A Gaussian-3 (G3) theoretical study of the interactions between alkali metal cations and polyhydroxyl ligands

Alkali and Alkaline-Earth Cations in Complexes with Small Bioorganic Ligands: Ab Initio Benchmark Calculations and Bond Energy Decomposition

Herein, we report a computational database for the complexes of alkali [Li(I), Na(I), K(I)] and alkaline-earth [Be(II), Mg(II) and Ca(II)] cations with 25 small ligands with varying charge and donor atoms ("O", "N", and "S") that provides geometries and accurate bond energies useful to analyze metal-ligand interactions in proteins and nucleic acids. The role of the ligand→metal charge transfer, the equilibrium bond distance, the electronegativity of the donor atom, the ligand polarizability, and the relative stability of the complexes are discussed in detail. The interacting quantum atoms (IQA) method is used to decompose the binding energy into electrostatic and quantum mechanical contributions. In addition, bond energies are also estimated by means of multipolar electrostatic calculations. No simple correlation exists between bond energies and structural/electronic descriptors unless the data are segregated by the type of ligand or metal. The electrostatic attraction of some molecules (H₂ O, NH₃ , CH₃ OH) towards the metal cations is well reproduced using their (unrelaxed) atomic multipoles, but the same comparison is much less satisfactory for other ligands (e. g. benzene, thiol/thiolate groups, etc.). Besides providing reference structures and bond energies, the database can contribute to validate molecular mechanics potentials capable of yielding a balanced description of alkali and alkaline-earth metals binding to biomolecules.

Interactions between alkaline earth cations and oxo ligands. DFT study of the affinity of the Mg²+ cation for phosphoryl ligands

DFT (B3LYP/6-31+G(d)) calculations of Mg(2+) affinities for a set of phosphoryl ligands were performed. Two types of ligands were studied: a set of trivalent [O = P(R)] and a set of pentavalent phosphoryl ligands [O = P(R)(3)] (R = H, F, Cl, Br, OH, OCH(3), CH(3), CN, NH(2) and NO(2)), with R either bound directly to the phosphorus atom or to the para position of a phenyl ring. The affinity of the Mg(2+) cation for the ligands was quantified by means of the enthalpy for the substitution of one water molecule in the [Mg(H(2)O)(6)](2+) complex for a ligand. The enthalpy of substitution was correlated with electronic and geometric parameters. Electron-donor groups increase the interaction between the cation and the ligand, while electron-acceptor groups decrease the interaction enthalpy.

Computational s-Block Thermochemistry with the Correlation Consistent Composite Approach

The correlation consistent composite approach (ccCA) is a model chemistry that has been shown to accurately compute gas-phase enthalpies of formation for alkali and alkaline earth metal oxides and hydroxides (Ho, D. S.; DeYonker, N. J.; Wilson, A. K.; Cundari, T. R. J. Phys. Chem. A 2006, 110, 9767). The ccCA results contrast to more widely used model chemistries where calculated enthalpies of formation for such species can be in error by up to 90 kcal mol-1. In this study, we have applied ccCA to a more general set of 42 s-block molecules and compared the ccCA DeltaHf values to values obtained using the G3 and G3B model chemistries. Included in this training set are water complexes such as Na(H2O)n+ where n = 1 - 4, dimers and trimers of ionic compounds such as (LiCl)2 and (LiCl)3, and the largest ccCA computation to date: Be(acac)2, BeC10H14O4. Problems with the G3 model chemistries seem to be isolated to metal-oxygen bonded systems and Be-containing systems, as G3 and G3B still perform quite well with a 2.7 and 2.6 kcal mol-1 mean absolute deviation (MAD), respectively, for gas-phase enthalpies of formation. The MAD of the ccCA is only 2.2 kcal mol-1 for enthalpies of formation (DeltaHf) for all compounds studied herein. While this MAD is roughly double that found for a ccCA study of >350 main group (i.e., p-block) compounds, it is commensurate with typical experimental uncertainties for s-block complexes. Some molecules where G3/G3B and ccCA computed DeltaHf values deviate significantly from experiment, such as (LiCl)3, NaCN, and MgF, are inviting candidates for new experimental and high-level theoretical studies.

Accurate Enthalpies of Formation of Alkali and Alkaline Earth Metal Oxides and Hydroxides: Assessment of the Correlation Consistent Composite Approach (ccCA)

Computing the enthalpies of formation for alkali metal and alkaline earth metal oxides (M(x)O) and hydroxides [M(OH)(n)] using the Gaussian-n (Gn) and Weismann-n (Wn) ab initio model chemistries is difficult due to an improper treatment of core-valence electron correlation effects. Using a new model chemistry called the correlation consistent Composite Approach (ccCA), enthalpies of formation were determined for eight different alkali/alkaline earth metal oxides and hydroxides. Unlike the Gn and Wn model chemistries, which must be modified to properly account for core-valence electron correlation, the standard implementations of the ccCA provide acceptable results, and all enthalpies of formation obtained with the ccCA are within the accepted range of recommended values.