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 ∼70% 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 ∼15 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 °C, the first-order rate constants are 1.8 × 10⁵ s^-1 in n-hexane, with the measured apparent activation energy of 41 ± 2 kJ mol^-1 (5−55 °C), and 4.3 × 10⁶ 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 × 10⁸ and 1.3 × 10⁸ s^-1, respectively, and as in acetonitrile, I₃^- and I₂^- 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 I₂ leaving group with the second-order rate constants ∼4.4 × 10⁵ M^-1 s^-1 in n-hexane, 3.4 × 10⁶ M^-1 s^-1 in dichloromethane, and 4.2 × 10⁶ M^-1 s^-1 in acetonitrile.