No abstract

Thermal reactions of the alkoxyamine diastereomers )-aminoxyl; RЈ: methoxy-carbonylethyl and phenylethyl] with (R,R) ϩ (S,S) and (R,S) ϩ (S,R) configurations have been investigated by 1 H NMR at 100°C. During the overall decay the diastereomers interconvert, and an analytical treatment of the combined processes is presented. Rate constants are obtained for the cleavage and reformation of DEPN-RЈ from NMR, electron spin resonance, and chemically induced dynamic nuclear polarization experiments also using 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO) as a radical scavenger. The rate constants depend on the diastereomer configuration and the residues RЈ. Simulations of the kinetics observed with styrene and methyl methacrylate containing solutions yielded rate constants for unimeric and polymeric alkoxyamines DEPN-(M) n -RЈ. The results were compatible with the known DEPN mediation of living styrene and acrylate polymerizations. For methyl methacrylate the equilibrium constant of the reversible cleavage of the dormant chains DEPN-(M) n -RЈ is very large and renders successful living polymerizations unlikely. Mechanistic and kinetic differences of DEPN-and TEMPO-mediated polymerizations are discussed.

A full kinetic analysis is presented for living polymerizations controlled by the reversible combination of growing propagating and persistent radicals. Analytical equations are derived for the concentrations of the radicals, the dormant and unreactive polymer chains, the monomer, the number-average degree of polymerization, and the polydispersity. In the absence of side reactions, these specify optimum ranges of rate constants of the dissociation and combination to ensure, for specific monomers, formation of products with preset molecular weights, predictable small fractions of unreactive polymer, and low polydispersities in predictable conversion times. The theoretical conclusions agree with experimental findings for alkoxyamine initiators. Consideration of borderline cases shows that a living and controlled radical polymerization can degenerate to give living polymers with no apparent control of molecular weight and polydispersity or can produce mainly unreactive polymers with regulated molecular weight and small polydispersity. The chain-length and viscosity dependence of the self-termination constant does not change the overall mechanism but does affect the polymerization rate.

Pair interaction potentials (IPs) were defined to describe the La(3+)-OH(2) interaction for simulating the La(3+) hydration in aqueous solution. La(3+)-OH(2) IPs are taken from the literature or parametrized essentially to reproduce ab initio calculations at the second-order Moller-Plesset level of theory on La(H(2)O)(8) (3+). The IPs are compared and used with molecular dynamics (MD) including explicit polarization, periodic boundary conditions of La(H(2)O)(216) (3+) boxes, and TIP3P water model modified to include explicit polarization. As expected, explicit polarization is crucial for obtaining both correct La-O distances (r(La-O)) and La(3+) coordination number (CN). Including polarization also modifies hydration structure up to the second hydration shell and decreases the number of water exchanges between the La(3+) first and second hydration shells. r(La-O) ((1))=2.52 A and CN((1))=9.02 are obtained here for our best potential. These values are in good agreement with experimental data. The tested La-O IPs appear to essentially account for the La-O short distance repulsion. As a consequence, we propose that most of the multibody effects are correctly described by the explicit polarization contributions even in the first La(3+) hydration shell. The MD simulation results are slightly improved by adding a-typically negative 1r(6)-slightly attractive contribution to the-typically exponential-repulsive term of the La-O IP. Mean residence times are obtained from MD simulations for a water molecule in the first (1082 ps) and second (7.6 ps) hydration shells of La(3+). The corresponding water exchange is a concerted mechanism: a water molecule leaving La(H(2)O)(9) (3+) in the opposite direction to the incoming water molecule. La(H(2)O)(9) (3+) has a slightly distorded "6+3" tricapped trigonal prism D(3h) structure, and the weakest bonding is in the medium triangle, where water exchanges take place.

Three strategies to improve the performance of living and controlled polymerizations mediated by persistent radicals are analyzed kinetically to explore their advantages and limits. First, rate enhancements by an additional conventional initiation or by the monomer self-initiation are found only compatible with high degrees of polymer livingness and control if the additional initiation rate is much smaller than the activation rate of the dormant polymer. Second, rate-enhancing reductions of the persistent radical concentration by reactions with additives or by a natural decay lead to living and controlled polymers even if these processes are surprisingly fast. The enhanced monomer conversion rates generally diminish the detrimental effects of a direct precursor decay or of the radical disproportionation to unreactive products. Third, a retarding initial excess of the persistent species can force unfavorably fast systems to livingness and control. The limits of these procedures are specified. Analytic equations are given which facilitate the extraction of kinetic data from conversions, degrees of polymerization and polydispersities. They agree with experimental findings.

A kinetic analysis of living polymerizations controlled by persistent nitroxide radicals is extended to cover the formation of hydroxylamines and alkene macromonomers. This reaction can occur by radical disproportionation during the cross-reaction of the carbon-centered propagating and the nitroxide radicals and/or by a direct fragmentation of the dormant alkoxyamines. For equal extents, both pathways cause nearly indistinguishable upper limits of the monomer conversion and a very similar increase of the polydispersity in the final stages of polymerization, but they do not markedly affect the linear increase of the degree of polymerization with conversion

This paper is concerned with issues related to the coupling between low-ffrequency nuclear modes and electronic emission in the bacterial reaction center of Rhodobacter sphaeroides. We have investigated the temporal evolution of the stimulated emission spectra of the excited state of the special pair bacteriochlorophyll dimer by means of computer simulation. To this purpose, we have carried out molecular dynamics simulations on a hydrated reaction center with the special pair in its ground and excited states, P and P*, respectively. Calculations of the time-dependent stimulated emission spectra has revealed an oscillatory behavior which is consistent with experiments (Vos et al.; Nature 1993, 363, 320; Biochemistry 1994, 33, 6750). The time scales of these nuclear vibrations are similar to those found experimentally. To a good approximation, both the protein environment and the special pair dimer couple to the P* → P transition with oscillations of similar frequencies. This finding offers an insight on the nature of the nuclear oscillations and might have important implications for their complete understanding.

Computers in chemistry V 0380 MED-SuMoLig: A New Ligand-Based Screening Tool for Efficient Scaffold Hopping. -(SPERANDIO, O.; ANDRIEU, O.; MITEVA, M. A.; VO, M.-Q.; SOUAILLE, M.; DELFAUD, F.; VILLOUTREIX*, B. O.; J. Chem. Inf. Model. (J. Chem. Inf. Comput. Sci.) 47 (2007) 3, 1097-1110; INSERM, Univ. Paris V, F-75006 Paris, Fr.; Eng.) -Lindner 34-203