The competition between intramolecular J couplings, radiation damping, and intermolecular dipolar couplings in two-dimensional solution nuclear magnetic resonance

Scalar couplings have long been used to observe forbidden transitions indirectly, e.g., the multiple-quantum transitions detected frequently in the indirect dimension of multidimensional NMR experiments. Recent work has shown that intermolecular dipolar couplings in solution will also permit the detection of multiple-quantum coherences, this time between separated spins (intermolecular multiple-quantum coherences, iMQCs). Analytical expressions are derived to describe spin evolution for an essentially arbitrarily complex coupled spin system when dipolar couplings or radiation damping also are significant. The results are tested on an A₂B₃+C_n system (methyl ethyl ketone). iMQCs that are forbidden from radiation damping or dipolar couplings alone can become allowed in the presence of J couplings, but the multiplet patterns differ dramatically from conventional multiplets. Scalar couplings open up new pathways for pumping the normally allowed transitions.