Chemical shift
NMR resonance frequencies of individual nuclei are quoted relative to the resonance frequency of the same nuclei in tetramethylsilane, (CH3)4Si, which is the universally accepted reference for 1H and 13C NMR spectroscopy. NMR data are given in terms of a chemical shift (δ) (ppm), over a scale of 0–10. The area under a resonance signal is proportional to the number of protons giving rise to the signal. If a signal is shifted towards the left of the spectrum, it is shifted ‘downfield’ (nuclei de-shielded) and if a signal is shifted towards the right of the spectrum, it is shifted ‘up-field’ (nuclei more shielded). Thus, NMR signals give information on the immediate environment of nuclei and consequently give structural information – chemical shift (δ) data correlate with the ‘type of hydrogen’ or ‘type of carbon’ present.