spinpaired

Spinpaired refers to a state in which all electrons in an atom, molecule, or solid occupy orbitals in pairs with opposite spins, giving a total electronic spin S of zero and a singlet spin multiplicity (2S+1 = 1). In such spin-paired systems, the overall magnetic moment is zero, and the material is typically diamagnetic.

In chemistry and physics, spin pairing describes closed-shell configurations where electrons fill orbitals with opposite spins according to the Pauli exclusion principle and Aufbau principle. Open-shell species, by contrast, contain unpaired electrons and can be paramagnetic or exhibit magnetic ordering. In computational chemistry, spin pairing is associated with restricted (closed-shell) methods, which assume paired electrons in each spatial orbital, whereas unrestricted methods allow spin polarization for open-shell states. The term is often used descriptively, with speakers noting that a molecule is spinpaired when all electrons are paired rather than radical.

Spin pairing also influences spectroscopic properties: diamagnetic materials are typically spinpaired, and paramagnetic behavior arises from unpaired electrons that can be detected by techniques such as electron spin resonance (ESR or EPR).

Common spinpaired systems include noble gases and many small molecules such as H2, N2, CH4, and CO2, as well as most closed-shell metal complexes in low-spin states. An important exception is molecular oxygen (O2), whose ground state has two unpaired electrons and is paramagnetic.

See also: spin multiplicity, closed shell, diamagnetism, electron spin resonance.