Electronic energy transfer in mixed organic solids: Anderson localization or delocalization?

A simple Anderson transition model, ignoring guest clusterization, excitation lifetime, sensor concentration, exciton-phonon coupling and thermalization, appears to be incompatible with the critical concentrations observed for triplet exciton transport in several ternary crystal systems. Dynamic percolation, involving hopping or tunneling through long-range clusters, remains our suggested model.     

Electronic delocalization contribution to the anomeric effect evaluated by computational methods

This study proposes the determination of the electronic delocalization contribution to the Anomeric Effect (EDCAE, Delta Delta E(deloc), eq 3) as a computational alternative in the evaluation of the excess of the axial preference shown by an electronegative substituent located at alpha position to the annular heteroatom of a heterocyclic compound (anomeric position) in both the presence and the absence of electronic delocalization retaining the same molecular geometry. The determination of the EDCAE is computationally accessible through the application of the natural bond orbital analysis (NBO). This type of analysis allows the comparison of hypothetical molecules lacking electronic delocalization (Lewis molecules, in which the electrons are strictly located in bonds and lone pairs) with the fully delocalized molecules retaining the same geometry and the evaluation of the anomeric effect in terms of eq 3. The role of the Lewis molecules is the same as the cyclohexane used experimentally to evaluate the anomeric effect. The advantage of doing this is that Lewis molecules are stereoelectronically inert. Applying this methology to cyclic and acyclic molecules at B3LYP/6-31G(d,p) and HF/6-31G(d,p)//B3LYP/6-31G(d,p) levels of theory, we found that the anomeric effect shown by Cl in 1,3-dioxane; F, Cl, SMe, PH(3), and CO(2)Me groups in 1,3-dithiane is of stereoelectronic nature while the preference of F, OMe, and NH(2) in 1,3-dioxane and the P(O)Me(2) group in 1,3-dithiane is not. Furthermore, this methodology shows that anomeric effects without stereoelectronic origin can modify the molecular geometry in agreement with the geometric pattern required by the double-bond no-bond model, as recently proposed by Perrin.     

One-bond C-C coupling constants in ethers are not primarily determined by N-sigma delocalization

One-bond carbon-carbon coupling constants, 1JCC, were measured for a series of cyclic and acyclic ethers. Surprisingly, the dependence on COCC dihedral angle, tau, parallels cos(tau), rather than the cos(2tau) characteristic of n-sigma* delocalization. These results complement recently calculated 1JCH values in three ethers. The variations in 1J are not primarily determined by delocalization but instead are attributed to a dipolar interaction that affects electron density, perhaps via the hybridization.     

Intraparticle charge delocalization of carbene-functionalized ruthenium nanoparticles manipulated by selective ion binding

Olefin metathesis reactions of carbene-stabilized ruthenium nanoparticles were exploited for the incorporation of multiple functional moieties onto the nanoparticle surface. When the nanoparticles were cofunctionalized with 4-vinylbenzo-18-crown-6 and 1-vinylpyrene, the resulting particles exhibited fluorescence characteristics that were consistent with dimeric pyrene with a conjugated chemical bridge, with three peaks observed in the emission spectra at 391, 410, and 485 nm. The behaviors were ascribed to intraparticle charge delocalization between the pyrene moieties afforded by the conjugated Ru═carbene interfacial linkages. Notably, upon the binding of metal ions in the crown ether cavity, the emission intensity of the nanoparticle fluorescence was found to diminish at 485 nm and concurrently increase at 391 and 410 nm rather markedly, with the most significant effects observed with K(+). This was accounted for by the selective binding of 18-crown-6 to potassium ions, where the positively charged ions led to the polarization of the nanoparticle core electrons that was facililated by the conjugated linkage to the metal surface and hence impeded intraparticle charge delocalization. Control experiments with a pyrene-crown ether conjugate (2) and with ruthenium nanoparticles cofunctionalized with 4-vinylbenzo-18-crown-6 and 1-allylpyrene suggested that the through-bond pathway played a predominant role in the manipulation of intraparticle electronic communication whereas the contributions from simple electrostatic interactions (i.e., through-space pathway) were minimal.     

Comparison of the localization of an electron as determined by the two-particle distribution function and by the single-particle sharing index

A comparison of the measure of the delocalization of a particle based on the two-particle distribution function and that based on the single-particle density matrix is made using a simple set of wave functions which span states ranging from single determinant ground and doubly excited states through states mimicking correlated states and which include the singly excited state for electrons and for bosons replacing electrons in H2. The comparison further includes an analysis of the application of the measures to a classical ideal gas and a compressible fluid. It is found that the values of the integrated atom-atom measures agree for a range of wave functions involving combinations of the two single determinant (and equivalent Bose) wave functions but disagree for a different range of these wave functions and for the singly excited wave functions. Aside from the single determinant (and equivalent Bose) wave functions, the two sets of point-point measures that underlie the integrated measures all differ. For the sets of wave functions considered, the values of the measures are identical for electrons and bosons. When applied to a closed classical ideal gas and to a closed compressible fluid, the delocalization measure based on the two-particle distribution has a residual long range term, whereas the sharing index in the classical limit gives a completely localized particle. In general, the two measures describe different aspects of the behavior of the particles. The measures based on the two-particle distribution function give only two-particle properties and the single-particle density, and the sharing quantities give only single-particle properties. The latter includes, however, the quantitative measures of the delocalization of a single particle, the point-point sharing index and the sharing amplitude.     

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     

Core localization and sigma* delocalization in the O 1s core-excited sulfur dioxide molecule

Electron-ion-ion coincidence measurements of sulfur dioxide at discrete resonances near the O 1s ionization edge are reported. The spectra are analyzed using a model based upon molecular symmetry and on the geometry of the molecule. We find clear evidence for molecular alignment that can be ascribed to symmetry properties of the ground and core-excited states. Configuration interaction (CI) calculations indicate geometry changes in accord with the measured spectra. For the SO(2) molecule, however, we find that the localized core hole does not produce measurable evidence for valence localization, since the transition dipole moment is not parallel to a breaking sigma* O-S bond, in contrast to the case of ozone. The dissociation behavior based upon the CI calculations using symmetry-broken orbitals while fixing a localized core-hole site is found to be nearly equivalent to that using symmetry-adapted orbitals. This implies that the core-localization effect is not strong enough to localize the sigma* valence orbital.     

Zinc chelation with hydroxamate in histone deacetylases modulated by water access to the linker binding channel

It is of significant biological interest and medical importance to develop class- and isoform-selective histone deacetylase (HDAC) modulators. The impact of the linker component on HDAC inhibition specificity has been revealed but is not understood. Using Born-Oppenheimer ab initio QM/MM MD simulations, a state-of-the-art approach to simulating metallo-enzymes, we have found that the hydroxamic acid remains to be protonated upon its binding to HDAC8, and thus disproved the mechanistic hypothesis that the distinct zinc-hydroxamate chelation modes between two HDAC subclasses come from different protonation states of the hydroxamic acid. Instead, our simulations suggest a novel mechanism in which the chelation mode of hydroxamate with the zinc ion in HDACs is modulated by water access to the linker binding channel. This new insight into the interplay between the linker binding and the zinc chelation emphasizes its importance and gives guidance regarding linker design for the development of new class-IIa-specific HDAC inhibitors.     

Flexibly capped β-cyclodextrins by the hydrogen-bonded nucleic acid base pair. The pH-control of binding ability by an on-off-switched capping

Preparation and characterization of flexibly capped β-cyclodextrins ($\text{3a-c}$) by the hydrogen bonded nucleobase pair (i.e., adenine-thymine) are described.     

Investigation of charge localization and charge delocalization in model molecules by multiphoton ionization photoelectron spectroscopy and DFT calculations

In this work we focus on the question to which degree a surplus charge is localized or delocalized in extended molecular systems. Molecules consisting of a flexible tail and the benzene chromophore, such as n-propylbenzene, 2-phenylethyl alcohol and 2-phenylethylamine, are used as model molecules. Their S0-S1 resonance enhanced multiphoton ionization (MPI) spectra containing origin transitions of different conformers appear at similar wavelengths. This shows, that in the neutral the electronic excitation is localized at the benzene chromophore. Geometry differences between the neutral and the cation can be qualitatively derived from intensities of vibrational transitions or the onset behavior in MPI high-resolution photoelectron (MPI-PE) spectra. We identify two possible reasons for structural changes: Charge-dipole interaction and charge delocalization. Whereas both effects can be active for the folded gauche conformers, the charge-dipole interaction is expected to be small for the extended anti conformers and geometry changes are attributed to charge delocalization. Density functional calculations of structures and energies qualitatively confirm the experimental results for all molecules and their conformers. They predict charge delocalization into the end group of below 20% for n-propylbenzene and 2-phenylethyl alcohol. In the case of 2-phenylethylamine the charge is equally shared by the near-isoenergetic charge sites of the benzene chromophore and the amine group.     

Relation between pi-electron localization/delocalization and H-bond strength in derivatives of omicron-hydroxy-schiff bases

Detailed investigations of electronic effects taking place within the molecular system of o-hydroxy Schiff bases have been performed. The analysis of geometry, local and global aromaticity, selected AIM-based parameters, and finally, pi-electron currents induced in the systems under consideration have been performed on the basis of quantum chemical calculations at the B3LYP/6-311+G** level of theory. The relation between localization/delocalization of pi-electrons within the whole system has been described. It has been shown that the character of the bond which is common to the phenylic ring and the quasi-ring formed as a result of H-bond formation has a crucial impact on the strength of H-bonding. The strongest H-bonds can be observed for the systems in which the sequence of formally single and double bonds within the H-bridged quasi-ring enable a pi-electronic coupling. These observations indicate that pi-electron effects play a fundamental role in the stabilization of the hydrogen bridge within omicron-hydroxy Schiff bases. Analysis of pi-ring currents induced by a magnetic field perpendicular to the molecular plane of selected analyzed systems confirms these conclusions.     

Control of delocalization and structural changes by means of an electric field

The strength and, mainly, the direction of a static electric field can be used to control delocalization effects occurring in a non-polar pi-system. The delocalization energy, the weights, and the probabilities of some local electronic structures, the behavior of electron pairs, and the electronic fluctuations are considered and examined in cis-butadiene, used as model system. The effects of the electric field are detected and evaluated in the basis of natural orbital spaces appropriate to investigate the behavior of one- and poly-electron distributions. The consequences of modifying the delocalization effects on structural changes are also investigated. Full geometry optimizations in both Hartree-Fock and MP2 levels show that the changes in bond lengths, guided by the changes of the behavior of the electronic assembly, can be controlled by means of the electric field.     

Synthesis of an isomer of the germacranolide isoaristolactone

Photocycloadditlon of (+)-isoplperltenone and cyclobutene-1-carboxylic acid gives an adduct which upon reduction with NaCNBH₃ followed by thermolysis yields an isomer of isoaristolactone in an overall yield of 26%.     

Manifestation of space-time electron delocalization in an x-ray study

We report a manifestation of the space-time electron delocalization in an x-ray diffraction structural analysis upon the condition that the relaxation time of the structure is less than the lifetime of the delocalized electron at any site, between which electron transfer occurs. In this case, the x-ray diffraction data may provide informative parameters characterizing the fraction of the time for the presence of an extra electron at each site. Such parameters are bond lengths in the coordination unit and mean square displacements of the atoms. The differences in the lengths of the bridging oxygen-iron bonds are explained for mixed-valence trinuclear iron carboxylates with the general formula [Fe₃O(COOR₁)₆3R₂]R₃. An expression was obtained, which gives the ratio of the lifetime of an electron at each of the sites using the bond lengths l = Fe–Ol₁:l₂:l₃ = ₁:₂:₃. The direction and magnitude of the displacement of the atoms upon rearrangement related to electron transfer (r) may be determined using this expression for the mean square atomic displacements u²=u₀ ²+(1–)·r² in the case of an experiment at two temperatures.Institute of Chemical Physics, Academy of Sciences of the USSR. Translated from Zhurnal Strukturnoi Khimii, Vol. 30, No. 5, pp. 90–95, September–October, 1989.     

Unpaired-electron localization and delocalization in macromolecules with a system of conjugated C=C bonds

The radical polymerization of acetylene monomers has been studied in a wide temperature range (77–340 K) using EPR spectroscopy, calorimetry, chromatography, and gravimetry It has been found that the growing macroradical is active, and its unpaired electron is localized at a terminal fragment of the molecule. However, it lost its activity even at small chain lengths (less than ten monomer units) because of delocalization of the unpaired electron along the system of conjugated double bonds.     

Lone pair versus bonding pair electrons: the mechanism of electronic polarization of water in the presence of positive ions

It is commonly accepted that the water molecules in the first solvation shell of a positive ion are strongly polarized because of an elongation of the oxygen lone pair orbitals along the ion-oxygen direction and this is commonly considered the dominant effect. Recent experimental and theoretical works have instead suggested that this is not the dominant aspect and that the problem is by far more complicated. Consistent with the picture given above, here we show that, in particular, an equally important role into the polarization process is played by the bonding pair electrons located along the internal oxygen-hydrogen bond. We also provide some arguments which suggest that the main reason of such a behavior is due to the distortion of the molecular orbitals caused by the interaction between non-hydrogen-bonded water molecules in the first solvation shell of the ion.     

Hydrogen bonding interaction and topological insights of the electron localization/delocalization of l- arginine acetate

The vibrational spectral studies of the semi-organic material l- arginine acetate (LAA) are carried out with the help of density functional calculations to derive the equilibrium geometry as well as the vibrational wavenumbers and intensities of the spectral bands. The vibrational spectrum assignments are performed using normal coordinate analysis (NCA) in accordance with the scaled quantum mechanical force field approach (SQMFF). Vibrational spectra confirm the COO- modes split due to intra- and intermolecular association based on C–O….H, N–H….O, and O–H?O hydrogen bonding in the molecule, which lowers carboxylate wavenumbers. The natural bond orbital (NBO) analysis and DFT computations also confirm the occurrence of strong intra and intermolecular N–H?O and O–H?O ionic hydrogen bonding between charged species, providing the non-centrosymmetric structure in the LAA crystal.     

Guanidiniocarbonyl-pyrrole-aryl conjugates as nucleic acid sensors: switch of binding mode and spectroscopic responses by introducing additional binding sites into the linker

Two novel guanidiniocarbonyl pyrrole-pyrene conjugates 3 and 4 as spectroscopic probes for ds-polynucleotides were synthesized and their interaction with different ds-DNAs/RNAs studied. Compared to a previously reported first set of conjugates (1 and 2) the significant extension and increased rigidity of the central part of the structure resulted in a switch of DNA binding mode from intercalative (previously studied derivatives 1 and 2 with a nonbinding and flexible linker) to minor groove binding of the two novel guanidiniocarbonyl-pyrrole-pyrene conjugates 3 and 4. These two compounds interact strongly with ds-DNAs, but only weakly with ds-RNA. The newly incorporated heterocyclic moieties within the central part of the structure of 3 and 4 were able to control by steric and hydrogen-bonding effects the alignment of the molecules within various, structurally different forms of DNA minor grooves, whereby even small differences in the position of the attached pyrene within the groove were reflected in different fluorimetric responses. In addition, 3 and 4 revealed intriguing in vitro selectivity among various human tumour cell lines.     

Identification of an isomer impurity in piperaquine drug substance

A significant contaminant of the antimalarial drug piperaquine (1,3-bis-[4-(7-chloroquinolyl-4)-piperazinyl-1]propane) has been identified using liquid chromatography-mass spectrometry (LC-MS) and 2D NMR spectroscopy (1H-1H COSY, 1H-13C HSQC, 1H-13C HMBC). The impurity was identified as the positional isomer 1-[(5-chloroquinolin-4)-piperazinyl]-3-[(7-chloroquinolin-4)-piperazinyl]propane. The impurity is formed because of contamination of batches of 4,7-dichloroquinoline (a precursor in the synthesis of piperaquine) with 4,5-dichloroquinoline. The amount of impurity (peak area impurity/peak area piperaquine using LC-UV at 347 nm) in old batches of piperaquine and in Artekin (the combination of dihydroartemisinin-piperaquine) ranged from 1.5 to 5%.