Spin-philicity and spin-donicity as auxiliary concepts to quantify spin-catalysis phenomena

For molecular systems susceptible to undergo a change of their spin state as a result of a chemical reaction with a given reactant, the spin-polarized density functional theory is used to define the concepts of "spin-philicity" (ω_S ⁺) and "spin-donicity" (ω_S ^-) as global reactivity indexes. They are defined as the maximum energy change when a molecular system acquires or donates a spin number ΔN_S to increase (ω_S ⁺) or decrease (ω_S ^-) its spin multiplicity. The spin transformation of chemically reactive species induced by the interaction of these molecules with external spin carriers-a phenomenon known as spin catalysis-is discussed on the basis of an absolute scale for ω_S ⁺ and ω_S ^-. As an illustration of the method, a selection of paramagnetic and diamagnetic molecules, commonly used as spin catalyst, is classified within this scale and the hierarchy obtained is compared with the available experimental information.