Paramagnetic relaxation time

Van Vleck, J.H. Paramagnetic Relaxation Times for Titanium and Chrome... 

The superscript" is used in Scheme 12.2 and throughout this chapter to represent spin polarization, a term applied to situations for which a paramagnehc species possesses a population of spin states that is different from the Boltzmann distribution at the temperature of the experiment. Polarization disappears during the radical paramagnetic relaxation time, usually in the microsecond timescale. Here and below, we will... 

During a microsecond time frame, which is the typical paramagnetic relaxation time of free radicals, polarized free radicals can participate in addition to multiple bonds, to dioxygen, in hydrogen (electron) transfer, in addition to polyradicals, etc. Products of these reactions are polarized in most cases, and they demonstrate TR ESR signals. 

The orthochromite, DyCrOa, is paramagnetic above the Neel temperature of 2-16 K, but at 4-2 K a well-resolved magnetic hyperfine spectrum is seen because of a long paramagnetic relaxation time [107]. The parameters (Table... 

Fig. 17.29 Yb spectra at 4 2 K showing the effect of a long paramagnetic relaxation time (Yb(N03)3.6H20 and Yb2(S04)3.8H20), short (YbCls and 6203), and intermediate (Yb acetate). Note that the splitting in 6203 is due to two sites with quadrupole interactions. [Ref. 178, Fig. 1]...

From Figure 8.9 together with the tabulated values in Table 8.8, it can be seen that the contribution of the hyperfine coupling makes no significant contribution to the overall relaxation mechanism, since the experimentally determined value for the paramagnetic relaxation time of 9 ns cannot be obtained. The results do however. 

The design of a pulsed EPR spectrometer depends heavily on tlie required pulse lengdi and pulse power which in turn are mainly dictated by the relaxation times of tlie paramagnetic species to be studied, but also by the type of experiment perfomied. When pulses of the order of a few nanoseconds are required (either to compete... 

Here Ti is the spin-lattice relaxation time due to the paramagnetic ion d is the ion-nucleus distance Z) is a constant related to the magnetic moments, i is the Larmor frequency of the observed nucleus and sis the Larmor frequency of the paramagnetic elechon and s its spin relaxation time. Paramagnetic relaxation techniques have been employed in investigations of the hydrocarbon chain... 

The addition of paramagnetic species, such as the metal ions Cu ", Mn, or CF", can have dramatic effects on both the observed spectmm and the relaxation behavior of a molecule. The added ion reduces nuclear relaxation times, and permitting more rapid data collection. In addition, faster relaxation rates minimize NOE effects in the spectra, which can be useful in obtaining quantitative intensity data. The most widely used reagent for this purpose is chromium acetylacetonate [13681 -82-8] known as Cr(acac)2. Practically speaking, the use of such reagents requires care, because at... 

The low intensities of nonprotonated carbons is usually due to their long relaxation times. The addition of a paramagnetic substance such as... 

Often the electronic spin states are not stationary with respect to the Mossbauer time scale but fluctuate and show transitions due to coupling to the vibrational states of the chemical environment (the lattice vibrations or phonons). The rate l/Tj of this spin-lattice relaxation depends among other variables on temperature and energy splitting (see also Appendix H). Alternatively, spin transitions can be caused by spin-spin interactions with rates 1/T2 that depend on the distance between the paramagnetic centers. In densely packed solids of inorganic compounds or concentrated solutions, the spin-spin relaxation may dominate the total spin relaxation 1/r = l/Ti + 1/+2 [104]. Whenever the relaxation time is comparable to the nuclear Larmor frequency S)A/h) or the rate of the nuclear decay ( 10 s ), the stationary solutions above do not apply and a dynamic model has to be invoked... 

To visualize the effects induced by paramagnetic relaxation, the time-dependent forward-scattering intensity has been calculated by implementing the stochastic relaxation between spin states i ) and j ) into the SYNFOS program package [30]. The transition rates from i) to j ) with , > Ej are described by [53]... 

The reported in situ NMR of combustion [2] served largely as proof of concept work. It was demonstrated that despite the presence of paramagnetic oxygen and radicals, the xenon relaxation times are sufficiently long for gas exchange studies. 

This approach was followed by Yushmanov for the localization of papaverine in ionic micelles.42 Another interesting application was reported by Chien43 who measured 19F NMR relaxation times of trifluor-omethyl labelled atrazine induced by paramagnetic probes gadolinium ethylenediamine tetraacetic acid and 2,2,6,6-tertramethyl-piperidine-N-oxyl. The results showed that atrazine solubilized by humic micelles occupied a hydrophobic domain accessible only to neutral hydrophobic molecules. 

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