15min:

C. A. BAUMANN, Department of Chemistry, The University of Scranton, Scranton, PA 18510-4626.

The ultraviolet irradiation of matrix-isolated biacetyl (C₄H₆O₂) results in the formation of a complex of trans -methylhydroxycarbene (CH₃COH) and ketene (CH₂CO). Studies in this laboratory using pulsed sources have confirmed that these products arise from a two-photon process: the first photon ( < 450 nm) places the molecule in the à ¹A_u state, followed by relaxation to the long-lived ã ³A_u state. Triplet-triplet excitation( < 550 nm) from this state leads to the observed products.

The apparent quantum efficiency for the formation of these products was measured as a function of matrix (N₂, O₂, Ar, Kr, and Xe), dilution ratio for the parent compound, and wavelength of irradiation (using a continuous source). For all matrices, the apparent efficiency decreases with increasing C₄H₆O₂ dilution, indicating that the products may arise from a bimolecular process. No such effect was seen in C₄D₆O₂ experiments, which exhibit much lower levels of efficiency at all dilutions. The matrix has an effect on the high-dilution efficiency (N₂, O₂ > Kr, Xe > Ar), which suggests that the matrix participates in the H-transfer process.

Increasing the wavelength of irradiation from 436 to 450 nm results in an increase in the measured efficiency. At 436 nm, the reverse intersystem crossing (ã ³A_u à ¹A_u) channel is available to drain the triplet population. At 450 nm, this channel is no longer available, and the resulting increase in the steady-state triplet population leads to the observed enhancement.