Dihydrogen Binding Affinity of Polyatomic Anions: A DFT Study

The dihydrogen-binding ability of polyatomic oxohalo anions ClO^–, ClO₂^–, ClO₃^–, ClO₄^–, BrO^–, BrO₂^–, BrO₃^–, and BrO₄^– has been studied at the M06L/6-311++G­(d,p) density functional theory and the CCSD­(T)/aug-cc-pVTZ//CCSD/aug-cc-pVDZ ab initio theory. The maximum number of dihydrogen adsorbed by the anions (n_max) varies from 17 to 24 in the first coordination shell. As the number of H₂ adsorbed varies from 1 to n_max, the oxochloro and oxobromo anions show a wide range for interaction energy (E_int), namely, 1.5–45.4 kcal/mol for the former and 1.4–46.0 kcal/mol for the latter. These results indicate that both series of anions show very similar and high affinity to bind with several dihydrogen molecules. Further, an increase in the coordination ability and a decrease in the strength of the dihydrogen interaction are observed with an increase in the number of oxygen atoms in the polyatomic anion. In contrast, the neutral oxohaloacids show negligible interaction with dihydrogen. The anion···H₂ noncovalent interactions along with H···H dihydrogen interactions within the complex are ascertained by locating the bond critical points (bcps) in the quantum theory of atoms in molecules analysis. The electron density at the bcp summed up for all of the anion···H₂ interactions (∑ρ_bcp) showed a strong linear relationship with E_int, indicating that the stability of the complex is due to the formation of a large network of noncovalent bonds in the complex. The amount of electron density donated by the anion to the dihydrogen during complex formation is also gauged from the molecular electrostatic potential values at the nuclei (V_n) of all of the atoms in the anion. The hydrogen uptake leads to a significant reduction in the negative character of V_n, and the total change in V_n from all of the anion atoms (∑ΔV_n) is found to be directly proportional to E_int. The polyatomic anions have a very high affinity toward dihydrogen binding, which can be utilized for the development of new hydrogen storage systems.