The reliability of density functional theory (DFT) in the determination of the isotropic hyperfine coupling constants (hfccs) of the ground electronic states of organic and inorganic radicals is examined. Predictions using several DFT methods and 6-31G, TZVP, EPR-III and cc-pVQZ basis sets are made and compared to experimental values. The set of 75 radicals here studied was selected using a wide range of criteria. The systems studied are neutral, cationic, anionic; doublet, triplet, quartet; localized, and conjugated radicals, containing 1H, 9Be, 11B, 13C, 14N, 17O, 19F, 23Na, 25Mg, 27Al, 29Si, 31P, 33S, and 35Cl nuclei. The considered radicals provide 241 theoretical hfcc values, which are compared with 174 available experimental ones. The geometries of the studied systems are obtained by theoretical optimization using the same functional and basis set with which the hfccs were calculated. Regression analysis is used as a basic and appropriate methodology for this kind of comparative study. From this analysis, we conclude that DFT predictions of the hfccs are reliable for B3LYP/TZVP and B3LYP/EPR-III combinations. Both functional/basis set scheme are the more useful theoretical tools for predicting hfccs if compared to other much more expensive methods.

A novel Fe-NHC catalytic system allows the alkyl-alkyl cross-coupling reaction of alkyl halides and alkylmagnesium reagents has been developed. To our knowledge, this is the first Fe-catalysed Kumadatype coupling for the formation of C(sp 3 )-C(sp 3 ) bonds in the presence of functional groups. The process takes place under mild conditions, avoiding the formation of b-elimination products.Mechanistic studies suggest the intermediacy of Fe(I) complexes, formed by reduction with the Grignard reagent, as the active species.

Nitrogen hyperfine coupling constants (hfccs) of organic radicals have been calculated by density functional theory (DFT) methodology. The capability of the B3LYP functional, combined with 6-31G*, TZVP and EPR-III basis sets, to reproduce experimental nitrogen coupling constant data has been analyzed for 109 neutral, cationic and anionic radicals, all of them containing at least one nitrogen atom. The results indicate that the selection of the basis set plays an important role in the accuracy of DFT calculations of hfccs, mainly in relation with the composition of the primitive functions and the quantum number of those functions. The main conclusion obtained is the high reliability of the scheme B3LYP/6-31G* for the prediction of nitrogen hfccs with very low computational cost.

Transposon mutagenesis of Anabaena sp. PCC7120 led to the isolation of a mutant strain, PHB11, which grew poorly at pH values above 10. The mutant strain exhibited pronounced Na+ sensitivity; this sensitivity was higher under basic conditions. Mutant PHB11 also showed an inhibition of photosynthesis that was much more pronounced at alkaline pH. Reconstruction of the transposon mutation of PHB11 in the wild-type strain reproduced the phenotype of the original mutant. The wild-type version of the mutated gene was cloned and the mutation complemented. In mutant strain PHB11, the transposon had inserted within an ORF that is part of a seven-ORF operon with significant sequence similarity to a family of bacterial operons that are believed to code for a novel multiprotein cation/proton antiporter primarily involved in resistance to salt stress and adaptation to alkaline pH. The Anabaena operon was denoted mrp (multiple resistance and pH adaptation) following the nomenclature of the Bacillus subtilis operon; the ORF mutated in PHB11 corresponded to mrpA. Computer analysis suggested that all seven predicted Anabaena Mrp proteins were highly hydrophobic with several transmembrane domains; in fact, the predicted protein sequences encoded by mrpA, mrpB and mrpC showed significant similarity to hydrophobic subunits of the proton pumping NADH : ubiquinone oxidoreductase. In vivo expression studies indicated that mrpA is induced with increasing external Na+ concentrations and alkaline pH; mrpA is also upregulated under inorganic carbon (Ci) limitation. The biological significance of a putative cyanobacterial Mrp complex is discussed.

In a previous paper (Hermosilla, L.; Calle, P.; Garcia de la Vega, J. M.; Sieiro, C. J. Phys. Chem. A 2005, 109, 1114), an adequate computational protocol for the calculation of isotropic hyperfine coupling constants (hfcc's) was proposed. The main conclusion concerns the reliability of the scheme B3LYP/TZVP//B3LYP/6-31G* in the predictions of hfcc's with low computational cost. In the present study, we gain insight into the behavior of the above functional/basis set scheme on nuclei of the third row, for which few systematic studies have been carried out up to the present date. The systems studied are neutral, cationic, anionic, localized, and conjugated radicals, containing (29)Si, (31)P, and (33)S nuclei. After carrying out a regression analysis, we conclude that density functional theory (DFT) predictions on the hfcc's of the third-row nuclei are reliable for B3LYP/TZVP by using an optimized geometry with B3LYP/6-31G* combination. By comparison with other much more computationally demanding schemes, namely, B3LYP/cc-pVTZ and B3LYP/cc-pVQZ, we conclude that the B3LYP functional in conjunction with the TZVP basis set is the most useful computational protocol for the assignment of experimental hfcc's, not only for nuclei of first and second rows, but also for those of the third row.

The radical photopolymerization of methyl methacrylate in the presence of lithium triflate is investigated by EPR and NMR spectroscopies in order to assess the effect of the ionic medium on the polymerization kinetics. The EPR spectra show a notorious increase in the concentration of propagating radical as result of a dramatic decrease in the average termination rate coefficient. IR spectroscopy experiments and theoretical studies confirm the formation of a complex between the lithium cation and the oxygen atom at the carbonyl position of the ester in the methyl methacrylate. The electrostatic repulsion between the complexed propagating radicals, specifically at the end of the chains, allows explaining the observed low termination rate. This strong interaction would lead to "quasi-living" polymer chains from the first steps of the polymerization.

The performance of DFT methodology to predict with accuracy the isotropic hyperfine coupling constants (hfccs) of aromatic radicals containing (14)N nucleus is investigated by an extensive study in which 165 hfccs, belonging to 38 radical species, are obtained from calculations with B3LYP and PBE0 functionals combined with 6-31G*, N07D, TZVP, and EPR-III basis sets, and are compared to the reported experimental data. The results indicate that the selection of the basis set is of fundamental importance in the calculation of (14)N hfccs, whereas there is not so great an influence on the accurate computation of that parameter for (1)H nuclei. The values of the calculated (14)N coupling constants of aromatic nitroxide radicals using DFT methodology are noticeably lower than the experimental ones. A very simple relation to predict these hfccs with high accuracy is proposed on the basis of the present results, as an interesting alternative to the highly computationally demanding integrated approaches so far used.

Exchange interactions, J, in polymethylene biradicals with chain lengths ranging from 8 to 16 carbon atoms, have been measured as a function of temperature in hydrocarbon solvents by time-resolved electron paramagnetic resonance (TREPR) spectroscopy. Fitting the EPR spectra gives accurate J values (*lo%), and the chainlength dependence at a particular temperature can be modeled successfully only by using a linear combination of through-bond and through-solvent coupling mechanisms. Temperature-dependent coefficients for each mechanism are extracted from fits of the distance dependence. Through-bond contributions are found to dominate for the CS biradicals, while the through-solvent mechanism is the major contributor to J in the Clt, chains. For C12 and C14 biradicals, intermediate contributions of the two mechanisms are found. The conformational dependence of the mechanisms and the scope and limits of the model are discussed.