Theoretical study of structural and thermodynamic properties of gaseous metal hydrides m2xh4 (m = li, na; x = be, mg)

Abstract

This study aimed to explore the properties of the complex hydrides which seem to be the promising candidates for hydrogen storage materials. The geometrical structure, vibrational spectra and thermodynamic characteristics of the gaseous complex hydrides, MXH3 and M2XH4 (M = Li, Na; X = Be, Mg) and the subunits LiH, NaH, Li2H+, Li2H+, Li2H2, Na2H2, BeH2, MgH2, MgH3−, BeH3− have been investigated. Quantum chemical methods of density functional theory and second order Moller−Plesset perturbation theory have been applied. According to the calculations, three isomers of M2XH4 have been proved to exist, polyhedral of C2v symmetry; two−cycled, D2d and hexagonal shape, C2v. For M2BeH4 polyhedral isomer was found to have the lowest energy and for M2MgH4 was cyclic hexagonal configuration. The abundance of the isomers in vapour was evaluated and the hexagonal isomer was observed to prevail in vapour both for beryllium and magnesium complexes. For smaller hydrides, MXH3, the cyclic equilibrium structure (C2v) was shown to exist, another possible configuration, the linear one (C∞v) was found to be of much higher energy (~130 kJ mol–1). Different pathways of gas−phase and heterophase dissociation reactions were examined; the enthalpies of formation of the complex hydrides were found; 105 ± 26 (LiBeH3), 63 ± 37 (Li2BeH4), 121 ± 27 (NaBeH3), 117 ± 39 (Na2BeH4), 114 ± 13 (LiMgH3), 113 ±12 (Li2MgH4), 162 ± 11 (NaMgH3), and 175 ± 26 (Na2MgH4) (in kJ mol 1). The assessment of thermal stability of the hydrides was done through Gibbs free energies for heterophase decomposition.