Ab initio and density functional theory calculations on the protonated species of As4 clusters

High level ab initio and density functional theory approaches have been used to study the structure and bonding of the stationary points of the ${\mathrm{As}}_{\mathrm{4}}{\mathrm{H}}^{\mathrm{+}}$ potential energy surface. The geometries and the harmonic vibrational frequencies of the different species investigated were obtained through the use of all-electron MP2(full) and B3LYP methods using $\text{6-311}\mathrm{G}\mathrm{(d)}$ basis expansions. Final energies were obtained in G2(MP2) and $\mathrm{B3LYP/6-311+G}\text{(3df,2p)}$ calculations. The global minimum is the result of the side protonation of ${\mathrm{As}}_{\mathrm{4}}$ tetrahedral molecule. The corner-protonated species is significantly less $\text{(∼23\hspace{0.17em}}\mathrm{kcal/mol})$ stable, while the face-protonated form is a second-order saddle point. The open structure 5 resulting from the fission of two As–As bonds lies 18 kcal/mol above the global minimum. In the global minimum the proton is covalently attached to two As atoms through the formation of a three-center delocalized bonding orbital. The relative stability of this “nonclassical” structure is enhanced with respect to that of the homologous ${\mathrm{P}}_{\mathrm{4}}{\mathrm{H}}^{\mathrm{+}}$ system, indicating that the heavier elements prefer divalent arrangements. As a consequence, the ${\mathrm{As}}_{\mathrm{4}}$ cluster is predicted to be about 8 kcal/mol more basic than ${\mathrm{P}}_{\mathrm{4}}$ tetramer in the gas phase. The harmonic vibrational frequencies of the ${\mathrm{As}}_{\mathrm{4}}{\mathrm{H}}^{\mathrm{+}}$ side-protonated species is consistent with the existence of the three-center As–H–As covalent bond.