The relation of Einstein D = BkT/e, relating the diffusivity and mobility of particles (ions or atoms) must be altered where D is the diffusion constant of tracers in solids, if a vacancy mechanism holds, because there will be a correlation between successive steps of a particle, even if the steps of the vacancies themselves are uncorrelated. Certain symmetry conditions being fulfilled, the relation of Einstein must be extended with a correlation factorwhere el , i + 1 is the angle between two successive steps of a particle.The diffusion problem can be translated into the theory of electrical networks and with the help of measurements in a resistor network the value of C o S i , i + 1 is easily evaluated. This has been done for several types of lattices. Other cases, where eqn. (1) does not hold, because of lack of symmetry, are treated by similar methods.Appropriate substitutions having been made, the diffusion of associated pairs has been treated as the diffusion of a single particle in a so-called transposed lattice. * Unfortunately in their eqn. (A2), which is our eqn. (l.Z), a factor 2 was erroneously overlooked, modifying their equation 1 + OlPr 1 -&P,' (AS) into f = -~ f = -1 -&P, 1 which is our eqn. (3.4). International Meeting on Reactivity of Solids, to be held in Madrid, April, 1956.-f A discussion of these applications will be published in the proceedings of the Third

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In order to address problems such as aging, cell death, and cancer, it is important to understand the mechanisms behind reactions causing DNA damage. One specific reaction implicated in DNA oxidative damage is hydroxyl free-radical attack on adenine (A) and other nucleic acid bases. The adenine reaction has been studied experimentally, but there are few theoretical results. In the present study, adenine dehydrogenation at various sites, and the potential-energy surfaces for these reactions, are investigated theoretically. Four reactant complexes [A···OH]* have been found, with binding energies relative to A+OH* of 32.8, 11.4, 10.7, and 10.1 kcal mol(-1). These four reactant complexes lead to six transition states, which in turn lie +4.3, -5.4, (-3.7 and +0.8), and (-2.3 and +0.8) kcal mol(-1) below A+OH*, respectively. Thus the lowest lying [A···OH]* complex faces the highest local barrier to formation of the product (A-H)*+H(2)O. Between the transition states and the products lie six product complexes. Adopting the same order as the reactant complexes, the product complexes [(A-H)···H(2)O]* lie at -10.9, -22.4, (-24.2 and -18.7), and (-20.5 and -17.5) kcal mol(-1), respectively, again relative to separated A+OH*. All six A+OH* → (A-H)*+H(2)O pathways are exothermic, by -0.3, -14.7, (-17.4 and -7.8), and (-13.7 and -7.8) kcal mol(-1), respectively. The transition state for dehydrogenation at N(6) lies at the lowest energy (-5.4 kcal mol(-1) relative to A+OH*), and thus reaction is likely to occur at this site. This theoretical prediction dovetails with the observed high reactivity of OH radicals with the NH(2) group of aromatic amines. However, the high barrier (37.1 kcal mol(-1)) for reaction at the C(8) site makes C(8) dehydrogenation unlikely. This last result is consistent with experimental observation of the imidazole ring opening upon OH radical addition to C(8). In addition, TD-DFT computed electronic transitions of the N(6) product around 420 nm confirm that this is the most likely site for hydrogen abstraction by hydroxyl radical.

Several possible mechanisms underlying isoguanine formation when OH radical attacks the C2 position of adenine (A) are investigated theoretically for the first time. Two steps are involved in this process. In the first step, one of two low‐lying A⋅⋅⋅OH reactant complexes is formed, leading to C2H2 bond cleavage. Between the two reactant complexes there is a small isomerization barrier, which lies well below separated adenine plus OH radical. The complex dissociates to free molecular hydrogen and an isoguanine tautomer (isoG 1 or isoG 2). The local and activation barriers for the two pathways are very similar. This evidence suggests that the two pathways are competitive. After dehydrogenation, there are two possible routes for the second step of the reaction. One is direct hydrogen transfer, via enol–keto tautomerization, which has high local barriers for both tautomers and is not favored. The other option is indirect hydrogen transfer involving microsolvation by one water molecule. The water lowers the reaction barrier by over 20 kcal mol−1, indicating that water‐mediated hydrogen transfer is much more favorable. Both A+OH.→isoG+H. reactions are exothermic and spontaneous. Among four isoguanine tautomers, isoG 1 has the lowest energy. Our findings explain why only the N1H and O2H tautomers of isolated isoguanine and isoguanosine have been observed experimentally.