Recoupling dipolar interactions with multiple I=1 quadrupolar nuclei: A ¹¹B{⁶Li} and ³¹P{⁶Li} rotational echo double resonance study of lithium borophosphate glasses

The case of rotational echo double resonance (REDOR) experiments on the observe nuclei ¹¹B and ³¹P interacting with multiple I=1 quadrupolar nuclei is analyzed in detail by SIMPSON simulations and experimental studies. The simulations define the region within the parameter space spanned by nutation frequency, quadrupolar coupling constant and spinning frequency where the parabolic analysis of the initial REDOR curve in terms of dipolar second moments has validity. The predictions are tested by experimental studies on the crystalline model compounds lithium diborate and lithium pyrophosphate, which are subsequently extended to measure dipolar second moments M₂(¹¹B{⁶Li}) and M₂(³¹P{⁶Li}) in three borophosphate glasses. The data indicate that the lithium cations interact significantly more strongly with the phosphate than with the borate species, despite the formally anionic character of four-coordinate boron and the formally neutral character of the ultraphosphate (P⁽³⁾) units to which they are linked.