Nuclear magnetic resonance spectra measurement

It is now accepted that the proper expression of hormone activity depends on the conformation of the peptide. Unfortunately, the conformation of peptides of 20 or more amino acids, which are the subject of this chapter, cannot easily be determined by spectroscopic methods, such as nuclear magnetic resonance, which have been used successfully for some smaller peptides. Crystallization of some of these molecules, however, has allowed the determination, using single-crystal X-ray diffraction, of their three-dimensional structures. These have been related to the solution structures by comparing the circular dichroism and nuclear magnetic resonance spectra measured in... 

Protonation of simple amides has been found to markedly decrease the double bond character of theC—N amide linkage. Thus Berger, et al. (1959) foimd that protonation of the C—N bond in iNT-methylacetamide and iV,AT -dimethylacetamide resulted in a depression of the double bond character as measured by nuclear magnetic resonance. In neutral aqueous solution the nuclear magnetic resonance spectrum shows the presence of two iV-methyl lines indicating the absence of free rotation about the C—N linkage. In the presence of acid the doublet is replaced by a single absorption band with the onset of free rotation. 

Nuclear magnetic resonance spectroscopy is a complex technique that is used to determine the constituents of foods. This method makes use of the fact that some compounds contain certain atomic nuclei which can be identified from a nuclear magnetic resonance spectrum, which measures variations in frequency of electromagnetic radiation absorbed. It provides more specific and detailed information of the conformational structure of compoimds than, for example, NIRS but is more costly and requires more time and skUl on the part of the operator. For these reasons, it is more suited to research work and for cases in which the results from simpler spectroscopy techniques require further investigation. Nuclear magnetic resonance spectroscopy has been useful in the investigation of the soluble and structural components of forages. 

Nuclear spin relaxation measurements were made of single crystals at between 150 and 1500K, under an O partial pressure of latm. The nuclear magnetic resonance spectrum consisted of a doublet which resulted from 2 different orientations of the electric field gradient tensor at 2 different Ti sites in the unit cell of the rutile lattice. The electric field gradient tensor was due to the 6 surrounding q2- ions, which formed a stretched octahedron. From the temporal evolution of the nuclear spin relaxation rate after a step-wise change in the 0 partial pressure, 2 different types of motion of the intrinsic defects were... 

Nuclear magnetic resonance spectroscopy Measures the absorption of light energy in the radio-frequency portion of the electromagnetic spectrum. NMR spectroscopy furnishes indirect information about the carbon skeleton of organic molecules. In C NMR peaks corresponding to all carbon atoms are recorded. 

Nuclear magnetic resonance measurements have led to the conclu-sion that 2-pyridones have about 35% of the aromaticity of benzene and that the formally related l,2-dihydro-2-methylenepy-ridine is not aromatic. A substantial contribution by such resonance is indicated by the electronic spectrum of 2-quinolone, which is... 

The small amount of available crystalline abscisin II limited this investigation to the measurement and interpretation of elemental analysis, mass spectrum, and infrared, ultraviolet, and nuclear magnetic resonance (NMR) spectra (11). 

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... 

The basic instrumentation used for spectrometric measurements has already been described in Chapter 7 (p. 277). The natures of sources, monochromators, detectors, and sample cells required for molecular absorption techniques are summarized in Table 9.1. The principal difference between instrumentation for atomic emission and molecular absorption spectrometry is in the need for a separate source of radiation for the latter. In the infrared, visible and ultraviolet regions, white sources are used, i.e. the energy or frequency range of the source covers most or all of the relevant portion of the spectrum. In contrast, nuclear magnetic resonance spectrometers employ a narrow waveband radio-frequency transmitter, a tuned detector and no monochromator. 

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). 

A nuclear magnetic resonance method for chlorpromazine has been reported [178]. For tablets, capsules, and bulk chemical, the sample is shaken with CHCI3 containing cyclohexane or piperanol as an internal standard. For injectable solutions, tetramethylammonium bromide was used as the internal standard. The NMR spectrum was recorded between 0 and 7.0 ppm, and the drug resonance at 2.7 ppm (relative to TMS) measured. The signals for the respective internal standards were at 1.5, 6.0, and 3.3 ppm. 

Solid-state C variable-amplitude cross polarization magic-angle spinning (VACP/MAS) nuclear magnetic resonance (NMR) spectra were acquired for the sorbitol samples. Proton decoupling was achieved by a two-pulse phase modulation (TPPM) sequence. Identical C spectra were measured for the y-form sorbitol samples, and a representative spectrum is shown in Figure 9. 

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