CHAPTER THIRTEEN Spectroscopy ample rt oscillator NMR Spectrum 13.5 Diagram of a nuclear magnetic resonance spectrometer(From S. H. Pine, J.B. son, D J Cram, and G S Hammond, Organic Chemistry, 4th edition, McGraw-Hill, New 980p.136) It turns out though that there are several possible variations on this general theme. We could, for example, keep the magnetic field constant and continuously vary the radiofrequency until it matched the energy difference between the nuclear spin states. Or we could keep the rf constant and adjust the energy levels by varying the magnetic field strength. Both methods work, and the instruments based on them are called continuous wave(CW) spectrometers. Many of the terms we use in NMR spectroscopy have their origin in the way CW instruments operate, but Cw instruments are rarely used anymore CW-NMR spectrometers have been replaced by a new generation of instruments called pulsed Fourier-transform nuclear magnetic resonance(FT-NMR) spectrometers FT-NMR spectrometers are far more versatile than CW instruments and are more com- plicated. Most of the visible differences between them lie in computerized data acquisi- tion and analysis components that are fundamental to FT-NMR spectroscopy. But there is an important difference in how a pulsed FT-NMR experiment is carried out as well Rather than sweeping through a range of frequencies(or magnetic field strengths ), the sample is irradiated with a short, intense burst of radiofrequency radiation(the pulse that excites all of the protons in the molecule. The magnetic field associated with the new orientation of nuclear spins induces an electrical signal in the receiver that decreases with time as the nuclei return to their original orientation. The resulting free-induction decay(FID) is a composite of the decay patterns of all of the protons in the molecule The free-induction decay pattern is stored in a computer and converted into a spectrum Richard r. ernst of the Swiss by a mathematical process known as a Fourier transform. The pulse-relaxation sequence Federal Institute of Technol- takes only about a second, but usually gives signals too weak to distinguish from back ground noise. The signal-to-noise ratio is enhanced by repeating the sequence many Prize in chemistry for devis- ing pulse-relaxation NMR times, then averaging the data. Noise is random and averaging causes it to vanish; sig nals always appear at the same place and accumulate. All of the operationsthe inter val between pulses, collecting, storing, and averaging the data and converting it to a pectrum by a Fourier transformare under computer control, which makes the actual taking of an FT-NMR spectrum a fairly routine operation Back Forward Main MenuToc Study Guide ToC Student o MHHE WebsiteIt turns out though that there are several possible variations on this general theme. We could, for example, keep the magnetic field constant and continuously vary the radiofrequency until it matched the energy difference between the nuclear spin states. Or, we could keep the rf constant and adjust the energy levels by varying the magnetic field strength. Both methods work, and the instruments based on them are called continuous wave (CW) spectrometers. Many of the terms we use in NMR spectroscopy have their origin in the way CW instruments operate, but CW instruments are rarely used anymore. CW-NMR spectrometers have been replaced by a new generation of instruments called pulsed Fourier-transform nuclear magnetic resonance (FT-NMR) spectrometers. FT-NMR spectrometers are far more versatile than CW instruments and are more com￾plicated. Most of the visible differences between them lie in computerized data acquisi￾tion and analysis components that are fundamental to FT-NMR spectroscopy. But there is an important difference in how a pulsed FT-NMR experiment is carried out as well. Rather than sweeping through a range of frequencies (or magnetic field strengths), the sample is irradiated with a short, intense burst of radiofrequency radiation (the pulse) that excites all of the protons in the molecule. The magnetic field associated with the new orientation of nuclear spins induces an electrical signal in the receiver that decreases with time as the nuclei return to their original orientation. The resulting free-induction decay (FID) is a composite of the decay patterns of all of the protons in the molecule. The free-induction decay pattern is stored in a computer and converted into a spectrum by a mathematical process known as a Fourier transform. The pulse-relaxation sequence takes only about a second, but usually gives signals too weak to distinguish from back￾ground noise. The signal-to-noise ratio is enhanced by repeating the sequence many times, then averaging the data. Noise is random and averaging causes it to vanish; sig￾nals always appear at the same place and accumulate. All of the operations—the inter￾val between pulses, collecting, storing, and averaging the data and converting it to a spectrum by a Fourier transform—are under computer control, which makes the actual taking of an FT-NMR spectrum a fairly routine operation. 492 CHAPTER THIRTEEN Spectroscopy Magnet rf input oscillator rf output signal amplifier NMR Spectrum rf output receiver rf input coil Sample tube 0 FIGURE 13.5 Diagram of a nuclear magnetic resonance spectrometer. (From S. H. Pine, J. B. Hendrickson, D. J. Cram, and G. S. Hammond, Organic Chemistry, 4th edition, McGraw-Hill, New York, 1980, p. 136.) Richard R. Ernst of the Swiss Federal Institute of Technol￾ogy won the 1991 Nobel Prize in chemistry for devis￾ing pulse-relaxation NMR techniques. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website