Probing the cyclic transition state in the reaction O(~3P)+alkyl iodides to form HOI:electronic, steric and thermodynamic factors influencing the reaction pathway

摘要

Electronic, steric and thermodynamic factors governing the reaction of O(~3P) with alkyl iodides to yield HOI are probed by time-resolved Fourier transform infrared emission spectroscopy. The reaction to produce HOI is known to proceed through a cyclic 5-membered transition state. Steric effects are examined by studying the nascent vibrational distribution of the HOI product in the reactions of O(~3P) with cyclopentyl iodide and cyclohexyl iodide. Little effect fo steric hindrance is observed with either of these reactants. ACF_3 electron withdrawing group on the carbon in the #beta#-position to the iodine atoms, probed by studying the precursor CH_2ICH_2CF_3,weakens the C-H bond participating in the cyclic transition state and therefore diminishes the partitioning of vibrational energy into the HOI product. The cyclic 5-membered transition state occurs not only with saturated hydrocarbon cains, but also when either the H atom or the I atom is abstracted from an olefinic carbon site to yield an allene or acetylene product. This is explored by probing the reactions of O(~3P) with CH_2=CHI and CH_2=CHCH_2I,vinyl and allyl iodide, respectively. The energetic driving force for these reactions is the formation of the carbon-carbon multiple bond in the corresponding product. If a strongly double bound product pathway is not available, such as in the reaction of O(~3P) with trimethyliodosiane, (CH_3)_3Sil,the reaction exothermicity is not sufficient to form vibrationally excited HOI. Preferential reaction through a 5-membered cyclic transitiona state to abstract an H atom from a carbon atom, rather than through a 6-memebered ring by abstraction of an H atom from an oxygen atom, appears to be the mechanism in the reaction of O(~3P) with 2-iodoethanol, CH_2ICH_2H.