[en] The first half of this dissertation studies reactive intermediates which were formed in rare-gas matrices using epr matrix isolation spectroscopy. In particular, the reactions of lithium with small molecules were studied at liquid helium temperatures in argon matrices. A theoretical study of the bonding involved in the Li-H₂O species was studied, as an example of the lithium-Lewis Base reactions, using restricted Hartree-Fock theory. In addition, the theory is used to calculate the change in spin density, relative to the free atom, which occurs when lithium forms a complex with water. Three different basis sets were used in order to determine the sensitivity of the spin density to the basis set selection. The second half of this disseratation involves theoretical studies of the interactions between a proton and rare-gases. The total interaction energy between a proton and one or two helium atoms is resolved into electrostatic, polarization, and charge-transfer energy components. A general model is described for determining the long-range interaction energies in proton rare-gas clusters by separately calculating the individual components