The IR Photochemistry of Organic Compounds. IV. The Infrared Multiple-Photon Decomposition of Cyclic Ethers Induced by a TEA CO2 Laser

Abstract

Abstract The mechanism of the infrared multiple-photon decomposition (IRMPD) of saturated cyclic ethers (1–7) has been systematically studied on the basis of product analysis, particularly with the aim of finding a suitable cyclic ether for oxygen-isotope separation. The main products are H2CO, CO, H2, and lower hydrocarbons. Acetaldehyde is additionally formed in the IRMPD of 1 and 3–6, while acetone is produced only in the IRMPD of 3b. The initial process is the homolytic cleavage of the C–O bond to yield the corresponding biradical with a high internal energy. The biradical decomposes sequentially of through secondary IRMPD to yield the primary products with high internal energies. The decomposition of the biradicals proceeds mainly via β-fission, but also partly via γ-fission. Some of the primary products further decompose sequentially or through secondary IRMPD into stable products. Several radical intermediates are trapped with Br2. The product distributions are clearly dependent on the irradiation parameters. This dependence and the branching ratio are discussed in terms of sequential decomposition, the collisional effect, and the internal energy of the transient species. On the basis of the experimental results, it is suggested that, among cyclic ethers, the best starting ether in the oxygen-isotope separation by a TEA CO2 laser is tetrahydropyran (4).