Ab-initiomolecular orbital theory of hydrogenation of LiAl andLi2Al2: The magic clusters(LiAlH4)and(LiAlH4)2

Abstract

Interaction of hydrogen atoms with light metal alloy clusters such as LiAl and ${\mathrm{Li}}_{2}{\mathrm{Al}}_{2}$ have been investigated under the linear combination of atomic and molecular orbital approach using the post-Hartree-Fock and density functional formalism under the Perdew-Burke-Erzerhof and Lee-Yang-Parr exchange correlation functional. A correlation consistent polar valance triple zeta basis set was employed for this purpose. The saturation composition for Li:Al:H is found to be 1:1:4, reflecting the bulk stoichiometry even in the smallest cluster. The sequential attachment of H atoms to the ${\mathrm{Li}}_{2}{\mathrm{Al}}_{2}$ cluster shows that for $n>6$, the Al-Al bond in the ${\mathrm{Li}}_{2}{\mathrm{Al}}_{2}{\mathrm{H}}_{n}$ cluster dissociates and tetrahedral ${(\mathrm{Al}{\mathrm{H}}_{4})}^{\ensuremath{\delta}\ensuremath{-}}$ moiety is formed. Other than inertness towards further reaction, the ${\mathrm{Li}}_{2}{\mathrm{Al}}_{2}{\mathrm{H}}_{8}$ cluster thus formed shows higher binding energy, ionization potential and low electron affinity, characteristics of a highly stable species. Based on the energetics it is found that the dimerization energy of $\mathrm{Li}\mathrm{Al}{\mathrm{H}}_{4}$ is $1.85\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, which is significantly higher than the interaction energy usually observed for molecules or stable clusters. The higher binding energy of the ${(\mathrm{Li}\mathrm{Al}{\mathrm{H}}_{4})}_{2}$ has been attributed to the increased coordination of Li, where additional bonds are formed between ${\mathrm{Li}}^{\ensuremath{\delta}+}$ and ${\mathrm{H}}^{\ensuremath{\delta}\ensuremath{-}}$ by the electrostatic force of attraction.