Cross-polarization Magic Angle Spinning Spectra Research Articles

Analysis of Humic Acids by Solution and Solid‐state Carbon‐13 Nuclear Magnetic Resonance

AbstractEighteen humic acids (HA's) were characterized by chemical analyses, infrared (IR) and electron spin resonance spectra, and by solution and solid‐state cross polarization magic‐angle spinning (CPMAS) 13C nuclear magnetic resonance (NMR). The main objectives of this investigation were: (i) to compare the solution and solid‐state 13C NMR spectra of the HA's; (ii) to examine effects of different delay times (D1) on the solution spectra; and (iii) to find reasons for the failure of 13C NMR to detect the presence of significant concentrations of phenolic C in unmethylated HA's. Compared to solution spectra, CPMAS spectra consisted of relatively few, broad bands, indicative of considerable overlapping of individual signals, especially in the aliphatic regions. Proportions of aliphatic C determined by solution 13C NMR at D1 = 0.74 s were greater than those obtained at D1 = 1.90 s and by CPMAS 13C NMR (where D1 = relaxation delay). By contrast, proportions of aromatic C and aromaticities were greatest at D1 = 1.90 s. The highest values for CO2H groups were obtained by CPMAS 13C NMR and approached total acidities; values measured by solution 13C NMR at D1 = 1.90 s were intermediate, while those determined by solution 13C NMR with D1 = 0.74 s were the lowest and closest to CO2H concentrations determined chemically. Because the presence in HA's of significant concentrations of phenolic OH groups has been confirmed by several independent methods, it is possible that in 13C NMR spectra of HA's, signals arising from phenolic C are shifted downfield so that they overlap with resonances of C in CO2H groups. The net result is that CO2H groups are overestimated and phenolic OH groups underestimated.

A solid-state 13C nuclear magnetic resonance study of the conformational states of penicillins

13 C Cross polarization–magic angle spinning spectra have been obtained for crystalline pencillins in which essentially all the resonances have been resolved and assigned. Comparison of the spectra of a series of penicillins of known structure has enabled separation of the effects of intermolecular and intramolecular interactions to be achieved. Two aspects of particular interest have emerged from the work. First, dynamic behaviour of the sidechain is evident in the spectra and this has been shown to depend critically on the crystalline environment of the penicillin molecules. Second, by correlating chemical shifts in the series of molecules, it has proved possible to predict features in the solid-state conformations of penicillins of unknown crystal structure. Further, on comparison with solution spectra, the existence of rapidly interconverting thiazolidine ring puckers in solution is suggested and their relative populations estimated for the different penicillins studied.

Solid-state structures of keto-disaccharides as probed by 13C cross-polarization, “magic-angle” spinning n.m.r. spectroscopy

The 13C cross-polarization, magic-angle spinning (CP-MAS) spectra of maltulose · H 2O and anhydrous lactulose were examined at different applied magnetic and proton-decoupling fields B 0 and B 1, respectively. Whereas the spectrum of lactulose was insensitive to these changes, that of maltulose showed significant responses. From selective-relaxation experiments, a mixture of three “forms” (based on the ratio of the C-2 resonances) of both lactulose and maltulose were shown to exist in the solid state. The 360-MHz 1H spectra of the unmutarotated disaccharides in dimethyl sulfoxide solution were used to establish the tautomeric composition of these crystalline solids. The tautomeric composition of lactulose, as determined by the 1H spectrum in dimethyl sulfoxide, correlated well with the CP-MAS data, but the spectrum of unmutarotated maltulose showed only a single, β-pyranose tautomer to be present. Based on the ratio of the lactulose tautomers determined from the 1H spectrum in dimethyl sulfoxide (referenced to fructose), the C-2 resonances corresponding to each anomeric form of lactulose were assigned in the CP-MAS spectrum. A “crossover” in chemical-shift positions of the anomeric resonances was observed in going from solution to the solid state. Furthermore, a pronounced increase in the proportion of the furanoid anomers was noted for crystalline lactulose relative to its solution.