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Lecture 13 - Carbenes Carey Sundberg Part B-650 Carbenes...

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Carbenes Carey & Sundberg , Part B, Chapter 10, 614-650.
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H H H H Carbene Electronic Structure Triplet (two unpaired e - ) Singlet (all e - paired) p σ σ p Often has radica l-like character Capale of both electrophilic and nucleophilic behavior 1.078 Å 133.8˚ Nitrene Electronic Structure R N Singlet (all e - paired) empty filled filled R N H empty filled Nitrenium ion H Carbenes: Introduction
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p Energy σ Due to electron repulsion, there is an energy cost in pairing both electrons in the σ orbital. If a small energy difference between the σ and p orbitals exists, the electrons will remain unpaired (triplet) . If a large gap exists between the σ and p orbitals the electrons will pair in the σ orbital ( singlet ). singlet 8–10 kcal/mol T 1 S 1 Carbene Configuration: Triplet vs. Singlet p σ triplet The History of the Singlet-Triplet Gap Method HCH Angle S–T Splitting kcal/mol Author Year Qual. 90-100° –– Muliken 1932 Grnd State singlet Thermochem 180° small Walsh 1947 triplet 160° 30 Gallup 1957 triplet Qual. QM 138° >33 Harrison 1969 triplet Ab initio –– 8–9 Hase 1971 triplet Kinetics 19 Pople 1971 triplet SCF 132° 8.7 Dewar 1974 triplet MINDO 134° 19.5 Lineberger 1976 triplet Expt 138° 19.7 Schaeffer 1976 triplet An Initio ––– 8.1 Zare 1978 triplet Expt ––– 8.5 Haydon 1982 triplet Expt ––– Carbenes: Introduction
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The p orbital of carbenes substituted with p-donor atoms (N, O, halogen) is raised high enough in energy to make the pairing of the electrons in the σ orbital energetically favorable. As a result, these carbenes are often in the singlet state.
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