The Alkali and Alkaline Earth Metals

Abstract

The first two groups of the Periodic Table have in common that their members exist principally in solution as cations, and their study is thus that of solute-solvent interactions and of ion-ion interactions. The elements Li, Be, and Mg have in addition a covalent chemistry and their covalently bonded compounds are also studied. From the point of view of NMR spectroscopy, all these elements have nuclei with quadrupole moments, though 6Li and 133Cs have two of the smallest moments known. In the case of the group I ions, the ion-solvent interactions are relatively weak, so that we think of the ions as existing in spherical s states with low field gradients. We would thus expect relaxation times to be a maximum in these solutions and modified by the interaction with the solvent. Weak interaction means fast exchange and so only singlets will be observed giving data for the ensemble average of all the possible states experienced by the ions. Where strong interactions occur, these are immediately detected by the metal NMR. In the case of the smaller group II ions, a definite solvation sheath is formed which can be detected by proton NMR for Be or Mg only, though it probably persists for the heavier ions, only being differentiated from the group I ions by the time scale of the solvent exchange.