Spin-exchange collisions

Such so-called spin-exchange collisions may be experimentally distinguished only if the experiment involves measurement of differences due to spin interactions. For instance, collision cross-sections for eq. (6-5) might be measured with crossed molecular beams using spin polarizing magnetic fields. 

During the collision of atoms which have unfilled electron shells, a spin exchange is possible [30]. The spin exchange substantially perturbs the hyperfine states of the colliding atoms and thus plays an important role in... 

We shall confine ourselves to considering the spin exchange during the collision of atoms which have one s-electron in the outer shell (atoms of H or those of alkaline metals). The wave function describing the atoms with oppositely directed spins, before the collision, has the form... 

Collision of nitroxides with fast-relaxing radicals, such as oxygen and metal ion complexes, causes spin exchange that effectively shortens the spin-lattice relaxation time T of the nitroxide (Hyde and Subczynski, 1989). This effect can be measured either by continuous wave (CW) power saturation techniques or by saturation recovery methods. Collision frequency is directly proportional to the accessibility of the paramagnetic reagent to the nitroxide radical and is defined as... 

Such measurements were first applied with considerable success to elastic scattering. Indeed one was able to discuss experiments which would determine all the theoretically calculable amplitudes (Bederson, 1970). For inelastic processes, such measurements necessitate the simultaneous application of spin selection techniques and the alignment and orientation measurements discussed in the previous chapter. The experiments have become feasible with the advancement of experimental techniques. The first successful differential electron impact excitation study with spin-polarised electrons and alignment and orientation measurements was performed by Goeke et al. (1983) for the e—Hg case. McClelland, Kelley and Celotta (1985, 1986) carried out a systematic study for superelastic scattering of polarised electrons from polarised laser-excited Na (3 P) atoms. This system is essentially a two-electron collision system in which spin exchange is the dominant spin-dependent interaction. It thus allows one to obtain... 

Block equations can be constructed that describe the macroscopic magnetization of the spin system including the relaxation mechanism involving spin exchange caused by molecular collisions. These equations were solved as a function of the exchange frequency, wex, for the nitroxide free radical case. Spectra were computed by assuming the degrees of motional freedom observed with the labeled androstane or cholestane molecules localized at the air-water interface. 

Fig. 8. Two-dimensional exchange spectroscopy (often called 2D-ELDOR) of the spin-labeled 3K-8 peptide with mixing time T = 296 nsec. Both the 2D surface and the contour map are shown. The peaks along the diagonal are related to the absorption spectrum of the spin label. The high-held M,= - line is weak because of experimental dead time artifacts. The cross-peaks, especially those between the outermost hyperfine lines, provide direct evidence of Heisenberg spin exchange. The cross-peak intensity can be used to determine the second-order rate constant for collisions between peptides.

The problem of line widths, as applied to relative atom concentration measurements, has been discussed in a recent paper." For all species (except S-state atoms) in dilute concentration, the true unsaturated line width is determined simply by the total pressure (i.e. by the frequency of collision with any particle), and the signal heights are thus directly proportional to the relative concentration of atoms. This theoretical result has been verified experimentally for 0( Pg i) and CK Pg/g) atoms." For S-state atoms (H( S) and N( S)), the true line width depends on the atom concentration via spin-exchange collisions, unless a paramagnetic diluent is present." " This complication probably does not lead to great difiiculty in the majority of commonly encountered chemical systems. 

The primary salt effect on rates of electron spin exchange in solutions of Fremy s salt is consistent with bimolecular reaction species with a charge product (za b) of + 4, which supports a mechanism involving collision between two [(S03)2N0] anions (za = Zr = — 2). ... 

In a similar way collisional spin exchange between unlike species can be studied, as has been demonstrated for Cu and Ni complexes of crown ethers and the nitroxide radical TEMPO [40]. Comparison of the rate constant of spin exchange with the rate constant of diffusion collisions gave information on the steric factor, which characterizes the accessibility of the ligated transition metal ion to the nitroxide radical. 

The ESR spectra of monoradicals and mononuclear transition ion complexes can also be influenced by spin exchange, because the wave functions of the electrons overlap for a short time during diffusional collisions of paramagnetic species. At moderate concentrations (1 A/ or larger), the collisions are so frequent that line broadening and a decrease of the hyperfme splitting can be observed. In macromolecular and supramolecular systems, this effect is sometimes perceptible at lower bulk concentrations, as diffusion may be restricted or local concentrations of some species strongly exceed their bulk concentration. Examples are discussed in Chapter 7. 

Crampton SB, Wang HTM. (1975) Duration of hydrogen atom spin-exchange collisions. Phys. Rev. A 12 1305-1312. 

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