Investigations of non-covalent bonding: synthesis-aided calculations
Engerer, Laura Kathryn
This work describes three separate projects, for which Density Functional Theory (DFT) computations provide a unifying theme. The DFT approach gives a theoretically sound way to model the chemical and physical properties of a system based its electron density. Although the exact functional has been proven to exist, its form is not completely known and so all calculations are completed with various approximations, which sometimes seriously affect the results. Conventional functionals, for example, fail to account for dispersion corrections adequately, and consequently dispersion-corrected functionals were used extensively in this work. Cation-π interactions involve the largely noncovalent attraction of a cation with a ligand's p- electrons, which are often those of an arene or a heteroarene. These interactions incorporate electrostatic, inductive, and charge transfer effects, and in some cases, dispersion forces. Factors that could affect the strength of cation-p interactions were examined with the use of a simple geometric model. A chief finding was that the relationship between the strength of the interaction and the number of π-bonds involved is not linear. Asymmetric cation-π interactions (i.e., those in which the cations are not in line with the centers of π-electron density) were also examined; despite their relative weakness compared to more symmetric arrangements, they can contribute considerably to the total bonding energy in molecules. The σ- and π-bonding in high-spin manganese(II) allyls was examined with the aid of DFT methods. Comparisons were made to structurally similar magnesium allyls, as neither of these similarly-sized cations (Mn2+/Mg2+) provide ligand field stability to their compounds. The structure and bonding of group 2 and 14 metallocenes were studied with dispersion- corrected functionals. Metallocene compounds of calcium, strontium, and barium commonly display non-linear Cp-M-Cp (Cp = cyclopentadienyl) angles. Of the various explanations for this phenomenon, dispersion interactions between the cyclopentadienyl rings are among the most difficult to model computationally. Presently available DFT methods are not able to provide unambiguous evidence for the source(s) of the non-linear structures.