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?