化学位移Chemical shift

外磁场 B₀ 使核外电子产生环流和感应磁场，削弱原子核实际感受到的磁场 B_eff = B₀(1 − σ)，σ 为屏蔽常数。核周围电子云密度越大，屏蔽越强，共振越在高场（δ 小）；电负性基团吸电子使去屏蔽，δ 增大。δ = (ν_样 − ν_TMS)/ν_仪器 × 10⁶，与仪器磁场强度无关，便于比较。苯环 π 电子环流产生的各向异性效应使芳环氢 δ 约 7–8，醛氢 δ 约 9–10；炔氢处于三键屏蔽区，δ 约 2–3。

The external field B₀ makes surrounding electrons circulate and induce a field that reduces the field felt by the nucleus, B_eff = B₀(1 − σ), with shielding constant σ. Higher electron density means stronger shielding and resonance at high field (small δ); electron-withdrawing, electronegative groups deshield and raise δ. δ = (ν_sample − ν_TMS)/ν_instrument × 10⁶ is independent of field strength, making spectra comparable. Ring currents of aromatic π electrons put aromatic protons at about 7–8 ppm and aldehyde protons at 9–10, while acetylenic protons lie in the shielding cone of the triple bond at about 2–3.

TMS 0；烷基 CH₃ 0.9、CH₂ 1.3、CH 1.5；与羰基相连 CH 2.0–2.5；苄位 2.3；与 N 相连 2.2–2.9；与 O 相连（醇、醚）3.3–4.0；酯的 OCH 3.7–4.1；烯氢 4.5–6.5；芳氢 6.5–8.5；醛 9–10；羧酸 10–13；醇 OH 和胺 NH 变化大（1–5，受浓度和氢键影响，可用 D₂O 交换识别）。¹³C 化学位移范围约 0–220 ppm，羰基碳 160–220。

TMS 0; alkyl CH₃ 0.9, CH₂ 1.3, CH 1.5; next to carbonyl 2.0–2.5; benzylic 2.3; next to N 2.2–2.9; next to O in alcohols and ethers 3.3–4.0; ester OCH 3.7–4.1; vinylic 4.5–6.5; aromatic 6.5–8.5; aldehyde 9–10; carboxylic acid 10–13; and alcohol OH and amine NH widely variable (1–5, depending on concentration and hydrogen bonding, identified by D₂O exchange). ¹³C shifts span about 0–220 ppm, with carbonyl carbons at 160–220.

1949–1950 年美国物理学家普罗克特和虞福春在测量硝酸铵中 ¹⁴N 的核磁共振时，发现 NH₄⁺ 和 NO₃⁻ 中的氮核共振频率不同；奈特发现金属中的奈特位移；1951 年阿诺德等观测到乙醇中三种不同的 ¹H 信号（CH₃、CH₂、OH），积分比 3:2:1，从此 NMR 成为化学家确定分子结构的强大工具。

In 1949–50 the American physicists Proctor and Fu Chun Yu, measuring ¹⁴N NMR in ammonium nitrate, found that nitrogen in NH₄⁺ and NO₃⁻ resonates at different frequencies, and Knight found the Knight shift in metals. In 1951 Arnold and colleagues observed three distinct ¹H signals in ethanol, CH₃, CH₂ and OH, in a 3:2:1 ratio, and NMR became a powerful tool for determining structure.