FT NMR Spectroscopy

The detector records this loss of spin generating a free induction decay (FID) (d), an intensity vs time plot. The FID is a measure of energy emitted by the spin system as it 

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The crystalline mineral silicates have been well characterized and their diversity of stmcture thoroughly presented (2). The stmctures of siHcate glasses and solutions can be investigated through potentiometric and dye adsorption studies, chemical derivatization and gas chromatography, and laser Raman, infrared (ftir), and Si Fourier transform nuclear magnetic resonance ( Si ft-nmr) spectroscopy. References 3—6 contain reviews of the general chemical and physical properties of siHcate materials. 

Laser Raman spectroscopy and Si ft-nmr spectroscopy have been used to examine direcdy the stmcture of sdicate species in solution (34—41). 

The spin-lattice relaxation time, T/, is the time constant for spin-lattice relaxation which is specific for every nuclear spin. In FT NMR spectroscopy the spin-lattice relaxation must keep pace with the exciting pulses. If the sequence of pulses is too rapid, e.g. faster than BT/max of the slowest C atom of a moleeule in carbon-13 resonance, a decrease in signal intensity is observed for the slow C atom due to the spin-lattice relaxation getting out of step. For this reason, quaternary C atoms can be recognised in carbon-13 NMR spectra by their weak signals. 

Belokon et al.83 have investigated the formation of the homo-and bimetallic titanium complexes with di-Schiff base ligands, by means of FT NMR spectroscopy. The ligands have been shown to adapt the ds-p configuration in titanium (IV) complexes. Analysis of the 1H NMR spectra has allowed determination of the population of the homobimetallic complexes derived from two different Ti(IV) complexes [34],... 

The cartoon-like drawing of the structure of the parent bicylobutonium ion C4H7+ 36 is adopted from an ingenious forward-looking paper of Olah and coworkers in 1972, 61) long before routine 13C-FT-NMR spectroscopy and routine ab initio quantum chemical calculations were available, which envisaged correctly the stabilization mode of the parent bicyclobutonium ion to arise from the interaction of the backside lobe of the Cy-Hendo sp3 orbital with the empty carbenium carbon p-orbital at Ca. 

Foustoukos DI, James RH, Seyfried Jr WE (2003) Lithium isotopic systematics of the Main Endeavor Field vent fluids. Northern Juan de Fuca Ridge. Geochim Cosmochim Acta 67 A101 Franklin KJ, Halliday JD, Plante LM, Symons EA (1986) Measurement of the Li-6 Li-7 isotope ratio for lithium-salts hy FT NMR-spectroscopy. J Magnet Resonance 67 162-165 Fritz SJ (1992) Measuring the ratio of aqueous diliusion coefficients between Li Ck and Li Cf by osmometry. Geochim Cosmochim Acta 56 3781-3789... 

Users of any NMR instrument are well aware of the extensive employment of what is known as pulse sequences. The roots of the term go back to the early days of pulsed NMR when multiple, precisely spaced RF excitation pulses had been invented (17,98-110) and employed to overcome instrumental imperfections such as magnetic field inhomogeneity (Hahn echo) or receiver dead time (solid echo), monitor relaxation phenomena (saturationrrecovery, inversion recovery, CPMG), excite and/or isolate specific components of NMR signals (stimulated echo, quadrupole echo), etc. Later on, employment of pulse sequences of increasing complexity, combined with the so-called phase-cycling technique, has revolutionized FT-NMR spectroscopy, a field where hundreds of useful excitation and detection sequences (111,112) are at present routinely used to acquire qualitatively distinct ID, 2D, and 3D NMR... 

The principal advantage of FT NMR spectroscopy is a great increase in sensitivity per unit time of the experiment. A CW scan generally takes of the order of one hundred... 

Because the application of NMR spectroscopy to environmental samples is relatively new, we focused our studies on the identification and characterization of DOP by 31P FT-NMR spectroscopy. Ultrafiltration and reverse osmosis concentration techniques were employed to increase the dissolved organic phosphorus concentrations to the detection level of 31P FT-NMR techniques (approximately 10-20 mg of P/L). With these concentration methods a DOP concentration factor of up to 2000 is obtainable. This chapter reports the use of 31P FT-NMR spectroscopy in the analysis of DOP. In... 

Figure 15. Seasonal changes in dissolved organic phosphorus detected by 31P FT-NMR spectroscopy. The peak height ratio was 4.0-5.0 ppm 0.0-2.0 ppm.

These techniques may further enhance the ability of 31P FT-NMR spectroscopy to be used for the identification of hydrosphere DOP. We have demonstrated already that 31P NMR spectroscopy is a viable technique for DOP identification and characterization. 31P NMR spectroscopy may eventually be useful through several of the techniques mentioned in this chapter for studying interactions of DOP with dissolved humic substances, colloids, and seston and particulate adsorption. Hence, 31P FT-NMR spectroscopy... 

Because NMR spectroscopy is a nuclei-specific technique and has the ability to distinguish between similar compounds, it is an excellent method for identifying similar species in complex matrices. Thus, 31P FT-NMR spectroscopy is ideal for the identification and characterization of the hydrosphere DOP. Even so, NMR spectroscopy is fairly insensitive and requires high sample concentrations. Low DOP concentrations are increased to 31P FT-NMR detection limits by using ultrafiltration and reverse osmosis membranes. Not only is the DOP concentrated, but it is fractionated according to its molecular size. Compared to other concentration and molecular size fractionation techniques for DOP, ultrafiltration and reverse osmosis are relatively rapid and easy. 

Blossey EC, Cannon RG, Ford WT, Periyasam YM, Mohanraj S, Synthesis, reactions and carbon-13 FT NMR spectroscopy of polymer-bound steroids, J. Org. Chem., 55 4664 1668, 1990. 

Aminolevulinic acid (ALA), labelled with 13C at C(1). C(2), C(3), C(4) and C(5), has been synthesized64 from 13C-labelled KCN, glycine, Meldrum s acid or bromoacetate, prepared in turn from 13C-sodium acetate or 13C-acetic acid (equations 26 and 27). It was used for direct observation of the enzymatic transformation of ALA to porphobilinogen (PBG) by 13C-high-field FT-NMR spectroscopy without chemical degradation of the intermediate and the compounds. 

In addition to the information available from FT NMR spectroscopy, one may gain considerable insight into molecular interactions and motion within a material by studying NMR relaxation, either spin-spin relaxation, where nuclei exchange spin states and thus dephase, or spin-lattice relaxation, where excited nuclei lose energy to the bulk sample reservoir surrounding them. Those readers interested in a more detailed introduction to NMR are referred to Farrar and Becker (1971) or to the comprehensive texts of Slichter (1990) and Abragam (1961). 

Modern Fourier transform (FT) NMR spectroscopy excites all the nuclei of interest at once with a radiofrequency (rf) pulse and detects the alternating current produced by the precessing spins in a coil surrounding the sample. This signal, or free induction decay (FID), is collected in the time domain and then processed to the frequency domain to generate the NMR spectrum. 

The NMR spectrum can be recorded in various ways. The earliest commercial NMR spectrometers operated in continuous-wave mode, in which the sample is irradiated at constant frequency v while the magnetic field is swept through a range of values. The rf power absorbed by the sample is recorded at each value of H. When the value of H satisfies the resonance condition, a peak appears in the spectrum. Newer instruments rely on Fourier transform (FT NMR) spectroscopy, in which a sample held in a fixed magnetic field is irradiated with a short, intense burst of... 

To describe Fourier transform (FT) NMR spectroscopy and pulse techniques, it is best to picture the nuclear spins aligned with and against the field as in Figure A. 1.2(a). The nuclear spin precesses about the --axis with the frequency, v, given in equation (A. 1.5). 

BE C Lossey, RG Cannon, WT Ford, M Periyasamy, S Mohanraj. Synthesis, reactions, and 13C FT NMR spectroscopy of polymer-bound steroids. J Org Chem 55 4664-4668, 1990. 

FID Free Induction Decay. Decay of the induction (transverse magnetisation) back to equilibrium, following excitation of a nuclear spin by a radiofrequency pulse, in a way which is free from the influence of the radiofrequency field signal used as the basis for FT-NMR spectroscopy)... 

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