Quantum superposition phenomena in chemistry within the perspective of quantum information theory

In light of the first quantum revolution, our understanding of chemical bonding was enhanced by explaining covalent bonds through a quantum superposition of overlapping atomic orbitals between two atoms. On the other hand, the second quantum revolution showed that quantum superposition and its special forms, such as quantum correlations, are essential for the deep understanding of some physical processes, and they can also be used as a resource, like energy, in emerging quantum technologies. This thesis aims to delve into understanding chemical phenomena using the latest tools of quantum information theory that drive the second quantum revolution. In the first part, we introduced the concept of orbital discord from the perspective of the fermionic quantum information theory, which enables us to present the first proper and additive decomposition of the total orbital correlations into its classical and quantum parts. Our analysis of three representative molecules showed that in some instances, quantum orbital correlations can surpass their classical counterparts and survive even without orbital entanglement. In the second part, we investigate the concept of aromaticity, a multi-atom bonding phenomenon arising from the delocalization of electrons in closed-loop atom structures. To do that, we describe the delocalization of electrons in the $\pi$-electronic structures of archetypal monocyclic aromatic molecules as the superposition in overlapping atomic orbitals. Our findings reveal that the biorthogonal representation of atomic orbitals accurately captures the aromaticity order of these molecules, whereas its superposition in nonorthogonal representation mostly falls short due to neglecting the overlaps between atomic orbitals.