Proton gyromagnetic ratio over 2π

Divide the proton gyromagnetic ratio by 2π and you get ordinary frequency instead of angular frequency: 42.577 MHz per tesla. This is the form used in practice, because spectrometers and scanners are specified in megahertz. A clinical 1.5 T MRI runs at about 63.9 MHz, a 3 T scanner at 127.7 MHz, and a chemist's "600 MHz" NMR instrument is announcing a 14.1 T superconducting magnet by way of its proton frequency.

The tightness of this constant is what makes magnetic resonance quantitative. Chemical shifts — the fingerprints that let a spectrum identify a molecule — are deviations of a few parts per million from this base rate, caused by the electron cloud partially shielding each nucleus from the applied field. Everything in NMR is measured against this number, which is why it is quoted to eleven digits for a technique most people meet as a noisy tube in a hospital.