Determination of NMR Spectra Acquisition Parameters

In the FT-NMR experiment, a number of acquisition parameters must be considered before NMR data are collected. The first of these parameters is spectral resolution which is controlled directly by the amount of time taken to acquire the signal. To distinguish two signals separated by An (in Hz), data must continue to be acquired for at least l/Ao seconds. For example, a desired resolution of 0.5 Hz in a spectrum requires an acquisition time of 1/0.5 Hz, or 2.0 s. Sampling for a longer time would improve the spectral resolution for example, acquisition for 4.0 s would yield a resolution of 0.25 Hz. Thus, longer acquisition times are necessary to produce narrower lines until the natural line width is reached. 

A second parameter concerns the range of frequencies (the spectral width) to be detected. The spectral width is determined by how often the detector samples the value of the FID—that is, the sampling rate. The FID is made up of a collection of sinusoidal signals 

If a signal is sampled 20,000 times per second, the detector spends 50 pis on each point. The reciprocal of the sampling rate is called the dwell time, which signifies the amount of time between samplings. Reducing the dwell time means that more data points are collected in the same period, so that a larger computer memory is required. If the acquisition time is 

0 s (for a resolution of 0.25 Hz) and the sampling rate is 20,000 times per second (for a spectral width of 10,000 Hz) the computer must store 80,000 data points. Making do with fewer points because of computer limitations would require either lowering the resolution of the spectrum or decreasing its spectral width. 

In the early days of FT-NMR, the dedicated spectrometer computers were quite slow by today s standards and had very limited memory. Consequently, NMR spectroscopists had to consider a trade-off between spectral width and resolution. Today, we are fortunate to have dedicated computers that are very fast and have abundant memory. For the most part, NMR spectroscopists now can let science, rather than computer limitations, dictate how they set the parameters of their experiments. 

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Strategies for NMR spectroscopic structure elucidation of organic compounds have been reviewed extensively.34-37 In this section, we briefly describe a set of the most commonly useful 2D NMR spectra that is sufficient for most (though certainly not all) organic structure determination problems, and we comment on specific modifications of acquisition parameters that facilitate the analysis of mixtures. 

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