Nuclear magnetic resonance spectra interpretation

The AlP nuclear magnetic resonance spectrum of a corundum or a-AlgOg single crystal is an orientation-dependent quintet arising from the quadrupole moment = - - 0.149 (136-139). O Reilly (140) obtained the Al dispersion mode envelope (141) powder pattern spectrum which results when y-alumina is heated to 1400°, and thereby eonverted to a-AlaOs. He interpreted the line shape in terms of the Redfield modification (142,143) of the Bloch equations. 

M. Saunders, A. Wishnia, J.G. Kirkwood, The nuclear magnetic resonance spectrum of ribonuclease, J. Amer. Chem. Soc. 79 3289-3290 (1957) O. Jardetzky, Ch.D. Jardetzky, An interpretation of the proton magnetic resonance spectrum of ribonuclease, J. Amer. Chem. Soc. 79 5322-5323 (1957)... 

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... 

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). 

For the characterisation of the biodegradation intermediates of C12-LAS, metabolised in pure culture by an a-proteobacterium, Cook and co-workers [23] used matrix-assisted laser desorption/ionisation (MALDI)-time of flight (TOF)-MS as a complementary tool to HPLC with diode array detection and 1H-nuclear magnetic resonance. The dominating signal in the spectrum at m/z 271 and 293 were assigned to the ions [M - H] and [M - 2H + Na]- of C6-SPC. Of minor intensity were the ions with m/z 285 and 299, interpreted to be the deprotonated molecular ions of C7- and C8-SPC, respectively. 

Infrared (IR) spectroscopy was the first modern spectroscopic method which became available to chemists for use in the identification of the structure of organic compounds. Not only is IR spectroscopy useful in determining which functional groups are present in a molecule, but also with more careful analysis of the spectrum, additional structural details can be obtained. For example, it is possible to determine whether an alkene is cis or trans. With the advent of nuclear magnetic resonance (NMR) spectroscopy, IR spectroscopy became used to a lesser extent in structural identification. This is because NMR spectra typically are more easily interpreted than are IR spectra. However, there was a renewed interest in IR spectroscopy in the late 1970s for the identification of highly unstable molecules. Concurrent with this renewed interest were advances in computational chemistry which allowed, for the first time, the actual computation of IR spectra of a molecular system with reasonable accuracy. This chapter describes how the confluence of a new experimental technique with that of improved computational methods led to a major advance in the structural identification of highly unstable molecules and reactive intermediates. 

Detailed descriptions of the theory and instrumentation of nuclear magnetic resonance Spectroscopy can be found elsewhere (see Bibliography). In this chapter the important features of the NMR spectrum and their use in the interpretation of spectra are described, together with details of certain special procedures which assist interpretation. 

An extremely sensitive technique able to detect the nature of radical pairs in a photochemical reaction is called chemically induced dynamic nuclear polarization (CIDNP), which depends on the observation of an enhanced absorption in a nuclear magnetic resonance (NMR) spectrum of the sample, irradiated in situ, in the cavity of a NMR spectrometer. The background to and interpretation of CIDNP are discussed by Gilbert and Baggott (28). 

Obtain infrared and nuclear magnetic resonance spectra following the procedures of Chapters 19 and 20. If these spectra indicate the presence of conjugated double bonds, aromatic rings, or conjugated carbonyl compounds obtain the ultraviolet spectrum following the procedures of Chapter 21. Interpret the spectra as fully as possible by reference to the sources cited at the end of the various spectroscopy chapters. 

The nuclear magnetic resonance (NMR) spectrometer has become in recent years one of the most popular and useful tools available to the chemist for structural studies. In the usual high-resolution NMR spectrum of a liquid the scalar parameter called the chemical shift is readily obtained and may be interpreted in an approximate and qualitative fashion to establish features of the overall structure. The task of placing the interpretation of the chemical shift on a more quantitative and rigorous basis is not an easy one but considerable progress has been made in this direction. The potential rewards of improved understanding in this area seem most attractive. 

This section provides correlation charts and operational information for the design and interpretation of ultraviolet-visible spectrophotometric (UV-Vis) measurements. While UV-Vis is perhaps not as information-rich as infrared or nuclear magnetic resonance, it nonetheless has value in structure determination and sample identification. Moreover, it is extremely valuable in quantitative work. Typical UV-Vis instruments cover not only the UV and visible spectrum, but the near-infrared as well. Although there is overlap among the ranges, the approximate breakdown is ... 

Another interesting spectroscopy performed on SmBg is nuclear magnetic resonance (NMR) where the temperature dependence of the B relaxation rate has been measured by Pena et al. (1981). Above 10 K the temperature dependence is exponential with a gap of 5.6 meV The authors interpret their results as the consequence of the fluctuations of 4f spins thus relating the measured line width to the contribution of the hyperfine field from these fluctuations. The NMR experiments thus measure directly a gap in the 4f spectrum, where the only other experiment, directly related to a gap in the 4f spectrum, was the IDS obtained by optical reflection by Travaglini and Wachter (1984a). Similar NMR results have been obtained by Takigawa et al. (1983). 

Nuclear magnetic resonance (NMR) spectroscopy is the most informative analytical technique and is widely applied in combinatorial chemistry. However, an automated interpretation of the NMR spectral results is difficult (3,4). Usually the interpretation can be supported by use of spectrum calculation (5-18) and structure generator programs (8,12,18-21). Automated structure validation methods rely on NMR signal comparison using substructure/ subspectra correlated databases or shift prediction methods (8,15,22,23). We have recently introduced a novel NMR method called AutoDROP (Automated Definition and Recognition of Patterns) to rapidly analyze compounds libraries (24-29). The method is based on experimental data obtained from the measured ID or 2D iH,i C correlated (HSQC) spectra. 

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