Bruker 400 MHz NMR Parameters | Bruker 400 MHz 核磁参数设定与谱图解析
1. 脉冲傅里叶核磁基本原理
- 基本概念：自旋在外磁场中产生能级分裂（Larmor 进动）；宏观磁化矢量。
- 脉冲激发与 FT：射频脉冲将磁化扳转到横向平面 → 采集 FID → 傅里叶变换得频域谱。
- 基本步骤：锁场 (
lock) → 匀场 (topshim) → 自动增益 (rga) → 施加射频脉冲 (P1) → 采集 FID (TD, SW, D1) → 累加 (NS, DS) → 傅里叶变换 → 相位/基线校正。
2. 常用参数含义与选定
| 参数 | 含义 | 典型值/选定方法 |
|---|---|---|
| NS | 扫描次数 | 信噪比 ∝ √NS；氢谱一般 16，碳谱一般 300 以上 |
| DS | 空扫次数 | 4 次，使样品充分弛豫稳态及探头温度稳定，不保存数据 |
| SW | 谱宽 (ppm) | 多数情形下氢谱 ~15 ppm；碳谱 ~240 ppm |
| O1P | 射频中心频率 (ppm) | 通常在 6.2 ppm 附近；溶剂压制时设于溶剂峰；1D-NOE 时设于受激峰（一些情形下需要保持 O1P 不变，而调整 SPOFFS2 值以激发特定峰） |
| TD | 时域点数 | 分辨率 = 谱宽 (Hz)/(TD/2)；一般为 65536 |
| D1 | 弛豫延迟 (s) | 需 ≥ 5×T₁；氢谱 2–5 s，精确定量 5–10 s；碳谱 2–5 s，定量 ≥ 10 s |
| P1 | 90° 脉冲宽 (μs) | 6–15 μs（400 MHz，需校准） |
| O2P | 第二通道中心频率 (ppm) | 一般在使用二维谱或去耦谱时使用 |
| P12 | 去耦通道 90° 脉宽 (μs) | 60–100 μs（需校准） |
| PLDB24 | 去耦功率 (dB) | 20–30 dB，从低功率优化 |
| L30 | 循环计数器 | 溶剂压制时指定压制峰数目（如 WET 序列） |
| TE | 温度 (K) | 一般情形下无需调整 |
| RO | 样品旋转频率 (Hz) | 一般为 0，即样品不旋转。二维谱/去耦谱不能旋转 |
3. 谱图解析
基本流程：基线/相位校正 → 化学位移校正 → 峰识别（标位移、求积分、裂分判定、积分的线性校正） → 结构推断（官能团、连接关系）与定量分析（积分比等）。
- 氢谱：利用化学位移、积分、裂分形式与裂分常数指认不同类型氢原子；注意溶剂峰、杂质峰（如水）和交换性氢（如羟基）的特征。
- 碳谱：需要合理利用窗函数（如 LB = 0.3–1 Hz）以平衡分辨率和信噪比；利用 DEPT 等实验区分碳的级数。
- 硼谱：需要合理利用线性预测功能对核磁管本身的信号进行处理，以获得更清晰的谱图。
- 二维谱：
- COSY：氢-氢邻接关系 (³J)。
- HSQC：直接相连的 ¹H–¹³C。
- HMBC：远程 2–3 键相关，连接片段。
- NOESY：空间接近 (< 5 Å)，立体构型或混合物互作。
1. Basic Principles of Pulsed FT NMR
- Concepts: Nuclear spins undergo Zeeman splitting in an external magnetic field (Larmor precession); macroscopic magnetization vector.
- Pulse excitation and FT: RF pulse tips magnetization into the transverse plane → FID acquisition → Fourier transform to frequency-domain spectrum.
- Workflow: Lock (
lock) → Shim (topshim) → Auto-gain (rga) → RF pulse (P1) → FID acquisition (TD, SW, D1) → Accumulation (NS, DS) → FT → Phase/baseline correction.
2. Key Parameters
| Parameter | Meaning | Typical Value / Selection |
|---|---|---|
| NS | Number of scans | S/N ∝ √NS; ¹H typically 16, ¹³C ≥ 300 |
| DS | Dummy scans | 4, to allow steady-state relaxation and probe temperature stabilization; data not saved |
| SW | Spectral width (ppm) | ¹H ~15 ppm; ¹³C ~240 ppm in most cases |
| O1P | Carrier frequency offset (ppm) | Usually near 6.2 ppm; set to solvent resonance for solvent suppression; set to target resonance for 1D-NOE (in some cases O1P is held constant while SPOFFS2 is adjusted to excite the desired peak) |
| TD | Time-domain data points | Resolution = SW (Hz)/(TD/2); commonly 65536 |
| D1 | Relaxation delay (s) | ≥ 5×T₁; ¹H 2–5 s (5–10 s for quantitation); ¹³C 2–5 s (≥ 10 s for quantitation) |
| P1 | 90° pulse width (μs) | 6–15 μs (400 MHz; requires calibration) |
| O2P | Second channel carrier frequency (ppm) | Used primarily in 2D or decoupled experiments |
| P12 | Decoupler 90° pulse width (μs) | 60–100 μs (requires calibration) |
| PLDB24 | Decoupler power (dB) | 20–30 dB; start from low power and optimize |
| L30 | Loop counter | Specifies number of suppression peaks in solvent-suppression sequences (e.g., WET) |
| TE | Temperature (K) | Generally does not require adjustment |
| RO | Sample rotation frequency (Hz) | Typically 0 (no rotation). Must not rotate for 2D or decoupled spectra |
3. Spectrum Interpretation
General workflow: Baseline/phase correction → Chemical shift referencing → Peak identification (shift assignment, integration, splitting analysis, linear correction of integrals) → Structural deduction (functional groups, connectivity) and quantitative analysis (integration ratios, etc.).
- ¹H NMR: Assign protons based on chemical shift, integration, splitting patterns, and coupling constants; watch for solvent peaks, impurities (e.g., water), and exchangeable protons (e.g., OH).
- ¹³C NMR: Apply an appropriate window function (e.g., LB = 0.3–1 Hz) to balance resolution and S/N; use DEPT to assign carbon multiplicities.
- ¹¹B NMR: Use linear prediction to process the probe background signal and obtain a cleaner spectrum.
- 2D NMR:
- COSY: ¹H–¹H scalar coupling (³J).
- HSQC: Direct ¹H–¹³C one-bond correlation.
- HMBC: Long-range 2–3 bond correlation; connects molecular fragments.
- NOESY: Through-space proximity (< 5 Å); stereochemistry or mixture interactions.
Ref: Bruker Official Manuals (https://www.bruker.com/protected/en/services/user-manuals/nmr):
- TopSpin Advanced NMR Experiments
- TopSpin Commands and Parameters