2-PHOTON CHEMISTRY IN THE LASER JET - PHOTOIONIZATION OF THE DIPHENYLMETHYL RADICAL GENERATED BY NORRISH TYPE-I PHOTOCLEAVAGE OF BENZHYDRYL PHENYL KETONE

It is shown that in the laser jet (LJ) mode (argon ion laser) the benzhydryl phenyl ketone (1) undergoes a two-photon reaction in CCl4 to yield benzhydryl chloride. The one-photon products are 1,1,2,2-tetraphenylethane (3), benzophenone, and 1,1,1-trichloro-2,2-diphenylethane (4). In methanol, on the other hand, our results suggest even a three-photon reaction in which benzhydryl methyl ether (5) is produced besides the one-photon products 3 and benzophenone. The ratios of the three-photon to one-photon products, i.e. 5 to 3, depended on the wavelength (333 or 351 nm). The three-photon mechanism was established by a competition experiment, in which ketone 1 was irradiated in mixtures of CCl4 and methanol. The ratio of the two high-intensity products ether 5 and benzhydryl chloride showed a strong intensity dependence. We propose that the electronically excited diphenylmethyl radical 2* is photochemically ionized to the diphenylmethyl cation 2+ under the high-intensity LJ photolysis conditions and the latter trapped with methanol and deuteriomethanol to give the identical benzbydryl methyl ether (5). The fact that no deuterium-incorporated ether 5 was observed eliminates the carbene mechanism proposed in the literature, at least under our LJ photolysis conditions.