 6 lists the polarization effects of the initial compounds, normal addition products and trichlorobutene observed in the photolysis of allyltriorganogermanes in the presence of CCl3Br. The analysis of the detected 1II CIDNP shows that, similarly to the reaction of allyltriethylstannane described above, the polarization is formed in the diffusion F-pair... 

The direct photolysis of the reaction mixture of 11 and 14 in the probe of the NMR spectrometer has allowed one to detect H CIDNP effects (Figure 14) of the methyl protons of (Me3Si)2NBr and hexamethyldisilazane, and the methylene protons of allene. The spectrum also demonstrates polarization effects in the region of olefin protons of precursor 14. 

Table 12 and Figure 18 show the polarization effects detected in the photolysis of 26 in the presence of thiacycloheptyne 21 which could be compared with the effects observed under analogous conditions in the photodecomposition of 7-germanorbornadiene 17 (Table 8). 

Similar to the case of 7-heteronorbornadienes, the application of the CIDNP method has allowed us to identify the elementary stages of the photolytic decomposition of 26 . In addition to 27, the main reaction products also include oligogermanes and the product of photorearrangement of the initial 26, namely 6,6, 7,7 -tetramethyl-2,5-diphenyl-3,4-benzo-6,7-digermatricyclo[3.3.0.0]octane . Table 11 and Figure 17 show the polarization effects of methyl and aromatic 5,6-protons of the initial 26 observed in the photolysis in CsDs similar effects were also observed in c-C Di2 and in CCLj-CsDs mixtnres. 

Selective formation of the azirane 11 from the triplet-excited azoalkanes 3 in polar media is rationahzed in terms of solvent stabilization of the more polar transition state for the (i-CC-bond cleavage vs.that for the a-CN-bond scission. This solvent polarity effect, observed for the liquid-phase photolysis of azoalkanes 3, is similar to the photochemical behavior of the azoalkane 3a in the interior of the zeoHtes. The polar zeohte environment enhances the formation of azirane 1 la compared to the solution photochemistry in benzene. In contrast to zeolites, the formation of azirane 11 a, a triplet-state product, is completely suppressed in the crystaUine state and the housane 10a, a singlet-state product, is formed selectively. ... 

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