Analytical techniques

Chromatography 色谱 separates a mixture using two "phases" — one that stays still and one that moves. In thin-layer chromatography 薄层色谱 (TLC):

• the stationary phase 固定相 stays still (for example aluminium oxide on a plate).
• the mobile phase 流动相 moves (a polar or non-polar solvent that travels up the plate).
• the baseline 基线 is the starting line where the spots are placed; the solvent front 溶剂前沿 is the highest level the solvent reaches.

The $R_{\text{f}}$ value compares how far a spot moves with how far the solvent moves:

It is always between 0 and 1. A substance that sticks more strongly to the stationary phase, or is less soluble in the mobile phase, moves less and has a smaller $R_{\text{f}}$.

Thin-layer chromatography: the $R_\text{f}$ value is how far the spot moved divided by how far the solvent front moved

A real TLC plate under UV light: each glowing spot is a separated component of the mixture

In gas/liquid chromatography 气液色谱 (GLC):

• the stationary phase is a high-boiling-point non-polar liquid on a solid support.
• the mobile phase is an unreactive carrier gas.
• the retention time 保留时间 is how long a component takes to pass through.

The area of each peak gives the percentage of that component in the mixture. A component that interacts more with the stationary phase has a longer retention time.

A gas–liquid chromatogram: each component gives a peak at its own retention time, and the peak area is its percentage in the mixture

NMR 核磁共振 (nuclear magnetic resonance) studies how certain nuclei behave in a strong magnetic field.

An NMR spectrometer: the large cylinder holds a superconducting magnet that makes the very strong magnetic field the technique needs

In carbon-13 NMR, each different chemical environment 化学环境 of carbon gives one peak. So:

• the number of peaks tells you how many different carbon environments there are (equivalent carbons share a peak).
• the position of each peak (its chemical shift) suggests the type of carbon, which helps you deduce possible structures.

Proton NMR looks at the hydrogen atoms (proton 质子 nuclei). From the spectrum you read off:

• chemical environments: protons in different environments appear at different chemical shift 化学位移 values.
• relative numbers: the relative peak areas give the ratio of each type of proton.
• splitting 裂分: a peak is split by the protons on the neighbouring carbon, following the $n+1$ rule — $n$ equivalent neighbours split a peak into $n+1$ lines:

The $n+1$ rule: $n$ equivalent neighbouring protons split a peak into $n+1$ lines (singlet, doublet, triplet, quartet)

Proton NMR of ethanol: three environments give a CH$_3$ triplet, a CH$_2$ quartet and an OH singlet, with areas in the ratio $3:2:1$

Practical points

• tetramethylsilane 四甲基硅烷 (TMS) is the standard, set at a chemical shift of $0$.
• a deuterated solvent 氘代溶剂 (such as $\text{CDCl}_3$) is used so that the solvent itself gives no proton signal.
• shaking the sample with $\text{D}_2\text{O}$ makes the O–H and N–H peaks disappear (their hydrogen is swapped for deuterium), which identifies those protons.

Worked example. A compound $\text{C}_4\text{H}_8\text{O}_2$ gives three proton NMR peaks: a triplet at $\delta\ 1.2$ (area 3), a quartet at $\delta\ 4.1$ (area 2), and a singlet at $\delta\ 2.0$ (area 3). Deduce the structure. Read the areas first, then the splitting. Areas $3:2:3$ give three proton environments holding 3, 2 and 3 hydrogens. Apply the $n+1$ rule in reverse: a triplet (area 3) is a $\text{CH}_3$ with 2 neighbours, and a quartet (area 2) is a $\text{CH}_2$ with 3 neighbours - each splitting the other, which is the classic ethyl group, $\text{CH}_3\text{CH}_2-$. That $\text{CH}_2$ lies far downfield at $\delta\ 4.1$, so it is attached to an oxygen. The remaining singlet (area 3) is a $\text{CH}_3$ with no neighbours at $\delta\ 2.0$, so it sits next to the C=O. Together: $\text{CH}_3\text{COOCH}_2\text{CH}_3$, ethyl ethanoate. A triplet-and-quartet pair is almost always an ethyl group - spot it first and the rest follows.

• In $^{13}\text{C}$ NMR the number of peaks equals the number of carbon environments — use symmetry to count them.
• In $^1\text{H}$ NMR use chemical shift (data booklet), integration (ratio of H's) and splitting (the $n+1$ rule): a triplet + quartet means an ethyl group.
• TMS is the reference ($\delta = 0$); a $\text{D}_2\text{O}$ shake removes O-H and N-H peaks.
• Combine IR, mass spectrum and NMR to deduce a structure, stating which evidence gives which feature.