Department of Chemistry, University of Pittsburgh , Pittsburgh, Pennsylvania 15260, United States.
J Am Chem Soc. 2013 Sep 18;135(37):13862-9. doi: 10.1021/ja406002n. Epub 2013 Sep 4.
The nature of the electronic ground state of the tetramethyleneethane (TME) diradical has proven to be a challenge for both experiment and theory. Through the use of quantum Monte Carlo (QMC) methods and multireference perturbation theory, we demonstrate that the lowest singlet state of TME is energetically lower than the lowest triplet state at all values of the torsional angle between the allyl subunits. Moreover, we find that the maximum in the potential energy curve for the singlet state occurs at a torsional angle near 45°, in contrast to previous calculations that placed the planar structure of the singlet state as the highest in energy. We also show that the CASPT2 method when used with a sufficiently large reference space and a sufficiently flexible basis set gives potential energy curves very close to those from the QMC calculations. Our calculations have converged the singlet-triplet gap of TME as a function of methodology and basis set. These results provide insight into the level of theory required to properly model diradicals, in particular disjoint diradicals, and provide guidelines for future studies on more complicated diradical systems.
四亚甲基乙烷（TME）自由基的电子基态本质一直是实验和理论的挑战。通过使用量子蒙特卡罗（QMC）方法和多参考微扰理论，我们证明在丙烯基亚基之间的扭转角的所有值处，TME 的最低单线态能量低于最低三重态能量。此外，我们发现单线态势能曲线的最大值出现在扭转角接近 45°处，与以前的计算结果相反，以前的计算结果认为单线态的平面结构能量最高。我们还表明，当使用足够大的参考空间和足够灵活的基组时，CASPT2 方法给出的势能曲线非常接近 QMC 计算的结果。我们的计算已经收敛了 TME 的单线态-三线态能隙作为方法和基组的函数。这些结果提供了对正确模拟自由基（特别是不相交自由基）所需理论水平的深入了解，并为未来更复杂自由基体系的研究提供了指导。
