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

Abstract

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 of steric hindrance is observed with either of these reactants. A CF₃ electron withdrawing group on the carbon in the β-position to the iodine atom, probed by studying the precursor CH₂ICH₂CF₃, 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 chains, 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 CH₂CHI and CH₂CHCH₂I, 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 doubly bound product pathway is not available, such as in the reaction of O(³P) with trimethyliodosilane, (CH₃)₃SiI, the reaction exothermicity is not sufficient to form vibrationally excited HOI. Preferential reaction through a 5-membered cyclic transition state to abstract an H atom from a carbon atom, rather than through a 6-membered 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, CH₂ICH₂OH.