ChemInform Abstract: Reactivity and Intersystem Crossing of Singlet Methylene in Solution.

Abstract

Evidence is reported which demonstrates that singlet methylene, produced from the photolysis of diazomethane or diazirine, undergoes intersystem crossing to form triplet methylene in perfluorohexane solvent. The results of triplet sensitized photolysis and of direct photolysis experiments with dilute concentrations of substrate (cis- and trans-2-pentene and chloroform) appear to be essentially identical. Stern-Volmer analyses of the competition kinetics between acetonitrile and 2-pentenes or chloroform for singlet methylene are consistent with the near diffusion controlled reactivity of singlet methylene. With the assumption of diffusion-controlled reactions for singlet methylene, plots of the quantum yield for singlet vs. triplet reaction for methylene allow the first estimate (58 X lo8 s-l) of the rate of intersystem crossing of singlet methylene in the condensed phase. This value is considerably smaller than the value that is extrapolated to the solution phase from results in the gas phase. The possible reasons for this difference are discussed. Methylene (CH,), the parent of the carbene family, has at- tracted the attention of organic chemists,' theorists,, spectro- scopist~,~ and chemical physicist^..^ Almost all of the quantitative information concerning this species is derived from gas-phase investigations,6 from which it has been concluded that CH2 is a ground state triplet (3CH2) and that a low-lying singlet state ('CH,) exists at about 9 kcal/mo17 above the ground state (Figure 1). Methylene possesses one carbon atom, two hydrogen atoms, and two nonbonding electrons. The two nonbonding electrons occupy the u and T orbitals with their spins parallel for the more linear VH, (6 = 136O), while they occupy the u orbital paired spins for the lowest energy singlet methylene (0 = 102O) (Figure 1). Both states have been observed spectroscopically in the gas phase,3 and from kinetic investigation^^.^ it has been concluded that 'CH2 is quenched by all additives, even inert gases, with high efficiency. Indeed, extrapolation of the gas-phase quenching data to the solution phase leads to the expectation that the lifetime of ICH2 will be of the order of picoseconds in the condensed phase. For example, 'CH, is quenched by He in the gas phase4 with a rate constant of ca. 4 X cm3 molecule-' s-', which translates into a bimolecular rate constant of ca. 2 X lo9 M-l s-I. For an organic solvent concentration of 5 M, the rate of quenching for a completely inert solvent (such as helium!) would be 1 X 1O'O s-I, implying a lifetime of the order of 50 ps or less for 'CH2 in solution. This expectation is based on the assumption that the rate of the 'CH2 deactivation is a linear function of pressure even at high pressure (or in the solution phase). Since quenching must involve either reaction or intersystem crossing to 3CH2, one is led