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

Electronic, steric and thermodynamic factors governing the reaction of O(P- 3) with alkyl iodides to yield HOI are probed by time-resolved Fourier tran sform 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 produ ct in the reactions of O(P-3) with cyclopentyl iodide and cyclohexyl iodide . Little effect of steric hindrance is observed with either of these reacta nts. A CF3 electron withdrawing group on the carbon in the beta-position to the iodine atom, probed by studying the precursor CH2ICH2CF3, weakens the C-H bond participating in the cyclic transition state and therefore diminis hes the partitioning of vibrational energy into the HOI product. The cyclic 5-membered transition state occurs not only with saturated hydrocarbon cha ins, but also when either the H atom or the I atom is abstracted from an ol efinic carbon site to yield an allene or acetylene product. This is explore d by probing the reactions of O(P-3) with CH(2)2CHI and CH(2)2CHCH(2)I, vin yl and allyl iodide, respectively. The energetic driving force for these re actions is the formation of the carbon-carbon multiple bond in the correspo nding product. If a strongly doubly bound product pathway is not available, such as in the reaction of O(P-3) with trimethyliodosilane, (CH3)(3)SiI, t he reaction exothermicity is not sufficient to form vibrationally excited H OI. Preferential reaction through a 5-membered cyclic transition state to a bstract 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 mechani sm in the reaction of O(P-3) with 2-iodoethanol, CH2ICH2OH.