NMR, accurate mass and chromatography

What you’ll learn

• Combine spectroscopic evidence to identify an organic structure.
• Interpret carbon and proton environments, integrals and splitting.
• Calculate and interpret Rf values.

Understand the idea

Accurate mass helps distinguish molecular formulae with the same nominal mass; calculate it by adding the supplied isotope masses, without rounding intermediate values. Infrared absorption identifies possible functional groups, while the number of carbon-13 NMR signals counts distinct carbon environments in the usual proton-decoupled spectrum. Symmetry can make several carbons equivalent, so signal count need not equal total carbon count.

In proton NMR, chemical shift suggests the local environment, integration gives relative proton numbers, and splitting provides neighbour information. The simple $n+1$ rule applies to coupling with $n$ equivalent adjacent protons; equivalent protons do not split one another, and exchangeable OH or NH signals often do not follow this simple pattern. Use all the evidence together rather than identifying a structure from one peak.

Chromatography separates substances through their different interactions with a stationary phase and a mobile phase. An $R_f$ comparison is meaningful only under the same conditions. A matching spot supports, but does not alone prove, identity; gas chromatography–mass spectrometry combines separation with a mass spectrum for each eluting component.

Key formulae

$$R_f=\frac{\text{distance travelled by spot}}{\text{distance travelled by solvent front}}$$

$$\text{simple splitting multiplicity}=n+1$$

$$\text{integral ratio}=\text{relative numbers of protons}$$

$$m_{\mathrm{molecule}}=\sum_i n_i m_i\quad\text{(using supplied isotope masses)}$$

Worked example

A compound has formula C₃H₆O₂, an ester carbonyl absorption, and three proton NMR signals: a 3H singlet, a 2H quartet and a 3H triplet. Is this complete dataset internally consistent?

1. Add the stated proton integrals: 3 + 2 + 3 = 8 hydrogen atoms.
2. The stated molecular formula contains only 6 hydrogens, so it conflicts with those integrals.
3. The singlet plus ethyl quartet–triplet pattern could fit ethyl ethanoate, whose formula is C₄H₈O₂, but the original formula must be checked.

Answer: No. The formula and integral totals disagree; do not force a structure to fit inconsistent data.