Nuclear magnetic resonance spectroscopy thermal equilibrium magnetization

The isothermal time dependence of relaxation and fluctuation due to molecular motions in liquids at equilibrium usually cannot be described by the simple linear exponential function exp(-t/r), where t is the relaxation time. This fact is well known, especially for polymers, from measurements of the time or frequency dependence of the response of the equilibrium liquid to external stimuli such as in mechanical [6], dielectric [7, 33], and light-scattering [15, 34] measurements, and nuclear-magnetic-resonance spectroscopy [14]. The correlation or relaxation function measured usually decays slower than the exponential function and this feature is often referred to as non-exponential decay or non-exponentiality. Since the same molecular motions are responsible for structural recovery, certainly we can expect that the time dependence of the structural-relaxation function under non-equilibrium conditions is also non-exponential. An experiment by Kovacs on structural relaxation involving a more complicated thermal history showed that the structural-relaxation function even far from equilibrium is non-exponential. For example (Fig. 2.7), poly(vinyl acetate) is first subjected to a down-quench from Tq = 40 °C to 10 °C, and then, holding the temperature constant, the sample... 

All measurements were made in the gas phase. The methods used are abbreviated as follows. UV ultraviolet (including visible) spectroscopy IR infrared spectroscopy R Raman spectroscopy MW microwave spectroscopy ED electron diffraction NMR nuclear magnetic resonance LMR laser magnetic resonance EPR electron paramagnetic resonance MBE molecular beam electric resonance. If two methods were used jointly for structure determination, they are listed together, as (ED, MW). If the numerical values listed refer to the equilibrium values, they are specified by and 6. In other cases the listed values represent various average values in vibrational states it is frequently the case that they represent the Tj structure derived from several isotopic species for MW or the r structure (i.e., the average internuclear distances at thermal equilibrium) for ED. These internuclear distances for the same atom pair with different definitions may sometimes differ as much... 

A major limitation for NMR spectroscopy is the intrinsically low sensitivity due to the rather unfavorable Boltzmann distribution for nuclear spins at thermal equilibrium. Thus, considerable effort in magnetic resonance spectroscopy is made towards sensitivity enhancement by hyperpolarization techniques, such as optical polarization, para-hydrogen-induced polarization enhancement, and dynamic nuclear polarization (DNP), a method which exploits the magnetization of unpaired electrons in stable radicals or transition metals to enhance nuclear polarization beyond the Boltzmann limit. In the chapter Dynamic Nuclear Hyperpolarization in Liquids, the fundamental theory for different polarization transfer... 

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