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Photochemistry of the Stilbenes in Methanol. Trapping the Common Phantom Singlet State
The Journal of Physical Chemistry A ( IF 2.7 ) Pub Date : 2018-06-21 00:00:00 , DOI: 10.1021/acs.jpca.8b04011 Jack Saltiel 1 , Shipra Gupta 1
The Journal of Physical Chemistry A ( IF 2.7 ) Pub Date : 2018-06-21 00:00:00 , DOI: 10.1021/acs.jpca.8b04011 Jack Saltiel 1 , Shipra Gupta 1
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A comparative study of the photochemistry of cis- and trans-stilbene in methanol shows that both isomers undergo methanol photoaddition giving similar yields of α-methoxybibenzyl in competition with cis–trans photoisomerization. Methanol addition occurs primarily following torsional relaxation of the lowest excited singlet states of each isomer, 1c* and 1t*, to a common twisted singlet excited state intermediate, 1p*, initially called the phantom singlet state. The addition is consistent with the zwitterionic character of 1p*. Ether forms by direct 1,2-addition of CH3OH to the central carbon atoms and by 1,1-addition following rearrangement to 1-benzyl-1-phenylcarbene. Use of CD3OD and GC/MS (gas chromatographic/mass spectroscopic) analysis of the ether products revealed that the ratio of carbene/direct addition pathways is higher starting from cis-stilbene. We conclude that 1p* formed from 1c* is hotter than 1p* formed from 1t*. Surprisingly, except for favoring the carbene pathway, the use of higher energy photons (254 vs 313 nm) does not affect the overall ether quantum yield starting from cis-stilbene but significantly enhances both pathways starting from trans-stilbene. It appears that carbene formation and direct methanol addition to higher trans-stilbene excited state(s) compete with relaxation to S1. Substitution of D for the vinyl Hs of stilbene enhances the direct addition pathway more than 2-fold and strongly suppresses the carbene insertion pathway, revealing a large, kpcd0/kped2 = 6.3, primary deuterium isotope effect in the carbene rearrangement. The 2-fold increase in the ether quantum yield is due primarily to a 2.75-fold increase in the lifetime of 1p* on deuterium substitution of the vinyl hydrogens.
中文翻译：
斯蒂苯类在甲醇中的光化学。陷害普通幻影单重态
对顺式和反式二苯乙烯在甲醇中的光化学的比较研究表明，两种异构体都经历了甲醇的光加成反应，在与顺式-反式光异构化竞争的情况下，α-甲氧基联苄的收率相近。甲醇的添加主要发生在每个异构体的最低激发单重态1 c *和1 t *扭转弛豫到常见的扭曲单重态激发态中间体1 p *的扭转弛豫之后，该中间态最初称为幻像单重态。该添加与1 p *的两性离子特性一致。通过直接1，2-加成CH 3形成醚OH到中心碳原子上，并在重排为1-苄基-1-苯基卡宾后通过1,1-加成反应。对醚产物的CD 3 OD和GC / MS（气相色谱/质谱）分析表明，卡宾/直接加成途径的比率从顺式-lb开始较高。我们的结论是1个p *从形成1个ç *是温度高于1个p *从形成1吨*。出乎意料的是，除了有利于卡宾途径外，使用更高能量的光子（254 vs 313 nm）不会影响从顺式-二苯乙烯开始的总醚量子产率，但会显着增强从卡宾途径开始的两种途径。反式-二苯乙烯。看起来，卡宾的形成和直接甲醇向更高的反式二苯乙烯激发态的加成与向S 1的弛豫竞争。用D取代二苯乙烯的乙烯基Hs可将直接加成途径提高2倍以上，并强烈抑制卡宾插入途径，从而揭示了在卡宾重排中较大的k pc d0 / k pe d2 = 6.3的主要氘同位素效应。醚量子产率提高2倍主要是由于乙烯基氢的氘取代使1 p *的寿命增加了2.75倍。
更新日期：2018-06-21
中文翻译：
斯蒂苯类在甲醇中的光化学。陷害普通幻影单重态
对顺式和反式二苯乙烯在甲醇中的光化学的比较研究表明，两种异构体都经历了甲醇的光加成反应，在与顺式-反式光异构化竞争的情况下，α-甲氧基联苄的收率相近。甲醇的添加主要发生在每个异构体的最低激发单重态1 c *和1 t *扭转弛豫到常见的扭曲单重态激发态中间体1 p *的扭转弛豫之后，该中间态最初称为幻像单重态。该添加与1 p *的两性离子特性一致。通过直接1，2-加成CH 3形成醚OH到中心碳原子上，并在重排为1-苄基-1-苯基卡宾后通过1,1-加成反应。对醚产物的CD 3 OD和GC / MS（气相色谱/质谱）分析表明，卡宾/直接加成途径的比率从顺式-lb开始较高。我们的结论是1个p *从形成1个ç *是温度高于1个p *从形成1吨*。出乎意料的是，除了有利于卡宾途径外，使用更高能量的光子（254 vs 313 nm）不会影响从顺式-二苯乙烯开始的总醚量子产率，但会显着增强从卡宾途径开始的两种途径。反式-二苯乙烯。看起来，卡宾的形成和直接甲醇向更高的反式二苯乙烯激发态的加成与向S 1的弛豫竞争。用D取代二苯乙烯的乙烯基Hs可将直接加成途径提高2倍以上，并强烈抑制卡宾插入途径，从而揭示了在卡宾重排中较大的k pc d0 / k pe d2 = 6.3的主要氘同位素效应。醚量子产率提高2倍主要是由于乙烯基氢的氘取代使1 p *的寿命增加了2.75倍。
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