H2-Binding by Neutral and Multiply Charged Titaniums: Hydrogen Storage Capacity of Titanium Mono- and Dications

Given that transition metal-hydrogen systems have been studied as a predecessor for hydrogen storage materials, we have investigated the neutral and multiply charged titanium-H2 systems (Ti-H2, Ti(+)-H2, Ti(2+)-H2, Ti(3+)-H2, and Ti(4+)-H2) using density functional theory (DFT) and high-level ab initio calculations, including coupled cluster theory with single, double, and perturbatively triple excitations [CCSD(T)]. These systems show different types of hydrogenation depending on their charged state. The neutral Ti-H2 system shows dihydride structure with covalent interaction where the Ti-H distance is 1.76 Å, while H2 is dissociated into two neigboring hydride ions by withdrawing electrons from Ti. The charged Ti(+)-H2, Ti(2+)-H2, and Ti(3+)-H2 systems show dihydrogen structures with noncovalent interaction, where the Ti(+)-H, Ti(2+)-H, and Ti(3+)-H distances are 2.00, 2.14, and 2.12 Å, respectively. The main binding energies in these systems arise from the hydrogen polarizability driven interaction by the positive charge of Ti(n+) (n = 1-3). Among Ti(n+)-H2 (n = 1-3) the Ti(+)-H2 has the shortest distance against our common expectation, while Ti(2+)-H2 has the longest distance. The Ti(+)-H2 distance is the shortest because of the d-σ* molecular orbital (MO) interaction which is not present in Ti(2+)-H2 and Ti(3+)-H2. The Ti(4+) ion does not bind H2. In this regard, we have investigated the maximal hydrogen binding capacity by Ti complexes. The coordination of titanium mono- and dications complexed with dihydrogen (H2) [Ti(+)(H2)n and Ti(2+)(H2)m] is studied along with their structures, binding energies, electronic properties, and spectra. The titanium monocations of the quartet ground state have up to the hexacoordinaton, while titanium dications of the triplet ground state have up to the octacoordination at very low temperatures. At room temperature, the monocations favor penta- to hexacoordination, while the dications favor hexacoordination. This information would be useful for the design of hydrogen storage devices of Ti complexes, such as Ti-decorated/dispersed polymer-graphene hybrid materials.