Thermohalochromism of phenolate dyes conjugated with nitro-substituted aryl groups.

The cationic halochromism and thermohalochromism of four phenolate dyes conjugated with aryl moieties substituted with one or two nitro groups were investigated in the presence of organic (tetra-n-butylammonium bromide and benzyltriethylammonium chloride) and inorganic (sodium perchlorate) salts, in hydrogen-bond donating (water, 1-propanol, 1-butanol and 2-propanol) and hydrogen-bond accepting (acetonitrile and dimethylsulfoxide) solvents. Although a positive halochromic response was observed in water for tetraalkylammonium salts, their thermohalochromic behavior was negligible. A negative halochromic behavior was observed for the dyes in all solvents, when the added cation was Na+. Plots of Δλmaxvs. c (Na+) allowed the apparent association constants for the solvated phenolate-cation pair to be estimated. In most cases, a positive thermohalochromism was observed in the range of 25-50°C, exceptions being the more sterically hindered phenolate dyes in the less polar solvents 2-propanol and acetonitrile. The observed variations were rationalized by invoking the effect of temperature on the phenolate-cation, phenolate-solvent and cation-solvent interactions.

Possibility of [1,5] sigmatropic shifts in bicyclo[4.2.0]octa-2,4-dienes.

The thermal equilibration of the methyl esters of endiandric acids D and E was subject to a computational study. An electrocyclic pathway via an electrocyclic ring opening followed by a ring flip and a subsequent electrocyclization proposed by Nicolaou [ Nicolaou , K. C. ; Chen , J. S. Chem. Soc. Rev. 2009 , 38 , 2993 ], was computationally explored. The free-energy barrier for this electrocyclic route was shown to be very close to the bicyclo[4.2.0]octa-2,4-diene reported by Huisgen [ Huisgen , R. ; Boche , G. ; Dahmen , A. ; Hechtl , W. Tetrahedron Lett. 1968 , 5215 ]. Furthermore, the possibility of a [1,5] sigmatropic alkyl group shift of bicyclo[4.2.0]octa-2,4-diene systems at high temperatures was explored in a combined computational and experimental study. Calculated reaction barriers for an open-shell singlet biradical-mediated stepwise [1,5] sigmatropic alkyl group shift were shown to be comparable with the reaction barriers for the bicyclo[4.1.0]hepta-2,4-diene (norcaradiene) walk rearrangement. However, the stepwise sigmatropic pathway is suggested to only be feasible for appropriately substituted compounds. Experiments conducted on a deuterated analogous diol derivative confirmed the calculated (large) differences in barriers between electrocyclic and sigmatropic pathways.

On the road to MM'X polymers: redox properties of heterometallic Ni···Pt paddlewheel complexes.

On the quest of heterometallic mixed-valence MM'X chains, we have prepared two stable discrete bimetallic compounds: the reduced (PPN)[ClNi(µ-OSCPh)4Pt] (PPN = bis(triphenylphosphine)iminium; OSCPh = benzothiocarboxylato) and the oxidized [(H2O)Ni(µ-OSCPh)4PtCl] species. The role of the aqua and chlorido axial ligands is crucial to facilitate oxidation of the {Ni(µ-OSCPh)4Pt} core. Experimental and theoretical analyses indicate that a NiPt-Cl/Cl-NiPt isomerization process occurs in the oxidized species. The electronic structure of the reduced system shows two unpaired electrons, one located in a d(x(2)-y(2)) orbital of the Ni(II) ion and a second in the antibonding d(z(2)-dz(2)) combination from the Ni(II) and Pt(II) centers. Oxidation occurs by removing one electron from this second multicenter molecular orbital. Although the mixed-valence character of the oxidized species makes the isolation of MM'X chains very attractive, such polymeric structure is prevented by the low Pt-Cl···Ni interaction energy and the high tendency of Ni centers to coordinate water molecules. Thus, this work offers valuable insights and hints to engage the production of heterometallic mixed-valence MM'X chains, which still is a challenging task.

Extension of the AMBER Force Field for Nitroxide Radicals and Combined QM/MM/PCM Approach to the Accurate Determination of EPR Parameters of DMPO-H in Solution.

A computational strategy that combines both time-dependent and time-independent approaches is exploited to accurately model molecular dynamics and solvent effects on the isotropic hyperfine coupling constants of the DMPO-H nitroxide. Our recent general force field for nitroxides derived from AMBER ff99SB is further extended to systems involving hydrogen atoms in ß-positions with respect to NO-moiety. The resulting force-field has been employed in a series of classical molecular dynamics simulations, comparing the computed EPR parameters from selected molecular configurations to the corresponding experimental data in different solvents. The effect of vibrational averaging on the spectroscopic parameters is also taken into account, by second-order vibrational perturbation theory involving semidiagonal third energy derivatives together first and second property derivatives.

Competitive reactions of organophosphorus radicals on coke surfaces.

The efficacy of organophosphorus radicals as anticoking agents was subjected to a computational study in which a representative set of radicals derived from industrially relevant organophosphorus additives was used to explore competitive reaction pathways on the graphene-like coke surface formed during thermal cracking. The aim was to investigate the nature of the competing reactions of different organophosphorus radicals on coke surfaces, and elucidate their mode of attack and inhibiting effect on the forming coke layer by use of contemporary computational methods. Density functional calculations on benzene and a larger polyaromatic hydrocarbon, namely, ovalene, showed that organophosphorus radicals have a high propensity to add to the periphery of the coke surface, inhibiting methyl radical induced hydrogen abstraction, which is known to be a key step in coke growth. Low addition barriers reported for a phosphatidyl radical suggest competitive aptitude against coke formation. Moreover, organophosphorus additives bearing aromatic substituents, which were shown to interact with the coke surface through dispersive π-π stacking interactions, are suggested to play a nontrivial role in hindering further stacking among coke surfaces. This may be the underlying rationale behind experimental observation of softer coke in the presence of organophosphorus radicals. The ultimate goal is to provide information that will be useful in building single-event microkinetic models. This study presents pertinent information on potential reactions that could be taken up in these models.

O-O bond activation in H2O2 and (CH3)3C-OOH mediated by [Ni(cyclam)(CH3CN)2](ClO4)2: different mechanisms to form the same Ni(III) product?

Reaction of [Ni(II)(cyclam)(CH(3)CN)(2)](ClO(4))(2) (1) with tert-butylhydroperoxide (TBHP) or H(2)O(2), in acidic media results in a formation of [Ni(III)(cyclam)(CH(3)CN)(2)](3+) species (2), the nature of which is characterized by UV-vis, EPR and XPS. The formation rate of 2 is much higher when H(2)O(2) is used as oxidant. In absence of acid, TBHP reacts with 1 generating the same Ni(III) species but, in contrast, no reaction is observed between H(2)O(2) and 1. Addition of cis-stilbene as an oxidable substrate quenches the formation of Ni(III) for reaction with H(2)O(2) only. Overall, these observations reveal a different reaction mechanism when reacting H(2)O(2) with 1 than when reacting the same metal complex with TBHP, despite the fact that the final product is the same. The proposed pathways arising from these observations consist in a homolytic O-O cleavage for the reaction of 1 with TBHP in CH(3)CN, but a proton assisted heterolysis for the O-O activation in H(2)O(2). Density functional calculations (B3LYP and OPBE) on the thermodynamic feasibility of the two reaction processes support the proposed mechanisms, since the O-O homolysis is strongly disfavored when H(2)O(2) is used as reactant.

Modeling of cw-EPR spectra of propagating radicals in methacrylic polymerization at different temperatures.

The temperature dependence of the cw-EPR spectra corresponding to the propagating radical responsible for the polymerization of methacrylic monomers has been simulated by an integrated computational approach that determines the structural and magnetic parameters (via quantum mechanical calculations) and diffusive properties (via hydrodynamic based methods), which represent the overall input of the stochastic Liouville equation that yields the spectrum. The system has been modeled as a rotator with only one relaxation process, the rotation around the C alpha-C beta bond. The simulations clearly indicate that the change of the spectral shape with the temperature is essentially related to the internal flexibility of the radical end.