At scan rates above 10 V s-l, the waves complementary to those for oxidation a t 1.1 V, and the reduction a t -0.06 V, appear. By application of the method of Nicholson and Shain,6 the specific rates for the S to 0 isomerization of the metastable 3+ species and the 0 to S isomerization of the metastable reduced species were determined as (5.0 f 0.5) X 10 s-I and (2.0 f 0.2) X 10 s-l, respectively. At low scan rates, <20 mV s-l, the reduction wave at E , = -0.06 V decreases in amplitude, as is seen by comparing Figure I C with Figure 1 b. Intramolecular electron transfer can cause such a decrease, but other processes, for example, intermolecular electron transfer and loss of the electroactive species from the diffusion layer, can also contribute, so that, in the absence' of a more complete study, we can only set the upper limit, 4.1 X s-I, on the rate of intramolecular electron transfer. The intervalence band for the stable form of the mixed-valence species, [S3+/(SO)2+], measured in acetone, has a maximum at -640 nm with c -4 X 10 M-l cm-I. The shift to high energies, as compared to substitutionally symmetric species, is a reflection of the difference in the redox potentials for the two sites. There is a second much stronger absorption at 452 nm which we attribute to ligand to metal charge transfer at R u ( I I I ) .~The more usual redox couples conform to the Marcus relat i~n s h i p ,~ a necessary condition for its validity being that the distortions that bring energy matching at the two sites are nearly harmonic. Those in which the potential profile for at least one oxidation state has two minima-in our case, this is true in both oxidation states-comprise an important class which has not been investigated systematically. Devices of the kind we have described provide a means of studying electron transfer for them in the intramolecular mode. They may also provide a means of locking in charge transfer brought about by light absorption and, as such, may find application in high-density storage of memory. The excited state [Sz+/(SO)3+]* which arises on absorption of light at -640 nm is expected to be quenched rapidly, and if intramolecular electron transfer is slow enough, isomerization to [Sz+/(OS)3+] will occur.Many examples of geometrical isomerization accompanying a change in oxidation states have been reported,'O particularly by Bond and co-workers." The appearance of linkage isomerizations, which depends on a change in back-bonding capacity attending a change in oxidation state, is predictable for many yet untested systems with a reasonable degree of certainty. The field has been greatly extended in the study of molecules in which the metal ion is bound by q2 to organic ligands,I2 including aromatic molecules, and additional chromophores can easily be built into mixed-valence molecules. It needs to be acknowledged that there is precedent for the "double square" potential diagram featured in Figure 2.13 The system that we describe is to be regarded as a prototype of others which can be devised to show "molecul...

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The structural characterization and physical properties of [Cp*M(pentalene)M'Cp*]"+ (Cp* = pentamethylcyclopentadiene; M, M' = Fe, Fe (la); Co, Co (lb); Ni, Ni (lc); Ru, Ru (Id); Fe, Ru (le); Fe, Co (If); n = 0, 1,2) and [Cp*M(s-indacene)M'Cp*]"+ (s = symmetric) and [Cp*M(as-indacene)M'Cp*]"+ (M, M' = Fe, Fe (2a, 3a); Co, Co (2b, 3b); Ni, Ni (2c, 3c); n = 0, 1, 2) (as = asymmetric) are reported. The local molecular structure of the organometallic complex does not change significantly with oxidation state; in all cases the Cp*M moieties reside on opposite faces of the fused ^-bridging ring systems, reflecting the dominance of steric effects. These complexes generally exhibit behavior consistent with significant electronic interactions between metal centers, including large electrochemical potential separations between successive one-electron redox events, and for the mixed valent (n = 1+) complexes, intervalent charge transfer absorption bands. The magnetic susceptibility data are consistent with intramolecular ferromagnetic coupling of spins for la2+ and 2c2+ and antiferromagnetic coupling of spins for lc, lc2+, lb, 2b, lc+, 2c+, and 3c+. In general, the paramagnetic complexes exhibit Curie-Weiss behavior, except for 2c and 3c, which possess singlet ground states and high spin excited states that are 0.036 and 0.056 eV (290 and 524 cm"1) above the ground state, respectively. Mixed-valent la+ and 2a+ are fully detrapped on the Mossbauer time scale (i.e., electron transfer rates >107 s-1) above 1.5 K, consistent with a negligible energy barrier to intramolecular electron transfer or complete delocalization. The EPR spectra of la+, 2a+, and le+ exhibit significantly reduced g-factor ansiotropies and more intense spectral features at ambient temperature compared to [FeCp*2]*+, implying intramolecular electron transfer rates >1010 s_1.

Reducing health disparities requires an understanding of the mechanisms that generate disparities. Life course approaches to health disparities leverage theories that explain how socially patterned physical, environmental, and socioeconomic exposures at different stages of human development shape health within and across generations and can therefore offer substantial insight into the etiology of health disparities. Life course approaches are informed by developmental and structural perspectives. Developmental perspectives emphasize how socially patterned exposures to risk factors during sensitive life stages shift health trajectories, whereas structural perspectives emphasize how social identity and position within socially patterned environments disproportionately allocate risk factors and resources, resulting in altered health trajectories. We conclude that the science of health disparities will be advanced by integrating life course approaches into etiologic and intervention research on health disparities. The following 4 strategies are offered to guide in this process: (1) advance the understanding of multiple exposures and their interactions, (2) integrate life course approaches into the understanding of biological mechanisms, (3) explore transgenerational transmission of health disparities, and (4) integrate life course approaches into health disparities interventions.