Hybrid Orbital Formation and Multicenter Bonding of Hydrogen Atoms and Molecules in Ti 3 C 2 ${\rm Ti}_{3}{\rm C}_{2}$ MXenes

The formation and stability of solids and molecules is not possible without chemical bonds, which are divided into covalent, ionic, metallic, and van der Waals bonds. A special type of intermolecular bond is hydrogen bonding, which plays a crucial role for chemical, biological, and physical processes. However, hydrogen shows a far more complex behavior when it is present in solids. In this paper, it is shown that the chemical bonding of hydrogen atoms and molecules extends far beyond the simple picture of conventional, ionic, covalent, and multicenter bonds. The interaction of H with its host material is particularly important for hydrogen storage in metallic materials such as ${\mathrm{Ti}}_3{C}_2$ MXenes. Hydrogen atoms and $H_2$ molecules form multicenter bonds in ${\mathrm{Ti}}_3{C}_2$. On the surface and between two ${\mathrm{Ti}}_3{C}_2$ sheets this is limited to the formation of H–Ti bonds. However, H and $H_2$ on interstitial sites form multicenter bonds not only with nearest neighbor Ti atoms but also with carbon atoms. Interestingly, the H–C bonds are characterized by the formation of s–p hybrid orbitals. For $H_2$ molecules, multicenter bond formation is accompanied by an increase of the bond length to 2.07 and 1.85 Å for $H_2$ on the surface and at the interstitial site, respectively. On the other hand, placing $H_2$ between two sheets of ${\mathrm{Ti}}_3{C}_2$ leads to dissociation. For all H and $H_2$ complexes the vibrational eigenmodes are calculated. Their frequencies are in the range of 890 to 1610 ${\mathrm{cm}}^{-1}$, which indicates that the bonds are remarkably strong.