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Individual polyglycans and their corresponding monomers have been studied separately for several decades. Attention has focused primarily on the modifications of these polyglycans instead of the simple relationship between the polyglycans themselves and their corresponding monomers. Two polyglycans, chitin and chitosan, were examined along with their respective monomeric units, N-acetyl-D-glucosamine (GlcNAc) and (+)D-glucosamine (GlcN) using solid-state proton decoupling Magic Angle Turning (MAT) techniques and X-Ray Powder Diffraction (XRPD). A down-field shift in isotropic (13)C chemical shifts was observed for both polymers in Cross Polarization/Magic Angle Spinning (CP/MAS) spectra. An explanation of misleading peak assignments in previous NMR studies for these polyglycans was determined by comparing sideband patterns of the polymers with their corresponding monomers generated in a 2D FIve pi REplicated Magic Angle Turning (FIREMAT) experiment processed by Technique for Importing Greater Evolution Resolution (TIGER). Structural changes in the crystalline framework were supported by XRPD diffraction data.

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.

The 13C chemical shift tensor principal values for the trigonal carbonate and thiocarbonate carbon atoms in the dialkyl carbonates, dimethyl carbonate, ethylene carbonate, and diphenyl carbonate, and in the trithiocarbonates, ethylene trithiocarbonate and dimethyl trithiocarbonate, respectively, were measured in various solid-state one-dimensional and two-dimensional nuclear magnetic resonance experiments. Furthermore, the chemical shift tensor principal values and orientations were calculated for the corresponding isolated molecules with quantum mechanically fully optimized geometries. Proton-optimized X-ray geometries of ethylene carbonate, ethylene trithiocarbonate, and diphenyl carbonate were used in embedded ion method (EIM) calculations and in calculations on the isolated molecules to obtain the theoretical principal values and to assign the chemical shift tensor orientations in these three compounds. Considerable improvement in the correlation between the experimental and calculated principal values is obtained when the electrostatic crystal potentials are included in EIM calculations. The chemical shift tensor orientations and principal values obtained for the dialkyl compounds in this study complement the previous data on a series of ionic potassium carbonates and thiocarbonates.

The principal values of the 13C chemical shift tensors in potassium carbonate (K2CO3), trithiocarbonate (K2CS3), bicarbonate (KHCO3), methylcarbonate (KO2COCH3), S-methyl-monothiocarbonate (KO2CSCH3), O-methyl-monothiocarbonate (KOSCOCH3), S-methyl-dithiocarbonate (KOSCSCH3), and O-methyl-dithiocarbonate (KS2COCH3), were measured in solid-state nuclear magnetic resonance experiments. Chemical shift tensor calculations on the corresponding isolated anions were used to assign the chemical shift tensor orientations in the molecular frames of all anions. The correlation between experimental and calculated principal values improves significantly when the calculations are performed on isolated anions with proton-optimized X-ray geometries rather than on isolated anions with fully optimized geometries. Further considerable improvement in the correlation is achieved by utilizing the embedded ion method, which was recently developed to include electrostatic crystal potentials in chemical shift tensor calculations on ionic compounds. Similarities and differences in the chemical shift tensor orientations and principal values of the trigonal sp2 carbon atoms in the carbonate and thiocarbonate anions are compared with those known for condensed polyaromatic hydrocarbons.

A 15N dynamic nuclear polarization (DNP) experiment is reported in which a 15N DNP enhancement factor of approximately 2.6 x 10(2) is obtained on free radical doped samples of 99% 15N labeled benzamide. The free radicals BDPA (1:1 complex of alpha, gamma-bisdiphenylene-beta-phenylallyl with benzene) and DPPH (2,2-Di(4-tert-octylphenyl)-1-picrylhydrazyl) are used as dopants and the spin relaxation effects of adding these dopants are studied by means of changes in proton and nitrogen T1 values of the samples. The combination in solids of a very low natural abundance, 0.37%, a small gyromagnetic ratio, and a long spin-lattice relaxation time for 15N nuclei create severe sensitivity problems that, in large part, are ameliorated by the signal enhancement observed in the 15N DNP experiment on samples containing free electrons.

A simple 1D modulation method is presented for determining the spatial correlation of chemical shifts in two different single-crystal orientations. This proposed chemical shift modulated correlation (CSMC) spectroscopy utilizes a Jeener pulse sequence to give 1D spectra containing peaks that are amplitude modulated by the chemical shifts associated with the single-crystal's evolution orientation. Conversely, the spectral frequencies in these 1D CSMC spectra designate the shifts in the crystal's orientation during the detection period. By observing the amplitude modulation of these spectral frequencies, the spatial correlation between the two chemical environments can be established for a specific nucleus from a single 1D spectrum. The CSMC procedure finishes by acquiring a normal 1D static spectrum for the crystal oriented in the evolution direction, thereby providing very accurate evolution frequencies approximated by the amplitude modulation in the CSMC spectra. The CSMC method improves the time efficiency over both the traditional goniometer method and the 2D chemical shift-chemical shift correlation method, and therefore is beneficial for the study of samples with longer T1 values.

Delayed acquisition of the proton NMR in selected organic molecular solids (L-alanine, durene, ethyl fumarate, and p-hydroxybenzoic acid) is shown to allow the observation of mobile species in the presence of relatively rigid bulk molecules. The mobility is found to be thermally activated. The combination of the thermally activated motion and magic-angle spinning leads to a fraction of these species moving nearly isotropically on the time scale of the inverse of the homonuclear dipolar splitting. In the case of ethyl fumarate and alanine, there exist populations with differing values of T1 and T1 row. This indicates the co-existence of relatively rigid and relatively mobile molecules in the same sample. The intensities under delayed acquisition cannot always be trusted to yield quantitative information. Comparison of spectra taken under delayed acquisition and under the CRAMPS (B.C. Gerstein, R.G. Pembleton, R.C. Wilson and L.M. Ryan, J. Chem. Phys., 66 (1977)361) technique is made.

A practical method is described for measuring the principal values of the chemical shift tensors in compounds with very long proton spin-lattice relaxation times (T1). This technique involves shortening the effective proton T1. by mixing a compound of interest with another compound having a much shorter T1 value. The doped mixture, partly consisting of a monophasic glass, allows efficient intermolecular spin diffusion between the two compounds. Using a slow magic-angle turning (MAT) experiment, we have successfully used such mixtures to measure the principal values of the chemical shift tensors of all the carbons in dibenzofuran in just four days. Without using this technique the experimental time required for the pure compound would have been several months.

The magic-angle turning (MAT) experiment introduced by Gan is developed into a powerful and routine method for measuring the principal values of 13C chemical shift tensors in powdered solids. A large-volume MAT probe with stable rotation frequencies down to 22 Hz is described. A triple-echo MAT pulse sequence is introduced to improve the quality of the two-dimensional baseplane. It is shown that measurements of the principal values of chemical shift tensors in complex compounds can be enhanced by using either short contact times or dipolar dephasing pulse sequences to isolate the powder patterns from protonated or non-protonated carbons, respectively. A model compound, 1,2,3-trimethoxybenzene, is used to demonstrate these techniques, and the 13C principal values in 2,3-dimethylnaphthalene and Pocahontas coal are reported as typical examples.

Several improvements to the magic angle hopping experiment first introduced by Bax et al. [J. Magn. Reson., 52 (1983) 147] are presented. A dc servo motor driven sample hopping mechanism which requires less than 60 ms to accomplish a 120 degrees sample rotation is described. Modifications to the data acquisition process, including starting the acquisition period immediately after the second hop and acquiring a hypercomplex data set, are also presented. Principal values of the 13C chemical shielding tensor are measured for 1,2,3-trimethoxybenzene and 2,6-dimethoxynaphthalene.

Taking advantage of the long 13C T1 values generally encountered in solids, selective saturation and inversion of more than one resonance in 13C CP/MAS experiments can be achieved by sequentially applying several DANTE pulse sequences centered at different transmitter frequency offsets. A new selective saturation pulse sequence is introduced composed of a series of 90 degrees DANTE sequences separated by interrupted decoupling periods during which the selected resonance is destroyed. Applications of this method, including the simplification of the measurement of the principal values of the 13C chemical shift tensor under slow MAS conditions, are described. The determination of the aromaticity of coal using a relatively slow MAS rate is also described.