delocalization

Delocalization is the spreading of a property, most commonly electronic density or a wavefunction, over multiple atoms rather than being confined to a single atom or bond. In chemistry, it refers to electrons that are shared among several atoms in a molecule rather than localized between a specific pair.

In chemistry, delocalization is closely connected to resonance structures; the real molecule is a hybrid of multiple contributing structures. Delocalized pi electrons stabilize molecules (aromaticity) and influence color, acidity, basicity, and reactivity. Common examples include benzene, where six pi electrons are distributed around a ring, the allyl cation and allyl radical, and many polyenes.

In solid-state and materials science, electron delocalization extends over many lattice sites, enabling electrical conduction. In metals, valence electrons form a conduction band described by Bloch waves; in insulators, localization suppresses transport. Delocalization also appears in conjugated polymers and organic semiconductors, which are important for dyes, photovoltaics, and organic light-emitting devices.

Theoretical treatments include molecular orbital theory and the Hückel method for conjugated systems; density functional theory and other quantum-chemical approaches model delocalization by the spread of electron density or by using delocalized molecular orbitals.

Delocalization can be limited by energy gaps, geometric constraints, or disorder that leads to localization (Anderson localization). Experimental indicators include red or blue shifts in UV-Vis absorption, aromatic stabilization, and characteristic NMR shielding patterns.