Exchange narrowing

Sack R A 1958 A contribution to the theory of the exchange narrowing of spectral lines Mol. Phys. 1 163-7... 

Knapp EW (1984) Lineshapes of molecular aggregates, exchange narrowing and intersite correlation. Chem Phys 85 73-82... 

The method of determination of the exchange rate is particularly suited to in situ electrogeneration methods, since one can add different, high concentrations of parent material to the cell, coulometrically generate a small amount of radical ion, and determine the exchange-narrowed line width. The occurrence of exchange reactions (Eq. 29.25) can also be a problem in routine electrochemical EPR studies, for unless the radical generation step is essentially complete, the spectral resolution will be less than ideal. 

The next step in approximation of the theory for EPR measurements was done by Atsarkin and co-workers in [138]. Basing on Fel dman s results, the researchers studied dipolar broadening and spin exchange narrowing of EPR lines from the paramagnetic centers distributed on the solid surface, and a simple formula for a signal of free precession S(t) was expressed [138] ... 

This single line (go = 1.935 0.005, AH 140 G) is usual for V4+ centers, which are coupled by strong spin-exchange interaction [23, 164], These V4+ ions are localized in some areas with high local concentration (V4+)s (the so called nano-phases - because of their probable size), which can be estimated from the exchange narrowing of a spectrum by the equation [23] ... 

Voigt parameter (also a parameter associated with exchange narrowing)... 

How fast should one spin the sample to achieve line narrowing Just as with chemical exchange narrowing, the criterion for motion to narrow the spectrum is that the rate of... 

The method is based on the ability of TCNQ to react with bonded nitrogen compounds (secondary, tertiary amines) to form stable ion-radical salts (IRS) under mild conditions and with high yields. Completeness of formation of stable IRS has been monitored by ESR and electronic spectroscopy. One of the most important features of this method is the formation of two-dimensional phase by TCNQ anion-radicals on the surface, if bonded donor molecules are fixed at a small distance between their anchoring points, i.e. when separation between points of fixation is in the range 1-1.5 nm. ESR spectra of such samples contain an exchange-narrowed singlet with AH = 0.5 — 1.1 Gs, and g-factor 2.0025. Such spectra are typical for crystalline samples of IRS of TCNQ [25]. If bonded donor molecules are separated by as much as 2 nm or more, a distinct phase of TCNQ IRS is not formed. ESR spectra of such samples reveal either dipole-dipole broadening or hyperfine structure. 

Before we proceed to the discussion of applications of the method, one more characteristic feature deserves mentioning. Exchange-narrowed singlet is assigned to occurrence of the TCNQ IRS phase on the surface, that is to clusters of bonded molecules with M > 3. This feature is an essential difference of the TCNQ method from the methods using luminescent or nitroxyl labels, in which the response is changed already in the case of occurrence of pairs of closely spaced molecules. Unfortunately, the theory of ESR spectra of IRS of TCNQ cannot predict how many species are sufficient to form the phase. In the present work triples of neighbouring molecules were believed to suffice. 

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