Carbon-13 nuclear magnetic resonance instrumentation

Electrochemical nuclear magnetic resonance (NMR) is a relatively new technique that has recently been reviewed (Babu et al., 2003). NMR has low sensitivity, and a typical high-held NMR instrument needs 10 to 10 NMR active atoms (e.g., spins), to collect good data in a reasonable time period. Since 1 cm of a single-crystal metal contains about 10 atoms, at least 1 m of surface area is needed to meet the NMR sensitivity requirement. This can be met by working with carbon-supported platinum... 

Proton and carbon-13 nuclear magnetic resonance (NMR) spectra were recorded on a IBM Instruments 270 MHz NMR Spectrometer on 6-8 weight percent solutions in deuterated chloroform. Ultraviolet spectra were recorded on an IBM Ultraviolet Spectropluitometer Model 9420 using chloroform solutions containing 2 x 10-5 g/ml of the copolymers. 

If one wishes to obtain a fluorine NMR spectrum, one must of course first have access to a spectrometer with a probe that will allow observation of fluorine nuclei. Fortunately, most modern high field NMR spectrometers that are available in industrial and academic research laboratories today have this capability. Probably the most common NMR spectrometers in use today for taking routine NMR spectra are 300 MHz instruments, which measure proton spectra at 300 MHz, carbon spectra at 75.5 MHz and fluorine spectra at 282 MHz. Before obtaining and attempting to interpret fluorine NMR spectra, it would be advisable to become familiar with some of the fundamental concepts related to fluorine chemical shifts and spin-spin coupling constants that are presented in this book. There is also a very nice introduction to fluorine NMR by W. S. and M. L. Brey in the Encyclopedia of Nuclear Magnetic Resonance.1... 

Today, organic chemists rely on an array of very powerful instruments that enable them to identify compounds in much less time. With use of these instruments, it is often possible to determine the structure of an unknown compound in less than an hour. Three of the most powerful techniques are presented in this and the following chapters. They are infrared spectroscopy and two related techniques proton and carbon-13 nuclear magnetic resonance spectroscopy. Spectroscopy is the study of the interaction of electromagnetic radiation (light) with molecules. 

GeneraUy, 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 instruments 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 namral products including steroids (269). 

NMR. Quantitative liquid-state carbon-13 nuclear magnetic resonance ( 3c NMR) spectra were recorded for humic and fulvic acid from Como Creek foam and for stream and foam fulvic- and humic- acid samples from the Suwannee River at the U.S. Geological Survey, laboratory in Arvada, CO. C NMR could not be performed on other humic substances due to insufficient sample or instrument availability. The acquisition parameters used were as follows C NMR spectra were recorded on a Varian XL-300 NMR spectrometer at 75 MHz. Each sample (200 mg of freeze-dried material) was dissolved in deuterated water and deuterated sodium hydroxide was added to ensure solution a total solution volume of approximately 6 to 7 mL. Spectra were recorded using a 30,000 Hz spectral window, a 45 pulse width, a 0.199 second acquisition time, and a pulse delay of 10 seconds for quantitative spectra. The number of transients was 10,000, and line broadening was 50 Hz. 

Today, a number of different instrumental techniques are used to identify organic compounds. These techniques can be performed quickly on small amounts of a compound and can provide much more information about the compound s structure than simple chemical tests can provide. We have already discussed one such technique ultraviolet/visible (UVA/is) spectroscopy, which provides information about organic compounds with conjugated double bonds. In this chapter, we will look at two more instrumental techniques mass spectrometry and infrared (IR) spectroscopy. Mass spectrometry allows us to determine the molecular mass and the molecular formula of a compound, as well as certain structural features of the compound. Infrared spectroscopy allows us to determine the kinds of functional groups a compound has. In the next chapter, we will look at nuclear magnetic resonance (NMR) spectroscopy, which provides information about the carbon-hydrogen framework of a compound. Of these instrumental techniques, mass spectrometry is the only one that does not involve electromagnetic radiation. Thus, it is called spectrometry, whereas the others are called spectroscopy. 

Chapter 13 introduced two instrumental techniques used to determine the structure of organic compounds mass spectrometry and IR spectroscopy. Now we will look at nuclear magnetic resonance (NMR) spectroscopy, another instrumental technique that chemists use to determine a compound s structure. NMR spectroscopy helps to identify the carbon-hydrogen framework of an organic compound. 

The carbon-13 nuclear magnetic resonance spectrum of lovastatin shown In Figure 3 was obtained using a Bruker instruments Model AM-300 NMR spectrometer and an approximately 4% w/v solution of the compound In deuterochloroform. Signal assignments are tabulated below and refer to the numbered structure shown in Section 4.2... 

See also Liquid Chromatography Liquid Chromatography-Nuclear Magnetic Resonance Spectrometry. Nu-ciear Magnetic Resonance Spectroscopy Principles Instrumentation. Nuclear Magnetic Resonance Spectroscopy-Applicable Elements Carbon-13 Nitrogen-15. Nuclear Magnetic Resonance Spectroscopy Techniques Nuclear Overhauser Effect. 

See also Atomic Absorption Spectrometry Principles and Instrumentation. Atomic Emission Spectrometry Inductively Coupled Plasma. Cosmetics and Toiletries. Derivatization of Analytes. Electrophoresis Is-otachophoresls. Environmental Analysis. Enzymes Overview. Extraction Supercritical Fluid Extraction Solid-Phase Extraction Solid-Phase Microextraction. Ion Exchange Ion Chromatography Applications. Liquid Chromatography Reversed Phase Liquid Chromatography-Mass Spectrometry. Nuclear Magnetic Resonance Spectroscopy - Applicable Elements Carbon-13 Phosphorus-31. Perfumes. 

Proton nuclear magnetic resonance (NMR) spectra (60 MHz) were recorded on a Varian EM-360L spectrometer. Carbon-13 (25 MHz) and 100 MHz proton NMR spectra were obtained on a Jeol JNM-FX-100 instrument. 1-H NMR (300 MHz) and 75 MHz 13-C spectra were determined on a Nicolet NT-300 instrument. Chemical shifts are given in parts per million (ppm) on a 6 scale downfield from tetramethylsilane (TMS) or solvent peaks [(dimethyl sulfoxide-d ) ... 

Nuclear Magnetic Resonance Spectroscopy Using an NMR instrument, obtain a proton NMR spectrum of your carvone. Compare your spectrum with the NMR spectra for (—)-carvone and (+)-limonene shown in this experiment. Attempt to assign as many peaks as you can. If your NMR instrument is capable of obtaining a carbon-13... 

Carbon-12, the most abundant isotope of carbon, does not possess spin (I = 0) it has both an even atomic number and an even atomic weight. The second principal isotope of carbon, however, does have fhe nuclear spin property (I = j). atom resonances are not easy to observe, due to a combination of two factors. First, the natural abundance of is low only 1.08% of all carbon atoms are Second, the magnetic moment fi of is low. For these two reasons, the resonances of are about 6000 times weaker than those of hydrogen. With special Fourier transform (FT) instrumental techniques, which are not discussed here, it is possible to observe nuclear magnetic resonance (carbon-13) spectra on samples that contain only the natural abundance of... 

Nuclear magnetic resonance (NMR) spectroscopy is an invaluable instrumental technique for the structural determination of all flavonoids including 3-deoxy-anthocyanins. As well as providing information on the chemical environment of each proton or carbon nucleus in the molecule, the technique can be employed to determine linkages among nearby nuclei, often enabling a complete structure to be assembled. The reader is referred to Refs. 44 and 45 for details of the principles of NMR and general interpretation of NMR spectra. 

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