Hybrid Orbital Formation and Multicenter Bonding of Hydrogen Atoms and Molecules in Ti3C2${\rm Ti}_{3}{\rm C}_{2}$ 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 Ti3C2${\rm Ti}_{3}{\rm C}_{2}$ MXenes. Hydrogen atoms and H2${\rm H}_2$ molecules form multicenter bonds in Ti3C2${\rm Ti}_{3}{\rm C}_{2}$. On the surface and between two Ti3C2${\rm Ti}_{3}{\rm C}_{2}$ sheets this is limited to the formation of H–Ti bonds. However, H and H2${\rm H}_2$ 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 H2${\rm H}_2$ molecules, multicenter bond formation is accompanied by an increase of the bond length to 2.07 and 1.85 Å for H2${\rm H}_2$ on the surface and at the interstitial site, respectively. On the other hand, placing H2${\rm H}_2$ between two sheets of Ti3C2${\rm Ti}_{3}{\rm C}_{2}$ leads to dissociation. For all H and H2${\rm H}_2$ complexes the vibrational eigenmodes are calculated. Their frequencies are in the range of 890 to 1610 cm−1${\rm cm}^{-1}$, which indicates that the bonds are remarkably strong. [ABSTRACT FROM AUTHOR]