An experimental and Ab initio study of the nature of the binding in gas-phase complexes of sodium ions

Fourier transform ion cyclotron resonance (FT-ICR) ligand exchange equilibrium experiments have been used to establish a relative scale of sodium binding free energies of about fifty organic molecules. Ab initio calculations yield accurate enthalpies and entropies of complexation for a new set of 30 molecules. These calculations establish an absolute basis for the relative experimental free energy scale. In addition, they provide structural information for the complexes which permits considerable insight into the nature of sodium ion binding. We found that when the binding site is a first row atom, the sodium ion aligns with the molecular dipole axis in order to maximize charge-dipole electrostatic interactions. Strong deviations from this behavior occur when the ion is attached to a heavier atom such as sulfur, chlorine or bromine. For flexible molecules such as the isomers of butyl chloride, there are several isomers of low energy, and differences exist between the enthalpy and free energy orders of stability. Finally, sodium ion affinities have been obtained for several aromatic molecules which lend support to the importance of charge-quadrupole interactions in such cation-pi complexes.