Nuclear magnetic resonance spectra acquisition

Chemically induced dynamic nuclear polarization (CIDNP) is a nuclear magnetic resonance method based on the observation of transient signals, typically substantially enhanced, in either absorption of emission. These effects are induced as a result of magnetic interactions in radical or radical ion pairs on the nanosecond time scale. This method requires acquisition of an NMR spectrum during (or within a few seconds of) the generation of the radical ion pairs. The CIDNP technique is applied in solution, typically at room temperature, and lends itself to modest time resolution. The first CIDNP effects were reported in 1967, and their potential as a mechanistic tool for radical pair reactions was soon recognized [117, 118]. Nuclear spin polarization effects were discovered in reactions of neutral radicals and experiments in the author s laboratory established that similar eflects could also be induced in radical ions [119-121]. 

Nuclear magnetic resonance (NMR) spectroscopy is regarded as one of the most important analytical techniques in chemistry for characterization of molecular structure. In addition to the structural information, NMR spectroscopy also gives quantitative information about the sample constituent. The induced current in the coil can be regarded as linearly dependent on the concentration of the nucleus in the sample. Therefore the resonance integrals in a simple one-dimensional spectrum measured with the excitation-acquisition scheme offer a way to measure absolute amounts of the chemicals present in the sample. Recently, the need for quantitative analysis of highly complex samples has led to a situation where resonance overlap in... 

In virtually all types of experiments in which a response is analyzed as a function of frequency (e.g., a spectrum), transform techniques can significantly improve data acquisition and/or data reduction. Research-level nuclear magnetic resonance and infra-red spectra are already obtained almost exclusively by Fourier transform methods, because Fourier transform NMR and IR spectrometers have been commercially available since the late 1960 s. Similar transform techniques are equally valuable (but less well-known) for a wide range of other chemical applications for which commercial instruments are only now becoming available for example, the first commercial Fourier transform mass spectrometer was introduced this year (1981) by Nicolet Instrument Corporation. The purpose of this volume is to acquaint practicing chemists with the basis, advantages, and applications of Fourier, Hadamard, and Hilbert transforms in chemistry. For almost all chapters, the author is the investigator who was the first to apply such methods in that field. 

Hgure 1 Nuclear magnetic resonance spectra of natural rubber. (A) 300 MHz H NMR spectrum. Pulse delay 8.0 s acquisition time 1.8s 32 transients co-added 50°C 0.5% wt/vol in CDCI3. (B) 75 MHz C NMR spectmm. Pulse delay 2.6 s acquisition time 0.4 s 2400 transients co-added 5% wWol in CDCI3 ambient temperature. (Provided by courtesy of The Goodyear Tire Rubber Company, Akron, OH.)... 

This low level of the NMR-active isotope of carbon makes it more difficult to obtain a suitable carbon nuclear magnetic resonance ( C NMR) spectrum. For example, whereas it is usually possible to measure a NMR spectrum in a few minutes, it may take tens of minutes or even hours to accumulate enough data to produce a NMR spectrum in which the signal-to-noise ratio is high enough for the resonances due to the carbon atoms to be seen. Nonetheless, modern spectrometers and the sophisticated computers associated with them allow acquisition of the data necessary for a NMR spectrum on samples of 1-5 mg, which is about an order of magnitude greater than the amount needed for a NMR spectrum. 

CIDNP - is a nuclear resonance method based on the observation of transient signals, substantially enhanced in either absorption or emission. These effects are induced as a result of magnetic interactions in radical or radical-ion pairs on the nanosecond timescale. This method requires acquisition of an NMR spectrum during (or within a few seconds of) the generation of the radical-ion pairs. 

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