A novel dipolar dephasing method for the slow magic angle turning experiment.

Complete suppression of the resonances from protonated carbons in a slow magic angle spinning experiment can be achieved using five dipolar dephasing (Five-DD) periods distributed in one rotor period. This produces a spectrum containing only the spinning sidebands (SSB) from the nonprotonated carbons. It is shown that the SSB patterns corresponding to the nonprotonated carbons are not distorted over a wide range of dipolar dephasing times. Hence, this method can be used to obtain reliable principal values of the chemical shift tensors for each nonprotonated carbon. The Five-DD method can be readily incorporated into isotropic-anisotropic 2D experiments such as FIREMAT and 2D-PASS to facilitate the measurement of the (13)C chemical shift tensors in complex systems.

Characterization of fine particulate matter produced by combustion of residual fuel oil.

Combustion experiments were carried out on four different residual fuel oils in a 732-kW boiler. PM emission samples were separated aerodynamically by a cyclone into fractions that were nominally less than and greater than 2.5 microns in diameter. However, examination of several of the samples by computer-controlled scanning electron microscopy (CCSEM) revealed that part of the PM2.5 fraction consists of carbonaceous cenospheres and vesicular particles that range up to 10 microns in diameter. X-ray absorption fine structure (XAFS) spectroscopy data were obtained at the S, V, Ni, Fe, Cu, Zn, and As K-edges and at the Pb L-edge. Deconvolution of the X-ray absorption near edge structure (XANES) region of the S spectra established that the dominant molecular forms of S present were sulfate (26-84% of total S) and thiophene (13-39% of total S). Sulfate was greater in the PM2.5 samples than in the PM2.5+ samples. Inorganic sulfides and elemental sulfur were present in lower percentages. The Ni XANES spectra from all of the samples agreed fairly well with that of NiSO4, while most of the V spectra closely resembled that of vanadyl sulfate (VO.SO4.xH2O). The other metals investigated (i.e., Fe, Cu, Zn, and Pb) also were present predominantly as sulfates. Arsenic was present as an arsenate (As+5). X-ray diffraction patterns of the PM2.5 fraction exhibit sharp lines due to sulfate compounds (Zn, V, Ni, Ca, etc.) superimposed on broad peaks due to amorphous carbons. All of the samples contain a significant organic component, with the loss on ignition (LOI) ranging from 64 to 87% for the PM2.5 fraction and from 88 to 97% for the PM2.5+ fraction. Based on 13C nuclear magnetic resonance (NMR) analysis, the carbon is predominantly condensed in graphitic structures. Aliphatic structure was detected in only one of seven samples examined.

A high-resolution (13)C 3D CSA-CSA-CSA correlation experiment by means of magic angle turning.

It is shown in this paper that a previously reported 90 degrees sample flipping (13)C 2D CSA-CSA correlation experiment may be carried out alternatively by employing constant slow sample rotation about the magic angle axis and by synchronizing the read pulse to 13 of the rotor cycle. A high-resolution 3D CSA-CSA-CSA correlation experiment based on the magic angle turning technique is reported in which the conventional 90 degrees 2D CSA-CSA powder pattern for each carbon in a system containing a number of inequivalent carbons may be separated according to the isotropic chemical shift value. The technique is demonstrated on 1,2,3-trimethoxybenzene in which all of the overlapping powder patterns that cannot be segregated by the 2D CSA-CSA experiment are resolved successfully by the 3D CSA-CSA-CSA experiment, including even the two methoxy groups (M(1) and M(3)) whose isotropic shifts, confirmed by high-speed MAS, are separated by only 1 ppm. A difference of 4 ppm in the principal value component (delta(33)) between M(1) and M(3) is readily obtained.

1H dynamic nuclear polarization in supercritical ethylene at 1.4 T.

(1)H dynamic nuclear polarization (DNP) has been measured in supercritical ethylene in the pressure range 60-300 bar in an external field of 1.4 T. A single-cell sapphire tube was used as a high-pressure cell, and powdered 1,3-bisdiphenylene-2-phenyl allyl (BDPA) free radicals were added and distributed at the wall of the cell. At all pressures the dominant DNP mechanism was a positive Overhauser enhancement, caused by proton-electron contact interactions at the fluid/solid radical interface. The observed enhancements varied from 12 at 60 bar to 17 at 300 bar. Besides the Overhauser enhancement, small solid state and thermal mixing enhancements also were observed, indicating that part of the ethylene was adsorbed at the radical surface for a prolonged time. The impacts of the experimental conditions on the Overhauser enhancement factors are discussed, and enhancements of at least 40-60 are estimated when the EPR saturation factor and the leakage factor become maximal. These data indicate that DNP-enhanced NMR has the potential of extending the impact of NMR in research areas involving supercritical fluids.

Modified spectral editing methods for (13)C CP/MAS experiments in solids.

The spectral editing approach of Zilm and coworkers utilizes polarization, polarization inversion, and spin depolarization methods for enhancing or suppressing NMR spectral lines in solids. The proposed pulse sequences allow nonprotonated C, CH, CH(2), and CH(3) types of carbon resonances to be separated from one another and identified accordingly. The former method tentatively separates the nonprotonated C and CH(3) peaks with a cutoff shift of 35 ppm. This shift is a reasonable demarcation shift for a preponderance of organic molecules, but exceptions do exist that could constitute a serious drawback in a few instances. The new approach separates the nonprotonated C and CH(3) carbon peaks unequivocally using modified pulse sequences similar to those of Zilm. Further, both the CH only and CH(2) only spectra, respectively, can be acquired directly from combining so called (+) and (-) sequences using different spectral delay periods and pulse parameters. The (+) and the (-) pulse sequences produce signals for the nonprotonated and methyl carbons that have essentially the same amplitude but opposite phases. These spectra, combined with the previously reported CH(3) and nonprontonated C only spectra, offer a complete spectral editing technique for solid samples. Examples of these spectral editing methods are provided for 3-methylglutaric acid, fumaric acid monoethyl ester, and two complex natural products: methyl o-methylpodocarpate and 10-deacetylbaccatin III.

Dynamic nuclear polarization of nitrogen-15 in benzamide.

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.

Technique for importing greater evolution resolution in multidimensional NMR spectrum.

A very simple and general procedure that extracts constant-evolution-frequency data from a truncated multidimensional (2D, 3D, 4D, etc.) FID is described, generalized, analyzed, and illustrated. The method replaces Fourier transformation of the evolution dimension with a linear model created from a separate, high-quality 1D FID. The equivalent of high resolution in the evolution dimension can be achieved without obtaining an extensive multidimensional FID. The analysis of the 1D FID can also be used to predict the signal to noise ratio of the extracted slices that will result from various evolution dimension sampling protocols, making it possible to develop a priori an optimal sampling strategy for the multidimensional FID. The evolution dimension need not be sampled periodically. The procedure has a potential signal-to-noise ratio advantage because it extracts usable information from a multidimensional FID at short evolution times before the magnetization has decayed significantly.

Use of relaxation agent doping to shorten very long spin-lattice relaxation times in a magic-angle turning experiment.

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.

Measurement of 13C chemical shift tensor principal values with a magic-angle turning experiment.

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.

Structure elucidation of three triterpenoid saponins from Alphitonia zizyphoides using 2D NMR techniques.

High-field nmr experiments have been used to determine the structures of three new saponins, zizyphoisides C, D, and E [1-3], isolated from Alphitonia zizyphoides, a medicinal plant found in the Samoan rain forest. The saponins consist of the aglycone, jujubogenin, to which are attached three sugar units and an unsaturated side-chain. The three compounds are similar except for the location of an acetyl group on one of the sugar units.

Improvements to the magic angle hopping experiment.

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.

Improvements in the computerized analysis of 2D INADEQUATE spectra.

The carbon skeleton of a molecule can be determined by using the powerful 2D INADEQUATE experiment, but the method suffers from very poor sensitivity at natural carbon-13 abundance. A computer program, described previously, has been significantly improved in its ability to recognize AB spectral patterns corresponding to carbon-carbon bonds which makes it possible to evaluate reliably spectra with rms S/N ratio as low as 2.5, i.e., nearly 1 order of magnitude below the level required for routine manual interpretation. Application of the INADEQUATE experiment to samples containing as little as 20 mumol of a compound of interest is now possible. The method is described in detail and critically evaluated by means of examples and simulations.

Applications of the improved computerized analysis of 2D INADEQUATE spectra.

This paper illustrates the use of the program CCBond to determine the carbon skeletons of bioorganic molecules in low concentration samples. Discussed is the structure elucidation of bistramide A, a compound extracted from a Fijian Lissoclinum sp. and cholesterol in 71- and 20-mumol samples, respectively. The detection limit of the automated bond extraction is shown to be dramatically improved compared to the manual interpretation of 2D INADEQUATE spectra.

Selective saturation and inversion of multiple resonances in high-resolution solid-state 13C experiments using slow spinning CP/MAS and tailored DANTE pulse sequences.

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.