Chemical shifts are measured relative to a reference compound, tetramethylsilane (TMS), Si(CH3)4. A small amount of TMS is added to the sample, and its peak is defined as δ = 0 ppm. Every other peak is measured from it.Si(CH3)4: four CH3 groups bonded to a central silicon atom TMS is used as the standard because:
• All 12 of its H atoms are in the same environment, and all 4 of its C atoms are in the same environment, so it gives a single, strong peak in both 1H and 13C NMR.
• Silicon is less electronegative than carbon, so the nuclei in TMS are strongly shielded. Its peak appears at a lower shift than the peaks of almost all organic compounds, so it does not overlap with them.
• It is inert, so it does not react with the sample.
• It is non-toxic.
• It is volatile (boiling point about 27 °C), so it can easily be removed from the sample by evaporation afterwards.When you read a spectrum, the peak at 0 ppm is the TMS peak. It is not part of the compound, so do not count it as one of the compound's environments.Example: a 13C spectrum of propanone shows peaks at 0, about 30 and about 205 ppm. The 0 ppm peak is TMS, so propanone has two carbon environments.
• All 12 of its H atoms are in the same environment, and all 4 of its C atoms are in the same environment, so it gives a single, strong peak in both 1H and 13C NMR.
• Silicon is less electronegative than carbon, so the nuclei in TMS are strongly shielded. Its peak appears at a lower shift than the peaks of almost all organic compounds, so it does not overlap with them.
• It is inert, so it does not react with the sample.
• It is non-toxic.
• It is volatile (boiling point about 27 °C), so it can easily be removed from the sample by evaporation afterwards.When you read a spectrum, the peak at 0 ppm is the TMS peak. It is not part of the compound, so do not count it as one of the compound's environments.Example: a 13C spectrum of propanone shows peaks at 0, about 30 and about 205 ppm. The 0 ppm peak is TMS, so propanone has two carbon environments.