Magnetic measurements electron paramagnetic resonance

Information relevant to the electronic configuration can be obtained from atomic emission spectroscopy, x-ray photoelectron spectroscopy, paramagnetic susceptibility measurements, electron paramagnetic resonance, electronic transition spectroscopy, crystal structure data, and atomic-beam experiments. Discussions of the theoretical and experimental aspects of atomic spectroscopy, magnetic properties, crystal structures of solids, and electronic absorption spectroscopy are to be found in the later chapters of this work. 

Other magnetic measurements of catalysts include electron paramagnetic resonance and magnetic susceptibility. Although those are not as common as NMR, they can be used to look at the properties of paramagnetic and ferromagnetic samples. Examples of these applications can be found in the literature [87. 

Nuclear magnetic resonance spectroscopy of the solutes in clathrates and low temperature specific heat measurements are thought to be particularly promising methods for providing more detailed information on the rotational freedom of the solute molecules and their interaction with the host lattice. The absence of electron paramagnetic resonance of the oxygen molecule in a hydroquinone clathrate has already been explained on the basis of weak orientational effects by Meyer, O Brien, and van Vleck.18... 

Electron paramagnetic resonance (EPR) and NMR spectroscopy are quite similar in their basic principles and in experimental techniques. They detect different phenomena and thus yield different information. The major use of EPR spectroscopy is in the detection of free radicals which are uniquely characterised by their magnetic moment that arises from the presence of an unpaired electron. Measurement of a magnetic property of a material containing free radicals, like its magnetic susceptibility, provides the concentration of free radicals, but it lacks sensitivity and cannot reveal the structure of the radicals. Electron paramagnetic resonance spectroscopy is essentially free from these defects. 

Electron-nuclear double resonance (ENDOR) spectroscopy A magnetic resonance spectroscopic technique for the determination of hyperfine interactions between electrons and nuclear spins. There are two principal techniques. In continuous-wave ENDOR the intensity of an electron paramagnetic resonance signal, partially saturated with microwave power, is measured as radio frequency is applied. In pulsed ENDOR the radio frequency is applied as pulses and the EPR signal is detected as a spin-echo. In each case an enhancement of the EPR signal is observed when the radiofrequency is in resonance with the coupled nuclei. 

For our purpose, it is convenient to classify the measurements according to the format of the data produced. Sensors provide scalar valued quantities of the bulk fluid i. e. density p(t), refractive index n(t), viscosity dielectric constant e(t) and speed of sound Vj(t). Spectrometers provide vector valued quantities of the bulk fluid. Good examples include absorption spectra A t) associated with (1) far-, mid- and near-infrared FIR, MIR, NIR, (2) ultraviolet and visible UV-VIS, (3) nuclear magnetic resonance NMR, (4) electron paramagnetic resonance EPR, (5) vibrational circular dichroism VCD and (6) electronic circular dichroism ECD. Vector valued quantities are also obtained from fluorescence I t) and the Raman effect /(t). Some spectrometers produce matrix valued quantities M(t) of the bulk fluid. Here 2D-NMR spectra, 2D-EPR and 2D-flourescence spectra are noteworthy. A schematic representation of a very general experimental configuration is shown in Figure 4.1 where r is the recycle time for the system. 

Figure 6.39(a) shows the vs. T curve, normalized to the RT value, for a 100 nm thick a-/ -NPNN/glass film obtained from electron paramagnetic resonance (EPR) measurements with the static magnetic field applied perpendicular to the substrate plane. As previously shown in Fig. 3.19, the molecular a -planes are parallel to the substrate s surface. The data points closely follow the Curie-Weiss law = (T — w)/C, where C stands for the Curie constant. In this case w — —0.3 K, indicating that the net intermolecular interactions are weakly anfiferromagnetic. No hint of a transition at low temperature is observed. These results coincide with those derived from SQUID measurements on a single a-p-NPNN crystal (Tamura etal, 2003), where 0.5 < w < 0, which are displayed in Fig. 6.40. 

The problem of bringing a large magnet into the field for ambient measurements has been overcome in electron paramagnetic resonance (EPR, also called electron spin resonance, ESR) by Mihelcic, Helten, and coworkers (93-99). They combined EPR with a matrix isolation technique to allow the sampling and radical quantification to occur in separate steps. The matrix isolation is also required in this case because EPR is not sensitive enough to measure peroxy radicals directly in the atmosphere. EPR spectroscopy has also been used in laboratory studies of peroxy radical reactions (100, 101). 

Information about RNA structure and movement is critical for our understanding of how RNA is able to carry out its multifaceted functions. One spectroscopic technique that has shown great promise to study RNA, as well as other biopolymers, is electron paramagnetic resonance (EPR) spectroscopy, also named electron spin resonance (ESR) spectroscopy. EPR is a magnetic resonance technique that monitors the behaviors of unpaired electrons, and has long been used to study structure and dynamics of biomolecules (see recent reviews by Klug and Feix, 2008 Sowa and Qin, 2008). Structural information can be obtained by distance measurements, that is, by determination of distances between two spin-centers, and is the topic of another chapter in this volume (see Chapter 16 in this volume). 

The experimental methods of Chapter 11, which contain the word "resonance" (e.g., "nuclear magnetic resonance" "electron paramagnetic resonance, etc.), refer to an allowed absorption or emission process (just as in optical spectroscopy), which is measured in a circuit electrically timed to the frequency for the quantum-mechanical transition. Of course, absorption or emission of light by an atom or molecule also occurs only if the light energy matches the energy level difference nevertheless, by tradition the term "resonance" is not used in that case. 

Lacshmi, K.V. and Brudvig, G.W. (2000) Electron paramagnetic resonance distance measurement in photosynthetic reaction centers, in Berliner, L, Eaton, S., and Eaton, G. (eds.),, Magnetic Resonance in Biology, V. 18, Kluwer Academic Publishers. Dordrecht, pp. 513-568. 

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