Spin multiplicities

The interval between the second and third pulse is called the mixing time, during which the spins evolve according to the multiple-spin version of equation B 1.13.2 and equation B 1.13.3 and the NOE builds up. The final pulse converts the longitudinal magnetizations, present at the end of the mixing time, into detectable transverse components. The detection of the FID is followed by a recycle delay, during which the equilibrium... 

Frequency-selective REDOR (fsREDOR) is a very powerful technique developed for the study of 13C and 15N uniformly labeled peptides or proteins [92]. The basic idea of this technique is to combine REDOR and soft n pulses to recouple a selected 13C-15N dipole-dipole interaction in a multiple-spin system. Usually one could use Gaussian shaped pulses to achieve the required selective n inversions. Other band selective shaped pulses have been developed for a more uniform excitation profile [93]. In its original implementation, fsREDOR was used to extract the intemuclear distances of several model crystalline compounds [92], In the past few years, this technique has proven to be very useful for the study of amyloid fibrils as well. For the Ure2p10 39 fibril samples containing 13C and 15N uniformly... 

Fluorescence Photon emission. Fluorescence involves a radiative transition between states of the same multiplicity (spin allowed), usually from the lowest vibrational level of the lowest excited singlet state, Si. Si(v = 0) —> S0 + hv... 

R. A. Marion, S. M. Klainer, Multiple Spin Echos in Pure Quadrupole Resonance, Journal of Chemical Physics, 67 (1977) 3388. 

As in the case of the single spin-echo detection, the multiple spin-echoes method attenuates magnetization components with short 72(5 ) values and the magnitude of the attenuation increases with increasing echo... 

Q and 3Q correlations can be utilized in combination with highest efficiency. In some cases more than one spectrum of each is worthwhile to acquire because cross peak intensity is an explicit function of all coupling constants involved [6, 27, 28]. Coherences of higher order than 3Q exhibit significantly reduced sensitivity. Also, few spin systems in biologically relevant molecules allow sufficient communication between multiple spins to allow observation of higher order spin systems. 

An elegant solution to analyse the REDOR response in the presence of multiple-spin interactions was introduced by Eckert and co-workers. As shown by the authors, the initial part of the REDOR evolution curves proves to be virtually independent on the exact spin geometry and distribution effects. Thus, when restricting the data analysis to AS/So values <0.3, the data can be analysed in terms of the second moment defined by... 

In the case of multiple-spin interactions, the second moment defined in Equation (2) may be approximated to first order by... 

In this variant of CT-REDOR (cf. Figure 3C), the duration of the f-channel dephasing pulses tpo is stepped from 0 to 2it while keeping the evolution time constant. This concept is quite similar to the 0-REDOR approach developed by Gullion and Pennington " but serves a different purpose. While the latter experiment aims at the determination of individual coupling constants in multiple-spin systems, the CT-VPD-REDOR enables... 

Figure 8 Samples used for the validation of the various CT-REDOR approaches. (A) and (B) second coordination sphere of Al in Al(P03)3 (A) and B in BPO4 (B) as examples for an S-/e and S-/4 multiple-spin system with strong dipolar coupling (C) doubly labelled glycine (in a 9 1 mixture unlabelled/labelled glycine) as an example for an isolated two-spin system.

As shown in the preceding two sections, the constant time version of REDOR, CT-REDOR, may be applied as an expedient alternative to the existing REDOR versions in the presence of strong heteronuclear dipolar couplings. In these cases, only few data points are available for the data analysis, which especially in the case of multiple-spin systems renders an evaluation of the second moments impossible. The efficiency of the dipolar recoupling may be intentionally reduced either via a dislocation of the dephasing Ti-pulses from the centre of the rotor period or via an application of non-Ti-dephasing pulses. A variation of the pulse position fpp... 

Carper, W. R., Pflug, J. L., and Wilkes, J. S., Multiple spin probe NMR-studies of ionic structure in l-methyl-3-ethylimidazolium chloride AlClj molten-salts, Inorg. Chim. Acta, 193,201,1992. 

Many TM ions can support multiple spin states. Usually, a change in spin is accompanied by significant geometrical changes. For example, high-spin d8 six-coordinate Nickel(II) amine... 

Lowry, D. F., Correlated Vector Mode of Multiple-Spin Systems, Concepts in Magnetic Resonance An Educational Journal, 1994, 6 (No I)... 

Figure 12.1—Energy diagram comparing fluorescence and phosphorescence. Short arrows correspond to internal conversion without the emission of photons. Fluorescence is an energy transfer between states of the same multiplicity (spin state) while phosphorescence is between states of diiferent multiplicity. The situation is more complex than that shown by this Jablonski diagram.

In the 1950s Hickman developed a centrifugal vapor compression evaporator for seawater desalination (53). This device consisted of multiple spinning discs. Seawater sprayed on one side of the disc evaporated, while the centrifugal force removed the residue from the plate surface. The vapor was compressed and returned to the opposite side of the plate, where condensation provided the heat for evaporation and the desired freshwater for recovery. Overall heat transfer coefficients of 18 kW/m2-K are about three times higher than those achieved in steam turbine condensers. 

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