自旋-自旋耦合J-coupling

一组氢若与 n 个化学等价的相邻氢耦合，其信号裂分为 n+1 重峰，各峰强度比符合二项式系数（帕斯卡三角）：双峰 1:1，三重峰 1:2:1，四重峰 1:3:3:1，五重峰 1:4:6:4:1。如乙醇 CH₃CH₂OH 中 CH₃ 为三重峰、CH₂ 为四重峰（OH 通常因快速交换不耦合）。若与两组不等价的氢耦合，则为 (n+1)(m+1) 重峰，如双二重峰 dd。

A set of protons coupled to n equivalent neighbours splits into n+1 lines with intensities following binomial coefficients (Pascal's triangle): doublet 1:1, triplet 1:2:1, quartet 1:3:3:1, quintet 1:4:6:4:1. In ethanol, CH₃CH₂OH, the CH₃ is a triplet and the CH₂ a quartet (the OH usually does not couple because it exchanges rapidly). Coupling to two inequivalent sets gives (n+1)(m+1) lines, such as a doublet of doublets (dd).

²J（同碳）约 −10 至 −15 Hz；³J（邻碳）一般 6–8 Hz，按卡普拉斯方程随二面角变化：二面角 0° 或 180° 时最大，90° 时接近零。因此可区分环己烷中直立-直立（约 10–13 Hz）和直立-平伏（约 2–5 Hz）关系，以及烯烃顺式（约 6–12 Hz）和反式（约 12–18 Hz）构型；芳环邻位约 7–9 Hz、间位约 2–3 Hz、对位 < 1 Hz。

Geminal ²J is about −10 to −15 Hz; vicinal ³J is typically 6–8 Hz and varies with dihedral angle by the Karplus equation, largest at 0° or 180° and near zero at 90°. This distinguishes axial–axial (about 10–13 Hz) from axial–equatorial (about 2–5 Hz) protons in cyclohexanes and cis (about 6–12 Hz) from trans (about 12–18 Hz) alkenes; aromatic ortho coupling is about 7–9 Hz, meta 2–3 Hz and para below 1 Hz.

1951 年古托夫斯基、麦考尔和斯利克特以及哈恩和麦克斯韦分别观测到与磁场无关的谱线裂分，提出耦合机制经由成键电子传递。1959 年卡普拉斯从理论上推导出邻位耦合常数与二面角的关系，卡普拉斯后来因多尺度模拟获 2013 年诺贝尔化学奖。

In 1951 Gutowsky, McCall and Slichter, and independently Hahn and Maxwell, observed field-independent splittings and proposed coupling transmitted through bonding electrons. In 1959 Karplus derived the relation between vicinal coupling and dihedral angle; he later shared the 2013 Nobel Prize in Chemistry for multiscale modelling.