Using periodic density functional theory calculations, the aldol condensation of acetaldehyde to 3-hydroxybutanal over dehydroxylated MgO surfaces with and without structure defects was investigated. Compared with the C-C coupling step, the enolization step via proton transfer of the α-hydrogen of acetaldehyde to the MgO surface or the proton back-transfer step to form the desired 3-hydroxybutanal has a higher energy barrier, indicating that the proton transfer process is the key step for the aldol condensation on MgO. To highlight the effect of water, we also calculated the proton transfer steps in the presence of water and studied the reaction pathways over the partially hydroxylated MgO surface. The results show that water can participate in the proton back-transfer step by donating a proton to the alkoxide anion to form the 3-hydroxybutanal, thus reducing the activation energy; the surface OH groups induce a lowering of the activation energy barriers for the overall reaction. The results of the electronic structure analysis indicate that a strong Lewis acid-weak/medium base pair may have the best performance for aldol condensation, such as Mg-O-D produced by divacancy defects and Mg-OH produced by the dissociative adsorption of water. A strong Lewis acid generated by low-coordinated Mg can adsorb and stabilize the acetaldehyde molecule near the catalyst surface which is beneficial for the abstraction of an α-proton from an acetaldehyde molecule, and a medium or weak Brønsted base is favorable for the proton back-transfer step.
Inter-anion hydrogen and halogen bonds have emerged as counterintuitive linkers and inspired us to expand the range of this unconventional bonding pattern. Here, the inter-anion chalcogen bond (IAChB) was proposed and theoretically analyzed in a series of complexes formed by negatively charged bidentate chalcogen bond donors with chloride anions. The kinetic stability of IAChB was evidenced by the minima on binding energy profiles and further supported by ab initio molecular dynamic simulations. The block-localized wave function (BLW) method and its subsequent energy decomposition (BLW-ED) approach were employed to elucidate the physical origin of IAChB. While all other energy components vary monotonically as anions get together, the electrostatic interaction behaves exceptionally as it experiences a Coulombic repulsion barrier. Before reaching the barrier, the electrostatic repulsion increases with the shortening Ch⋯Cl− distance as expected from classical electrostatics. However, after passing the barrier, the electrostatic repulsion decreases with the Ch⋯Cl− distance shortening and subsequently turns into the most favorable trend among all energy terms at short ranges, representing a dominating force for the kinetic stability of inter-anions. For comparison, all energy components exhibit the same trends and vary monotonically in the conventional counterparts where donors are neutral. By comparing inter-anions and their conventional counterparts, we found that only the electrostatic energy term is affected by the extra negative charge. Remarkably, the distinctive (nonmonotonic) electrostatic energy profiles were reproduced using quantum mechanical-based atomic multipoles, suggesting that the crucial electrostatic interaction in IAChB can be rationalized within the classical electrostatic theory just like conventional non-covalent interactions.
Perfluorinated cycloparaphenylenes (F‐[n]CPP, n = 5–8), boron nitride nanohoop (F‐[5]BNNH), and buckybowls (F‐BBs) were proposed as anion receptors via anion‐π interactions with halide anions (Cl−, Br− and I−), and remarkable binding strengths up to −294.8 kJ/mol were computationally verified. The energy decomposition approach based on the block‐localized wavefunction method, which combines the computational efficiency of molecular orbital theory and the chemical intuition of ab initio valence bond theory, was applied to the above anion‐π complexes, in order to elucidate the nature and selectivity of these interactions. The overall attraction is mainly governed by the frozen energy component, in which the electrostatic interaction is included. Remarkable binding strengths with F‐[n]CPPs can be attributed to the accumulated anion‐π interactions between the anion and each conjugated ring on the hoop, while for F‐BBs, additional stability results from the curved frameworks, which distribute electron densities unequally on π‐faces. Interestingly, the strongest host was proved to be the F‐[5]BNNH, which exhibits the most significant anisotropy of the electrostatic potential surface due to the difference in the electronegativities of nitrogen and boron. The selectivity of each host for anions was explored and the importance of the often‐overlooked Pauli exchange repulsion was illustrated. Chloride anion turns out to be the most favorable anion for all receptors, due to the smallest ionic radius and the weakest destabilizing Pauli exchange repulsion.
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