While studying NMR theory, my textbook explained that only nuclei with odd mass numbers are NMR active because they have non-integer spin quantum numbers and nuclei with an even mass number and atomic number have I= 0 spin.
This doesn't make much sense to me. NMR works by generating a magnetic field which interacts with spinning charges (like a proton or electron) that also generate a magnetic field. So how come the nucleus of Carbon-12 is "invisible" to NMR?
In other words, if we treat each proton as miniature bar magnet that is capable of flipping between alpha and beta states (parallel and antiparallel), what is stopping the protons from "reversing spin" in a carbon-12 nucleus?
Best Answer
The interaction between nucleons in a nucleus is very strong so the energy changes associated with flipping a spin are very high. The first few energy levels of a carbon nucleus can be found in this document. The energy spacing between the ground state $J=0$ and the first excited state $J=2$ is 4.44 MeV. This isn't much by the standards of modern accelerators, but it is far, far too big for the transition to be caused by an external magnetic field.
So as far as NMR is concerned the carbon atom behaves as an elementary particle of spin zero with no internal excitations.