A limitation in the above kinetic analysis is that the quasitwo-dimensional nature of the reaction in multilayer samples has not been taken into account. There are two ways in which this circumstance may manifest itself.(1) The spin label, which has a diffusion coefficient ~10_7-10~8 cm2/s in the fluid phase,4 is essentially "immobile" with respect to the alkyl radicals. Thus a "diffusioncontrolled" rate constant may be lowered by a factor of 2 compared to the case where both molecules are equally mobile.(2) The spin label is constrained to move strictly in the plane of the membrane head-group region, and the free water-soluble molecules are constrained to move in a strip which is only ~20 Á wide, hence nearly but not strictly two-dimensional. The collision probability per unit time, and hence the rate constant, is then enhanced over the three-dimensional case.15-18 It is difficult to predict the magnitude of this effect precisely but it is not likely to be more than a factor of the order of 4.18Considering the magnitudes of these effects, and the fact that they work in opposite directions, we conclude that the order of magnitude variations we have observed cannot be due to such effects. In more detailed studies they should be considered quantitatively. Non-membrane-bound, water-soluble spin labels can be readily included in multilayer preparations, so that the kinetics with such species can be compared to the membrane-bound case. It is interesting to note that this affords a system for the study of two-dimensional reaction kinetics. [15][16][17][18] There are other useful properties of this reaction which are not dealt with here experimentally, but which the kinetic analysis makes explicit. The fact that the homolysis is revers-ible in a step of rate comparable to the "forward" irreversible step (kf) is of the utmost importance, since it allows one to control the lifetime of the alkyl radicals by varying the concentration of Co(CN)53-.19 This feature makes the reaction suitable for time-resolved studies of fast processes by flash photolysis.
Transients absorbing in the 400-550-nm range have been observed in the flash photolysis of a series of 1,1-diarylindenes. These transients are assigned as isoindenes, and have been studied by ultraviolet spectroscopy, low-temperature NMR, chemical trapping, and flash photolysis techniques. Irradiation of 1,1,3-triphenylindene at 254 nm in cyclopentane at -70 °C gives an orange solution (Xmax 478 nm) which contains 1,2,3-triphenylisoindene.NMR (-70 °C) resonances at 4.45 and 6.80 are assigned to the methine and vinylic protons of 1,2,3-triphenylisoindene. Reaction with 4-phenyl-1,2,4-triazoline-3,5dione at -70°C gave the Diels-Alder adduct of the azo compound and 1,2,3-triphenylisoindene. The structure of the adduct was assigned from mass and NMR (*H and 13 *C) spectra and elemental analysis. The isoindene formed 1,2,3-triphenylindene
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