NMR 2

When a nucleus is shielded by electrons, these circulate to partially counteract an external magnetic field. This is circulation is called a diamagnetic current. The shielding effect produced  by this current is called diamagnetic shielding.

So heavily shield protons experience a reduced magnetic field, hence a reduced energy gap, hence they absorb light at a lower frequency. Deshielding therefore increases the frequency of absorbed radiation.

Rather then measuring frequency of absorption directly, NMR usually measures absorption relative a reference compound. The most common reference is tetramethylsilane:

This is chosen because its protons are more shielded than usual. This prevents overlap with protons on most organic molecules.

The frequency which a proton absorbs at also depends on the magnetic field strength of the NMR machine, which varies according to research budgets, since strong magnets increase the resolution, but are more expensive. To allow easy comparison of specta, the field-independent frequency can be measured using chemical shift notation (δ).

The (shift in Hz) part refers to the shift relative to tetramethylsilane or another standard. Since this is divided by MHz, then chemical shift could also be described as "shift from tetramethylsilane in parts per million of the spectrometer's operating frequency".