Panda Sunanda, Sivadasan Devipriya, Job Nisha, Sinjari Aland, Thirumoorthy Krishnan, Anoop Anakuthil, Thimmakondu Venkatesan S
Department of Chemistry, Indian Institute of Technology Kharagpur, Kharagpur 721 302, West Bengal, India.
Department of Chemistry, School of Advanced Sciences, Vellore Institute of Technology, Vellore 632 014, Tamil Nadu, India.
J Phys Chem A. 2022 Jul 14;126(27):4465-4475. doi: 10.1021/acs.jpca.2c02220. Epub 2022 Jun 29.
Considering the recent findings of linear doublet (Σ) MgCH isomers ( = 2, 4, and 6) in the evolved carbon star IRC+10216, various structural isomers of MgCH and MgCH are theoretically investigated here. For MgCH, 11 doublet and 8 quartet stationary points ranging from 0.0 to 71.8 and 0.0 to 110.1 kcal mol, respectively, have been identified initially at the UωB97XD/6-311++G(2d,2p) level. To get accurate relative energies, further energy evaluations are carried out for all isomers with coupled cluster methods and thermochemical modules such as G3//B3LYP, G4MP2, and CBS-QB3 methods. Unlike the even series, where the global minima are linear molecules with a Mg atom at one end, in the case of MgCH, the global minimum geometry turns out to be a cyclic isomer, 2-magnesabicyclo[1.1.0]but-1,3,4-triyl (, , ). In addition, five low-lying isomers, magnesium-substituted cyclopropenylidene (, , '), 1-magnesabut-2,3-dien-1-yl-4-ylidene (, , ″), 1-magnesabut-2-yn-1-yl-4-ylidene (, , ″), 2λ-magnesabicyclo[1.1.0]but-1,3-diyl-4-ylidene (, ;, ), and 1-magnesabut-2,3-dien-2-yl-4-ylidene (, , Σ), were also identified. The doublet linear isomer of MgCH, 1-magnesabutatrienyl (, , Σ) turns out to be a minimum but lies 54.1 kcal mol above at the ROCCSD(T)/cc-pVTZ level. The quartet (Σ) electronic state of was also found to be a minimum, but it lies 8.0 kcal mol above at the same level. Among quartets, isomer is the most stable molecule. The next quartet electronic state (of isomer ) is 34.4 kcal mol above , and all other quartet electronic states of other isomers are not energetically close to low-lying doublet isomers to . Overall, the chemical space of MgCH contains more cyclic isomers (, , and ) on the low-energy side unlike their even-numbered MgCH counterparts ( = 2, 4, and 6). Though the quartet electronic state of is linear, it is not the global minimum geometry on the MgCH potential energy surface. Isomerization pathways among the low-lying isomers (doublets of - and a quartet of ) reveal that these molecules are kinetically stable. For the cation, MgCH, the cyclic isomers (, , and ) are on the low-energy side. The singlet linear isomer, , is a fourth-order saddle point. The low-lying cations are quite polar, with dipole moment values of >7.00 D. The current theoretical data would be helpful to both laboratory astrophysicists and radioastronomers for further studies on the MgCH isomers.
鉴于在演化碳星IRC+10216中最近发现了线性双重态（Σ）MgCH异构体（=2、4和6），本文对MgCH和MgCH的各种结构异构体进行了理论研究。对于MgCH，最初在UωB97XD/6-311++G(2d,2p)水平上确定了11个双重态和8个四重态驻点，其能量范围分别为0.0至71.8千卡/摩尔和0.0至110.1千卡/摩尔。为了获得准确的相对能量，使用耦合簇方法和热化学模块（如G3//B3LYP、G4MP2和CBS-QB3方法）对所有异构体进行了进一步的能量评估。与偶数系列不同，偶数系列的全局最小值是一端有Mg原子的线性分子，而对于MgCH，全局最小几何结构是一种环状异构体，即2-镁杂双环[1.1.0]丁-1,3,4-三基（，，）。此外，还确定了五个低能异构体，即镁取代的环丙烯叉（，，'）、1-镁杂丁-2,3-二烯-1-基-4-亚基（，，″）、1-镁杂丁-2-炔-1-基-4-亚基（，，″）、2λ-镁杂双环[1.1.0]丁-1,3-二基-4-亚基（，；，）和1-镁杂丁-2,3-二烯-2-基-4-亚基（，，Σ）。MgCH的双重态线性异构体1-镁杂丁三烯基（，，Σ）在ROCCSD(T)/cc-pVTZ水平上是一个最小值，但比高54.1千卡/摩尔。还发现的四重态（Σ）电子态是一个最小值，但在同一水平上比高8.0千卡/摩尔。在四重态中，异构体是最稳定的分子。下一个四重态电子态（异构体的）比高34.4千卡/摩尔，其他异构体的所有其他四重态电子态在能量上与低能双重态异构体至不接近。总体而言，MgCH的化学空间在低能侧包含更多的环状异构体（，，和），这与它们的偶数MgCH对应物（=2、4和6）不同。尽管的四重态电子态是线性的，但它不是MgCH势能面上的全局最小几何结构。低能异构体（-的双重态和的四重态）之间的异构化途径表明这些分子在动力学上是稳定的。对于阳离子MgCH，环状异构体（，，和）在低能侧。单重态线性异构体是一个四阶鞍点。低能阳离子具有很强的极性，偶极矩值>7.00 D。当前的理论数据将有助于实验室天体物理学家和射电天文学家对MgCH异构体进行进一步研究。
