Nuclear magnetic resonance characteristic absorption region

Both absorption and emission may be observed in each region of the spectrum, but in practice only absorption spectra are studied extensively. Three techniques are important for analytical purposes visible and ultraviolet spectrometry (electronic), infrared spectrometry (vibrational) and nuclear magnetic resonance spectrometry (nuclear spin). The characteristic spectra associated with each of these techniques differ appreciably in their complexity and intensity. Changes in electronic energy are accompanied by simultaneous transitions between vibrational and rotational levels and result in broadband spectra. Vibrational spectra have somewhat broadened bands because of simultaneous changes in rotational energy, whilst nuclear magnetic resonance spectra are characterized by narrow bands. 

In practice, UV absorption spectroscopy (in the region from 200 to 380 nm) is for the most part yields information only about the conjugated system present in the molecule. However, when taken in conjunction with the wealth of detail provided by infrared (IR) and nuclear magnetic resonance (NMR) bands may lead to successful structural elucidations. The principal characteristics of an UV absorption band are its position (2max) and intensity (emax or log emax). 

Nuclear magnetic resonance (NMR) spectrometry is basically another form of absorption spectrometry, akin to IR or UV spectrometry. Under appropriate conditions in a magnetic field, a sample can absorb electromagnetic radiation in the radio frequency (rf) region at frequencies governed by the characteristics of the sample. Absorption is a function of certain nuclei in the molecule. A plot of the frequencies of the absorption peaks versus peak intensities constitutes an NMR spectrum. This chapter covers proton magnetic resonance ( H NMR) spectrometry. 

Certain functional groups in a molecule (e.g., hydroxyl, carbonyl, and amine) absorb IR radiation and exhibit absorption bands at characteristic frequencies regions regardless of the structure of the rest of the molecule. These bands are termed group frequencies. They are predictable and allow the analyst to deduce important structural information about an unknown molecule. An IR spectrum can be rapidly recorded for any phase, i.e., solid, liquid, or vapor. By coupling IR spectroscopy with other analytical techniques such as nuclear magnetic resonance (NMR)... 

The selection rules help to predict the probability of a transition but are not always strictly followed. If the transition obeys the rules it is allowed, otherwise it is forbidden. A molecule can become excited in a variety of ways, corresponding to absorption in different regions of the spectrum. Thus certain properties of the radiation that emerges from the sample are measured. The fraction of the incident radiation absorbed or dissipated by the sample is measured in optical (ultraviolet and visible) absorption spectroscopy and some modes of nuclear magnetic resonance spectrometry (NMR). Because the relative positions of the energy levels depend characteristically on the molecular structure, absorption spectra provide subtle tools for structural investigation. 

Nuclear magnetic resonance (NMR) spectrometry is based on the net absorption of energy in the radiofrequency region of the electromagnetic spectrum by the nuclei of those elements that have spin angular momentum and a magnetic moment. For the nuclei of a particular element, characteristic absorption, or resonance frequencies, and other spectral features provide useful information on identity and molecular structure. 

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