Jackson Benjamin A, Miliordos Evangelos
Department of Chemistry and Biochemistry, Auburn University, Auburn, Alabama 36849-5312, USA.
J Chem Phys. 2022 May 21;156(19):194302. doi: 10.1063/5.0089815.
Beryllium ammonia complexes Be(NH) are known to bear two diffuse electrons in the periphery of a Be(NH) skeleton. The replacement of one ammonia with a methyl group forms CHBe(NH) with one peripheral electron, which is shown to maintain the hydrogenic-type shell model observed for Li(NH). Two CHBe(NH) monomers are together linked by aliphatic chains to form strongly bound beryllium ammonia complexes, (NH)Be(CH)Be(NH), n = 1-6, with one electron around each beryllium ammonia center. In the case of a linear carbon chain, this system can be seen as the analog of two hydrogen atoms approaching each other at specific distances (determined by n). We show that the two electrons occupy diffuse s-type orbitals and can couple exactly as in H in either a triplet or singlet state. For long hydrocarbon chains, the singlet is an open-shell singlet nearly degenerate with the triplet spin state, which transforms to a closed-shell singlet for n = 1 imitating the σ-covalent bond of H. The biradical character of the system is analyzed, and the singlet-triplet splitting is estimated as a function of n based on multi-reference calculations. Finally, we consider the case of bent hydrocarbon chains, which allows the closer proximity of the two diffuse electrons for larger chains and the formation of a direct covalent bond between the two diffuse electrons, which happens for two Li(NH) complexes converting the open-shell to closed-shell singlets. The energy cost for bending the hydrocarbon chain is nearly compensated by the formation of the weak covalent bond rendering bent and linear structures nearly isoenergetic.
已知铍氨配合物Be(NH)在Be(NH)骨架外围有两个弥散电子。用一个甲基取代一个氨形成具有一个外围电子的CHBe(NH)，结果表明其保持了在Li(NH)中观察到的类氢壳层模型。两个CHBe(NH)单体通过脂肪族链连接在一起，形成强键合的铍氨配合物(NH)Be(CH)Be(NH)，n = 1 - 6，每个铍氨中心周围有一个电子。在碳链为线性的情况下，该体系可视为两个氢原子以特定距离（由n决定）相互靠近的类似物。我们表明这两个电子占据弥散的s型轨道，并且可以像在H₂中一样精确地以三重态或单重态耦合。对于长烃链，单重态是一个近简并于三重态自旋态的开壳层单重态，对于n = 1时它转变为闭壳层单重态，类似于H₂的σ共价键。分析了该体系的双自由基特性，并基于多参考计算估计了单重态 - 三重态分裂作为n的函数。最后，我们考虑弯曲烃链的情况，对于较大的链，这使得两个弥散电子更靠近，并在两个弥散电子之间形成直接共价键，这发生在两个Li(NH)配合物将开壳层转变为闭壳层单重态时。烃链弯曲的能量成本几乎被弱共价键的形成所补偿，使得弯曲和线性结构几乎等能。
