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首页> 外文期刊>Chemistry: A European journal >An ab Initio/RRKM Study of Product Branching Ratios in the Photodissociation of Buta-1,2- and -1,3-dienes and But-2-yne at 193 nm
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An ab Initio/RRKM Study of Product Branching Ratios in the Photodissociation of Buta-1,2- and -1,3-dienes and But-2-yne at 193 nm

机译：从头开始/ RRKM研究在193 nm处Buta-1,2-和-1,3-二烯和But-2-yne光解离中的产物分支比

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摘要

Ab initio G2M(MP2)//B3LYP/6-311G~** calculations have been performed to investigate the reaction mechanism of photodissociation of buta-1,2-and -1,3-dienes and but-2-yne after their internal conversion into the vibrationally hot ground electronic state. The detailed study of the potential-energy surface was followed by microcanonical RRKM calculations of energy-dependent rate constants for individual reaction steps (at 193 nm photoexcitation and under collision-free conditions) and by solution of kinetic equations aimed at predicting the product branching ratios. For buta-1,2-diene, the major dissociation channels are found to be the single C-C bond cleavage to form the methyl and propargyl radicals and loss of hydrogen atoms from various positions to produce the but-2-yn-1-yl (p1), buta-1,2-dien-4-yl (p2), and but-1-yn-3-yl (p3) isomers of C_4H_5. The calculated branching ratio of the CH_3 + C_3H_3/C_4H_5 + H products, 87.9:5.9, is in a good agreement with the react experimental value of 96:4 (ref. [21]) taking into account that a significant amount of the C_4H_5 product undergoes secondary dissociation to C_4H_4 + H. The isomerization of buta-1,2-diene to buta-1,3-diene or but-2-yne appears to be slower than its one-step decomposition and plays only a minor role. On the other hand, the buta-1,3-diene -> buta-1,2-diene, buta-1,3-diene-> but-2-yne, and buta-1,3-diene-> cyclobutene rearrangements are significant in the dissociation of buta-1,3-diene, which is shown to be a more complex process. The major reaction products are still CH_3 + C_3H_3, formed after the isomerization of buta-1,3-diene to buta-1,2-diene, but the contribution of the other radical channels, C_4H_5 + H and C_2H_3 + C_2H_3, as well as two molecular channels, C_2H_2 + C_2H_4 and C_4H_4 + H_2,significantly increases. The overall calculated C_4H_5 + H/CH_3 + C_3H_3/C_2H_3/C_2H_3/C_4H_4 + H_2/C_2H_2 + C_2H_4 branching ratio is 24.0:49.6:6.1:15.2, which agrees with the experimental value of 20:50:8.2:20~[22] within 5% margins. For but-2-yne, the one-step decomposition pathways, which include mostly H atom loss to produce p1 and, to a minor extent, molecular hydrogen elimination to yield methylethynylcarbene, play an approximately even role with that of the channels that invovle the isomeriation of but-2-yne to buta-1,3-dienes p1 + H are the most important reaction products, with a branching ratio of 56.6%, followed by CH_3 + C_3H_3 (23.8%). The overall C_4H_5 + H/CH_3 + C_3H_3/C_2H_3 + C_2H_3/C_4H_4 + H_2/C_2H_2 + C_2H_4 branching ratio is predicted as 62.0:23.8:2.5:5.7:5.6. Contrary to buta-1,2-and-1,3-dienes, photodissociation of but-2-yne is expected to produce more hydrogen atoms than methyl radicals. The isomerization mechanisms between various isomers of the C_4H_6 molecule including buta-1,2-and-1,3-dienes, but-2-yne, 1-methylcyclopropene, dimethyl-vinylidene, and cyclobutene have been also characterized in detail.
机译：从头算G2M（MP2）// B3LYP / 6-311G〜**已进行计算，以研究丁二酸，丁二酮和丁二烯和丁-2-炔的光解离反应机理。转换为振动热态的电子状态。对势能表面的详细研究之后，对单个反应步骤（在193 nm光激发和无碰撞条件下）的能量依赖性速率常数进行微规范RRKM计算，并通过动力学方程求解来预测产物的分支比。 。对于buta-1,2-diene，主要的解离通道被发现是单个CC键裂解形成甲基和炔丙基基团，氢原子从各个位置损失而产生but-2-yn-1-yl（ p1），C_4H_5的buta-1,2-dien-4-yl（p2）和but-1-yn-3-yl（p3）异构体。考虑到大量的C_4H_5，CH_3 + C_3H_3 / C_4H_5 + H产物的计算支化比87.9：5.9与96：4的反应实验值（参考[21]）非常吻合。产物经历二次离解为C_4H_4 +H。丁一1，2-二烯异构化为丁1，3-二烯或丁2炔的异构化似乎比其一步分解慢，并且仅起次要作用。另一方面，buta-1,3-diene-> buta-1,2-diene，buta-1,3-diene-> but-2-yne，and buta-1,3-diene->环丁烯重排丁烯-1,3-二烯的解离反应非常重要，这被证明是一个更复杂的过程。主要反应产物仍是CH_3 + C_3H_3，是在buta-1,3-diene异构化为buta-1,2-diene之后形成的，但其他自由基通道C_4H_5 + H和C_2H_3 + C_2H_3的贡献也是如此C_2H_2 + C_2H_4和C_4H_4 + H_2这两个分子通道显着增加。计算得出的总C_4H_5 + H / CH_3 + C_3H_3 / C_2H_3 / C_2H_3 / C_4H_4 + H_2 / C_2H_2 + C_2H_4分支比为24.0：49.6：6.1：15.2，与实验值20：50：8.2：20〜[22 ]在5％的利润范围内。对于but-2-yne，一步分解路径（主要包括氢原子的损失以生成p1）和较小程度的分子氢消除以生成甲基乙炔基卡宾，与参与该过程的通道近似起作用。最重要的反应产物是丁-2-炔异构化成丁1,3-二烯p1 + H，支化率为56.6％，其次是CH_3 + C_3H_3（23.8％）。整个C_4H_5 + H / CH_3 + C_3H_3 / C_2H_3 + C_2H_3 / C_4H_4 + H_2 / C_2H_2 + C_2H_4分支比预计为62.0：23.8：2.5：5.7：5.6。与buta-1,2-and-1,3-dienes相反，but-2-yne的光解离可产生比甲基更多的氢原子。还已经详细表征了C_4H_6分子的各种异构体之间的异构化机理，包括丁1，2，2-和-1，3-二烯，丁2-炔，1-甲基环丙烯，二甲基-亚乙烯基和环丁烯。

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