Search for: All records

Abstract The pathways for the reactions of aluminum oxide cluster ions with ethane have been measured. For selected ions (Al₂O⁺, Al₃O₂⁺, Al₃O₄⁺, Al₄O₇⁺) the structure of the collisionally‐stabilized reaction intermediates were explored by measuring the photodissociation vibrational spectra from 2600 cm⁻¹–3100 cm⁻¹. Density functional theory was used to calculate features of the potential energy surfaces for the reactions and the vibrational spectra of intermediates. Generally, more than one isomer contributes to the observed spectrum. The oxygen‐deficient clusters Al₂O⁺and Al₃O₂⁺have large C−H activation barriers, so only the entrance channel complexes in which intact C₂H₆binds to aluminum are observed. This interaction leads to a substantial (~200 cm⁻¹) red shift of the C−H symmetric stretch in ethane, indicating significant weakening of the proximal C−H bonds. In Al₃O₄⁺, the complex formed by interactions with three C₂H₆is investigated and, in addition to entrance channel complexes, the C−H activation intermediate Al₃O₄H⁺(C₂H₅)(C₂H₆)₂is observed. For oxygen‐rich Al₄O₇⁺, the C₂H₆is favored to bind at an aluminum site far from the reactive superoxide group, reducing the reactivity. As expected, oxygen‐rich species and open‐shell cluster ions have smaller barriers for C−H bond activation, except for Al₃O₄⁺which is predicted and observed to be reactive. 

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. 

Vibrational spectra of a series of gas-phase metal 1+ and 2+ ions solvated by acetone molecules are collected to investigate how the metal charge, number of solvent molecules and nature of the metal affect the acetone. The spectra of Cu+(Ace)(N2)2, Cu+(Ace)4, and M2+(Ace)4, where M = Co, Ni, Cu, and Zn are measured via photodissociation by monitoring fragment ion signal as a function of IR wavenumber. The spectra show a red shift of the C=O stretch and a blue shift of the C–C antisymmetric stretch. DFT calculations are carried out to provide the simulated spectra of possible isomers to be compared with the observed vibrational spectra, and specific structures are proposed. The red shift of the C=O stretch increases as the number of acetone molecules decreases. Higher charge on the metal leads to a larger red shift in the C=O stretch. Although all of the M2+ complexes have very similar red shifts, they are predicted to have different geometries due to their different electron configurations. Unexpectedly, we find that the calculated red shift in the C=O stretch in M+/2+(Ace) is highly linearly correlated with the ionization energy of the metal for a wide range of metal cations and dications. 

The interaction between aluminum cations and acetone is studied in the gas phase via photodissociation vibrational spectroscopy from 1100 to 2000 cm-1. Spectra of Al+(acetone)(N2) and ions with the stoichiometry of Al+(acetone)n (n=2-5) were measured. The experimental results are compared to DFT calculated vibrational spectra to determine the structures of the complexes. The spectra show a red shift of the C=O stretch and a blue shift of the CCC stretch which decrease as the size of the clusters increases. The calculations predict that the most stable isomer for n≥3 is a pinacolate in which oxidation of the Al+ enables reductive C-C coupling between two acetone ligands. Experimentally, pinacolate formation is observed for n=5, as evidenced by a new peak observed at 1185 cm-1 characteristic of the pinacolate C-O stretch.