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Mary T. Rodgers


Cation-Π interactions play important roles in biological structure and function. These interactions arise primarily from ion-quadrupole, ion-induced dipole, and ion-dipole interactions. The strength of a cation-Π interaction depends upon various factors such as the size and electronic structure of the cation, nature of the Π-ligand, and extent of ligation. This dissertation study has focused on achieving a more detailing understand of influence that these factors exert on the strength and specificity of cation-Π interactions via elucidation of the fundamental interactions that contribute to cation-Π interactions in a wide variety of complexes that bind via cation-Π interaction.

The kinetic energy dependence of the collision-induced dissociation (CID) of a wide variety of cation-Π complexes with Xe is studied using guided ion beam tandem mass spectrometry techniques. The cations examined in these studies include: Li+, Na+, K+, Rb+, and Cs+. The ligands investigated include: N-methylaniline, N,N-dimethylaniline, chlorobenzene, bromobenzene, and iodobenzene. The dominant dissociation processes observed for most complexes is simple CID corresponding to endothermic loss of a single neutral ligand to produce the metal cation (or metal cation bound ligand) and neutral ligand. Sequential dissociation of additional ligands is also observed at elevated energies for all complexes containing more than one ligand.

The cross section thresholds for the primary dissociation pathways are interpreted to yield 0 and 298 K BDEs after accounting for the effects of multiple ion-neutral collisions, the kinetic and internal energy distributions of the reactants, and dissociation lifetimes. Density functional theory calculations are performed to obtain model structures, vibrational frequencies, and rotational constants for the neutral Π-ligands and alkali metal-ΠΠ-ligand complexes. The relative stabilities of the various conformations of these neutral Π-ligands and alkali metal-Π-ligand complexes as well as theoretical alkali metal-Π-ligand BDEs are determined from single point energy calculations using the optimized geometries. Reasonably good agreement between the measured and computed BDEs of cation-Π complexes are found in most cases. The absolute BDEs of the cation-Π complexes are observed to monotonically decrease as the size of the alkali metal cation increases reflects the electrostatic nature of the cation-Π interactions. Previously established binding parameter model analyses were extended by assuming the linear correlation of fundamental interactions i.e., ion-quadrupole, ion-induced dipole, and ion-induced dipole interactions with quadrupole moment, polarizability, and dipole moment component perpendicular to the plane of the Π-ligand. These analyses performed using the measured binding energies of cation-Π complexes combined with theoretical studies has allowed estimates for the quadrupole moments of the Π-ligands to be determined and the contributions of fundamental interactions to cation-Π interactions to be quantitatively assessed. Effects of the size of the metal cation and the nature of the Π-ligand were examined by investigating a wide variety of cation-Π complexes. Ion-dipole interactions were found to increase with amination on B but decrease with increasing extent of methylation on amino substituent. Inductive effects of the halogen substituent result in cation-Π interactions that are weaker than to B and were found to correlate with the relative electron withdrawing ability of the halogen substituent. Extended binding parameter model analyses were used to investigate the influence of the electronic structure of the metal cation, multiple electron withdrawing substituents on the Π-ligand, and sequential ligation on the strength of cation-Π interactions.

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