Infrared Spectroscopic Study on Photolysis of Ethyl Iodide in Solid Parahydrogen: Perdeuterated Iodide System † Norihito Sogoshi, ‡ Tomonari Wakabayashi, ‡ Takamasa Momose,* ,‡ and Tadamasa Shida § DiVision of Chemistry, Graduate School of Science, Kyoto UniVersity, Kyoto 606-8502, Japan, and Kanagawa Institute of Technology, 1030 Shimo-Ogino, Atsugi 243-0292, Japan ReceiVed: October 31, 2000 Perdeuterated ethyl iodide in solid parahydrogen is photolyzed at 4.4 K to find the formation of all deuterated ethylene, ethane, and ethyl radical and deuterium iodide. The temporal change in the intensity of the vibrational spectra upon UV irradiation reveals that the initial ethyl iodide exists in both monomeric and dimeric units. The monomeric unit is subjected to the following competitive reactions: C 2 D 5 I + hν f •C 2 D 5 +•I and C 2 D 5 I + hν f CD 2 dCD 2 + DI. The ethylene produced thereby is loosely complexed with the counterpart DI. The dimeric unit undergoes the following one-photonic parallel reactions I and II: (I) (C 2 D 5 I) 2 + hν f 2•C 2 D 5 + I 2 to be followed by a gradual disproportionation, 2•C 2 D 5 f CD 2 dCD 2 + C 2 D 6 , which proceeds by quantum tunneling of a D atom between the radicals in the experimental time scale. The possible recombination of the two radicals to butane is not observed at all. (II) (C 2 D 5 I) 2 + hν f CD 2 dCD 2 + C 2 D 6 + I 2 , which is a direct molecular process to give the same products as (I). The ethylene produced by both (I) and (II) tends to form complexes with C 2 D 6 and with I 2 . Prolonged irradiation induces the following secondary photolysis of the three primary photoproducts: •C 2 D 5 + hν f CD 2 dCD 2 +•D, DI + hν f •D +•I, and I 2 + hν f 2•I. Introduction Solid parahydrogen (p-H 2 ) matrix is a useful medium for the observation of photoinduced elementary processes. This is not only because the matrix is optically transparent but also because photofragments are separated enough by virtue of the extreme softness of the solid p-H 2 , which is characteristic of a quantum solid. 1 The second feature cannot be overemphasized because photofragments in conventional rare gas matrixes suffer severe cage effects to leave no appreciable photolytic result in most cases. 2,3 With the use of this advantageous feature of the p-H 2 matrix, we studied some time ago the photolysis of normal ethyl iodide, C 2 H 5 I, 4 to obtain further information on the ethyl radical, which had been studied by Pacansky et al. for a system of dipropionyl peroxide, CH 3 CH 2 CO-O 2 -COCH 2 CH 3 , in rare gas matrixes. 5,6 The authors had to use the peroxide rather than ethyl iodide because the simpler ethyl iodide in rare gas matrixes is subjected to the recombination by the cage effect as has been confirmed by many authors. 2,3 The peroxide photolyzes into two ethyl radicals and two intervening carbon dioxide molecules as H 5 C 2• + (CO 2 ) 2 +•C 2 H 5 . The carbon dioxide molecules function as a spacer, and the recombination of the radicals is prevented. However, in Pacansky’s system, there is a possibility that CO 2 perturbs the ethyl radical in one way or the other. Moreover, byproducts containing oxygen atoms may have been produced. In our previous system of ethyl iodide/p-H 2 , the ethyl radical and the counterpart iodine atom can elude the recombination and the reaction mechanism is expected to be simpler than that of the peroxide system. The major primary processes found in our previous work were as follows: 4 C 2 H 5 I + hν f •C 2 H 5 +•I and (C 2 H 5 I) 2 + hν f 2•C 2 H 5 + I 2 and/or CH 2 dCH 2 + C 2 H 6 + I 2 . These primary processes were followed by secondary photolysis of the radical as •C 2 H 5 + hν f CH 2 dCH 2 +•H. The present work on the C 2 D 5 I system is intended to extend the previous study to obtain supporting evidence for the previous work and to find any isotopic effects anticipated. It is also desirable to find an answer to the unsolved problem of whether the above dimer photolysis proceeds concurrently or only one of the two reactions occurs. As a result, the above ambiguity of the “and/or” is removed to conclude that the two reactions proceed in parallel; i.e., it is confirmed that under irradiation both the homolytic reaction, (C 2 D 5 I) 2 + hν f 2•C 2 D 5 + I 2 , and the disproportionation, (C 2 D 5 I) 2 + hν f CD 2 dCD 2 + C 2 D 6 + I 2 , take place parallelwise. The two near-lying radicals produced by the former reaction gradually disproportionate in the dark to the pair CD 2 dCD 2 + C 2 D 6 by a quantum tunneling of a deuterium between the two radicals. No deuterated butane is ever formed nor any H-D mixed products, which might be expected for the system of C 2 D 5 I embedded in the p-H 2 matrix. As for the monomeric iodide, a molecular process of C 2 D 5 I + hν f CD 2 dCD 2 + DI is found in addition to the familiar C-I bond dissociation of C 2 D 5 I + hν f •C 2 D 5 +•I. All the experimental findings are explained by a few key reactions quite consistently. Experimental Section The experimental procedure is similar to that of the previous work on the C 2 H 5 I system 4 except that perdeuterated ethyl iodide † A preliminary result was reported at The Third International Conference on Cryocrystals and Quantum Crystals, July 28 to Aug 4, 2000, Szklarska Poreba, Poland. * To whom correspondence should be addressed. E-mail:momose@ kuchem.kyoto-u.ac.jp. ‡ Kyoto University. § Kanagawa Institute of Technology. 3077 J. Phys. Chem. A 2001, 105, 3077-3086 10.1021/jp004027g CCC: $20.00 © 2001 American Chemical Society Published on Web 03/10/2001