Magnetic field, NMR spectroscopy

NMR spectroscopy is a powerful technique to study molecular structure, order, and dynamics. Because of the anisotropy of the interactions of nuclear spins with each other and with their environment via dipolar, chemical shift, and quadrupolar interactions, the NMR frequencies depend on the orientation of a given molecular unit relative to the external magnetic field. NMR spectroscopy is thus quite valuable to characterize partially oriented systems. Solid-state NMR... 

The field of steroid analysis includes identification of steroids in biological samples, analysis of pharmaceutical formulations, and elucidation of steroid stmctures. Many different analytical methods, such as ultraviolet (uv) spectroscopy, infrared (ir) spectroscopy, nuclear magnetic resonance (nmr) spectroscopy, x-ray crystallography, and mass spectroscopy, are used for steroid analysis. The constant development of these analytical techniques has stimulated the advancement of steroid analysis. 

Generally, the most powerful method for stmctural elucidation of steroids is nuclear magnetic resonance (nmr) spectroscopy. There are several classical reviews on the one-dimensional (1-D) proton H-nmr spectroscopy of steroids (267). C-nmr, a technique used to observe individual carbons, is used for stmcture elucidation of steroids. In addition, C-nmr is used for biosynthesis experiments with C-enriched precursors (268). The availability of higher magnetic field instmments coupled with the arrival of 1-D and two-dimensional (2-D) techniques such as DEPT, COSY, NOESY, 2-D J-resolved, HOHAHA, etc, have provided powerful new tools for the stmctural elucidation of complex natural products including steroids (269). 

Nuclear magnetic resonance (NMR) spectroscopy is, next to X-ray diffraction, the most important method to elucidate molecular structures of small molecules up to large bio macromolecules. It is used as a routine method in every chemical laboratory and it is not the aim of this article to give a comprehensive review about NMR in structural analysis. We will concentrate here on liquid-state applications with respect to drugs or drug-like molecules to emphasize techniques for conformational analysis including recent developments in the field. 

A full range of spectral data was routinely reported for each of the new compounds isolated. Nuclear magnetic resonance (NMR) spectroscopy and X-ray crystallography have essentially only been used as methods of structure determination/ confirmation and the results are unexceptional. The use of mass spectrometry in these series of compounds has been mainly confined to molecular ion determination. Ultraviolet (UV), infrared (IR), and Raman techniques have been used for confirmation of structures, but no special report has been published. The major data in this field are well documented in CHEC-II(1996) and will not be reproduced in this chapter. Over the last decade, all these methods played a major role in establishing the structure, but did not provide new interesting structural information on these bicyclic systems. In consequence, these methods are not considered worthy of mention in detail here. 

Nuclear magnetic resonance (NMR) spectroscopy is used to study the behavior of the nuclei in a molecule when subjected to an externally applied magnetic field. Nuclei spin about the axis of the externally applied magnetic field and consequently possess an angular momentum. The group of nuclei most commonly exploited in the structural... 

In a traditional magnet for NMR spectroscopy, the field Bq of the magnet is much higher than the field components originating from outside sources. Moreover, devices such as efficient NMR field stabilizers are used to suppress all interfering external fields. Consequently, the presence of such field components can be usually ignored. On the contrary, during a FFC NMR measurement the sample may be subject to very low fields (ideally down to zero) which is practically impossible when the relaxation field value becomes comparable to the environmental fields. The amplitude of such fields, if not compensated, represents the lower relaxation field limit for a reliable NMRD profile. 

Nuclear magnetic resonance (NMR) spectroscopy In NMR technique, a sample is placed in a magnetic field which forces the nuclei into alignment. When the sample is bombarded with radiowaves, they are absorbed by the nuclei. The nuclei topple out of alignment with the magnetic field. By measuring the specific radiofrequencies that are emitted by the nuclei and the rate at which the realignment occurs, the spectroscope can obtain the information on molecular structure. 

Nuclear magnetic resonance (NMR) spectroscopy is a most effective and significant method for observing the structure and dynamics of polymer chains both in solution and in the solid state [1]. Undoubtedly the widest application of NMR spectroscopy is in the field of structure determination. The identification of certain atoms or groups in a molecule as well as their position relative to each other can be obtained by one-, two-, and three-dimensional NMR. Of importance to polymerization of vinyl monomers is the orientation of each vinyl monomer unit to the growing chain tacticity. The time scale involved in NMR measurements makes it possible to study certain rate processes, including chemical reaction rates. Other applications are isomerism, internal relaxation, conformational analysis, and tautomerism. 

Nuclear magnetic resonance (NMR) spectroscopy is a method of absorption spectroscopy that has some characteristics similar to ultraviolet and visible spectroscopy but also some that are unique. In NMR, a molecular sample, usually dissolved in a liquid solvent, is placed in a magnetic field... 

Nuclear magnetic resonance (NMR) spectroscopy is also largely used to characterize C02 complexes. The 13C NMR spectrum of C02 dissolved in a nonpolar solvent shows a resonance at 124ppm, which is shifted when C02 is bonded to a metal center. Depending on the mode of bonding, the shift may be up or down field, and may vary from a few ppm up to several hundreds of ppm. A few examples are given below for different types of bonding. 

---