Fachbereich Chemie, Philipps-Universität Marburg, Hans-Meerwein-Strasse, D-35032 Marburg, Germany.
J Am Chem Soc. 2011 Mar 16;133(10):3557-69. doi: 10.1021/ja109812r. Epub 2011 Feb 18.
Quantum-chemical calculations using DFT and ab initio methods have been carried out for 32 carbenes RR'C which comprise different classes of compounds and the associated ketenes RR'C═C═O. The calculated singlet-triplet gaps ΔE(S-T) of the carbenes exhibit a very high correlation with the bond dissociation energies (BDEs) of the ketenes. An energy decomposition analysis of the RR'C-CO bond using the triplet states of the carbene and CO as interacting fragments supports the assignment of ΔE(S-T) as the dominant factor for the BDE but also shows that the specific interactions of the carbene may sometimes compensate for the S/T gap. The trend of the interaction energy ΔE(int) values is mainly determined by the Pauli repulsion between the carbene and CO. The stability of amino-substituted ketenes strongly depends on the destabilizing conjugation between the nitrogen lone-pair orbital and the ketene double bonds. There is a ketene structure of the unsaturated N-heterocyclic carbene parent compound NHC1 with CO as a local energy minimum on the potential-energy surface. However, the compound NHC1-CO is thermodynamically unstable toward dissociation. The saturated homologue NHC2-CO has only a very small bond dissociation energy of D(e) = 3.2 kcal/mol. The [3]ferrocenophane-type compound FeNHC-CO has a BDE of D(e) = 16.0 kcal/mol.
使用 DFT 和从头算方法对 32 种卡宾 RR'C 进行了量子化学计算，这些卡宾包括不同类别的化合物和相关的烯酮 RR'C═C═O。卡宾的单重态-三重态能隙ΔE(S-T)与烯酮的键离解能（BDE）之间存在非常高的相关性。使用卡宾的三重态和 CO 作为相互作用片段对 RR'C-CO 键进行的能量分解分析支持将ΔE(S-T)分配为 BDE 的主要因素，但也表明卡宾的特定相互作用有时可以补偿 S/T 间隙。相互作用能ΔE(int)值的趋势主要由卡宾和 CO 之间的 Pauli 排斥决定。取代氨基的烯酮的稳定性强烈取决于氮孤对轨道和烯酮双键之间的失稳共轭。在势能表面上，存在不饱和 N-杂环卡宾母体化合物 NHC1 与 CO 作为局部能量最低点的烯酮结构。然而，化合物 NHC1-CO 在热力学上不稳定，易于离解。饱和同系物 NHC2-CO 的键离解能 D(e)仅为 3.2 kcal/mol。[3]ferrocenophane 型化合物 FeNHC-CO 的 BDE 为 D(e) = 16.0 kcal/mol。
