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 equilibr ium experiments have been used to establish a relative scale of sodium bind ing 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 infor mation 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 maxi mize charge-dipole electrostatic interactions. Strong deviations from this behavior occur when the ion is attached to a heavier atom such as sulfur, c hlorine or bromine. For flexible molecules such as the isomers of butyl chl oride, there are several isomers of low energy, and differences exist betwe en the enthalpy and free energy orders of stability. Finally, sodium ion af finities have been obtained for several aromatic molecules which lend suppo rt to the importance of charge-quadrupole interactions in such cation-pi co mplexes.