1. Ca²⁺ Reactivity in the Gas Phase. Bonding, Catalytic Effects and Coulomb Explosions

In this chapter we present an overview of what is known today about the reactivity of Ca²⁺ with different species, both from the theoretical and the experimental points of view. We have paid particular attention to the bonding characteristics of Ca²⁺ complexes and to the electron density re-distribution undergone by the neutral interacting with this doubly charged metal, due to the quite intense electric field around it, which results sometimes in, dramatic polarization effects. Another important question is related with the stability of Ca²⁺ complexes in the gas phase, in clear contrast with the instability of analogous complexes involving other metal dications, such as Cu²⁺ or Pb²⁺, which only produce singly charged species when interacting with neutral bases. The last part of this chapter will be devoted to review the different combined experimental and theoretical studies of several reactions involving Ca²⁺. This survey will include a comparison of the reactivity exhibited by Ca²⁺ with respect to the reactivity shown by other divalent metal ions, as well as some preliminary results of the reaction of Ca²⁺ with uracil. Particular attention will be also paid to the catalytic effect of Ca²⁺ in the tautomeric process of these kinds of biochemical compounds.