Why is water diamagnetic, but water protons align with external field in NMR?

Question

I recently re-learned today that water is diamagnetic. I don't believe I totally understand why, but I will accept it as true.

However, I remember that in NMR of water, the protons of water form magnetic dipoles that align with the direction of the main magnetic field.

How can we say that water is diamagnetic (meaning that the bulk magnetization, $\mathbf{M}$, aligns opposite to the direction of the applied field), but the protons of water have a magnetic dipole moment that aligns with the direction of the applied field? Doesn't this seem somewhat contradictory?

Thank you for any and all help.

Answer 1

Macroscopic diamagnetism in static field is explained as due to electrons. Even if protons behaved paramagnetically, they have much smaller magnetic moment than electrons, so it is not surprising if in static field, electrons' diamagnetism wins over protons' paramagnetism.

In NMR, oscillating EM field is used, and low enough frequency is used so that its interaction with nuclei is more noticeable (in its effect on EM field at that frequency) than its interaction with the electrons. In other words, by using low frequencies close to resonance with the nucleus spin transitions we can make interaction with nuclei more visible than interaction with the electrons, despite the much smaller magnetic moment of the nucleus.