Chemical heterogeneity in phosphosilicate gels by NMR magnetisation exchange

Nanometre scale chemical heterogeneity has been revealed in a phosphosilicate gel by 31P spin exchange. Monomeric phosphorus was found near to phosphorus singly cross-linked with phosphorus. Phosphorus doubly cross-linked with phosphorus was found to be remote from both other types of phosphorus species. Radiofrequency driven magnetisation exchange through 31P-31P dipolar coupling was rather ineffective, even with recoupling times of up to 30 ms, demonstrating the weak 31P-31P dipolar coupling and the effect of residual 1H-31P dipolar coupling. Effective 31P spin exchange was possible in the laboratory frame through the 1H spin interactions, when mixing times from 100 ms to 2 s were used