Determination of J coupling constants between spin-1/2 and quadrupolar nuclei in inorganic solids from spin echo and refocused INEPT experiments: A case study on AlPO₄ berlinite

A systematic study utilizing rotor-synchronized homonuclear ( ³¹P, ²⁷Al) and heteronuclear ({³¹P} ²⁷Al and {²⁷Al}³¹P) spin echo, and { ²⁷Al}³¹P refocused INEPT experiments (employing soft pulses for selective excitation of the central transition for the quadrupolar ²⁷Al (I=5/2)) have been performed on AlPO₄ berlinite at 30 kHz MAS to better understand the J modulation behavior involving half-integer quadrupolar nuclei in solid materials with framework structure. Analyses of the J modulation on either the ²⁷Al or ³¹P coherence in both the {³¹P}²⁷Al and {²⁷Al}³¹P spin echo experiments, and both periods of the refocused INEPT experiment yield consistent results for the ²J_AlP (AlOP) coupling constant (ca. 25 Hz). It is noted that the coupling of each ²⁷Al to four ³¹P spins during the first (²⁷Al) evolution period of the refocused INEPT, and the populations of ³¹P coupled to different numbers (04) of ²⁷Al in the ±1/2 Zeeman states during ³¹P coherence evolution, which have been neglected in previous studies, must be taken into account for proper treatment. Analysis of J modulation on the spin (²⁷Al) coupled to spin-1/2 nuclei in general gives more accurate results. Weak long-range homonuclear ⁴J _PP (POAlOP) coupling was also observed from the ³¹P spin echo and INADEQUATE experiments.