Multispin moments edited by multiple-quantum NMR:: application to elastomers

The spin system response to the five-pulse sequence used for measurements of double-quantum and triple-quantum buildup curves is evaluated in the initial excitation/reconversion regime. The multispin dipolar network that is present also in many soft solids like elastomers was considered. It is proved rigorously that the relevant quantity for analysis of double-quantum build-up curves in the initial regime is the second van Vleck moment. The higher-order moments edited by double-quantum as well as higher-order coherences in the multiple-quantum build-Lip experiments are different from van Vleck moments. These results can be applied to compare H-1 residual moments edited by double-quantum and triple-quantum experiments with those measured by other NMR methods. The sensitivity of multiple-quantum coherences to the changes in the values of residual dipolar couplings for crosslinked natural rubber under Uniaxial elongation is also discussed. Under Such conditions H-1 second van Vleck moments were measured for different elongation ratios of a cross-linked natural rubber. Moreover, H-1 triple-quantum edited moments were also measured for the same sample under uniaxial compression. The dependence of the second van Vleck moment and the time of the maximum of the double-quantum buildup Curve on the cross-link density of natural rubber measured at low magnetic field was also investigated. (C) 2004 Elsevier Inc. All rights reserved.