Flash pyrolysis of ethyl, n-propyl, and isopropyl iodides as monitored by supersonic expansion vacuum ultraviolet photoionization time-of-flight mass spectrometry.

The thermal decomposition of ethyl and propyl iodides, along with select isotopomers, up to 1300 K was performed by flash pyrolysis with a 20-100 mus time scale. The pyrolysis was followed by supersonic expansion to isolate the reactive intermediates and initial products, and detection was accomplished by vacuum ultraviolet single photon ionization time-of-flight mass spectrometry (VUV-SPI-TOFMS). The products monitored, such as CH(3), CH(3)I, C(2)H(5), C(2)H(4), HI, I, C(3)H(7), C(3)H(6), and I(2), provide for the simultaneous and direct observation of molecular elimination and bond fission pathways in ethyl and propyl iodides. In the pyrolysis of ethyl iodide, both C-I bond fission and HI molecular elimination pathways are competitive at the elevated temperatures, with C-I bond fission being preferred; at temperatures >or=1000 K, the ethyl radical products further dissociate to ethene + H atoms. In the pyrolysis of isopropyl iodide, both HI molecular elimination and C-I bond fission are observed and the molecular elimination channel is more important at all the elevated temperatures; the isopropyl radicals produced in the C-I fission channel undergo further decomposition to propene + H at temperatures >or=850 K. In contrast, bond fission is found to dominate the n-propyl iodide pyrolysis; at temperatures >or=950 K the n-propyl radicals produced decompose into methyl radical + ethene and propene + H atom. Isotopomer experiments characterize the extent of surface reactions and verify that the HI molecular eliminations in ethyl and propyl iodides proceed by a C1, C2 elimination mechanism (the 1,2 intramolecular elimination).