![]() The distortion/interaction model of reactivity explains why (1) there is a monotonic decrease of ∼6 kcal/mol in the activation energy along the series oxides, imine, and ylide for the diazonium, nitrilium, and azomethine betaine classes of 1,3-dipoles (2) nitrilium and azomethine betaines with the same trio of atoms have almost identical cycloaddition barrier heights (3) barrier heights for the cycloadditions of a given 1,3-dipole with ethylene and acetylene have the same activation energies (mean absolute deviation of 0.6 kcal/mol) in spite of very different reaction thermodynamics (ΔΔ H rxn range = 14−43 kcal/mol) and frontier molecular orbital (FMO) energy gaps. Quantum chemical calculations of activation barriers and reaction energies for 1,3-dipolar cycloadditions by the high-accuracy CBS-QB3 method reveal previously unrecognized quantitative trends in activation barriers.
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