Linewidths, Lineshapes, and Spin Relaxation in the One and Two-Dimensional Esr of Organic Radicals and Spin-Labels

Abstract

When I was a graduate student with George Fraenkel at Columbia University from 1958-1962, his laboratory was teeming with interest and activity in the area of spin-relaxation, mainly of organic free radicals in liquid solution. In 1959 George, working with an excellent theoretical postdoc Mike Stephen, had developed a new theory (1) which could successfully account for the fact that the measured Ti's from each hyperfine line of semiquinone spectra obtained by Jan Schreurs in George's lab were different in magnitude (2). Dan Kivelson, who had been stimulated by his visits to the Fraenkel Lab, was completing his theory of unsaturated linewidths (3) that he developed from the seminal Kubo and Tomita theory of lineshapes (4). The Stephen-Fraenkel theory deriving more from the Wangness-Bloch (5)-Redfield (6) theory (more commonly known as Redfield theory today) also incorporated components of Kubo and Tomita theory. In fact, the Stephen-Fraenkel theory of ESR saturation and the Kivelson theory of unsaturated linewidths were complementary in the insight and understanding they provided into spin-relaxation of organic radicals in solution.My own interest in these matters became focused by the project I was undertaking of trying to extend to free radicals in the liquid state George Feher's great invention of ENDOR in solids. It seemed to me that I needed to understand steady-state saturation in these systems and how irradiating the NMR transitions affects the saturated ESR signal. Mike Stephen encouraged me to study Redfield's 1957 paper as well as his and George Fraenkel's saturation theory, and he was a major source of advice as I continued my efforts. The important concept here was that of coupled first-order rate equations that emerge from the spin-relaxation of coupled spin-eigenstates. This is an idea that was already made clear in the early Wangness-Bloch (WB) papers (1953)(1954)(1955)(1956). Al Redfield showed (1957) how the concepts of spectral densities arising from random motions in liquids that are central to the pioneering paper of Bloembergen, Purcell, and Pound (7) could be incorporated into the WB theory. In addition, by irradiating the NMR transitions one could then "short out" the nuclear-spin transitions induced by the random motions and thereby enhance the coupled relaxation of a saturated ESR line.Needless to say, when I inserted estimates of spin-relaxation terms from Schreurs and Fraenkel's saturation study on semiquinones I concluded that the 658 Foundations of Modern EPR Downloaded from www.worldscientific.com by NANYANG TECHNOLOGICAL UNIVERSITY on 10/04/15. For personal use only.

Relaxation Times of Organic Radicals and Transition Metal Ions

“…Direct measurements of Tl by saturation recovery by Rengen (1971Rengen ( , 1974aRengen ( , 1974b and by Hyde and coworkers Hyde 1974a,b, 1976;Hyde 1974Hyde , 1998, and then of T2 by Freed and coworkers (Gorchester et aI., 1990) and by Schwartz et aI. (1979;Stillman and Schwartz, 1979) solidified understanding of the rates and mechanisms of relaxation of small organic radicals in fluid solution (Freed 1972(Freed , 1976(Freed , 1998Schneider and Freed, 1989). The direct measures of Tl and T2 were then used by these labs to explore motions in fluid solution and collisions with other nitroxyl radicals and with O2 (Subczynski and Hyde, 1981;Hyde et aI., 1990;Subczynski et aI., 1992a,b).…”

“…When tumbling motion dominates echo dephasing Tm = T2. (Freed 1976(Freed , 1998Stillman and Schwartz, 1979;Schwartz et aI., 1979).…”

“…3. Dynamics of a radical can be studied by analysis of contributions to T2 due to rotational modulation of hyperfine and Zeeman anisotropies (Freed, 1998). 4.…”