When we talk about an 1D-NMR spectrum (2-dimensional graph), what does the abcissa axis show?

The nuclei have a nuclear spin that gives rise to different energy levels and resonance frequencies in a magnetic field.

The magnetic field experienced by a nucleus consists of the external magnetic field and local magnetic fields caused by electrons in the molecular orbitals.

The electron distribution around each nucleus varies according to the local geometry, so the local magnetic field (and the resonance frequency) also varies.

This variation in resonance frequencies, is called the chemical shift #δ#.

The size of the chemical shift is measured against a reference substance such as tetramethylsilane (TMS), #("CH"_3)_4"Si"#.

The chemical shift is usually expressed in parts per million (ppm) by frequency.

#δ = (ν_0 - ν)/(ν_s) × 10^6#

where ν is the frequency of the peak, #ν_0# is the frequency of the reference, and #ν_s# is the spectrometer frequency.

Thus, an NMR signal observed at a frequency 660 Hz lower than the signal from TMS in a 300 MHz spectrometer has a chemical shift of:

#δ = (660 cancel("Hz"))/(300 × 10^6 cancel("Hz")) × 10^6 = "2.2 ppm"#