One- and Two-Dimensional NMR Experiments

In a one-dimensional (1-D) NMR experiment, t, is kept constant, t2 varies and the FT is taken with respect to t2 only in two-dimensional (2-D) NMR experiments, both t, and t2 are varied and the FT is taken with respect to both time variables. 

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GC analyses of the pupal secretion of E. borealis have indicated the presence of vitamin E acetate and other tocopherol derivatives [49,50]. However, in tests with ants, these compounds proved to be essentially inactive, whereas the secretion itself was potently deterrent. To find and identify the active components in the pupal Epilachna borealis secretion, NMR spectroscopic studies on the fresh secretion were carried out. One and two-dimensional NMR experiments revealed that the tocopheryl acetates account for only a relatively small percentage of the beetles5 total secretion (20%), whereas the major components represented a group of previously undetected compounds. By analysis of the COSY, HSQC and HMBC spectra of the mixture, these components were shown to be esters and amides derived from three (co-l)-(2-hydroxyethylamino)alka-noic acids 44-46. HPLC analyses coupled to a mass spectrometric detector revealed that the secretion contain a highly diverse mixture of macrocyclic polyamines, the polyazamacrolides (PAMLs) 47-52 (Fig. 8). 

With current commercially available equipment the ideal set-up for online analysis would be an HPLC-SPE system, a cryogenic flow probe (30 p,l active volume) that is in permanent use within an actively shielded magnet operating at 500 MHz or higher. The system would offer the optimum LC-NMR sensitivity (no dependency on LC peak volumes), and complex impurities as low as 0.1% could be identified by one- and two-dimensional NMR experiments, provided that the impurities are sufficiently stable to permit isolation on the SPE cartridges,... 

The development of carbon-13 NMR during the last eight years has been characterized by a continual increase in the sensitivity and quality of spectra. A reduction in measuring time - equivalent to an enhancement in sensitivity has been achieved mainly by cryomagnet technology. The efficiency with which NMR information can be obtained has been substantially improved by new computer-controllable pulse sequences for one-and two-dimensional NMR experiments. A selection of these new methods, in particular, those used for multiplicity analysis and homo- or heteronuclear shift correlations, is presented in chapter 2 of this edition. 

NMR spectroscopy has proven to be an invaluable tool in structure determination of hydrogenated fullerenes ever since the first synthesis of C60H36 (Haufler et al. 1990) and the first structure determination by NMR was reported for C60H2 by Henderson and Cahill in 1993 (Henderson and Cahill 1993). Today, a multitude of both one- and two-dimensional NMR experiments are available for this purpose. Recent developments in NMR technology with higher magnetic field strengths and cryoprobes have dramatically increased the sensitivity and improved the usefulness of NMR in this field even further. 

Today, a number of one- and two-dimensional NMR experiments are available for the detection of homonuclear Li, Li and Li, Li couplings. Aside from the COSY experiment, the double quantum filtered COSY (COSY-DQF), the TOCSY, and the ID and 2D INADEQUATE experiments [24] have been successfully employed. An attractive feature of all these experiments is their sensitivity for small scalar interactions which give rise to crosspeaks even if line splittings in the corresponding ID spectra are not resolved. This was first demonstrated with COSY experiments for a paramagnetic nickel complex [82] and for quadrupolar nuclei in the case of boron-11 [83]. 

Very recently an isotopic finger print method which rests on deuterium-induced isotope shifts for Li resonances has been proposed by Gunther and exemplified with applications from the aggregation behavior of cyclopropyllithium systems and mixed aggregates formation between methyllithium and lithium salts. By using one- and two-dimensional NMR experiments based on scalar spin-spin coupling and nuclear overhauser effects the structural aspects associated with benzyllithium and the formation of polylithium systems by lithium reduction of biphenylenes have been discussed. 

One- and two-dimensional NMR experiments were performed to examine the interactions between a hydrophobic pollutant, pyrene and cuticular material. Carbon-13 NMR experiments on labelled pyrene were performed. MAS... 

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