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 2JAlP (Al-O-P) 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 (0-4) 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 4JPP (P-O-Al-O-P) coupling was also observed from the 31P spin echo and INADEQUATE experiments.