Attah Isaac Kwame, Soliman Abdel-Rahman, Platt Sean P, Meot-Ner Mautner Michael, Aziz Saaudallah G, Samy El-Shall M
Department of Chemistry, Virginia Commonwealth University, Richmond, VA 23284-2006, USA.
Phys Chem Chem Phys. 2017 Mar 1;19(9):6422-6432. doi: 10.1039/c6cp08731k.
Polycyclic aromatic hydrocarbons (PAHs) and polycyclic aromatic nitrogen heterocyclics (PANHs) are present in ionizing environments, including interstellar clouds and solar nebulae, where their ions can interact with neutral PAH and PANH molecules leading to the formation of a variety of complex organics including large N-containing ions. Herein, we report on the formation of a covalently-bonded (benzene·pyrimidine) radical cation dimer by the gas phase reaction of pyrimidine with the benzene radical cation at room temperature using the mass-selected ion mobility technique. No ligand exchange reactions with benzene and pyrimidine are observed indicating that the binding energy of the (benzene·pyrimidine)˙ adduct is significantly higher than both the benzene dimer cation and the proton-bound pyrimidine dimer. The (benzene·pyrimidine)˙ adduct shows thermal stability up to 541 K. Thermal dissociation of the (CD·CHN)˙ adduct at temperatures higher than 500 K produces CHND (m/z 82) suggesting the transfer of a D atom from the CD moiety to the CHN moiety before the dissociation of the adduct. Mass-selected ion mobility of the (benzene·pyrimidine)˙ dimer reveals the presence of two families of isomers formed by electron impact ionization of the neutral (benzene·pyrimidine) dimer. The slower mobility peak corresponds to a non-covalent family of isomers with larger collision cross sections (76.0 ± 1.8 Å) and the faster peak is consistent with a family of covalent isomers with more compact structures and smaller collision cross sections (67.7 ± 2.2 Å). The mobility measurements at 509 K show only one peak corresponding to the family of stable covalently bonded isomers characterized by smaller collision cross sections (66.9 ± 1.9 Å at 509 K). DFT calculations at the M06-2X/6-311++G** level show that the most stable (benzene·pyrimidine)˙ isomer forms a covalent C-N bond with a binding energy of 49.7 kcal mol and a calculated collision cross section of 69.2 Å, in excellent agreement with the value obtained from the faster mobility peak of the (benzene·pyrimidine)˙ dimer. Formation of a C-N covalent bond displaces a hydrogen atom from a C-H bond of the benzene cation which is transferred to the second pyrimidine nitrogen atom, thus preserving the pyrimidine π system and yielding the most stable (benzene·pyrimidine)˙ isomer. The calculations also show less stable non-covalent electrostatically bonded perpendicular isomers of the (benzene·pyrimidine)˙ dimer with a binding energy of 19 kcal mol and a calculated collision cross section of 74.0-75.0 Å in excellent agreement with the value obtained from the slower mobility peak of the (benzene·pyrimidine)˙ dimer.
多环芳烃（PAHs）和多环芳族氮杂环化合物（PANHs）存在于电离环境中，包括星际云和气态星云，在这些环境中它们的离子可以与中性PAH和PANH分子相互作用，导致形成各种复杂有机物，包括含氮大离子。在此，我们报道了使用质量选择离子迁移技术，在室温下嘧啶与苯自由基阳离子通过气相反应形成共价键合的（苯·嘧啶）自由基阳离子二聚体。未观察到与苯和嘧啶的配体交换反应，这表明（苯·嘧啶）˙加合物的结合能明显高于苯二聚体阳离子和质子键合的嘧啶二聚体。（苯·嘧啶）˙加合物在高达541 K的温度下表现出热稳定性。（CD·CHN）˙加合物在高于500 K的温度下热解离产生CHND（m/z 82），这表明在加合物解离之前，一个D原子从CD部分转移到了CHN部分。（苯·嘧啶）˙二聚体的质量选择离子迁移率揭示了由中性（苯·嘧啶）二聚体的电子碰撞电离形成的两个异构体家族的存在。迁移率较慢的峰对应于具有较大碰撞截面（76.0±1.8 Å）的非共价异构体家族，而较快的峰与具有更紧凑结构和较小碰撞截面（67.7±2.2 Å）的共价异构体家族一致。在509 K下的迁移率测量仅显示一个峰，对应于以较小碰撞截面（509 K时为66.9±1.9 Å）为特征的稳定共价键合异构体家族。在M06 - 2X/6 - 311++G**水平的密度泛函理论（DFT）计算表明，最稳定的（苯·嘧啶）˙异构体形成了一个共价C - N键，结合能为49.7 kcal/mol，计算得到的碰撞截面为69.2 Å，与从（苯·嘧啶）˙二聚体较快迁移率峰获得的值非常吻合。C - N共价键的形成使苯阳离子的一个C - H键上的氢原子位移，该氢原子转移到第二个嘧啶氮原子上，从而保留了嘧啶π体系并产生了最稳定的（苯·嘧啶）˙异构体。计算还表明，（苯·嘧啶）˙二聚体存在稳定性较低的非共价静电键合垂直异构体，其结合能为19 kcal/mol，计算得到的碰撞截面为74.0 - 75.0 Å，与从（苯·嘧啶）˙二聚体较慢迁移率峰获得的值非常吻合。
