DFT calculations on the relative stability of various nucleobase radicals induced by e(aq)(-) and (*)OH have been carried out for assessing the energetics of rearrangements and water elimination reactions, taking the solvent effect of water into account. Uracil and thymine radical anions are protonated fast at O2 and O4, whereby the O2-protonated anions are higher in energy (50 kJ mol(-1), equivalent to a 9-unit lower pK(a)). The experimentally observed pK(a)=7 is thus that of the O4-protonated species. Thermodynamically favored protonation occurs slowly at C6 (driving force, thymine: 49 kJ mol(-1), uracil: 29 kJ mol(-1)). The cytosine radical anion is rapidly protonated by water at N3. Final protonation at C6 is disfavored here. The kinetically favored pyrimidine C5 (*)OH adducts rearrange into the thermodynamically favored C6 (*)OH adducts (driving force, thymine: 42 kJ mol(-1)). Very similar in energy is a water elimination that leads to the Ura-5-methyl radical. Purine (*)OH adducts at C4 and C5 (plus C2 in guanine) eliminate water in exothermic reactions, while water elimination from the C8 (*)OH adducts is endothermic. The latter open the ring en route to the FAPY products, an H transfer from the C8(*)OH to N9 being the most likely process.

The pore stuffing method is studied with the objective of improving the plasma induced damage for porous organo-silicate glass low-k dielectrics. Experiments on blanket films show that, pore stuffing reduces the low-k degradation compared to non-protected porous films during plasma etch. The post etch surface roughness is also improved. The protection mechanism is attributed to reduced radical penetration, mainly fluorine and oxygen. The resistance against vacuum UV degradation is also improved by pore stuffing, since polymers used as filling agents have a higher VUV absorption coefficient than Si-O based dielectric network. The protection effect against fluorocarbon-based gas discharges is extensively studied with both blanket and patterned samples. For patterning of porous, non-stuffed low-k films, the addition of polymerizing gas reduces surface plasma damage but this polymerizing protection effect doesn't work well on trench sidewalls. Pore stuffing enables an efficient sidewall protection even with oxidizing gas discharges.

Abstract:The radical cation of s-trioxane, radiolytically generated in a freon (CF3CCl3) matrix, was studied in the 10-140 K temperature region. Reversible changes of the EPR spectra were observed, arising from both ring puckering and ring inversion through the molecular plane. The ESREXN program based on the Liouville density matrix equation, allowing the treatment of dynamical exchange, has been used to analyze the experimental results. Two limiting conformer structures of the s-trioxane radical cation were taken into account, namely "rigid" half-boat and averaged planar ones, differing strongly in their electron distribution. The spectrum due to the "rigid" half-boat conformer can be observed only at very low (<60 K) temperatures, when the exchange of conformers is very slow. Two transition states for interconversion by puckering and ring-inversion were identified, close in activation energy (2.3 and 3.0 kJ/mol calculated). Since the energy difference is very small, both processes set on at a comparable temperature. In the case of nearly complete equilibration (fast exchange) between six energetically equivalent structures at T > 120 K in CF3CCl3, a septet due to six equivalent protons (hfs splitting constant 5.9 mT) is observed, OPEN ACCESSMolecules 2014, 19 17306 characteristic of the dynamically averaged planar geometry of the radical cation. DFT quantum chemical calculations and spectral simulation including intramolecular dynamical exchange support the interpretation.

No abstract

It is well known in supramolecular chemistry that cyclodextrin host molecules (CDs) are capable of including and binding guest molecules in their hydrophobic cavities via non-covalent interactions. The unique recognition capability of CDs depends on their inherent asymmetric cavities. We explored here the impacts of the chiral recognition of excited triplet state of guest 1,1 -Binaphthyl-2,2 -diylhydrogenphosphate (BNP) enantiomers by host native -cyclodextrin ( -CD) and chemically modified Heptakis(2,3,6-tri-O-methyl)--cyclodextrin (TMe--CD). For this regard, UV-Vis, fluorescence, and laser flash photolysis spectroscopy were performed and our experimental results were flanked by density functional theory (DFT) calculations. Triplet decay dynamics of BNP enantiomers were investigated in the absence and presence of cyclodextrins under dif-1832 | Y. M. Riyad et al. ferent conditions and the chiral recognition behavior is discussed based on the obtained kinetic and spectral data of excited triplet state. Here we show that the long lived triplet excited states (as opposed to the singlet states) and its relaxation in solution may be a sensitive probe to detect chiral recognition. The ability of TMe--CD to recognize axial chirality is superior to that of -CD. These findings may be attributed to the asymmetric cavities of cyclodextrins, which determines the degree of interaction. The differences in the triplet behavior of R-and S-BNP(T 1 )-TMe--CD may arise from the different orientations of the guest molecule within the twisted and restricted TMe--CD cavity. We assume here that the orientation of the more favorable enantiomer (S-BNP(T 1 )) is geometrically preferable, while a similar orientation of the less favorable enantiomer (R-BNP(T 1 )) might be hindered by steric reasons. In contrast, the triplet dynamics of BNP enantiomers within the unrestricted macrocyclic -CD ring are similar. This might be ascribed to a similar orientation and interaction and ultimately relaxation of both enantiomers within a round and undistorted -CD cavity, suggesting the formation of similar inclusion complexes. These findings are evidence for the possibility to detect chiral recognition and interaction as well as tiny structural differences in the BNP complexes within cyclodextrin cavities via laser flash photolysis technique.