Challenges in predicting ΔrxnG in solution: the mechanism of ether-catalyzed hydroboration of alkenes.

Ab initio (coupled-cluster and density-functional) calculations of Gibbs reaction energies in solution, with new entropy-of-solvation damping terms, were performed for the ether-catalyzed hydroboration of alkenes. The goal was to test the accuracy of continuum-solvation models for reactions of neutral species in nonaqueous solvents, and the hope was to achieve an accuracy sufficient to address the mechanism in the "Pasto case": B2H6 + alkene in THF solvent. Brown's SN2/SN1 "dissociative" mechanism, of SN2 formation of borane-ether adducts followed by SN1 alkene attack, was at odds with Pasto's original SN2/SN2 hypothesis, and while Brown could prove his mechanism for a variety of cases, he could not perform the experimental test with THF adducts in THF solvent, where the higher THF concentrations might favor an SN2 second step. Two diboranes were tested: B2H6, used by Pasto, and (9BBN)2 (9BBN = 9-borabicyclo[3.3.1]nonane, C8H15B), used by Brown. The new entropy terms resulted in improved accuracy vs traditional techniques (∼2 kcal mol(-1)), but this accuracy was not sufficient to resolve the mechanism in the Pasto case.

Semicontinuum Solvation Modeling Improves Predictions of Carbamate Stability in the CO2 + Aqueous Amine Reaction.

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.

Molecular Dynamics Simulations of Proposed Intermediates in the CO2 + Aqueous Amine Reaction.

Ab initio molecular dynamics simulations of up to 210 ps have been performed on various aqueous intermediates postulated in the CO2 + amine reaction, important for CO2 capture. Observations of spontaneous deprotonation of aqueous carbamate zwitterions R1R2NHCOO(±) by bulk water (instead of additional amine, or via umbrella sampling) are reported apparently for the first time. Carbamic acid structures R1R2NCOOH were observed in some simulations, arising from zwitterions not via classical 1,3-H-shifts but via Grotthuss-style multiple-H(+) transfer pathways that involve bulk H2O and require carbamate anions R1R2NCOO(-) as an intermediate stage along the way. H(+)-bridging complexes, including not only Zundel ions [water·H(+)·water](+) but neutral carbamate complexes [carbamate(-)·H(+)·water], were observed in simulation. These results should assist efforts in improving underlying mechanisms for kinetic modeling.