Spin paramagnetic compounds

Powder spectra of paramagnetic compounds measured with applied fields are generally more complicated than those shown in Fig. 4.14. Large internal fields at the Mossbauer nucleus that are temperature- and field-dependent give rise to this complication. If, however, the measurement is performed at sufficiently high temperature, which is above ca. 150 K, the internal magnetic fields usually collapse due to fast relaxation of the electronic spin system (vide infra, Chap. 6). Under... 

The procedure of spin counting is then to use the EPR spectrum of another paramagnetic compound as an external standard (which we will label K to avoid confusion with the spin S) of known concentration (cK) to obtain the unknown (U) concentration (cv) of the paramagnetic compound of interest as... 

Ahlin P. et al., The effect of lipophilicity of spin-labeled compounds on their distribution in solid lipid nanoparticle dispersions studied by electron paramagnetic resonance, J. Pham. Set, 92, 58, 2003. 

Acceleration of relaxation of a nuclear spin by paramagnetic compounds is the greater the slower the nucleus relaxes. For example, the nonprotonated carbon nuclei of... 

The pronounced relaxation acceleration observed for slowly relaxing nuclei in the presence of paramagnetic compounds is exploited in Fourier transform, 3C NMR spectroscopy. On use of the fast pulse sequences that are frequently necessary, the spin-lattice relaxation of slow 13C nuclei can no longer follow excitation, and the corresponding 13C signals have low intensities. In such cases, addition of small amounts of relaxation accelerators, such as radicals or transition metal salts, to the sample amplifies these signals [153]. 

The hyperfine shifts in the proton NMR spectra of paramagnetic hemes and hemoproteins are closely related to the electronic structures of these molecules. At present the most extensive NMR studies of the electronic spin distribution in the heme groups have been done with low spin ferric compounds, which will be discussed in this section. Procedures similar to those described here would apply to the analysis of the NMR spectra of hemoproteins in the other paramagnetic states (Table 1). 

Kochany and Bolton (1992) studied the primary rate constants of the reactions of hydroxyl radicals, benzene, and some of its halo derivatives based on spin trapping using detection by electron paramagnetic resonance (EPR) spectroscopy. The competitive kinetic scheme and the relative initial slopes or signal amplitudes were used to deduce the kinetic model. Based on a previously published rate constant (4.3 x 109 M 1 s ) in the pH range of 6.5 to 10.0 for the reaction of hydroxyl radicals with the spin trap compound 5,5 -d i methy I pyrro I i ne N-oxide (DMPO), rate constants for the reaction of hydroxyl radicals with benzene and its halo derivatives were determined. 

Paramagnetic compounds can also be difficult NMR subjects because the unpaired electron(s) couple(s) with the nuclear spin to provide rapid relaxation. Relaxation times can, however, yield useful information about a system certain parameters which contribute to the magnitude of 7) are directly related to the oxidation number and spin state of the metal ion, the nature of the metal ion and its coordination geometry. For example, high-spin Com in octahedral complexes has a relaxation time more than an order of magnitude faster than the same ion in a tetrahedral arrangement of ligands (Banci et at., 1992). 

Muonium (Mu) is the lightest hydrogen-like atom (mMu = 0.11 mH) available for chemical research it has a positive muon (/jl+, t = 2.2 fisec) as the nucleus. The muon spin resonance (/tSR) technique is described in several review articles (16, 99—102). Most of the research is performed in the condensed phases, but because of the development of the surface muon beams (103, 104), experiments in the gaseous phase have received more attention. At present three muonic fractions can be detected (1) fan, free muonium (2) fa, free n+, or Mu bound in a diamagnetic compound and (3) fa, Mu bound in a paramagnetic compound. In liquid phases, there is quite often a missing fraction, fa = 1 -fau - fa - fa ... 

The electron spin resonance of certain paramagnetic compounds e. g., diphenyl picryl hydrosil (DPPH) and the allyl radical are said to exhibit negative spin density. The negative spin density is determined by the freeon unpaired... 

It has been known since the earliest days of magnetic resonance spectroscopy that paramagnetic compounds, containing one or more unpaired spins, enhance the relaxation rates for the water protons in which they are dissolved." The extent of this enhancement, termed relaxivity, is highly dependent on the magnitude of the dipole-dipole interactions between the electron spin on the paramagnetic metal complex and the proton spin on the water molecule in question. These interactions are often quite complex. They can be treated on a formal (theoretical) level by the Solomon-Bloembergen equations." On a strictly practical level, however, the devel-... 

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