RESUMO
The role of protonated cyclopropane (PCP(+)) structures in carbocation rearrangement is a decades-old topic that continues to confound. Here, quantum-chemical computations (PBE molecular dynamics, PBE and CCSD optimizations, CCSD(T) energies) are used to resolve the issue. PCP(+) intermediates are neither edge-protonated nor corner-protonated (normally) but possess "closed" structures mesomeric between these two. An updated mechanism for hexyl ion rearrangement is presented and shown to resolve past mysteries from isotope-labeling experiments. A new table of elementary-step barrier heights is provided. The mechanism and barrier heights should be useful in understanding and predicting product distributions in organic reactions, including petroleum modification.
RESUMO
Quantum chemistry computations with a semicontinuum (cluster + continuum) solvation model have been used to cure long-standing misprediction of aqueous carbamate anion energies in the industrially important CO2 + aqueous amine reaction. Previous errors of over 10 kcal mol(-1) are revealed. Activation energies were also estimated with semicontinuum modeling, and a refined discussion of the competing hypothetical mechanisms for CO2 + monoethanolamine (MEA) is presented. Further results are also presented to demonstrate that the basicity of an amine (aqueous proton affinity) correlates only with CO2 affinity within an amine class: secondary amines have an extra CO2 affinity that primary amines do not have.