The remarkable ability of anions to bind dihydrogen

Abstract

The structural features and hydrogen binding affinity of anions F⁻, Cl⁻, Br⁻, OH⁻, NH₂⁻, NO₂⁻, CN⁻, and ClO⁻ have been explored at the CCSD(T)/aug-cc-pVTZ//CCSD/6-311++G(d,p) level of coupled cluster theory and the M06L/6-311++G(d,p) level of density functional theory along with a two-point extrapolation to the complete basis set limit and a benchmark study at CCSD(T) and MP2 levels. The coupled cluster, MP2 and DFT methods yield comparable results and show that anions have very high capacity to store hydrogen as the weight percent of H₂ in the highest H₂-coordinated state of F⁻, Cl⁻, Br⁻, OH⁻, NH₂⁻, NO₂⁻, CN⁻, and ClO⁻ is 56.0, 47.6, 33.5, 64.0, 65.4, 41.2, 55.4, and 40.0 wt%, respectively. The CCSD(T)/aug-cc-pVTZ//CCSD/6-311++G(d,p) results are presented for anions coordinated with up to nine or ten H₂ molecules, while up to the entire first coordination shell is computed using the M06L method which revealed H₂ coordination numbers of 12, 16, 20, 15, 15, 16, 16, and 17, respectively, for F⁻, Cl⁻, Br⁻, OH⁻, NH₂⁻, NO₂⁻, CN⁻, and ClO⁻. An increase in the total interaction energy (E_int) and a decrease in the interaction energy per H₂ molecule (E_int/H2) with an increase in the number of coordinated H₂ molecules are observed. However, the decrease in E_int/H2 is very less and even in the highest coordinated anions, substantially good values of E_int/H2 are observed, viz. 4.24, 2.59, 2.09, 3.32, 3.07, 2.36, 2.31, and 2.63 kcal mol⁻¹ for F⁻, Cl⁻, Br⁻, OH⁻, NH₂⁻, NO₂⁻, CN⁻, and ClO⁻, respectively, which are comparable with the values obtained for complexes with lesser H₂ coordination. The stability of the complexes is attributed to the formation of a large number of non-covalent X⁻⋯H bonds as revealed by the identification of bond critical points in the quantum theory of atoms in molecules (QTAIM) analysis. Further, critical features of molecular electrostatic potential (MESP) have been used to correlate the stability of X⁻(H₂)_n complexes with the charge delocalization in the complexes. These results show that anions have a remarkable ability to bind with a large number of hydrogen molecules and this property can be utilized for the development of novel salt systems for hydrogen storage.