ungepaarter

Ungepaarter is a German adjective meaning not paired. In scientific contexts the term describes electrons or other particles that do not have a partner with opposite spin in the same region of an atom or molecule. The concept is central in chemistry and physics, because unpaired electrons give rise to radicals and to paramagnetic behavior.

In chemistry, unpaired electrons occur in radicals and many reactive intermediates. They often reside in singly occupied molecular orbitals, which can make species highly reactive and influence reaction pathways. Hund’s rule and electron correlation play a role in why certain electronic configurations favor unpaired spins. Common examples include the methyl radical (CH3) with one unpaired electron and the dioxygen molecule in its ground state, O2, which has two unpaired electrons and a triplet spin state. Transition-metal ions such as Cu2+ or Fe3+ can also harbor unpaired electrons in their d-orbitals, contributing to characteristic magnetic properties.

Detection and study of unpaired electrons rely on techniques such as electron paramagnetic resonance (EPR, also called ESR) spectroscopy, which directly probes magnetic moments associated with unpaired spins. Magnetic susceptibility measurements and related spectroscopic methods provide complementary information about the number and environment of unpaired electrons in a system.

Spin states associated with unpaired electrons determine the spin multiplicity: a single unpaired electron gives a doublet state (multiplicity 2), while two unpaired electrons with parallel spins form a triplet state (multiplicity 3). More complex systems can exhibit higher multiplicities and coupling between spins.

See also: radical, paramagnetism, unpaired electron.