Recording an NMR spectrum

Continuous wave A method of recording an NMR spectrum in which the field B] is applied continuously and either the magnitude of Bo or the radiofrequency is varied so that the nuclei are brought successively into resonance. 

The NMR spectrum measured in one solvent may be slightly different from that measured in another solvent. This difference arises due to difference in polarity of the solvent. This is why recording an NMR spectrum, the solvent is mentioned in the spectrum. 

The information that may be obtained from an NMR spectrum depends to some extent on the experimental parameters used to record the spectrum. Before recording an NMR spectrum a number of acquisition parameters have to be defined which depend on both the type of sample and the pulse sequence being used. Often the various parameters interact with each other making the optimization of the acquisition parameters difficult. Once the data has been acquired it may be processed and analysed in a number of different ways to extract the required information. 

For quantitative studies, the areas under each of the absorptions are required. These areas are evaluated by an integrator and so are called integrations. Provided that the procedures for recording an NMR spectrum are chosen properly (Section 3.20.2), the ratios of the integrations for the different absorptions are equal to the ratios of the numbers of the respective nuclei present in the molecules, and can be used, for example, to evaluate relative molar compositions. 

The response of an atom to the strength of the external magnetic field is different for different elements, and for different isotopes of the same element. The resonance frequencies of most nuclei are sufficiently different that an NMR experiment is sensitive only to a paiticulai isotope of a single element. The frequency for H is 200 MHz at 4.7 T, but that of is 50.4 MHz. Thus, when recording the NMR spectrum of an... 

The procedure for conducting a CSA-NMR experiment consists simply of mixing the CSA and the substrate in an appropriate solvent and recording the NMR spectrum. For best results, severd precautions entailing only minimal effort should be taken. 

In the conventional NMR experiment, a radio-frequency field is applied continuously to a sample in a magnetic field. The radio-frequency power must be kept low to avoid saturation. An NMR spectrum is obtained by sweeping the rf field through the range of Larmor frequencies of the observed nucleus. The nuclear induction current (Section 1.8.1) is amplified and recorded as a function of frequency. This method, which yields the frequency domain spectrum f(ai), is known as the steady-state absorption or continuous wave (CW) NMR spectroscopy [1-3]. 

Nuclear Magnetic Resonance (NMR) spectroscopy is one of the most powerful analytical techniques in organic chemistry for elucidating the molecular structures of chemicals (1,2). Moreover, an NMR spectrum may be used like a fingerprint to identify a chemical by comparing it with its reference spectrum recorded from the authentic chemical under comparable conditions. The spectrum also reveals information on molecular conformation, isomerism, molecular dynamics, and diastereomers (3 6). 

For experimentalists, NMR spectroscopy is one of the most important tools to investigate a newly synthesized compound. Nowadays, NMR measurements are standard in synthetic chemistry and, therefore, it is common that the first experimental information on an unknown chemical compound is obtained by recording its NMR spectrum. By using measured chemical shifts and coupling constants, and comparing these magnetic properties of the molecule under investigation with those of suitable reference molecules, it is in most cases possible to provide useful structural information on the new compound. 

How does the rotation around carbon-carbon o bonds and the ring flip of cyclohexane rings affect an NMR spectrum Because these processes are rapid at room temperamre, an NMR spectrum records an average of all conformations that interconvert. 

In many cases it is necessary to monitor closely the progress of a small scale reaction by methods other than tic one very useful alternative is nuclear magnetic resonance (nmr). With larger scale reactions this is simply done by removing an aliquot of the reaction mixture and recording its nmr spectrum. Obviously this approach cannot be applied to reactions involving relatively small quantities of material. The answer is to carry out the reaction in an nmr tube. Both 5mm and lOmm diameter nmr tubes can conveniently be used as reaction vessels (Fig. 12.5) in the same way as test-tubes were employed in Section 12.2, but without magnetic stirrer bars. 

Figure 5-54 (A) An NMR spectrum of the 76-residue E. coU tRNA containing 5-fluorouracil in 14 positions. Recorded at 47°C. The numbers above the resonances indicate the position in the sequence. (The sequence is not identical to that for the yeast tRNA shown in Fig. 5-30.) Modified from Chu et Courtesy of Jack Horowitz.

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