Computer-aided molecular design for ion receptors and H2 and CO2 storages

Abstract

With the development of modeling chemistry at the molecular level, there has been a paradigm shift in the design of receptor and reservoir systems targeting specific molecules or ions. When designing a storage system with high selectivity, the binding mechanism at the molecular level is one of the most critical factors. Their potential applications are diverse, spanning a wide range of industries. Some storing techniques are expected to be core technologies seeding a futuristic economy. Anion recognition chemistry has grown as an important research area because of the central roles of anions in many biological and chemical systems and also the environmental problems. Designing the neutral anion-receptors using Lewis acidic aromatic rings is a recent research field. Among the ？-interactions, the anion-？ interaction has been a novel type of interaction. The theoretically predicted and experimentally observed anion-？ structures are quite different from the most frequently observed crystal structures. We investigate the structural changes in complexation of the anions with arenes as the number of solvent molecules increases. The covalent bonding and the hydrogen bonding types, which are the lowest energy structures in the gas phase, change to the solvent mediated anion-？ type or displaced anion-？ type complexes. This study explains why the (displaced) anion-？ type complexes with some flexible orientations are most common in many crystal structures.There have not been many studies to bind polyatomic anions with electron-deficient ？ rings by maximizing the binding strength. Extended arenes have advantages to target polyatomic anions strongly and selectively. Polyatomic anions have their unique charge distributions. Arenes are possible to be extended to involve an enough area for large size of polyatomic ions. Extended arenes are allowed to make suitable charge distribution for the best geometrical fit of guest and host towards the selective recognition. Trigonal-planar and tetrahedral anions most strongly interact with the C6N7 nucleus, forming the structure like the ？Lego block？. The Y-shaped and Linear anions most strongly interact with the positive charged C6 nucleus with the negatively charged edge, including local electrostatic linear-shaped bowl.Alkali metal amidoboranes are recently highlighted as a material satisfying many criteria demanded for the hydrogen storage media. It is thus crucial to understand the dehydrogenation mechanism of these materials for further applications and developments towards successful hydrogen storage. We attempt to shed light on the mechanisms involved in the hydrogen loss from solid lithium amidoboranes using high level ab initio calculations. In the lithium amidoboranes, hydrogen releases first by the formation of LiH, followed by the redox reaction of the dihydrogen bond. In the dehydrogenation process, the Li cation catalyzes the intermolecular N-B bond formation, indicating new pathways for the N-B polymerization.While group I and II metal amidoboranes have been identified as one of the promising families of materials for efficient H2 storage, we propose a metal cation hydride transfer mechanism for these materials. In this mechanism, with increasing ionic character of the metal hydride bond in the intermediate, the stability of the intermediate decreases, while the dehydrogenation process involving ionic recombination of the hydridic H with the protic H proceeds with a reduced barrier. Such correlations lead directly to a U-shaped relationship between the energy barrier for H2 elimination and the ionicity of metal hydride bond. The kinetic rates at low temperatures are determined by the maximum barrier height in the pathway, showing a ？-shaped relation, while those at moderately high temperatures are determined by most of multiple-barriers. At the operating temperatures of proton exchange membrane fuel cells, the metal amidoboranes with lithium and sodium release H2 along both oligomerization and non-oligomerization paths. The sodium amidoboranes show the most accelerated rates, while others release H2 at similar rates. In addition, we predict that the novel metal amidoborane-based adducts and mixtures would release H2 with accelerated rates as well as with enhanced reversibility. This comprehensive study is useful for further developments of active metal-based better hydrogen storage materials.The development of novel low-carbon technologies such as capturing and storing CO2 for various commercial applications in industry has been an important issue for green chemistry. While the current state-of-the-art technology for CO2 capture is the aqueous ammine process, the aqueous ammonia process has recently received much attention as an alternative to the amine process, due to its many advantages over the amine process. There has been controversy concerning the reaction mechanism of CO2 in aqueous ammonia solutions. Originally two-step zwitterion mechanism is proposed. Then, recently single-step trimolecular mechanism is suggested. However, the understanding is only limited to the formation of carbamates, while the catalytic effect on the reaction has been misunderstood. Towards novel development of the futuristic CO2 capture as well as the improved ammonia capturing process, a clear understanding of the reaction mechanism of CO2 in an aqueous ammonia will be definitely very important. Thus, we detail the comprehensive reaction mechanism of CO2 in an ammonia solution.