Nuclear Magnetic Resonance Pulse Analysis

One of the main innovations in NMR spectroscopy became a practical reality around 1980 when spectrometers changed from scanning spectrometers to pulse-analyzing spectrometers.  

Going back to the idea of representing a function by a Fourier series [8] as 

FIGURE 19.5 A schematic of how the tip of the magnetic moment of the spin-1/2 nucleus is flipped from one quantized state to the other hy a 180° side magnetic field. The +1, 0, —1 refer to energy in energy units of S/2. (Drawing by Dr. Walter Scott, University of British Columbia.) 

Source Lide, D.R., CRC Handbook of Chemistry and Physics, 90th Edn., CRC Press,  

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Vlahov, G., Shaw, A. D., and Kell, D. B. (1999). Use of 13C nuclear magnetic resonance distortionless enhancement by polarization transfer pulse sequence and multivariate analysis to discriminate olive oil cultivars. JAOCS 76,1223-1231. 

Methods such as nuclear magnetic resonance (NMR), electron spectroscopy for chemical analysis (ESCA), electron spin resonance (ESR), infrared (IR), and laser raman spectroscopy could be used in conjunction with rate studies to define mechanisms. Another alternative would be to use fast kinetic techniques such as pressure-jump relaxation, electric field pulse, or stopped flow (Chapter 4), where chemical kinetics are measured and mechanisms can be definitively established. 

Gounarides JS, Chen A, Shapiro MJ, Nuclear magnetic resonance chromatography applications of pulse gradient diffusion NMR to mixture analysis and ligand-receptor interactions, J. Chromatogr., B Biomed. Sci. Applic., 725 79-90, 1999. 

Schaefer J, High-resolution pulsed carbon-13 nuclear magnetic resonance analysis of some crosslinked polymers, Macromolecules, 4 110-112, 1971. 

Nuclear magnetic resonance (NMR) can be used as a rapid alternative to differential scanning calorimetry in the determination of the solid fat content and studies on the melting behaviour. The determination is based on detection of the different populations of protons in solid and liquid phases, which indicates the hardness of the fat. Hernandez and Rutledge (1994b) used low resolution pulse NMR to compare melting curves of roasted and non-roasted cocoa butters from Africa, Indonesia and South America. Discriminant analysis techniques showed... 

Methods that utilized derivatives (DNP and 7-hydroxyquinoline) combined with colorimetric or fluorimetric detection were not specific for acrolein and consistently did not correlate with those obtained from bioassays. Certain direct methods of detection (nuclear magnetic resonance (NMR), fluorescence, and differential pulse polarography) gave the best correlation to the bioassay results (see discussion of analysis of environmental samples in Section 6.2). 

In addition to the specific references given in the chapter, much of the classic treatment of relaxation comes from the book by Abragam,33 but there are many other discussions of this subject in almost every book on NMR. Additional details along the lines presented here are given in Pulse and Fourier Transform NMR by Thomas C. Farrar and Edwin D. Becker,97 Nuclear Magnetic Resonance Spectroscopy by Robin K. Harris,32 and The Nuclear Overhauser Effect in Structural and Conformational Analysis by D. Neuhaus and M. P. Williamson.98... 

The dilatometric method is time-consuming and subject to the bias introduced by the convention described. More recently pulsed Nuclear Magnetic Resonance (pNMR) has been used to measure the relative amounts of liquid and solid fat in a sample, based upon the difference in rates of relaxation of protons in the two phases after the sample has been pulsed (AOCS Method Cd 16-81). With proper calibration this gives a direct determination of the percentage of solid fat, and the results are termed sohd fat content (SFC). The analysis takes less time than dilato-metry, but the equipment is more expensive. 

Fig. 2. Cooling curves for samples in pulsed nuclear magnetic resonance (pNMR), tur-bidimetric, light-scattering, and image analysis experiments.

The independence of Hgff on the time interval of the sequence under the condition that this interval lies between T2 and Tie can be used for determining proper Heff. This approach was first used in multi-pulse nuclear magnetic resonance in paper, and later, in NQR, for the analysis of quasi-stationary state in multi-pulse spin locking in the nuclear system with the spin of 5/tF at exact resonance. We extended this approach to the arbitrary offset of the pulse carrier frequency in relation to the resonance transition. 

Nuclear Magnetic Resonance Spectroscopy. Like IR spectroscopy, NMR spectroscopy requires little sample preparation, and provides extremely detailed information on the composition of many resins. The only limitation is that the sample must be soluble in a deuterated solvent (e.g., deuterated chloroform, tetrahydro-furan, dimethylformamide). Commercial pulse Fourier transform NMR spectrometers with superconducting magnets (field strength 4-14 Tesla) allow routine measurement of high-resolution H- and C-NMR spectra. Two-dimensional NMR techniques and other multipulse techniques (e.g., distortionless enhancement of polarization transfer, DEPT) can also be used [10.16]. These methods are employed to analyze complicated structures. C-NMR spectroscopy is particularly suitable for the qualitative analysis of individual resins in binders, quantiative evaluations are more readily obtained by H-NMR spectroscopy. Comprehensive information on NMR measurements and the assignment of the resonance lines are given in the literature, e.g., for branched polyesters [10.17], alkyd resins [10.18], polyacrylates [10.19], polyurethane elastomers [10.20], fatty acids [10.21], cycloaliphatic diisocyanates [10.22], and epoxy resins [10.23]. 

Rg. 2.16 The pulse sequence used to determine spectra under dipolar decoupling conditions. Note that only the envelope curve of the FID is shown, not the high-frequency oscillations within it. This is what is usually recorded Fourier analysis then yields the spectrum as frequency shifts from the irradiating frequency. For all pulse sequences, the recovery time Tr between repeated applications of the sequence must be long compared with T. (Adapted from Nuclear Magnetic Resonance in Solid Polymers by V. J. McBrierty and K. J. Packer. Cambridge University Press, 1993.)... 

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