Guo Minggang, Wang Zhifan, Wang Fan
Institute of Atomic and Molecular Physics, Key Laboratory of High Energy Density Physics and Technology, Ministry of Education, Sichuan University, Chengdu, People's Republic of China, 610065.
College of Chemistry and Life Science, Chengdu Normal University, Chengdu, People's Republic of China, 611130.
J Phys Chem A. 2021 May 20;125(19):4079-4088. doi: 10.1021/acs.jpca.1c01007. Epub 2021 May 11.
Cyclic CH is the simplest cyclic hydrocarbon, but its configuration is complicated. The ground E″ state at the equilateral triangle geometry with symmetry undergoes both Jahn-Teller (JT) distortion and pseudo-Jahn-Teller (PJT) distortion to structures with , , , or symmetry. Previous works using complete active space self-consistent field (CASSCF) or multiconfiguration SCF (MCSCF) differ on the characteristics of these structures. To clarify the characteristics of these stationary points on the potential energy surface (PES) of this radical, coupled-cluster methods CCSD, CCSD(T), and EOMEA/IP-CCSD as well as density functional theory are employed to calculate their geometries, harmonic frequencies, and relative energies. Deformations between these stationary points due to JT and PJT effects are analyzed in detail. Most of the results with these methods are consistent with each other, except for the b mode of the B state with symmetry. CCSD and CCSD(T) provide an imaginary frequency for this vibrational mode, while it is calculated to be real with the other methods as well as with EOM-CCSDT-3. This may be related to unstable reference in CCSD and CCSD(T) calculations. On the other hand, most of the employed exchange-correlation functionals provide reliable results on the characteristics of these stationary points. Our results show that the A' state with symmetry is the only minimum structure on the PES of cyclic CH. The A and B states of the structure are second-order saddle points, while both the A state of the structure and the A″ state of the structure are transition states connecting the global minimum A' state.
环型CH是最简单的环烃，但其结构复杂。具有 对称性的等边三角形几何构型的基态E″会经历 Jahn-Teller（JT）畸变和赝 Jahn-Teller（PJT）畸变，转变为具有 、 、 或 对称性的结构。先前使用完全活性空间自洽场（CASSCF）或多组态自洽场（MCSCF）的研究在这些结构的特征上存在差异。为了阐明该自由基势能面（PES）上这些驻点的特征，采用耦合簇方法CCSD、CCSD(T)和EOMEA/IP-CCSD以及密度泛函理论来计算它们的几何结构、谐振频率和相对能量。详细分析了由于JT和PJT效应导致的这些驻点之间的变形。除了具有 对称性的B态的b模式外，这些方法得到的大多数结果相互一致。CCSD和CCSD(T)为该振动模式提供了一个虚频，而其他方法以及EOM-CCSDT-3计算得出该频率为实频。这可能与CCSD和CCSD(T)计算中不稳定的参考有关。另一方面，大多数所采用的交换相关泛函在这些驻点的特征上提供了可靠的结果。我们的结果表明，具有 对称性的A'态是环型CH势能面上唯一的极小结构。 结构的A态和B态是二阶鞍点，而 结构的A态和 结构的A″态都是连接全局极小A'态的过渡态。
