“…The efficiency of photodecarboxylation of benzyl phenylacetates (18)(19)(20) varies considerably with substituents and solvent, but is always lower than that for the decarbonylation of dibenzyl ketones; for example, +-c02 for benzyl phenylacetate, 2, is 0.31 in dioxane (19a) and 0.19 in methanol (18). Although it has been concluded that the photolysis proceeds by a radical pathway, the multiplicity of the excited state involved and the nature of the decarboxylation step are less certain.…”
. 62, 403 (1984).Singlet and triplet benzyl radical pairs have been generated on silica gel by photolysis of a benzyl phenylacetate, a dibenzyl ketone, and a dibenzyl sulfone. The extent of geminate radical recombination has been measured and requires that translational motion of radicals occur on the silica gel surface. This motion was affected by the radical pair multiplicity and the photolysis temperature, but was relatively insensitive to the state of hydration of the silica gel and the presence of coadsorbates. The presence of certain rearranged starting materials, which are not formed in solution, amongst the products from photolysis of both dibenzyl ketone and dibenzyl sulfone on silica gel indicates the restrictions on radical movement on the surface on the shorter tirnescale of the benzyl-phenylacetyl and benzyl -benzyl sulfonyl radical pairs.
“…The efficiency of photodecarboxylation of benzyl phenylacetates (18)(19)(20) varies considerably with substituents and solvent, but is always lower than that for the decarbonylation of dibenzyl ketones; for example, +-c02 for benzyl phenylacetate, 2, is 0.31 in dioxane (19a) and 0.19 in methanol (18). Although it has been concluded that the photolysis proceeds by a radical pathway, the multiplicity of the excited state involved and the nature of the decarboxylation step are less certain.…”
. 62, 403 (1984).Singlet and triplet benzyl radical pairs have been generated on silica gel by photolysis of a benzyl phenylacetate, a dibenzyl ketone, and a dibenzyl sulfone. The extent of geminate radical recombination has been measured and requires that translational motion of radicals occur on the silica gel surface. This motion was affected by the radical pair multiplicity and the photolysis temperature, but was relatively insensitive to the state of hydration of the silica gel and the presence of coadsorbates. The presence of certain rearranged starting materials, which are not formed in solution, amongst the products from photolysis of both dibenzyl ketone and dibenzyl sulfone on silica gel indicates the restrictions on radical movement on the surface on the shorter tirnescale of the benzyl-phenylacetyl and benzyl -benzyl sulfonyl radical pairs.
“…Next, we compare these k 2 /ε Max from the TG measurement with those reported previously from the transient absorption measurements. The values of k 2 and ε Max have been reported by many groups: ε Max = 1100 cm -1 M -1 and k 2 = 1.8 × 10 7 M -1 s -1 by McClarthy and MacLachlan reported in a mixed solvent of ethanol and glycol, k 2 = 2.3 × 10 9 M -1 s -1 and ε Max = 12 000 cm -1 M -1 by Hagemann and Schwartz in cyclohexane, ε Max = 1500 cm -1 M -1 , k 2 = 6.8 × 10 9 M -1 s -1 by Meiggs et al in methanol, ε Max = 8800 cm -1 M -1 , k 2 = 1.8 × 10 9 M -1 s -1 ,32a or k 2 = 2.3 × 10 9 M -1 s -1 32b by Fischer and co-workers in cyclohexane, ε Max = 5500 cm -1 M -1 and k 2 = 1.55 × 10 9 M -1 s -1 in water by Christensen et al, k 2 = 3.5 × 10 9 M -1 s -1 in benzene by Lauter and Dreeskamp, and k 2 = 4 × 10 9 M -1 s -1 by Burkhart . As shown above, the reported ε Max and k 2 are so much scattered.…”
Section: Discussionmentioning
confidence: 58%
“…It has been reported that the reaction process of the self-termination of BR is a pseudo-diffusion-controlled reaction and the steric factor of this reaction is 0.8 . The rate constant (2 k 2 ) of such a second ordered reaction has been measured in various solvents and it is reported to be ∼10 9 M -1 s -1 as discussed in a later section. − 1 …”
Section: Introductionmentioning
confidence: 94%
“…The photodissociation process of DBK has been studied extensively in various solvents. − The reaction scheme is shown in Scheme . The lowest excited triplet (T 1 ) state of DBK is created by the intersystem crossing from the lowest excited singlet (S 1 ) state within a picosecond time scale after the UV irradiation (process a−c).…”
Translational diffusion processes of the benzyl radical (BR)
created by the photodissociation from dibenzyl
ketone (DBK) were studied by using the transient grating (TG) method in
organic solvents (hexane,
cyclohexane, ethanol, and 2-propanol). The values of the diffusion
constants (D) of BR, DBK, and carbon
monoxide (CO) and the rates for the self-termination reaction of BR in
these solvents are accurately determined
from the TG signals. The ratios of D of BR to that of
DBK are independent of the solvent, and they are
close to 1.25, which is expected from the ratio of the molecular volume
of BR to that of DBK. This result
is contrary to those of radicals created by the photoinduced hydrogen
abstraction reaction, which show
anomalously slow diffusions. A possible origin of this difference
is discussed by comparison of the spin
densities, dipole moments, and polarizabilities of BR and the radicals
produced by hydrogen abstraction.
“…The rate constants of the benzyl radical selftermination reaction were determined by many groups and in various solvents: 2.3 × 10 9 M −1 s −1 in cyclohexane [2,7], 6.8 × 10 9 M −1 s −1 in methanol [8], 1.55 × 10 9 M −1 s −1 in water [9], 3.5 × 10 9 M −1 s −1 and 4 × 10 9 M −1 s −1 in benzene [10,11], 2.6 × 10 9 M −1 s −1 in toluene [12]. Less common methods like photomodulated voltammetry can also be applied to obtain kinetic parameters for the self-termination reaction of radicals (3.9 × 10 9 M −1 s −1 in acetonitrile [13] for the benzyl radical) as well as for the electron transfer reaction.…”
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
