NMR chemical shifts are measured relative to a standard, tetramethylsilane, TMS, formula Si(CH3)4. Its signal is defined as δ = 0 ppm.In TMS, four methyl groups are bonded to a central silicon atom, arranged tetrahedrally. All 4 carbon atoms are equivalent, and all 12 hydrogen atoms are equivalent.TMS is chosen because:• it gives a single, sharp peak, since all its H atoms (and all its C atoms) are in the same environment
• the peak is at a lower chemical shift than almost all organic compounds, because silicon is less electronegative than carbon, so the nuclei are highly shielded; it does not overlap sample peaks
• it is inert, so it does not react with the sample
• it is volatile (low boiling point), so it can easily be removed from the sample afterwards
• it is non-toxicA small amount of TMS is added to the sample. The spectrometer measures every peak's frequency relative to the TMS peak, so chemical shifts do not depend on the machine used.The TMS peak appears on both 1H and 13C spectra, at the right-hand end. When you count peaks to find the number of environments, do not count the TMS peak.Worked example: a 13C NMR spectrum of an unknown shows peaks at 0, 25 and 62 ppm. The peak at 0 ppm is TMS, so the compound has 2 carbon environments.
• the peak is at a lower chemical shift than almost all organic compounds, because silicon is less electronegative than carbon, so the nuclei are highly shielded; it does not overlap sample peaks
• it is inert, so it does not react with the sample
• it is volatile (low boiling point), so it can easily be removed from the sample afterwards
• it is non-toxicA small amount of TMS is added to the sample. The spectrometer measures every peak's frequency relative to the TMS peak, so chemical shifts do not depend on the machine used.The TMS peak appears on both 1H and 13C spectra, at the right-hand end. When you count peaks to find the number of environments, do not count the TMS peak.Worked example: a 13C NMR spectrum of an unknown shows peaks at 0, 25 and 62 ppm. The peak at 0 ppm is TMS, so the compound has 2 carbon environments.