Pandian Joshua, Vu Khanh, Muya Jules Tshishimbi, Parker Anna, Ancajas Christine Mae F, Saldana-Greco Diomedes, Yewer Tabitha, Parish Carol
Department of Chemistry, Gottwald Center for the Sciences, University of Richmond, Richmond, Virginia, USA.
Department of Chemistry, Faculty of Sciences, University of Kinshasa, Kinshasa, Democratic Republic of the Congo.
J Comput Chem. 2025 Jan 30;46(3):e70044. doi: 10.1002/jcc.70044.
The energies and geometries of the lowest lying singlet and triplet states of the four diradicals formed by removing two H atoms from thiophene have been characterized. We utilized the highly correlated, multireference methods configuration interaction with single and double excitations with and without the Pople correction for size-extensivity (MR-CISD+Q and MR-CISD) and averaged quadratic coupled cluster theory (MR-AQCC). CAS (8,7) and CAS (10,8) active spaces involving σ, σ*, π, and π* orbitals were employed along with the cc-pVDZ and cc-pVTZ basis sets. The larger active space included the two electrons in the nonbonding sp hybrid orbital on sulfur. We find that all didehydro isomers exist as planar, stable ground state singlets. The singlet-triplet (S-T) adiabatic gaps range from 15 to 25 kcal/mol while the vertical splittings are 21-35 kcal/mol. The 2,3 isomer has the lowest absolute ground state singlet energy and the largest adiabatic and vertical S-T splitting. The ground states of the 2,3-, and 2,5-didehydrothiophene isomers are predicted to exhibit the smallest and largest diradical character, respectively, based on their electronic structures, spin densities and bonding analysis. To our knowledge, no experimental excitation energies of any of the didehydrothiophene isomers are available, and our computed MR-AQCC/cc-pVTZ data are believed to be among the most accurate computed results. This extensive study shows a competitive performance between MR-AQCC and MR-CISD+Q.
通过从噻吩中移除两个氢原子形成的四个双自由基的最低单重态和三重态的能量及几何结构已得到表征。我们使用了高度相关的多参考方法，即带和不带普适校正以考虑尺寸扩展性的单双激发组态相互作用（MR-CISD+Q和MR-CISD）以及平均二次耦合簇理论（MR-AQCC）。采用了涉及σ、σ*、π和π*轨道的CAS(8,7)和CAS(10,8)活性空间，并结合了cc-pVDZ和cc-pVTZ基组。较大的活性空间包括硫上非键合sp杂化轨道中的两个电子。我们发现所有二脱氢异构体均以平面、稳定的基态单重态形式存在。单重态-三重态（S-T）绝热能隙范围为15至25 kcal/mol，而垂直分裂为21 - 35 kcal/mol。2,3-异构体具有最低的绝对基态单重态能量以及最大的绝热和垂直S-T分裂。基于2,3-和2,5-二脱氢噻吩异构体的电子结构、自旋密度和键合分析，预测它们的基态分别表现出最小和最大的双自由基特征。据我们所知，尚无任何二脱氢噻吩异构体的实验激发能数据可用，我们计算得到的MR-AQCC/cc-pVTZ数据被认为是最准确的计算结果之一。这项广泛的研究展示了MR-AQCC和MR-CISD+Q之间的竞争性能。
