Electron spin label rotational correlation time

Functional groups attached to solvent-swollen polymer chains exhibit free rotational motion as indicated by electron spin resonance rotational correlation times 132-134) These studies using nitroxide spin labels covalently bound to polystyrene matrices indicated that the mobility of the substituent is a function of the cross-link density and degree of swelling. The rotational correlation time of nitroxide within 2% cross-linked beads was about 100 times shorter in dichloromethane or benzene than in ethanol, and 2-3 times longer than nitroxide bound to non-cross-linked polystyrene. The latter observation shows that the heterogeneous reaction involving 2% cross-linked polystyrene is 2-3 times slower than the same reaction in solution. 

Two methods are useful to measure slow motion spin label spectra saturation transfer (ST-EPR) and two-dimensional electron spin echo spectroscopy (2D ESE). In the ST-EPR experiment, " a cw spectrometer is operated at high microwave power, and this causes partial saturation of the nitroxide spectrum. As the field is scanned, molecular motion carries this saturation to nearby regions of the spectrum, yielding spectra that are very sensitive to the rotational correlation time. One of the great advantages of ST-EPR is that little instrumentation is required beyond the conventional cw EPR instrument. Most any laboratory equipped to perform EPR can also perform ST-EPR. 

Spin probes I and II are studied in the system CTAB-hexanol-water [3,21]. Spin probe I, with its nitroxyl radical quite distant from the polar headgroup, probes the nonpolar region of the interface whereas spin probe II, with the nitroxyl radical close to the polar headgroup, explores the polar region of the interface. From the heights (/i) and the linewidths (A//) of the different multiplets (due to the electron-nitrogen hyperfine coupling), one can compute the rotational correlation times (t ) of these spin labels [84] from the relationship... 

Electron spin resonance (5) measures the difference in the relaxation process between the segments in the loops and trains. The spectra of an adsorbed layer is deconvoluted into contributions from the loops, the tails, and the trains of the adsorbed layer by comparison with the spectra obtained in solution or melt state (mobile components) and on adsorbed oUgomers at low temperatures (immobile components). It is also possible to measure the rotational correlation time of the mobile and immobile segments. The drawback of this technique is that a label has to be attached to the polymer and this might change the adsorption behavior of the chain. 

Regen (1974) described a method of assessing the mobility of the resin sites by covalent attachment of a spin-label probe [2,2,6,6-tetramethyl-4-piperidinyl-l-oxy group (Tempo) (Fig. 3-1. R = (P)—)] to the chlo-romethylated co(polystyrene-DVB). Rotational correlation times (t) were calculated from observed room-temperature electron paramagnetic resonance (epr) spectra. The degree of swelling, q, (swelled volume/ dry volume) was determined from the measured density of the dry resin and the weight of the imbibed solvent. The data indicate that those... 

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