Nuclear magnetic resonance spectra elucidation

Nuclear magnetic resonance spectra could have been useful in the structural elucidation and identification of the K vitamins. This is demonstrated in Fig. 4 by the nuclear magnetic resonance spectrum of vitamin Kgcst). The different resonances in Fig. 4 can be attributed to all the nonequivalent protons in the molecule. The resonance at —12 cycles per second (c.p.s.) is... 

Other methods of identification include the customary preparation of derivatives, comparisons with authentic substances whenever possible, and periodate oxidation. Lately, the application of nuclear magnetic resonance spectroscopy has provided an elegant approach to the elucidation of structures and stereochemistry of various deoxy sugars (18). Microcell techniques can provide a spectrum on 5-6 mg. of sample. The practicing chemist is frequently confronted with the problem of having on hand a few milligrams of a product whose structure is unknown. It is especially in such instances that a full appreciation of the functions of mass spectrometry can be developed. 

Mass spectrometry is an analytical technique to measure molecular masses and to elucidate the structure of molecules by recording the products of their ionization. The mass spectrum is a unique characteristic of a compound. In general it contains information on the molecular mass of an analyte and the masses of its structural fragments. An ion with the heaviest mass in the spectrum is called a molecular ion and represents the molecular mass of the analyte. Because atomic and molecular masses are simple and well-known parameters, a mass spectrum is much easier to understand and interpret than nuclear magnetic resonance (NMR), infrared (IR), ultraviolet (UV), or other types of spectra obtained with various physicochemical methods. Mass spectra are represented in graphic or table format (Fig. 5.1). 

Nuclear Magnetic Resonance (NMR) spectroscopy is one of the most powerful analytical techniques in organic chemistry for elucidating the molecular structures of chemicals (1,2). Moreover, an NMR spectrum may be used like a fingerprint to identify a chemical by comparing it with its reference spectrum recorded from the authentic chemical under comparable conditions. The spectrum also reveals information on molecular conformation, isomerism, molecular dynamics, and diastereomers (3 6). 

A mtyor alkaloid (251F) in the small Colombian poison frog Minyabates bombetes was unusual in exhibiting a base peak at an odd mass (mh 111) in its mass spectrum (151). It was clearly unrelated to other dendrobatid alkaloids and appears to be unique to this species. The structure of alkaloid 251F has been elucidated by nuclear magnetic resonance and FTIR spectroscopy (752) it is a cyclopenta[7>]quinolizidine as shown in XI. 

Nuclear magnetic resonance analysis with double and triple resonance was used to elucidate the structure as 3, 4 -dideoxykanamycin B 2"-adenylate. The spectrum of the inactivated 3, 4 -dideoxykanamycin B in deuterium oxide at pH 8.0, with tetramethylsilane as the external reference standard (8 =0), showed signals at 8 8.63 and 8.85 attributable to the adenine-ring protons, and at 8 6.53, 5.28, 4.98, 4.83 and 4.6 (H-2) the latter were assigned to the D-ribose-ring protons by successive doubleresonance experiments, and by comparison with disodium 5 -adenylate in deuterium oxide. Therefore, these observations confirmed the presence of one molecular proportion of 5 -adenylic acid in the molecule. Irradiation at 8 5.45 (7 3.6, H-1") caused the complex signal at 8 4.3 (H-2")... 

Part Nuclear Magnetic Resonance Spectroscopy by W. Robien focuses on structure elucidation of organic compounds. Spectra similarity searches, spectrum prediction (from a given chemical structure), recognition of substructures and automatic isomer generation are the main topics they are still areas of scientific research in computer-assisted structure elucidation. 

If the objective is identification (qualitative analysis), it suffices to compare the spectrum of the analyte with that of a standard, both recorded in the same solvent and at an identical pH. This is not the main application of UV-Vis spectrophotometry as the best results in this context are provided by spectroscopic methods considered more effective for the study of the molecular structure of organic compounds (infrared, nuclear magnetic resonance, mass spectrometry, and X-ray diffraction). However, UV-Vis spectrophotometry is a source of relevant supplementary information that helps in the elucidation of molecular structures of drugs, impurities, metabolites, intermediate compounds of degradation, etc. 

Nuclear magnetic resonance (NMR) spectroscopy has great relevance in the area of investigation of molecular structures. Many studies using NMR spectroscopy and microscopy have been reported in elastomer based systems. Due to the possibihty of reuse of samples after the analysis (NMR is nondestructive) NMR attracts many material scientists to select this technique for characterization. In the case of polymers NMR is specifically useful in finding out the crosslink density. Since the crosslink density is related to the size of pores or cavities inside solid polymers, this method points towards the structural elucidation of polymers. From the parameters such as magnetic relaxation and the dipolar correlation effect obtained from the NMR spectrum, crosslink density can be calculated. In addition to the crosslink density, the behaviour of small particles inside the polymer matrices can also be... 

An NMR signal arises from an interaction between the nuclear spin (I) of an atom with an external magnetic field (Bq). Elements with even atomic mass and number have a zero nuclear spin, where only nuclei with 1 0 produce an observable NMR signal. Nuclei with I > A generate broad resonance lines in an NMR spectrum and are not generally useful for structure elucidation. Fortunately, most of the elements that comprise metabolites correspond to nuclei that are observable by NMR. They are listed in Table 12.1 along with their relevant NMR properties. 

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