Modulating the Photochemistry of Bipyridylic Compounds by Symmetric Substitutions

A quantum electronic study of the effect of substituents on (2,2′‐bipyridyl)‐3,3′‐diol and (2,2′‐bipyridyl)‐3,3′‐diamine is presented. A large difference in the photochemical behavior between the original and the substituted selected systems is expected. For the sake of simplicity, the study is restricted to the symmetrically bi‐substituted compounds: fluorine, the more electronegative atom and thus a strong σ‐acceptor but also a weak π‐donor group, and NO₂, a strong π‐acceptor substituent. Among the large set of compounds studied, two receive special attention: 5,5′‐dinitro‐(2,2′‐bipyridyl)‐3,3′‐diamine and 6,6′‐difluoro‐(2,2′‐bipyridyl)‐3,3′‐diol. While in the former case the nitro substitution transforms (2,2′‐bipyridyl)‐3,3′‐diamine, previously suggested to behave as a photomemory material, into a simple fluorescent species, the latter substitution turns (2,2′‐bipyridyl)‐3,3′‐diol into a fresh new candidate for a photomemory device.     

Electron-pairing analysis from localization and delocalization indices in the framework of the atoms-in-molecules theory

This article presents an overview of recent advances in the study of electron pairing through the use of localization and delocalization indices obtained from double integration over atomic basins of the exchange–correlation density in the framework of the atoms-in-molecules theory. These localization and delocalization indices describe the intra- and interatomic distribution of the electron pairs in a molecule. The main results of the application of these second-order indices to the analysis of molecular structure and chemical reactivity are briefly reviewed. It is shown that localization and delocalization indices represent a powerful tool to describe the electron-pair structure of molecules, which, in turn, provides deeper insight into relevant chemical phenomena such as electron correlation effects and the formation of localized α, β electron pairs. Received: 8 April 2002 / Accepted: 26 June 2002 / Published online: 6 September 2002 Acknowledgements. Financial help was furnished by the Spanish DGES projects no. PB98-0457-C02-01 and BQU2002-04112-C02-02. J.P. thanks the Departament d'Universitats, Recerca i Societat de la Informació de la Generalitat de Catalunya for benefiting from a doctoral fellowship, no. 2000FI-00582. M.S. is indebted to the Departament d'Universitats, Recerca i Societat de la Informació of the Generalitat de Catalunya for financial support through the Distinguished University Research Promotion, 2001. We also thank the Centre de Supercomputació de Catalunya for providing us with computing facilities. Correspondence to: M. Solà e-mail: miquel.sola@udg.es     

Kähler tubes of constant radial holomorphic sectional curvature

We determine (up to holomorphic isometries) the family of Kähler tubes, around totally geodesic complex submanifolds, of constant radial holomorphic sectional curvature when the centreP of the tube is either simply connected or a complex hypersurface withH ¹ (P, R)=0. In the last case, these tubes have the topology of tubular neighbourhoods of the zero section of the complex lines bundles over symplectic manifolds (when they are Kähler) of the Kostant-Souriau prequantization.     

Observation and mechanistic study of facile C-O bond formation between a well-defined aryl-copper(III) complex and oxygen nucleophiles

A well-defined macrocyclic aryl–Cu(III) complex (2) reacts readily with a variety of oxygen nucleophiles, including carboxylic acids, phenols and alcohols, under mild conditions to form the corresponding aryl esters, biaryl ethers and alkyl aryl ethers. The relationship between these reactions and catalytic C-O coupling methods is demonstrated by the reaction of the macrocyclic aryl–Br species with acetic acid and p-fluorophenol in the presence of 10 mol% Cu(I). An aryl-Cu(III)-Br species 2(Br) was observed as an intermediate in the catalytic reaction. Investigation of the stoichiometric C-O bond-forming reactions revealed nucleophile-dependent changes in the mechanism. The reaction of 2 with carboxylic acids revealed a positive correlation between the log(k(obs)) and the pK(a) of the nucleophile (less-acidic nucleophiles react more rapidly), whereas a negative correlation was observed with most phenols (more-acidic phenols react more rapidly). The latter trend resembles previous observations with nitrogen nucleophiles. With carboxylic acids and acidic phenols, UV-visible spectroscopic data support the formation of a ground-state adduct between 2 and the oxygen nucleophile. Collectively, kinetic and spectroscopic data support a unified mechanism for aryl-O coupling from the Cu(III) complex, consisting of nucleophile coordination to the Cu(III) center, deprotonation of the coordinated nucleophile, and C-O (or C-N) reductive elimination from Cu(III).     

Protonation of water clusters induced by hydroperoxyl radical surface adsorption

We have investigated the HO(2) adsorption and acid dissociation process on the surface of (H(2)O)(20) and (H(2)O)(21) clusters by using quantum-chemistry calculations. Our results show that the radical forms a stable hydrogen-bond complex on the cluster. The HO(2) acid dissociation is more favorable in the case of the (H(2)O)(21) cluster, for which the inner water molecule plays a crucial role. In fact, acid dissociation of HO(2) is found to occur in two steps. The first step involves H(2) O autoionization in the cluster, and the second one involves the proton transfer from the HO(2) radical to the hydroxide anion. The presence of the HO(2) radicals on the surface of the cluster facilitates water autoionization in the cluster.     

Intramolecular [2+2+2] cycloaddition reactions of yne-ene-yne and yne-yne-ene enediynes catalysed by Rh(I): experimental and theoretical mechanistic studies

N-tosyl-linked open-chain yne-ene-yne enediynes 1 and 2 and yne-yne-ene enediynes 3 and 4 have been satisfactorily synthesised. The [2+2+2] cycloaddition process catalysed by the Wilkinson catalyst [RhCl(PPh(3))(3)] was tested with the above-mentioned substrates resulting in the production of high yields of the cycloadducts. Enediynes 1 and 2 gave standard [2+2+2] cycloaddition reactions whereas enediynes 3 and 4 suffered β-hydride elimination followed by reductive elimination of the Wilkinson catalyst to give cycloadducts, which are isomers of those that would be obtained by standard [2+2+2] cycloaddition reactions. The different reactivities of these two types of enediyne have been rationalised by density functional theory calculations.     

Synthetic strategies for the surface functionalisation of gold nanoparticles with metals and metal clusters

The reaction of the new ditopic thiol-phosphine compound HS(CH(2))(11)OOCC(6)H(4)PPh(2) (L) with an excess of dodecanethiol-protected gold nanoparticles gave the asymmetric gold complex [CH(3)(CH(2))(11)SAuPPh(2)C(6)H(4)COO(CH(2))(11)SH] (4), but no phosphine-protected gold nanoparticles were formed. However, by blocking the phosphine function in L with metal fragments, we have been able to produce gold nanoparticles functionalised with AuCl- and cluster [Fe(2)(CO)(7)Au] units on the surface by the method of ligand-place exchange reaction.     

Ostrowski type methods for solving systems of nonlinear equations

Four generalized algorithms builded up from Ostrowski’s method for solving systems of nonlinear equations are written and analyzed. A development of an inverse first-order divided difference operator for functions of several variables is presented, as well as a direct computation of the local order of convergence for these variants of Ostrowski’s method. Furthermore, a sequence that approximates the order of convergence is generated for the examples and it confirms in a numerical way that the order of the methods is well deduced.