Multiple refocusing in NMR spectroscopy: Compensation of pulse imperfections by scalar couplings

When applying multiple refocusing pulses to characterize the cross-correlated relaxation of heteronuclear multiple quantum coherence 2N(x)H(x) in biomolecules, the unavoidable effects of pulse imperfections are compensated by the scalar couplings between nitrogen atoms and protons. The experiment, which is useful as a tool for studying slow internal dynamics of biomolecules, greatly benefits from this compensation. The underlying effects is a manifestation of an interchange between three non-commuting components of the density operator. One perturbing Hamiltonian is counteracted by another, which leads to a nearly complete suppression of the perturbation. The effect proves to be an example of a hitherto unknown phenomenon in NMR spectroscopy.