Finding "Invisible" Protons with 27Al Decoupling

Sometimes when protons are coupled to quadrupolar nuclei, Q, the ¹H signals can range from being sharp singlets, to very broad or "invisible" signals, to sharp multiplets. The appearance of the proton signal depends on the relaxation properties of the quadrupolar isotope coupled to the proton and can also be broadened by chemical exchange. When the relaxation rate between the energy states of the quadrupolar isotope is fast with respect to the coupling constant, J_H-Q, the ¹H signal will be a sharp singlet. When the relaxation rate between the energy states of the quadrupolar isotope is slow with respect to J_H-Q, the ¹H signal will be a sharp multiplet governed by the spin of Q. If the relaxation rate is of the same order as J_H-Q, the ¹H signal will be broad and may even be "invisible". Such is the case for the complex below where a hydride proton is coupled to ²⁷Al. In this case the hydride signal can be "found" by applying ²⁷Al decoupling while observing protons.