Fourier transform infrared emission study of the mechanism and dynamics of HOI formed in the reaction of alkyl iodides with O(³P)

Vibrationally excited hypoiodous acid (HOI) is observed as a product in the reaction of alkyl iodides with O(³P). Fourier transform infrared emission techniques are used to detect the excited v₁. OH, stretch of the HOI product, to determine the mechanism of HOI production, and to measure the vibrational product state distributions. The HOI product is formed by O atom reaction with two-carbon and larger straight or branched chain alkyl iodides and cyclic alkyl iodides, e.g., C₂H₅I, n-C₃H₇I, i-C₃H₇I, (CH₃)₃CI, n-C₆H₁₃I, and c-C₆H₁₁I, but not with CH₃I. Experiments with selectively deuterated ethyl iodides provide direct evidence that HOI is formed in a beta-elimination mechanism involving a five-membered ring transition state. The O atom attacks the iodine and then abstracts a hydrogen from the beta carbon during the lifetime of the complex. Time-resolved experiments allow the extraction of nascent vibrational state distributions for the v₁ stretch of HOI (v =1:v = 2:v = 3) using different alkyl iodides and assuming the radiative rates are given by A_v→v-1 = v A_1→0: C₂H₅I, 0.53(4):0.39(3):0.08(3); n-C₃H₇I, 0.61(6):0.34(5):0.05(2); and i-C₃H₇I, 0.54(6):0.38(4):0.08(3). These distributions are nonstatistical with the v = 2 states having only slightly less population than those with v = 1. For product molecules with up to three quanta of v₁ excitation, more than 50% of the reaction exothermicity is deposited into the OH stretch.