(1)H/(29)Si cross-polarization NMR experiments of silica-reinforced polydimethylsiloxane elastomers: probing the polymer-filler interface

Polydimethylsiloxane (PDMS) elastomers reinforced with fumed silica exhibit unusual strength characteristics that are necessary for their designed applications. The microscopic details of the surface interaction between the polymer and silica are not well characterized. (1)H/(29)Si cross-polarization (cp) experiments are used to characterize cured and uncured samples of Dow Corning silastic 745. Changes to the cp dynamics upon curing are evident by the variation in peak intensities in the variable contact-time spectra of the two samples. Estimates of the cp relaxation parameters are reported for the cured sample. Additional information can be obtained by expanding the (1)H/(29)Si cp to a two-dimensional heteronuclear correlation experiment. Dramatic differences between the cured and uncured (1)H/(29)Si HetCor spectra are observed that are not visible in the 1D spectra. These changes can be rationalized as a dehydration of the silica surface and an increased hardening of the polymer after the curing process. Furthermore, isolation of the NMR signal corresponding to nuclei at or near the polymer-filler interface may be achieved in the 2D (1)H/(29)Si HetCor spectrum.     

Ring-chain tautomerism in solid-phase erythromycin A: evidence by solid-state NMR

Chemical shift modeling, utilizing the DFT B3LYP/D95** method, provides the spectral assignment of the 35 visible 13C resonances from the solid-phase erythromycin A dihydrate. A new resonance at 110.8ppm is observed in the high-resolution 13C CP/MAS spectrum upon the application of heat or sample desiccation. With the use of the dipolar-dephasing spectral editing technique, this resonance is identified as a hemiketal carbon and the alternative hypothesis, a conformational change to the anomeric carbon of the desosamine sugar, is ruled out. Hence, the formation of a cyclic hemiketal in erythromycin A while in the solid phase is proven by solid-state NMR. The principal components of the 13C chemical-shift tensor corresponding to this hemiketal are reported. This is the first measurement of hemiketal 13C principal values. The delta11 and delta22 components are unique compared to anomeric carbon values reported in the literature.