自旋多重度Multiplicity (chemistry)

多重度等于在外磁场中能级分裂的数目（总自旋在磁场方向投影 M_S = −S…+S 的个数）。一个未成对电子为二重态（自由基，如 NO、CH₃·）；两个为三重态或单重态；三个平行时为四重态。洪特第一规则指出，在能量相同的轨道中，多重度最高的状态能量最低，因此 O₂、碳原子和氧原子基态都是三重态。

Multiplicity equals the number of levels into which a state splits in a magnetic field, the number of projections M_S = −S…+S. One unpaired electron gives a doublet (a radical such as NO or CH₃·); two give a triplet or a singlet; three parallel give a quartet. Hund's first rule says that among degenerate orbitals the state of highest multiplicity lies lowest, which is why the ground states of O₂ and of carbon and oxygen atoms are triplets.

基态 O₂ 是三重态（³Σg⁻），而大多数有机分子是单重态，二者直接反应自旋禁阻，这正是有机物在空气中能稳定存在的重要原因。光敏剂可把 O₂ 激发为高活性的单线态氧（¹Δg，约 94 kJ/mol），用于光动力疗法。卡宾有单重态和三重态两种，反应立体化学不同：单重态卡宾立体专一地加成到烯烃上，三重态则非立体专一。

Ground-state O₂ is a triplet (³Σg⁻), while most organic molecules are singlets, so their direct reaction is spin-forbidden, a key reason organic matter survives in air. Photosensitisers excite O₂ to highly reactive singlet oxygen (¹Δg, about 94 kJ/mol above), used in photodynamic therapy. Carbenes come as singlets and triplets with different stereochemistry: singlet carbenes add to alkenes stereospecifically, triplets do not.

光谱项符号 ²ˢ⁺¹L_J 中的左上角即多重度。单重态与三重态之间的跃迁自旋禁阻，所以磷光（T₁ → S₀）寿命远长于荧光（S₁ → S₀）。电子顺磁共振只能检测 S > 0 的顺磁性物种；磁化率测量可测定未成对电子数。

Multiplicity is the left superscript in the term symbol ²ˢ⁺¹L_J. Transitions between singlets and triplets are spin-forbidden, so phosphorescence (T₁ → S₀) lasts far longer than fluorescence (S₁ → S₀). Electron paramagnetic resonance detects only paramagnetic species with S > 0, and magnetic susceptibility measurements count unpaired electrons.