Photochemistry of diiodomethane in solution studied by femtosecond and nanosecond laser photolysis.: Formation and dark reactions of the CH2I-I isomer photoproduct and its role in cyclopropanation of olefins

Femtosecond and nanosecond photolysis of CH2I2 in acetonitrile at an excitation wavelength of 266-nm yield the ground-state CH2I-I isomer product in similar to70% quantum yield. High isomer formation quantum yields (> 70%) were measured also in n-hexane, dichloromethane, methanol, and ethanol using nanosecond photolysis. The CH2I-I product is formed biphasically within similar to15 ps after excitation and can survive on a nanosecond to microsecond time scale before it decays via a mixture of first- and second-order processes. At 21 degreesC, the first-order rate constants are 1.8 x 10(5) s(-1) in n-hexane, with the measured apparent activation energy of 41 +/- 2 kJ mol(-1) (5-55 degreesC), and 4.3 x 10(6) s(-1) in acetonitrile. These rate constants correspond to intramolecular decomposition Of CH2I-I into a CH2I. radical and an I atom, with the additional formation of I- in acetonitrile. In methanol and ethanol, the CH2I-I isomer decays faster with the pseudo-first-order rate constants of 2.0 x 10(8) and 1.3 x 10(8) s(-1), respectively, and as in acetonitrile, I-3(-) and I-2(-) ion products dominate the absorption of photolyzed CH2I2 samples at long times. The thermodynamics of the CH2I2 system and its implication for the CH2I-I formation mechanism are discussed. A possible role of the CH2I-I isomer as a methylene transfer agent in photocyclopronation of olefins is investigated. Vibrationally relaxed CH2I-I is demonstrated to react with cyclohexene yielding an I-2 leaving group with the second-order rate constants similar to4.4 x 10(5) M-1 s(-1) in n-hexane, 3.4 x 10(6) M-1 s(-1) in dichloromethane, and 4.2 x 10(6) M-1 s(-1) in acetonitrile.