Chemical Interactions and Spin Structure in (O2)4: Implications for the ε-O2 Phase

The chemical interactions and spin structure of (O2)4 in its ground singlet state are analyzed by means of Quantum Chemical Topology descriptors. The energetic contributions of the Interacting Quantum Atoms approach are used to obtain information about the class of interactions displayed along the dissociation path of (O2)4. The exchange-correlation contribution to the binding energy is non-negligible for the O2-O2 interactions at intermolecular distances close to those found for the pressure induced ε phase of solid (O2) and this strengthening of the intermolecular bonding is built up from a simultaneous weakening of the intramolecular bond. This result is of interest in connection with the observed softening of the IR vibron frequency in the lower pressure range of the ε phase. The spin structure in the real space along the dissociation process is interpreted with the help of the so-called electron number distribution functions. At large distances, the four triplet O2 molecules are arranged in a way consistent with an antiferromagnetic structure, whereas at short distances, a significant spin redistribution is driven by the exchange process and it involves a propensity toward a null magnetic moment per molecule. Such probability behavior can be related with the magnetic evolution of solid oxygen across the δ → ε phase transition. Additional calculations of (O2)4 excited states support the conclusion that the relative stabilization and magnetic features of the ground singlet state are due to the onset of the new intermolecular bonds, and not to an exclusive modification of the electronic character within the O2 molecules.