Solvent effect on reaction mechanism and kinetics of triplet ion pairs between chloranil and acenaphthene.
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概要
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A comparative study on decay dynamics of triplet ion pairs has been performed for the chloranil (CA) and acenaphthene (ACN) system in both the nonpolar solvent benzene (BZ) and the moderately polar 1,2-dichloroethane (DCE) by means of nanosecond laser photolysis. Triplet chloranil (<SUP>3</SUP>CA) is quenched by ACN to yield the (1:1)triplet ion pair (IP<SUB>1</SUB>), <SUP>3</SUP>(CA<SUP>\newdot</SUP>, ACN<SUP>\underset.+</SUP>), in BZ, while the (1:2) ion pair (IP<SUB>2</SUB>), <SUP>3</SUP>(CA<SUP>\newdot</SUP>, ACN<SUB>2</SUB><SUP>\underset.+</SUP>), in DCE as well as IP<SUB>1</SUB> in the concentration range less than 1 M (1 M=1 mol dm<SUP>−3</SUP>) of ACN. Temperature dependence of the transient spectra indicates that IP<SUB>2</SUB> is energetically lower than IP<SUB>1</SUB> in DCE. In BZ, IP<SUB>1</SUB> decays through both back electron transfer (back ET) to the ground state of the donor-acceptor pair and intra-ion-pair proton transfer (PT) leading to formation of the chloranil semiquinone radical (CAH·). In DCE, both IP<SUB>1</SUB> and IP<SUB>2</SUB> disappear through the back ET and ionic dissociation (ID) to free anion and cation radicals, while no PT is observed. The efficiency of ID is greater in IP<SUB>2</SUB>(0.70) than in IP<SUB>1</SUB>(0.33). From temperature dependence on decay profiles, Arrhenius parameters are obtained for each decay process of the ion pairs. The PT process in BZ requires a considerably high activation energy (30 kJ mol<SUP>−1</SUP>), while the ID process in DCE proceeds with practically no activation energy for both IP<SUB>1</SUB> and IP<SUB>2</SUB>. The activation barrier for the back ET of IP<SUB>1</SUB> in BZ is obtained to be 13 kJ mol<SUP>−1</SUP> which is significantly greater than that (∼0 KJ Jmol<SUP>−1</SUP>) in DCE. This solvent effect may be attributed to the difference in the ion pair states: i.e., a contact type in BZ and solvent-separated type in DCE.
- 公益社団法人 日本化学会の論文
著者
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Shizuka Haruo
Deparment Of Chemistry Faculty Of Engineering Gunma University
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Ohsugi Youichi
Department of Chemistry, Faculty of Engineering, Gunma University
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Kobashi Harumichi
Department of Chemistry, Faculty of Engineering, Gunma University
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Okabe Shunichi
Department of Chemistry, Faculty of Engineering, Gunma University
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Kobashi Harumichi
Department of Chemistry, Faculty of Engineering Gunma University
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