Highly efficient gas-phase reactivity of protonated pyridine radicals with propene,Physical Chemistry Chemical Physics

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Highly efficient gas-phase reactivity of protonated pyridine radicals with propene
Physical Chemistry Chemical Physics ( IF 2.9 ) Pub Date : 2017-11-13 00:00:00 , DOI: 10.1039/c7cp06644a
Cameron C. Bright _{1,

2,

3,

4} , Matthew B. Prendergast _{1,

2,

3,

4} , Patrick D. Kelly _{1,

2,

3,

4} , James P. Bezzina _{1,

2,

3,

4} , Stephen J. Blanksby _{4,

5,

6,

7,

8} , Gabriel da Silva _{4,

9,

10,

11} , Adam J. Trevitt _{1,

2,

3,

4}

Affiliation

Small nitrogen containing heteroaromatics are fundamental building blocks for many biological molecules, including the DNA nucleotides. Pyridine, as a prototypical N-heteroaromatic, has been implicated in the chemical evolution of many extraterrestrial environments, including the atmosphere of Titan. This paper reports on the gas-phase ion–molecule reactions of the three dehydro-N-pyridinium radical cation isomers with propene. Photodissociation ion-trap mass spectrometry experiments are used to measure product branching ratios and reaction kinetics. Reaction efficiencies for 2-dehydro-N-pyridinium, 3-dehydro-N-pyridinium and 4-dehydro-N-pyridinium with propene are 70%, 47% and 41%, respectively. The m/z 106 channel is the major product channel across all cases and assigned 2-, 3-, and 4-vinylpyridinium for each reaction. The m/z 93 channel is also significant and assigned the 2-, 3-, and 4-N-protonated-picolyl radical cation for each case. H-Abstraction from propene is not competitive under experimental conditions. Potential energy schemes, at the M06-2X/6-31(2df,p) level of theory and basis set, are described to assist in rationalising observed product branching ratios and elucidating possible reaction mechanisms. Reaction barriers to the production of vinylpyridinium (m/z 106) + CH₃ are the lowest identified for the 3- and 4-dehydro-N-pyridinium reactions, in support of the observed dominance of the m/z 106 ion signal. Ethylene loss via ring-mediated H-transfer along the propyl group is found to be the lowest energy pathway for the 2-dehydro-N-pyridinium reaction, suggesting a preference toward m/z 93 (N-protonated-picolyl radical cation) over the experimentally observed products. Entropic bottle-necks along the m/z 93 pathway however, associated with ring-mediated H-atom transfer, are responsible for the dominance of m/z 106 in the 2-dehydro-N-pyridinium + propene reaction. For all three isomers, computed barriers for all observed reaction channels were below the entrance channel, suggesting these reactions can contribute to molecular weight growth in extraterrestrial environments with accelerated reaction rates in low temperature regions of space.

中文翻译：

质子化吡啶自由基与丙烯的高效气相反应性

少量的含氮杂芳族化合物是许多生物分子（包括DNA核苷酸）的基本组成部分。吡啶是一种典型的N-杂芳族化合物，已与许多地球外环境（包括泰坦大气）的化学演化有关。本文报道了三种脱氢-N-吡啶鎓自由基阳离子异构体与丙烯的气相离子分子反应。光解离离子阱质谱实验用于测量产物的分支比和反应动力学。2-脱氢-N-吡啶鎓，3-脱氢-N-吡啶鎓和4-脱氢-N-吡啶鎓与丙烯的反应效率分别为70％，47％和41％。的米/z 106通道是所有情况下的主要产品通道，并为每个反应分配了2-，3-和4-乙烯基吡啶鎓。所述米/ ž 93信道也显著和分配了2-，3-，和4- Ñ对于每种情况-protonated吡啶甲基自由基阳离子。在实验条件下，从丙烯中提取H并不具有竞争力。在M06-2X / 6-31（2df，p）的理论和基础水平上描述了势能方案，以帮助合理化所观察到的产物支化比并阐明可能的反应机理。乙烯基吡啶（m / z 106）+ CH ₃的生产的反应障碍是3-和4-脱氢-N的最低-吡啶鎓反应，以支持所观察到的m / z 106离子信号的优势。乙烯损失经由沿着丙基环介导的H-转移被发现是用于2脱氢最低能量途径Ñ -吡啶鎓反应，这表明朝向偏好米/ ž 93（Ñ -protonated吡啶甲基自由基阳离子）过实验观察到的产品。然而，沿m / z 93途径的熵瓶颈与环介导的H原子转移相关，是导致2-脱氢-N中m / z 106占主导地位的原因。-吡啶鎓+丙烯反应。对于所有三种异构体，所有观察到的反应通道的计算壁垒均在入口通道以下，这表明这些反应可促进地球外环境中分子量的增长，并在空间的低温区域加快反应速率。

更新日期：2017-11-20

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