Aluminum monocation basicity and affinity scales

Al+ Activates Acetone to Form Pinacolate

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.

On the Significance of Lone Pair/Lone Pair and Lone Pair/Bond Pair Repulsions in the Cation Affinity and Lewis Acid/Lewis Base Interactions

Interaction of H₂O, H₂S, H₂Se, NH₃, PH₃, and AsH₃ with cations H⁺, CH₃ ⁺, Cu⁺, Al⁺, Li⁺, Na⁺, and K⁺ was studied from the energetic and structural viewpoint using B3LYP/6-311++G(d,p) method. The charge transfer from the Lewis bases to the cations reduces lone pair/lone pair (LP/LP) repulsion in H₂O, H₂S, and H₂Se and LP/bond pair (LP/BP) repulsion in NH₃, PH₃, and AsH₃. In parallel, changes in the H-M-H angles (M = O, S, Se, N, P, and As) are observed. The change in the H-M-H angle during the interactions was proportional to the amount of charge transferred from the bases to the cations and electron density (ρ) at the molecule/cation bond critical point. Also, the opposite trend for proton affinities of these two families, that is, NH₃ > PH₃ > AsH₃ and H₂O < H₂S < H₂Se, was interpreted on the basis of LP/BP repulsion in their neutral and protonated forms. Interaction of the Lewis bases with neutral Lewis acids including BeH₂, BeF₂, and BH₃ was studied energetically and structurally. The calculated energies for interactions of H₂O and NH₃ with BeH₂, BeF₂, and BH₃ are larger than the corresponding values for H₂S, H₂Se, PH₃, and AsH₃. This difference was interpreted on the basis of the lower stability of H₂O and NH₃ because of large LP/LP and LP/BP repulsion in H₂O and LP/BP repulsion in NH₃.

Directionality of Cation/Molecule Bonding in Lewis Bases Containing the Carbonyl Group

Relationship between the C═O-X⁺ (X = H, Li, Na, K, Al, Cu) angle and covalent characteristic of the X⁺-M (M = CH₂O, CH₃CHO, acetone, imidazol-2-one (C₂H₂N₂O), cytosine, γ-butyrolactone) was investigated, theoretically. The calculated electron densities ρ at the bond critical points revealed that the covalency of the M-X⁺ interaction depended on the nature of the cation and varied as H⁺ > Cu⁺ > Al⁺ > Li⁺ > Na⁺ > K⁺. The alkali cations tended to participate in electrostatic interactions and aligned with the direction of the molecule dipole or local dipole of C═O group to form linear C═O-X geometries. Because of overlapping with lone-pair electrons of the sp² carbonyl oxygen, the H⁺ and Cu⁺ formed a bent C═O-X angle. Al⁺ displayed an intermediate behavior; the C═O-Al angle was 180° in [CH₂O/Al]⁺ (mainly electrostatic), but when the angle was bent (146°) under the effect of local dipole of an adjacent imine group in cytosine, the covalency of the CO-Al⁺ interaction increased. The C═O-X angles in M/X⁺ adduct ions were scanned in different O-X bond lengths. It was found that the most favorable C═O-X angle depended on the O-X bond length. This dependency was attributed to variation of covalent and electrostatic contributions with O-X distance. In addition, the structures of [CH₂S/X]⁺ and [CH₂Se/X]⁺ were studied, and only bent C═S-X and C═Se-X angles were obtained for all cations, although the dipole vectors of CH₂S and CH₂Se coincide with the C═S and C═Se bonds. The bending of the C═S-X and C═Se-X angles was attributed to the covalent characteristic of S-X and Se-X interactions due to high polarizability of S and Se atoms.

Effect of the Number of Methyl Groups on the Cation Affinity of Oxygen, Nitrogen, and Phosphorus Sites of Lewis Bases

The effect of number of CH₃ groups (n) on the cation (H⁺, Li⁺, Na⁺, Al⁺, CH₃⁺) affinity, polarizability, and dipole moment of 14 simple molecules was investigated. Linear correlations were observed between the polarizabilities and the number of methyl groups. The variations of the cation affinities and dipole moments with the number of methyl groups (n) were not linear, and a quadratic function was proposed for obtaining a good fit of the experimental data. Also, because the proton affinities (PA), lithium cation affinities (LCA), sodium cation affinities (SCA), aluminum cation affinity (AlCA), and methyl cation affinity (MCA) varied quadratically with polarizabilities (α), a formula of the form [cation affinities] = a + bα + cα² was proposed. After correction of the PAs, LCAs, SCAs, AlCA, and MCA for the dipole/charge interaction (E_μ), linear relationships were observed between the corrected cation affinities and n or α. The contribution of E_μ to PA and MCA was small (less than 20%), and its contribution to LCA and SCA was large (>50%). The electrostatic contribution to AlCA was considerable (20-50%); however, it was smaller than the electrostatic contribution to LCA and SCA.

Cationic Noncovalent Interactions: Energetics and Periodic Trends

In this review, noncovalent interactions of ions with neutral molecules are discussed. After defining the scope of the article, which excludes anionic and most protonated systems, methods associated with measuring thermodynamic information for such systems are briefly recounted. An extensive set of tables detailing available thermodynamic information for the noncovalent interactions of metal cations with a host of ligands is provided. Ligands include small molecules (H2, NH3, CO, CS, H2O, CH3CN, and others), organic ligands (O- and N-donors, crown ethers and related molecules, MALDI matrix molecules), π-ligands (alkenes, alkynes, benzene, and substituted benzenes), miscellaneous inorganic ligands, and biological systems (amino acids, peptides, sugars, nucleobases, nucleosides, and nucleotides). Hydration of metalated biological systems is also included along with selected proton-based systems: 18-crown-6 polyether with protonated peptides and base-pairing energies of nucleobases. In all cases, the literature thermochemistry is evaluated and, in many cases, reanchored or adjusted to 0 K bond dissociation energies. Trends in these values are discussed and related to a variety of simple molecular concepts.