Thermal and photochemistry of tert-butyl iodide on ice films

The thermal and photochemistry of tert-butyl iodide (t-buI) on ice films grown on Cu(111) at 100 K were studied using time-of-flight (TOF) temperature programmed desorption (TPD) mass spectroscopy and X-ray photoelectron spectroscopy (XPS). The thermal reaction products include isobutane (i-C4H10), isobutene (i-C4H8) and minor t-butyl alcohol. In the rising edge of the water desorption at ∼145 K, the parent t-buI molecule desorbs from the ice as well as other thermal reaction products. Above the falling edge of the water desorption at ∼180 K, the reaction products and the parent molecule desorb from the Cu(111). For t-buI on bare Cu(111) dosed at 100 K, dominant i-C4H8, some i-C4H10 and tiny t-buI desorption peaks were observed. This indicates that t-buI promptly dissociates on Cu(111) to form C4H9 and chemisorbed I at 100 K. The C4H9 quickly rearranges, followed by C-H scission to form mainly i-C4H8, or abstracts H to form some i-C4H10. The I directly chemisorbs on the Cu(111), evidenced by I(3d) XPS peaks. On ice layer, the most dominant i-C4H10 desorption peak indicates that the C4H9 abstracts H from ice to form C4H10. For photoirradiation of t-buI on ice, we observed an enhanced t-buI dissociation followed by ejection of reaction products, evidenced by decrease in TPD and C(1s) XPS intensities. No ejection of I-containing molecules was observed. We attribute the t-buI dissociation to solvated electrons in ice. During photoirradiation, I radicals seem to diffuse into the bulk ice to chemisorb on Cu(111), plausibly due to self-diffusion of water. In addition, a preferential desorption of C4H10 and/or an enhanced reaction channel to i-C4H8 by photoirradiation is evidenced by a faster TPD decay rate of C4H10 with photoirradiation time. An increased i-C4H8 TPD peak at 113 K and an appearance of 2 amu desorption could indicate the enhanced channel to i-C4H8.