Covalency in highly polar bonds.: Structure and bonding of methylalkalimetal oligomers (CH3M)n (M = Li-Rb; n=1, 4)
Covalency in highly polar bonds.: Structure and bonding of methylalkalimetal oligomers (CH3M)n (M = Li-Rb; n=1, 4)
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DOI：
10.1021/ct050333s
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发表时间：
2006-07-11
影响因子：
5.5
通讯作者：
Guerra, Celia Fonseca
中科院分区：
文献类型：
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作者：
Bickelhaupt, F. Matthias;Sola, Miquel;Guerra, Celia Fonseca
我们用密度泛函理论(DFT)在BP86/TZ2P水平上对M=Li、Na、K、Rb的甲基碱金属单体CH3M和四聚物(CH3M)(4)进行了理论计算，并与具有X=F、Cl、Br和I的甲基卤化物CH3X进行了比较。我们的目的是确定有机碱金属化合物的结构和热化学(例如，C-M键的长度和强度，齐聚能)如何依赖于金属原子，并了解定量Kohn-Sham分子轨道(KS-MO)理论方面的新趋势。如果从元素周期表从Li下降到Rb，无论是单体还是四聚物，C-M键都变得更长、更弱。定量成键分析表明，这一趋势不仅取决于静电吸引的减弱，而且在更大程度上还取决于轨道相互作用的减弱。后者在Li-Rb方向的稳定性变差，因为CH3中心点的单占据分子轨道(Somos)和M中心点自由基之间的键重叠随着金属ns原子轨道(AO)变得更大和更分散而减少。因此，C-M键在甲基自由基和碱金属原子之间表现为典型的电子对键，在这方面，它是共价的。结果还表明，这种电子对键仍然可以是高极性的，这与大的偶极矩是一致的。有趣的是，在甲基碱金属四聚体中，C-M键变得不那么极性，因为金属-金属相互作用稳定了碱金属轨道，从而使碱金属有效地降低了正电性。
We have carried out a theoretical investigation of the methylalkalimetal monomers CH3M and tetramers (CH3M)(4) with M = Li, Na, K, and Rb and, for comparison, the methyl halides CH3X with X = F, Cl, Br, and I, using density functional theory (DFT) at BP86/TZ2P. Our purpose is to determine how the structure and thermochemistry (e.g., C-M bond lengths and strengths, oligomerization energies) of organoalkalimetal compounds depend on the metal atom and to understand the emerging trends in terms of quantitative Kohn-Sham molecular orbital (KS-MO) theory. The C-M bond becomes longer and weaker, both in the monomers and tetramers, if one descends the periodic table from Li to Rb. Quantitative bonding analysis shows that this trend is not only determined by decreasing electrostatic attraction but also, even to a larger extent, by the weakening in orbital interactions. The latter become less stabilizing along Li-Rb because the bond overlap between the singly occupied molecular orbitals (SOMOs) of CH3 center dot and M center dot radicals decreases as the metal ns atomic orbital (AO) becomes larger and more diffuse. Thus, the C-M bond behaves as a typical electron-pair bond between the methyl radical and alkalimetal atom, and, in that respect, it is covalent. It is also shown that such an electron-pair bond can still be highly polar, in agreement with the large dipole moment. Interestingly, the C-M bond becomes less polar in the methylalkalimetal tetramers because metal-metal interactions stabilize the alkalimetal orbitals and, in that way, make the alkalimetal effectively less electropositive.