โ Scribed by Eduardo R deAzevedo; Tito J Bonagamba; Klaus Schmidt-Rohr
No coin nor oath required. For personal study only.
Three exchange nuclear magnetic resonance (NMR) techniques are presented that yield 13 C NMR spectra exclusively of slowly reorienting segments, suppressing the often dominant signals of immobile components. The first technique eliminates the diagonal ridge that usually dominates two-dimensional (2D) exchange NMR spectra and that makes it hard to detect the broad and low off-diagonal exchange patterns. A modulation of the 2D exchange spectrum by the sine-square of a factor which is proportional to the difference between evolution and detection frequencies is generated by fixed additional evolution and detection periods of duration , yielding a 2D pure-exchange (PUREX) spectrum. Smooth off-diagonal intensity is obtained by systematically incrementing and summing up the resulting spectra. The related second technique yields a static one-dimensional (1D) spectrum selectively of the exchanging site(s), which can thus be identified. Efficient detection of previously almost unobservable slow motions in a semicrystalline polymer is demonstrated. The third approach, a 1D pure-exchange experiment under magic-angle spinning, is an extension of the exchange-induced sideband (EIS) method. A TOSS (total suppression of sidebands) spectrum obtained after the same number of pulses and delays, with a simple swap of z periods, is subtracted from the EIS spectrum, leaving only the exchange-induced sidebands and a strong, easily detected centerband of the mobile site(s).
๐ SIMILAR VOLUMES
A series of uni-and multidimensional variants of the dipolar exchange-assisted recoupling (DEAR) NMR experiment is described and applied to determinations of 13 C-14 N dipolar local field spectra in amino acids and dipeptides. The DEAR protocol recouples nearby nuclei by relying on differences in th