One-dimensional NMR techniques

Unlike the one dimensional NMR techniques to which this book is largely devoted, those 2D 19F NMR techniques to be briefly discussed below will not generally be required for day to day structure elucidation by the working organic chemist. However, there will inevitably be situations where these techniques will be indispensible in determining the detailed 3-dimensional structure of compounds that contain fluorine, and at such times it may be necessary for the synthetic chemist to turn to an NMR specialist for assistance. 

The development of new and improved NMR techniques is continual and fast-paced, with the range of systems that are suitable for study and the information that can be elucidated from them increasing aU the time. Recent years have seen a shift away from one-dimensional NMR techniques towards two-dimensional techniques, such as MQMAS, which are often capable of providing higher resolution. In particular, the recent development of the SATRAS technique opens up new possibilities. In addition to the applications outlined above, Antonijevic and coworkers [28] have reported on the use of SATRAS to study host-guest interactions and guest dynamics in framework solids. Nuclei that could be studied by this method include 0, Na, Al and Ga. 

S.K. Sarkar and A. Bax, A simple and sensitive one-dimensional NMR technique for correlation of proton and carbon chemical shifts, J. Magn. Reson., 62 (1985) 109-112. 

In the one-dimensional NMR experiments discussed earlier, the FID was recorded immediately after the pulse, and the only time domain involved (ij) was the one in which the FID was obtained. If, however, the signal is not recorded immediately after the pulse but a certain time interval (time interval (the evolution period) the nuclei can be made to interact with each other in various ways, depending on the pulse sequences applied. Introduction of this second dimension in NMR spectroscopy, triggered byjeener s original experiment, has resulted in tremendous advances in NMR spectroscopy and in the development of a multitude of powerful NMR techniques for structure elucidation of complex organic molecules. 

Special emphasis has been given to the more important techniques for solving practical problems related to the interpretation of the spectral data obtained from one- and two-dimensional NMR techniques. An introductory... 

NMR spectroscopy is the most powerful method for structural elucidation in solution and advances in NMR techniques have made significant impacts on anthocyanin studies. Complete structural characterization of anthocyanins is possible with one- and two-dimensional NMR techniques. However, relatively large quantities of... 

NOESY NMR spectroscopy is a homonuclear two-dimensional experiment that identifies proton nuclei that are close to each other in space. If one has already identified proton resonances in one-dimensional NMR spectroscopy or by other methods, it is then possible to determine three dimensional structure through NOESY. For instance, it is possible to determine how large molecules such as proteins fold themselves in three-dimensional space using the NOESY technique. The solution structures thus determined can be compared with solid-state information on the same protein obtained from X-ray crystallographic studies. The pulse sequence for a simple NOESY experiment is shown in Figure 3.23 as adapted from Figure 8.12 of reference 19. 

First, various advanced multipulse techniques have been developed since the mid-1970s, and nowadays are routinely applicable on spectrometers of the latest generation. Particularly innovative and ingenious among these methods are two-dimensional NMR techniques (506-508) and double quantum transition measurements (INADEQUATE) (507-509), which allow one to determine connectivities between carbon atoms within a molecule. 

NMR is a remarkably flexible technique that can be effectively used to address many analytical issues in the development of biopharmaceutical products. Although it is already more than 50 years old, NMR is still underutilized in the biopharmaceutical industry for solving process-related analytical problems. In this chapter, we have described many simple and useful NMR applications for biopharmaceutical process development and validation. In particular, quantitative NMR analysis is perhaps the most important application. It is suitable for quantitating small organic molecules with a detection limit of 1 to 10 p.g/ml. In general, only simple one-dimensional NMR experiments are required for quantitative analysis. The other important application of NMR in biopharmaceutical development is the structural characterization of molecules that are product related (e.g., carbohydrates and peptide fragments) or process related (e.g., impurities and buffer components). However, structural studies typically require sophisticated multidimensional NMR experiments. 

New epibatidine analogues were characterized with the aid of one and two dimensional NMR techniques and IR, GC-MS spectroscopies (Fig. 38.1). All signals in the NMR spectra appeared in pairs due to the presence of rotamers that are resulted from the rotation between the unpaired electrons and benzoyl substituent on the nitrogen atom (Scheme 38.2). 

Identification of constitutive monosaccharides two-dimensional homonuclear NMR techniques such as DQF-COSY and TOCSY are used to assign chemical-shift values for all C-bonded protons in each individual monosaccharide (96). One-dimensional NMR spectra provide useful information about the chemical shifts and scalar couplings of such well-resolved signals as methyl groups for 6-deoxy monosaccharides (fucose, quinovose, and rhamnose) at 6 1.1-1.3 ppm. 

The aggregation behaviour and the structure in solution of two closely related dilithium compounds 90 and 91 (Figure 16) was studied by Gunther, Maercker and coworkers, using one- and two-dimensional NMR techniques ( H, C, Li) . While in diethyl ether, both compounds exist as a dimer, the dimeric structure is partly broken up in THF or by the addition of TMEDA. Also, activation barriers and thermodynamic parameters of aggregate exchanges were determined by temperature-dependent NMR studies. 

Two-Dimensional NMR—Basically, the two-dimensional NMR techniques of nuclear Overhauser effect spectroscopy (NOESY) and correlation spectroscopy (COSY) depend on the observation that spins on different protons interact with one another. Protons that are attached to adjacent atoms can be directly spin-coupled and thus can be studied using the COSY method. This technique allows assignment of certain NMR frequencies by tracking from one atom to another. The NOESY approach is based on the observation that two protons closer than about 0.5 nm perturb one another s spins even if they are not closely coupled in the primary structure. This allows spacial geometry to be determined for certain molecules. 

Basic One- and Two-Dimensional NMR Techniques. H. Frebolin, ed. Wiley Interscience, Chichester... 

Apart from all of this, multi-dimensional NMR finds considerable and still growing applications in more traditional areas of chemistry. Even if most organometallic and coordination compounds are smaller in size and exhibit simpler spectra than biopolymers, they are composed of a large pool of building blocks whose spectroscopic characteristics are less well known or unknown at all, and the bond connectivity patterns are much more diverse and intricate. Consequently, NMR spectra of organometallic and coordination compounds are less predictable, and multi-dimensional techniques are in many cases indispensable as analytical tools when structural assignments derived from the analysis of one-dimensional NMR spectra remain ambiguous or even incomplete. 

A. M. Seller and S. S. Gerritz, Using one- and two-dimensional NMR techniques to characterize reaction products bound to Chiron SynPhase crowns, J. Comb. Chem., 2000, 2, 127-133. 

H is particularly important in NMR experiments because of its high sensitivity and natural abundance. For macromolecules, 1H NMR spectra can become quite complicated. Even a small protein has hundreds of 1H atoms, typically resulting in a one-dimensional NMR spectrum too complex for analysis. Structural analysis of proteins became possible with the advent of two-dimensional NMR techniques (Fig. 3). These methods allow measurement of distance-dependent coupling of nuclear spins in nearby atoms through space (the nuclear Overhauser effect (NOE), in a method dubbed NOESY) or the coupling of nuclear spins in atoms connected by covalent bonds (total correlation spectroscopy, or TOCSY). 

New techniques for data analysis and improvements in instrumentation have now made it possible to carry out stmctural and conformational studies of biopolymers including proteins, polysaccharides, and nucleic acids. NMR, which may be done on noncrystalline materials in solution, provides a technique complementary to X-ray diffraction, which requires crystals for analysis. One-dimensional NMR, as described to this point, can offer structural data for smaller molecules. But proteins and other biopolymers with large numbers of protons will yield a very crowded spectrum with many overlapping lines. In multidimensional NMR (2-D, 3-D, 4-D), peaks are spread out through two or more axes to improve resolution. The techniques of correlation spectroscopy (COSY), nuclear Overhausser effect spectroscopy (NOESY), and transverse relaxation-optimized spectroscopy (TROSY) depend on the observation that nonequivalent protons interact with each other. By using multiple-pulse techniques, it is possible to perturb one nucleus and observe the effect on the spin states of other nuclei. The availability of powerful computers and Fourier transform (FT) calculations makes it possible to elucidate structures of proteins up to 40,000 daltons in molecular mass and there is future promise for studies on proteins over 100,000... 

In most cases, structural characterization of carbosilane dendrimers is accomplished by multinuclear one-dimensional NMR spectroscopy (1H, 13C and 29Si). However, as larger dendrimers are characterized standard spectroscopic methods become less useful due to the overlap of signals. This problem has been elegantly circumvented as described in a recent paper by Tessier, Rinaldi and coworkers56. In this paper the researchers described the use of 1 H/13C/29Si triple resonance, 3D and pulse field gradient NMR techniques to... 

Nuclear magnetic resonar ce techniques have advanced dramatically in the last few years, and are now more powerful and more versatile than ever before. To exploit these techniques efficiently, the chemist must have both an understanding of their theoretical basis, and the ability to interpret the spectia accurately. This workbook aims to develop the latter skill to an advanced level by a combination of worked examples and set problems that cover one- and two-dimensional NMR techniques applied to organic and inorganic systems. 

Distinguish clearly between a one-dimensional NMR experiment that uses a time increment, such as the inversion-recovery technique (Section 2.9 and Fig. 8.8a), and a two-dimensional NMR experiment, such as NOESY. 

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