CAIE A-Level Chemistry 37.4.3 Tetramethylsilane as the NMR Standard

Practise identifying tetramethylsilane and explaining its role as the δ 0 chemical-shift reference.

Exam points

identify the small signal at δ 0 as tetramethylsilane, TMS, with formula Si(CH3)4
state that TMS defines the zero point from which other proton chemical shifts are measured
distinguish the TMS reference signal from a peak produced by the sample molecule

[Maximum number: 1]

Lactic acid, $\mathrm{CH}_{3} \mathrm{CH}(\mathrm{OH}) \mathrm{COOH}$ , is the only monomer needed to form the polymer polylactic acid, PLA.

The proton $\left({ }^{1} \mathrm{H}\right) \mathrm{NMR}$ spectrum of $\mathrm{CH}_{3} \mathrm{CH}(\mathrm{OH}) \mathrm{COOH}$ in $\mathrm{CDCl}_{3}$ is shown in Fig. 8.1. The proton NMR chemical shift ranges are shown in Table 8.1.

Lactic acid

Table 8.1

Name the substance responsible for the peak at $\delta=0.0$ .

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37.4.3—Of tetramethylsilane, TMS, as the standard question 1