Structure and vibrational spectroscopy of complexes of Al+ and Al2+ with ethane.

The binding motifs of clusters of Al+ and Al2+ with ethane, Alx+(C2H6)n (x = 1, 2; n = 1-3), are determined using vibrational photodissociation spectroscopy in the C-H stretching region (2550-3100 cm-1) in conjunction with spectra calculated using density functional theory. The relative energies of candidate structures are determined with the B3LYP-D3 and ωB97X-D density functionals and the 6-311++G(d,p) basis set. Local mode Hamiltonian calculations are better able to reproduce the spectra than scaled harmonic calculations, due to contributions from bending overtones and combination bands. Vibrational photodissociation spectra show a red shift in the stretching frequencies of C-H bonds that are proximate to the cation. This red shift decreases as the number of ethanes increases. For Al+(C2H6)n (n = 1-3), side-on (T-shaped) binding of the metal is preferred to end-on binding, and subsequent ligands bind on the same side of the cation. Similarly, for Al2+(C2H6)n (n = 1-3), T-shaped configurations in which the C-C and Al-Al bonds are approximately perpendicular and the ethane binds side-on to the Al2+ are preferred. In Al2+(C2H6)n (n = 1-3) complexes, intense bands are observed, which are due to overtones and combinations of symmetric deformations in Fermi resonance with the red-shifted C-H stretches.