Role of singlet diradicals in reactions of 2-carbenabicyclo[3.2.1]octa-3,6-diene.

The generation of 2-carbenabicyclo[3.2.1]octa-3,6-diene (1) results in the formation of C(8)H(8) hydrocarbons endo-6-ethynylbicyclo[3.1.0]hex-2-ene (4), semibullvalene (5), and 5-ethynyl-1,3-cyclohexadiene (6), and C(8)H(10) hydrocarbons bicyclo[3.2.1]octa-2,6-diene (7), tricyclo[3.2.1.0(4,6)]oct-2-ene (8), and tetracyclo[3.3.0.0(2,8)0(4,6)]octane (9). Focus is placed on three mechanistic pathways for the formation of the C(8)H(10) hydrocarbon fraction: (a) abstraction of hydrogen by triplet carbene 1T to produce an equilibrating set of monoradicals, (b) interconversion of triplet carbene 1T into tricyclic triplet diradical 19T and tetracyclic triplet diradical 20T, and (c) interconversion of singlet 1S with analogous singlet diradical 19S and 20S. Ab initio calculations at the (U)B3LYP/6-311+G(3df,2p)//(U)B3LYP/6-31G(d,p) and broken spin symmetry UBS B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d,p) levels rule out choices (a) and (b) and are consistent with the singlet diradical process.

The C-N rotation barrier of the lithium enolate of acetamide: an ab initio and density functional theory investigation.

Semiempirical (PM3), ab initio (HF/6-31+G(d) and MP2/6-31+G(d)), and density functional (pBP/DN) calculations are used to investigate the rotation barrier of the carbon-nitrogen bond in a simple enolate anion: lithium acetamide, 1. For comparison, the amidate anion 2, vinylamine 3, and a simulated dimer 4 were also calculated. In all systems, the barrier to rotation was found to be less than 10 kcal x mol(-1) in agreement with experiment. The correlated calculations show the barrier to be lowest for the anion 2. The results show conjugation effects in 1 and 2 comparable to that in vinylamine 3 and imply that polarization effects are more important than charge transfer in amine conjugation.