Combined photoelectron spectroscopy and ab initio study of the hypermetallic Al3C molecule

The chemical structure and bonding of the hypermetallic ${\mathrm{Al}}_{\mathrm{3}}\mathrm{C}$ and ${\mathrm{Al}}_{\mathrm{3}}{\mathrm{C}}^{\mathrm{-}}$ species have been studied by photoelectron spectroscopy and ab initio calculations. ${\mathrm{Al}}_{\mathrm{3}}{\mathrm{C}}^{\mathrm{-}}$ is found to have a planar triangular ${\mathrm{(D}}_{\text{3h}},$ ${}^1{A}_1^{\prime })$ structure (when averaged over zero-point vibrational modes) and ${\mathrm{Al}}_{\mathrm{3}}\mathrm{C}$ is found to have a triangular distorted planar structure ${\mathrm{(C}}_{\text{2v}},$ ${}^2{B}_2)$ with one elongated Al–C bond. Four peaks in the photoelectron spectra of ${\mathrm{Al}}_{\mathrm{3}}{\mathrm{C}}^{\mathrm{-}}$ were identified at 2.56, 2.69, 3.23, and 4.08 eV. Assignment of the observed features was made on the basis of the ab initio calculations. The experimental adiabatic electron affinity of ${\mathrm{Al}}_{\mathrm{3}}\mathrm{C}$ was measured to be $\text{2.56±0.06\hspace{0.05em}}\mathrm{eV},$ compared to 2.47 eV calculated at the $\mathrm{CCSD(T)+OVGF/6-311+G(2df)}$ level of theory. The excellent agreement between the calculated and experimental electron affinity, vibrational frequencies, and excitation energies allowed us to completely elucidate the geometrical and electronic structure of the ${\mathrm{Al}}_{\mathrm{3}}\mathrm{C}$ molecule and its anion.