8.2: NMR Spectroscopy: Chemical Shift Overview
The position of the absorption signal of a sample is reported relative to the position of the signal of tetramethylsilane (TMS), which is added as an internal reference while recording spectra. The difference between the absorption frequencies of the sample and TMS (in Hz) is divided by the spectrometer operating frequency (in MHz) to obtain a dimensionless quantity called the chemical shift. It is reported on the δ (delta) scale and expressed in parts per million.
For instance, the proton signal from benzene is 436 Hz higher than the TMS signal in a 60 MHz spectrometer, while the difference is 2181 Hz in a 300 MHz instrument. In both cases, the obtained chemical shift is 7.27 ppm, indicating that it is independent of the instrument operating frequency. The low chemical shifts on the right side of the spectrum correspond to low-frequency upfield signals from shielded nuclei in electron-dense environments. In contrast, the higher chemical shifts correspond to high-frequency downfield signals from deshielded nuclei in electron-poor settings.
