Nuclear magnetic resonance energy separation

The basic instrumentation used for spectrometric measurements has already been described in Chapter 7 (p. 277). The natures of sources, monochromators, detectors, and sample cells required for molecular absorption techniques are summarized in Table 9.1. The principal difference between instrumentation for atomic emission and molecular absorption spectrometry is in the need for a separate source of radiation for the latter. In the infrared, visible and ultraviolet regions, white sources are used, i.e. the energy or frequency range of the source covers most or all of the relevant portion of the spectrum. In contrast, nuclear magnetic resonance spectrometers employ a narrow waveband radio-frequency transmitter, a tuned detector and no monochromator. 

Nuclear magnetic resonance (NMR) is based on a phenomenon that nuclei which possess both magnetic and angular moments (i.e. have odd mass number or odd atomic number) interact with an applied magnetic field B0 yielding 21 + 1 (where 1 is the nuclear spin quantum number) energy levels with separation AE ... 

The components in a mixture separate in the column and exit from the column at different times (retention times). As they exit, the detector registers the event and causes the event to be recorded as a peak on the chromatogram. A wide range of detector types are available and include ultraviolet adsorption, refractive index, thermal conductivity, flame ionization, fluorescence, electrochemical, electron capture, thermal energy analyzer, nitrogen-phosphorus. Other less common detectors include infrared, mass spectrometry, nuclear magnetic resonance, atomic absorption, plasma emission. 

Recall that nuclear spin states in nuclear magnetic resonance are typically separated by energies of 2 X 10 kj mol to 2 X 10 " kJ mol . What are the ratios of occupation probability between a pair of such levels at thermal equilibrium and a temperature of 25°C ... 

Nuclear magnetic resonance NMR Quantized orientation of nuclear spin in a magnetic field. Energy separations sampled with radiofrequency radiation. Identification of histidine by deuterium exchange (N—H vs. N—D) at or near metal, especially if paramagnetic. 

The Eo Coulombic interaction alters the energy separation between the ground state and the excited state of the nucleus, thereby causing a slight shift in the position of the observed resonance line. The shift will be different in various chemical compounds, and for this reason is generally known as the chemical isomer shift. It is also frequently referred to as the isomer shift or chemical shift, but in view of the earlier use of these terms in optical spectroscopy and nuclear magnetic resonance spectroscopy respectively, the longer expression is preferred. A less frequently used synonym is centre shift. 

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