<P> Ξ (%) = 100 (υ X o b s / υ T M S o b s) (\ displaystyle \ Xi (\%) = 100 (\ upsilon _ (X) ^ (obs) / \ upsilon _ (TMS) ^ (obs))) </P> <P> The use of the deuterium (lock) channel, so the H signal of the deuterated solvent, and the Ξ value of the absolute scale is a form of internal referencing and is particularly useful in heteronuclear NMR spectroscopy as local reference compounds may not be always be available or easily used (i.e. liquid NH for N NMR spectroscopy). This system, however, relies on accurately determined H NMR chemical shifts enlisted in the spectrometer software and correctly determined Ξ values by IUPAC . A recent study for F NMR spectroscopy reveiled that the use of the absolute scale and lock - based internal referencing led to errors in chemical shifts . These may be negated by inclusion of calibrated reference compounds . </P> <P> The electrons around a nucleus will circulate in a magnetic field and create a secondary induced magnetic field . This field opposes the applied field as stipulated by Lenz's law and atoms with higher induced fields (i.e., higher electron density) are therefore called shielded, relative to those with lower electron density . The chemical milieu of an atom can influence its electron density through the polar effect . Electron - donating alkyl groups, for example, lead to increased shielding while electron - withdrawing substituents such as nitro groups lead to deshielding of the nucleus . Not only substituents cause local induced fields . Bonding electrons can also lead to shielding and deshielding effects . A striking example of this is the pi bonds in benzene . Circular current through the hyperconjugated system causes a shielding effect at the molecule's center and a deshielding effect at its edges . Trends in chemical shift are explained based on the degree of shielding or deshielding . </P> <P> Nuclei are found to resonate in a wide range to the left (or more rare to the right) of the internal standard . When a signal is found with a higher chemical shift: </P>

Chemical shift of proton have a frequency range