Microwave magnetic resonance method

Electron spin resonance (e.s.r.) spectroscopy, applied to free radicals in condensed phases, is a long established technique with several commercially available spectrometers. The gas phase applications we will describe have little in common with condensed phase studies, and are much more a part of rotational spectroscopy. However, the experimental methods used for condensed phase studies can be applied to the study of gases with rather little change, so it is appropriate first to describe a typical microwave magnetic resonance spectrometer, as illustrated schematically in figure 9.1. 

Historically the first open shell molecule to be studied by magnetic resonance methods was nitric oxide, NO. The 2n1/2 fine-structure component is lower in energy than the 2n3/2 component by 123 cm 1 and is only weakly magnetic. However, the 2n3/2 component is substantially populated at room temperature and its microwave magnetic resonance spectrum is readily recorded. Spectra of the lowest rotational level,... 

These, then, are the reasons why magnetic resonance methods, microwave or far-infrared laser, have had limited success with 2A diatomic radicals. Similar considerations apply to nonlinear polyatomic radicals in doublet states success in far-infrared laser magnetic resonance depends upon the magnitude of the spin-rotation coupling, and the size of the energy mismatch between the transition frequency and the laser frequency, since the mismatch has to be magnetically tuned. This becomes less of a limitation as more laser frequencies become available, except that one then needs to know in advance which laser frequency to choose. It becomes part of the search problem ... 

The general method of the experiment to measure ground n=l state energy levels is microwave magnetic resonance spectroscopy as applied to muonium(21,22). It relies on parity nonconservation in the decay to... 

The first microwave spectrum of CF was observed by Carrington and Howard [195] using magnetic resonance methods. The J = 11/2 9/2 rotational transition... 

The magnetic resonance methods are applied to investigate triplet states [1]. To obtain these triplet states in reaction centers of photosynthetic bacteria, the electron transfer has to be blocked by reducing the primary acceptor, an ubiquinone, or by removing it [2]. Microwave radiation induces the spin transitions and the resulting absorption changes of the molecules involved can be detected. 

ESR is a magnetic resonance method using microwave frequencies of the order of 10 GHz. This means that ESR has a different time scale compared to NMR (electromagnetic waves of the order of 100 MHz are common in most NMR spectrometers). A few examples of motional narrowing of line width, which is connected with the time scales will be discussed in Sect. 8. 

The technique of LMR is very similar to other magnetic resonance methods such as EPR and NMR. While NMR uses radiofrequency radiation to produce transitions between nuclear spin levels, and EPR uses microwave radiation to produce transitions between electron spin levels, LMR uses... 

Various theoretical methods (self-consistent field molecular orbital (SCF-MO) modified neglect of diatomic overlap (MNDO), complete neglect of differential overlap (CNDO/2), intermediate neglect of differential overlap/screened approximation (INDO/S), and STO-3G ab initio) have been used to calculate the electron distribution, structural parameters, dipole moments, ionization potentials, and data relating to ultraviolet (UV), nuclear magnetic resonance (NMR), nuclear quadrupole resonance (NQR), photoelectron (PE), and microwave spectra of 1,3,4-oxadiazole and its derivatives <1984CHEC(6)427, 1996CHEC-II(4)268>. 

Modem structural chemistry differs from classical structural chemistry with respect to the detailed picture of molecules and crystals that it presents. By various physical methods, including the study of the structure of crystals by the diffraction of x-rays and of gas molecules by the diffraction of electron waves, the measurement of electric and magnetic dipole moments, the interpretation of band spectra, Raman spectra, microwave spectra, and nuclear magnetic resonance spectra, and the determination of entropy values, a great amount of information has been obtained about the atomic configurations of molecules and crystals and even their electronic structures a discussion of valence and the chemical bond now must take into account this information as well as the facts of chemistry. 

Muller, N., ibid. In Interferometry, Light Scattering, Microscopy, Microwave and Magnetic Resonance Spectroscopy Optical, Spectroscopic and Radioactivity Methods, Chapter VII. 

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