Can. J. Chem. Vol 85 Number 1-2 Update

Biradical radiationless decay channel in adenine and its derivativesMarek Z. Zgierski, Serguei Patchkovskii, and Edward C. Lim Can. J. Chem./Rev. can. chim. 85(2): 124-134 (2007)
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Abstract: Coupled-cluster calculations of increasing accuracy (approximate doubles: CC2; doubles: EOM-CCSD; connected triples: CR-EOM-CCSD(T)) for CIS-optimized potential energy profiles of adenine and its derivatives indicate that the ultrafast internal conversion of the optically excited π π* state occurs through a state switch to a biradical state, which intersects the ground state at a lower energy. The electronic nature of the biradical state is defined by an electronic configuration in which one unpaired electron occupies a π* orbital confined to the five-membered ring. The second unpaired electron is localized very strongly on a p-type C2 atomic orbital of the six-membered ring. The biradical state minimum has a strongly puckered six-membered ring and a C2–H bond, which is twisted nearly perpendicular to the average ring plane. Consistent with the biradical-mediated internal conversion, the π π* state lifetime is extremely short in adenine and 9-methyladenine, which have barrierless crossing to the biradical state. The lifetime is slightly longer in N,N-dimethyladenine, which has a small barrier for the state switch. In 2-aminopurine the biradical state is found above the π π* state, preventing the biradical state switch and dramatically increasing the lifetime. These results, combined with an earlier work on pyrimidine bases, strongly suggest the importance of a direct decay of the doorway π π* state via a biradical state switch in the photophysics of DNA, even though the nature of the biradical state is somewhat different in purines and pyrimidines.
Key words: adenine, guanine, DNA damage, radiationless decay, biradical, ab initio, coupled clusted.

Heterogeneities in sol–gel-derived paramagnetics-doped forsterites and willemites — Electron microprobe analysis and stretched-exponential 29Si MAS NMR spin–lattice relaxation studiesJ. Stephen Hartman, Arjun Narayanan, Suzie S. Rigby, David R. Sliwinski, Norman M. Halden, and Alex D. Bain Can. J. Chem./Rev. can. chim. 85(1): 56-65 (2007)
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Abstract: We report the synthesis and analysis of sol–gel-derived samples of forsterite (Mg2SiO4) and willemite (Zn2SiO4), doped with paramagnetic Cu2+, Ni2+, and Co2+, at a range of dopant concentrations. Electron probe microanalysis and backscattered electron imaging show the presence of major micrometre-scale heterogeneities in the distribution of paramagnetic centres. Despite the inhomogeneities, the 29Si NMR spin–lattice relaxation behaviour is well-behaved and is consistent with the stretched-exponential expression Mz(t)=Mz(∞){1 – a exp[–(t/T′)n]}. The exponent n is 0.5 within the experimental error in some samples. This value is consistent with relaxation by immobile isolated paramagnetic impurities with negligible 29Si spin diffusion from the impurity centres, but careful curve fitting confirms that n is significantly larger than 0.5 in other samples. Relaxation efficiency is highly dependent on the dopant ion and its concentration. Although the purely empirical stretched-exponential function does not provide a unique physical picture, it is noteworthy that it is sufficiently robust to describe spin–lattice relaxation even in highly inhomogeneous systems. Spin–lattice relaxation is a useful probe of paramagnetics-doped solid samples, but NMR does not provide information on homogeneity. Careful sample characterization on the micrometre scale is highly desirable, as a complement to NMR studies.
Key words: MAS NMR, spin–lattice relaxation, 29Si, forsterite, willemite, stretched-exponential relaxation, sol–gel, minor-component heterogeneity, backscattered electron analysis.