C-13-P-31 CROSS POLARIZATION BETWEEN ISOLATED SPIN PAIRS IN ORGANOPHOSPHORUS SUBSTANCES - INTRAMOLECULAR STRUCTURE AND INTERNUCLEAR DISTANCE DETERMINATIONS IN SPINNING SOLIDS

Cross polarization between dilute effectively isolated 13C-31P spin pairs provides a unique method to examine selectively the microenvironment around the phosphorus atom in organophosphorus substances. Strong oscillations occur during cross polarization between these isolated spin pairs in MAS experiments on microcrystalline powders. The oscillation frequency is proportional to the 13C-31P dipolar interaction and hence allows specific internuclear distance determinations on powder samples. Maximum carbon signal intensity and experimental simplicity are achieved in double cross polarization (DCP)/MAS 13C NMR experiments that impose a short T1pon the 31P magnetization by 31P radio frequency phase modulation. This experimental stratagem suppresses cross-polarization oscillations and yields simple exponential kinetics for the transfer process. It also relaxes the tolerance on the match condition and MAS speed for cross polarization between weakly coupled dipole pairs. © 1990, American Chemical Society. All rights reserved.