Benchmarking Binding and Barrier Energies for Modeling Accelerated Organic Reactions via the “On Water” Mechanism

Eduardo Romero-Montalvo and Gino DiLabio

Department of Chemistry, The University of British Columbia, 3247 University Way, Kelowna, British Columbia, Canada, V1V 1V7.

In 2005 Sharpless and co-workers defined the term "on water" reactions (OWRs) to describe a family of accelerated organic reactions of water-insoluble reagents in aqueous suspension (Narayan et al. Angewandte Chemie, 2005, 44, 3275-3279). In some cases, the reaction rates were several times higher than their counterparts in the absence of water. In the following years, numerous chemical systems were reported to behave similarly and, despite the immediate application of OWRs, the origin of this effect is still unclear. To provide insight into this problem, we analyzed hydrogen bonding interactions between small water clusters (38 oligomers, from the dimer to the decamer) and selected common organic molecules. We used a screening process, progressively increasing the level of theory, to choose the lowest in energy cluster-probe complexes, followed by a DLPNO-CCSD(T)/CBS approach to generate benchmark data. Our results allowed us to classify the different types of hydrogen bonds present in these models and provide an understanding of how these interactions scale with cluster size, which is necessary to design better models for OWRs. In addition, a diverse set of DFT methods was also assessed against our dataset, basis-set incompleteness error and dispersion effects were also tested. The obtained results provide a more accessible computational alternative to quantum chemistry calculations of aqueous organic chemistry systems. The obtained data helped us build models for evaluating energy barriers (EBs) in OWRs reactions; namely, the Claisen rearrangement and Diels-Alder cycloaddition were investigated, and it was possible to analyze the effects of cluster size and charge transfer into the EBs of these reaction models. We expect that the presented results herein contribute to the understanding of chemical reactions in heterogeneous oil-water systems that have important relevance in the fields of green, prebiotic and atmospheric chemistry.

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