Heteronuclear correlation (HETCOR) is a widely used experiment in solid-state NMR which provides through-bond and through-space information via scalar-J and dipolar (D) couplings. There are two categories of methods for establishing heteronuclear correlation. The first kind is based on the insensitive nuclei enhanced by polarization transfer (INEPT) and heteronuclear multiple-quantum coherence (HMQC) experiments originally developed for solution NMR spectroscopy. The second type uses cross-polarization (CP) with spinlock in the rf rotating frame first introduced for solid-state NMR spectroscopy . The two categories use different parts of the heteronuclear coupling Hamiltonian, the z spin-operator terms for INEPT and HMQC vs. the x and y terms for CP. Thus, the transfer processes are subject to different relaxation mechanisms (T2 vs. T1). For spins S = 1/2 like 13 C and 15 N in solids, the CP method is usually preferred given the fact that T1  T2 in the presence of strongly coupled protons.