Fourier transform FT NMR spectra

Chapter 1 focusses on the setting up and processing of spectra. It consists of a set of spectra that are wrongly processed or acquired, and its title reflects the experience of almost everyone who has tried to run Fourier transform (FT) NMR spectra. All the spectra are of the same sample of camphor, a convenient compound that should be readily available to anyone who wishes to repeat the experiments for themselves. If you do not run your own spectra, you should still find it instructive to work through these examples as they illustrate some of the dangers of simply accepting the appearance of a spectrum that has been run for you. 

A recent refinement of time-averaging signal enhancement is Fourier-transform (FT) NMR spectroscopy. With an FT-accessory spectra can be accumulated in the memory banks of a small computer at the rate of one or more per second, reducing the data accumulation time by factors of 250-500. 

Nuclear magnetic resonance (NMR) spectroscopy is at present one of the most widely applied physical techniques in biology, and its potential applications increase day by day, as more sophisticated instrumentation becomes available and deeper theoretical knowledge is obtained. The phenomenon of NMR was discovered simultaneously by Purcell and his associates at Harvard University and by Bloch and co-workers at Stanford University, for which they were jointly awarded the Nobel prize in physics in 1952. In the lipid field there are two main types of NMR spectroscopy that are of interest broad-line experiments, concerned mainly with the spectra obtained from samples in the solid state, or from oriented phases, and narrow-line, or high-resolution, experiments carried out with samples in the liquid, solution or gas phases. Both types of NMR spectroscopy are extremely useful in the study of the lipids. In addition, Fourier transform (FT) NMR has helped increase the sensitivity of the technique and the so-called pulse method of recording spectra has literally widened the prospect of NMR applications in the field of lipid research and industry. The application of NMR to solid fats is still in its infancy (Pines et aL, 1973 Schaefer and Stejskal, 1979 BocieketaL, 1985). 

With the advent of Fourier transform (FT) NMR spectrometers, NMR spectroscopy is now available as a simple and routine tool for the structure determination of organic molecules. Since is of low natural abundance (1.1%), addition of many spectra is required to obtain acceptable signal-to-noise (S/N) levels. With modem spectrometers, spectra can often be acquired simply by issuing software commands in some instruments a different probe is inserted into the magnet. Since resonates at roughly 25% of the proton operating frequency of a spectrometer system, an instrument that acquires H spectra at 300 MHz wiB be reset to about 75 MHz for work. 

The advent of pulse Fourier Transform (FT) NMR techniques in the middle 1970s set the stage for the use of Si NMR for qualitative and quantitative analysis in liquids. However, for solid samples the effects of H- Si magnetic dipole-dipole interactions and Si chemical shift anisotropies and the time bottleneck of long Si spin-lattice relaxation times render the direct application of the liquid-state Si NMR technique essentially useless yielding broad, featureless spectra of low intensity. For undo tanding the aspects for solid-state Si NMR, those for solid-state NMR will be briefly presented. 

NMR analyses were conducted using a Varian Unity Plus 400 Fourier Transform (FT) NMR spectrometer, which operates at 400 MHz for H observation and at 100 MHz for 13C observation. All spectra were obtained at probe temperature (22 1° C) with double precision data accumulation. Samples were provided in CDCI3, D2O or water. All samples run in CDCI3 were referenced to internal tetramethylsilane (TMS) using the CHCI3 resonance (d H, 7.24 dl c, 77.2) as a secondary reference. IH spectra in D2O and water were referenced to external sodium 3-trimethylsilylpropanoate-2,2,3,3-d4 (TSP) in D2O. Quantitative data were obtained by digital integration of peak areas. 

Like NMR spectrometers some IR spectrometers oper ate in a continuous sweep mode whereas others em ploy pulse Fourier transform (FT IR) technology All the IR spectra in this text were obtained on an FT IR instrument... 

NMR analyses were done on an IBM Instruments NR-300 spectrometer and an Oxford 7 Tesla superconducting narrow-bore magnet. Silicon-29 (Si-29) NMR spectra were recorded at 59.6 MHz and hydrogen (also commonly called proton or H-l) NMR spectra at 300.13 MHz. Spectra were recorded using conventional single-pulse techniques with proton decoupling for Si-29 acquisitions. Si-29 experiments were structured so as to suppress nuclear Overhauser enhancement (NOE). For Si-29 acquisitions, spectral widths were 50 kHz and Fourier transform (FT) sizes were 4K points. For protons, spectral widths were 7.5 kHz and FT sizes were 16K points. Si-29 rf pulse widths were approximately 12 fits and proton rf pulse widths were 8 jj.s. 

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