A COMPARISON OF PHOTODIMERIZATIONS OF 2-CYCLOPENTENONE AND OF 2-CYCLOHEXENONE IN ACETONITRILE

被引:127
作者
WAGNER, PJ
BUCHECK, DJ
机构
[1] Chemistry Department, Michigan State University, East Lansing
关键词
D O I
10.1021/ja01046a026
中图分类号
O6 [化学];
学科分类号
0703 ;
摘要
In acetonitrile, head-to-head: head-to-tail photodimer ratios are independent of ketone concentration, being 4:5 for cyclopentenone and 2:1 for cyclohexenone. The variation in dimer quantum yields as a function of ketone concentration indicates ki/ka ratios of 0.06 and 2.7, respectively, where ka is the bimolecular rate constant for reaction of triplet ketone with ground-state ketone and ki is the rate of unimolecular triplet decay. Stern-Volmer quenching studies with conjugated dienes yield measures of triplet lifetimes as a function of ketone concentration. These studies indicate ka values of 6.9 X 108 and 1.1 X 108 M−1[sec−1 and ki values of 4 X 107 and 3 X 108 sec−1, respectively, for cyclopentenone and cyclohexenone. For both ketones dimerization occurs from the lowest triplet; the more rapid decay rate of the reactive cyclohexenone triplet probably reflects a greater ease of t isting. The maximum efficiency of dimerization is only 36% for cyclopentenone and 74% for cyclohexenone. Since intersystem crossing is 100% efficient for both ketones, these results indicate that both dimeri zations proceed through metastable dimeric species, significant percentages of which revert to two ground-state monomers. A comparison of these results with others in the literature suggests that dimerization occurs from π,π* triplet states. © 1969, American Chemical Society. All rights reserved.
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页码:5090 / &
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