Spectroscopy magnetic resonance

In MRS techniques, the magnetic field is focussed on small selected areas or volumes of tissue and the NMR spectra recorded. 

Changes in the concentration of various metabolic P compounds can be recorded and related to the functioning of the particular part of the body from which they were obtained. MRS can be used to establish the presence or absence of those P compounds which are characteristic of certain diseases, and the technique shows considerable promise in this area of use [71], 

One of the simplest applications of MRS is in the measurement of intracellular pH. The chemical shifts characteristic of H2PO4 and HPO differ by only 2.4 ppm and the equilibrium between the two types of anion in the body results in only one observed resonance peak. The exact position of this resonance peak depends on the ratio of the two anionic species, and thus it can be used to estimate pH. 

Another illustration of the application of MRS is provided by the P spectra of human forearm muscle, taken before and during exercise. Both spectra show resonance peaks characteristic of CP, ATP and orthophosphate ions (Pi). A comparison of peak intensities indicates considerable changes of CP and P, while the ATP content remains nearly constant [64]. 

There is equipment already in use which can carry out whole-body scans, and combine H MRI with 3 P MRS. Instrumentation is presently undergoing vay rapid development and it seems not unlikely that the two techniques (both utilising P resonance) will become leading methods for the diagnosis of disease, assessment of drug effects and the observation of the results of surgery, etc. Widespread application and use has so far been somewhat limited by the high initial cost of equipment. 

As indicated previously, NMR may be used simply as an analytical technique for monitoring the decomposition of a reactant or formation of a product. In addition, NMR and ESR merit a special mention due to their importance in studying the dynamics of systems at equilibrium these so-called equilibrium methods do not alter the dynamic equilibrium of the chemical process under study. They have been used to study, for example, -transfer reactions, valence isomerisations, conformational interconversions, heteronuclear isotopic exchange processes (NMR) and electron-transfer reactions (ESR). These techniques can be applied to the study of fast or very fast reactions by analysis of spectral line broadening [16,39], 

Two objectives are fulfilled when using NMR (a) the functional groups involved in the chemical change are characterised and (b) kinetic parameters are determined. For a system A B, a single rate constant, k=(kf+kb),is determined from line shape analysis, kf and kb being the rate constants for the forward and back reactions, respectively. Individual values of kf and kb can then be calculated from the equilibrium constant, K = kf /kb, if that is independently measured from the equilibrium concentrations of A and B [40]. 

The higher concentrations of the sample needed for the NMR method compared with other physical methods is a drawback, as also is the lower precision in the determination of rate constants. The latter is usually because the temperature of the sample in the NMR probe is controlled by a flow of heated or cooled nitrogen which does not normally provide highly accurate temperature control and measurement. Sometimes, the need for isotopically labelled substrates and solvents can be an additional drawback. 

Electron spin resonance (ESR) spectroscopy is of application to organic species containing unpaired electrons radicals, radical ions and triplet states, and is much more sensitive than NMR it is an extremely powerful tool in the field of radical chemistry (see Chapter 10). Highly unstable radicals can be generated in situ or, if necessary, trapped into solid matrices at very low temperatures. Examples of the application of this techniques include study of the formation of radical cations of methoxylated benzenes by reaction with different strong oxidants in aqueous solution [45], and the study of the photodissociation of N-trityl-anilines [46], 

The major limitations to the use of ESR other than for fundamental studies of the radical and other trapped species formed during reactive processing are the experimental requirements of the apparatus. There has been success in using ESR to monitor the concentration of the propagating free radicals during the emulsion batch polymerization of methyl methacrylate (Parker et al, 1996) by using a time-sweep method for data acquisition and 

The low abundance of the NMR-active isotope of the most useful element, carbon, means that spectral acquisition times have to be long in order to obtain spectra with a good SNR. Analyses are therefore required off-line in order to determine the species formed in the process cycle. Unless the polymer may be dissolved in a suitable solvent, the NMR spectra are extremely broad. 

Studies of passivation layers and corrosion phenomena, particularly with iron and magnesium using EXAFS, NEXAFS and related X-ray absorption spectroscopies have been reviewed elsewhere [589]. Using GIXAFS, depth profiling has become possible and spatially resolved studies of silver dissolution and lithium intercalation have been reported. 

The formation and dissociation of S-S bonds in poly(tricyanuric acid), which is proposed as electrode material for lithium batteries, has been studied [590,591]. The reversibility of the process essential for the use of this material in a secondary battery could be established. Further studies of battery materials have been reported [592, 593]. X-ray absorption near edge structure spectroscopy has been successfully employed in studies of inhibiting species in passive films and the adjacent electrolyte solutions. 

Because in many electrochemical reactions, particularly in electroorganic ones, radicals are formed as reactive intermediates, ESR has been applied frequently to studies of the mechanism and the kinetics of these reactions [594-596]. Although possible, NMR spectroscopy has been used infrequently and only in very recent experiments, mainly because of the considerably larger experimental effort [597]. With NMR spectroscopy, information about surface structure, surface diffusion and electron spillover from the metal electrode onto an adsorbate can be obtained. So 

This method has frequently been called electron paramagnetic resonance spectroscopy (EPR) because the presence of one or several unpaired electrons, being a precondition for this spectroscopy, is also closely related to the phenomenon of paramagnetism. 

When considering an atom containing these nucleons in numbers characteristic for a given element, the resulting properties of the atom are slightly more complex. The resulting spin of the atomic nucleus depends upon the number of protons and neutrons and the relationship between both as listed in Table 5.1. 

---

Brown, J.K. and W.R. Ladner Jr (1960), Distribution in coallike materials by high-resolution nuclear magnetic resonance spectroscopy . Fuel, Vol. 39, p. 87. 

Lynden-Bell R M and Harris R K 1969 Nuclear Magnetic Resonance Spectroscopy (London Nelson) pp 81-3... 

Flaw J F 1992 Nuclear magnetic resonance spectroscopy Ana/. Chem. 64 R243-54... 

No molecule is completely rigid and fixed. Molecules vibrate, parts of a molecule may rotate internally, weak bonds break and re-fonn. Nuclear magnetic resonance spectroscopy (NMR) is particularly well suited to observe an important class of these motions and rearrangements. An example is tire restricted rotation about bonds, which can cause dramatic effects in the NMR spectrum (figure B2.4.1). 

Freeman R and Hiii H D W 1975 Determination of spin-spin reiaxation time in high-resoiution NMR Dynamic Nuclear Magnetic Resonance Spectroscopy e6 L M Jaokman and F A Cotton (New York Aoademio) p 131-62... 

Jaokman L M and Cotton F A 1975 Dynamic Nuclear Magnetic Resonance Spectroscopy (New York Aoademio)... 

Gutowsky H S and Holm C H 1975 Time-dependent magnetic perturbations Dynamic Nuclear Magnetic Resonance Spectroscopy ed L M Jackman and F A Cotton (New York Academic) pp 1-21... 

Karge FI G, Flunger M and Beyer FI K 1999 Characterization of zeolites—infrared and nuclear magnetic resonance spectroscopy and x-ray diffraction Catalysis and Zeolites, Fundamentals and Applications ed J Weitkamp and L... 

Present day techniques for structure determination in carbohydrate chemistry are sub stantially the same as those for any other type of compound The full range of modern instrumental methods including mass spectrometry and infrared and nuclear magnetic resonance spectroscopy is brought to bear on the problem If the unknown substance is crystalline X ray diffraction can provide precise structural information that m the best cases IS equivalent to taking a three dimensional photograph of the molecule... 

Monomer (Section 6 21) The simplest stable molecule from which a particular polymer may be prepared Monosaccharide (Section 25 1) A carbohydrate that cannot be hydrolyzed further to yield a simpler carbohydrate Monosubstituted alkene (Section 5 6) An alkene of the type RCH=CH2 in which there is only one carbon directly bonded to the carbons of the double bond Multiplicity (Section 13 7) The number of peaks into which a signal IS split in nuclear magnetic resonance spectroscopy Signals are described as singlets doublets triplets and so on according to the number of peaks into which they are split... 

Most hydrocarbon resins are composed of a mixture of monomers and are rather difficult to hiUy characterize on a molecular level. The characteristics of resins are typically defined by physical properties such as softening point, color, molecular weight, melt viscosity, and solubiHty parameter. These properties predict performance characteristics and are essential in designing resins for specific appHcations. Actual characterization techniques used to define the broad molecular properties of hydrocarbon resins are Fourier transform infrared spectroscopy (ftir), nuclear magnetic resonance spectroscopy (nmr), and differential scanning calorimetry (dsc). 

Nuclear Magnetic Resonance Spectroscopy. Bmker s database, designed for use with its spectrophotometers, contains 20,000 C-nmr and H-nmr, as weU as a combined nmr-ms database (66). Sadder Laboratories markets a PC-based system that can search its coUection of 30,000 C-nmr spectra by substmcture as weU as by peak assignments and by fiiU spectmm (64). Other databases include one by Varian and a CD-ROM system containing polymer spectra produced by Tsukuba University, Japan. CSEARCH, a system developed at the University of Vieima by Robien, searches a database of almost 16,000 C-nmr. Molecular Design Limited (MDL) has adapted the Robien database to be searched in the MACCS and ISIS graphical display and search environment (63). Projects are under way to link the MDL system with the Sadder Hbrary and its unique search capabiHties. 

Both vapor-phase chromatography and high performance Hquid chromatography, along with nuclear magnetic resonance spectroscopy, have been used for isomer and composition analysis. 

Instmmental methods of analysis provide information about the specific composition and purity of the amines. QuaUtative information about the identity of the product (functional groups present) and quantitative analysis (amount of various components such as nitrile, amide, acid, and deterruination of unsaturation) can be obtained by infrared analysis. Gas chromatography (gc), with a Hquid phase of either Apiezon grease or Carbowax, and high performance Hquid chromatography (hplc), using siHca columns and solvent systems such as isooctane, methyl tert-huty ether, tetrahydrofuran, and methanol, are used for quantitative analysis of fatty amine mixtures. Nuclear magnetic resonance spectroscopy (nmr), both proton ( H) and carbon-13 ( C), which can be used for quaHtative and quantitative analysis, is an important method used to analyze fatty amines (8,81). 

A detailed account is given in Reference 20. The techniques giving the most detailed 3-D stmctural information are x-ray and neutron diffraction, electron diffraction and microscopy (qv), and nuclear magnetic resonance spectroscopy (nmr) (see Analytical methods Magnetic spin resonance X-ray technology). 

Nuclear Magnetic Resonance Spectroscopy. The hterature up to 1984 has been reviewed (14). 

There are a variety of analytical methods commonly used for the characterization of neat soap and bar soaps. Many of these methods have been pubUshed as official methods by the American Oil Chemists Society (29). Additionally, many analysts choose United States Pharmacopoeia (USP), British Pharmacopoeia (BP), or Pood Chemical Codex (FCC) methods. These methods tend to be colorimetric, potentiometric, or titrametric procedures. However, a variety of instmmental techniques are also frequendy utilized, eg, gas chromatography, high performance Hquid chromatography, nuclear magnetic resonance spectroscopy, infrared spectroscopy, and mass spectrometry. 

Nuclear Magnetic Resonance Spectroscopy. Nmr is a most valuable technique for stmeture determination in thiophene chemistry, especially because spectral interpretation is much easier in the thiophene series compared to benzene derivatives. Chemical shifts in proton nmr are well documented for thiophene (CDCl ), 6 = 7.12, 7.34, 7.34, and 7.12 ppm. Coupling constants occur in well-defined ranges J2-3 = 4.9-5.8 ... 

Boron s electron deficiency does not permit conventional two-electron bonds. Boron can form multicenter bonds. Thus the boron hydrides have stmctures quite unlike hydrocarbons. The B nucleus, which has a spin of 3/2, which has been employed in boron nuclear magnetic resonance spectroscopy. 

---