Flash Photolysis of Ketene and Diazomethane: The Production and Reaction Kinetics of Triplet and Singlet Methylene

Abstract

Ketene and diazomethane have been flash photolyzed in their strongest absorption continua in the vacuum ultraviolet and far ultraviolet. Triplet methylene was monitored by kinetic spectroscopy at 141.5 nm. Singlet methylene was not observed directly, but the growth of the triplet absorption as a function of inert-gas pressure indicates that singlet methylene was the major primary product in both the ketene and diazomethane systems. Various inert additives quantitatively quenched the singlet to the triplet state, while reactive additives were able to intercept the singlet before quenching occurred. A number of reactions and deactivations involving singlet methylene (1CH2) and triplet methylene (3CH2) have been investigated at 298°k, and their rate constants are listed below (cm3 molecule·−1 second−1):CH23+3CH2→C2H2+H2 (5.3 ± 1.5) × 10−11 CH21+H2→CH4*→CH3+H (7.0 ± 1.5rparl × 10−12 CH21+H2→3CH2+H2 < 1.5 × 10−12 CH21+CH4→C2H6*→CH3+CH3 (1.9 ± 0.5) × 10−12 CH21+CH4→3CH2+CH4 (1.6 ± 0.5) × 10−12 CH21+He→3CH2+He (3.0 ± 0.7) × 10−13 CH21+Ar→3CH2+Ar (6.7 ± 1.3) × 10−13 CH21+N2→3CH2+N2 (9.0 ± 2.0) × 10−13 CH23+H2→CH3+H < 5 × 10−14 CH23+CH4→CH3+CH3 < 5 × 10−14. The absolute reaction rates for the singlet are apparently independent of the source (i.e., ketene or diazomethane), suggesting that the reactions occur via vibrationally cold singlet methylene. No evidence was found for deactivation of the triplet to the singlet, indicating that the energy difference between the two states is greater than kT (T = 298°K).