Carbon-supported base metal nanoparticles: cellulose at work.

Pyrolysis of base metal salt loaded microcrystalline cellulose spheres gives a facile access to carbon-supported base metal nanoparticles, which have been characterized with temperature-dependent XRD, SEM, TEM, ICP-MS and elemental analysis. The role of cellulose is multifaceted: 1) it facilitates a homogeneous impregnation of the aqueous base metal salt solutions, 2) it acts as an efficacious (carbonaceous) support material for the uniformly dispersed base metal salts, their oxides and the metal nanoparticles derived therefrom, and 3) it contributes as a reducing agent via carbothermal reduction for the conversion of the metal oxide nanoparticles into the metal nanoparticles. Finally, the base metal nanoparticles capable of forming metastable metal carbides catalytically convert the carbonaceous support into a mesoporous graphitic carbon material.

Planar homotropenylium cation: a transition state with reversed aromaticity.

In contrast to the equilibrium structure of the homoaromatic C(s) homotropenylium cation, C8H9(+) (1), which supports a pinched diatropic ring current, the C(2v) transition state (2) for inversion of the methylene bridge of 1 is antiaromatic and supports a two-lobe paratropic π current, as detected by plotting B3LYP/6-31G** ipsocentric current maps. Participation of the bridge CH bonds is crucial for the change in global character of the current in the transition state, as shown by the quenching of its paratropicity on substitution of H by F. Orbital-based arguments allow rationalization of this transition between homoaromaticity and hyper(conjugative) antiaromaticity.

The homotropenylium cation: a system with a pinched π ring current.

The homotropenylium cation (1, C8H9(+)) is a key species in the discussion of homoaromaticity. Constrained optimisations around the minimum structure have been performed, varying the size of the gap spanned by the CH2-bridge and optimising all other geometrical parameters. At each bridging distance, ab initio current-density maps have been calculated and plotted using the ipsocentric approach.

Relativistic ring currents in metallabenzenes: an analysis in terms of contributions of localised orbitals.

Ring currents calculated in the ipsocentric CTOCD-DZ formalism are presented for four representative metallabenzenes, compounds in which a benzene CH group is formally replaced by a transition metal atom with ligands. Aromaticity is probed using ring currents computed using non-relativistic and relativistic orbitals (derived with relativistic effective core potentials or ZORA). Maps computed at different levels of relativistic theory turn out to be similar, showing that orbital nodal character is the main determinant of ring current.

Aromaticity of strongly bent benzene rings: persistence of a diatropic ring current and its shielding cone in [5]paracyclophane.

Direct evaluation of the induced π current density in [5]paracyclophane (1) shows that, despite the significant non-planarity (α = 23.2°) enforced by the pentamethylene bridge, there is only a modest (ca. 17%) reduction in the π ring current, justifying the use of shielding-cone arguments for the assignment of (1)H NMR chemical shifts of 1 and the claim that the non-planar benzene ring in 1 retains its aromaticity (on the magnetic criterion).

Synthesis of and evidence for electronic communication within heteromultimetallic tetrakis(NCN-pincer metal)-(metallo)porphyrin hybrids.

Several heteromultimetallic pincer-porphyrin hybrids have been prepared in excellent yields by stepwise metalation of a general precursor, [2H(Br(NCN))(4)], which was designed in such a way so as to guarantee selectivity for either the porphyrin or pincer sites during the metalation steps. First, a metal was introduced in the porphyrin cavity using a metal(II) salt, followed by metalation of the pincer units through oxidative addition to an appropriate metal(0) complex. The resulting multimetallic complexes show an appreciable amount of intramolecular communication between the meso-pincer metal groups and the central metalloporphyrin component.

Assignment of phantom bands in the solid-state infrared and Raman spectra of coronene: The importance of a minute out-of-plane distortion.

The molecular geometry and the normal modes properties of coronene are investigated by means of DFT(B3LYP) and restricted/Hartree-Fock calculations utilizing basis sets of triple zeta+polarization quality. The interpretation of the infrared and Raman spectra of coronene, especially in solid state, is critically revised. The phantom bands in the solid state, previously not understood, are readily assigned after considering a minute out-of-plane molecular distortion from D(6h) to C(2h).

Weak distance dependence of through-bond interactions in tetrahydro-4H-thiopyran-4-ylidene end-capped oligo(cyclohexylidenes); a computational survey.

Calculations on members of the oligo(cyclohexylidene) series [(n), n = 1-5)] and related tetrahydro-4H-thiopyran end-capped analogues [(n), n = 1-4)] show a strong through-bond coupling between their pi bonds and sulfur lone pairs (Lp(pi)S). This coupling is mediated by an interaction between the H(ax)-C-C-H(ax) structural sub-units and the pi bonds connecting the cyclohexyl moieties. A comparison of the length dependency of the through-bond coupling via an oligo(cyclohexylidene) and an alkane bridge [divinyl alkanes (n)] shows that oligo(cyclohexylidenes) are more efficient in mediating through-bond couplings over large distances.

Current-density maps as probes of aromaticity: global and Clar pi ring currents in totally resonant polycyclic aromatic hydrocarbons.

Calculation and visualisation of induced current density are important aids to the study of both aromaticity and observable molecular magnetic response properties. The ipsocentric method offers an accurate and economical approach to calculation of induced current density, and a physical interpretation in terms of occupied-orbital contributions. In monocyclic systems, these contributions allow rationalisation of the existence, sense, and strength of ring current using simple symmetry and node-counting arguments.

On the interpretation of valence bond wavefunctions.

Valence bond wavefunctions are naturally geared to the chemist's idea of chemical bonding. In a structure one may distinguish different electron pair bonds and possible radical character. A structure may correspond to a covalent bond, where all electrons are equally divided over the atoms, or may describe an excess charge on a discrete part of the molecule, which indicates ionic bond character.

The phenalenyl motif: a magnetic chameleon.

The 12pi cation (3) and 14pi anion (4) derived from the phenalenyl radical (2) support diatropic ("aromatic") perimeter ring currents, but isoelectronic replacement of the central atom by either boron (5) or nitrogen (6) leads to paratropic ("antiaromatic") current; the ipsocentric approach to molecular magnetic response accounts for all four patterns in terms of competition between translationally and rotationally allowed virtual pi-pi* excitations.

On the lack of ring-current aromaticity of (heteroatom) [N]radialenes and their dianions.

Current-density maps, calculated at the ab initio RHF//6-31G**/CTOCD-DZ level, show no significant pi ring current in planar equilateral geometries of neutral and dianionic [N]radialenes, oxocarbons and thiocarbons C(N)Y(N) (q-) (Y=CH(2), O, S; N=4, 5, 6; q=0 (1 a-12 a), 2 (1 b-12 b)). Only the N=3 deltate dianions C(3)Y(3) (2-) (Y=CH(2), O, S (1 b, 5 b and 9 b)) have discernible pi ring current, and then with at most 20-25 % of the strength of the standard benzene current. On the magnetic criterion, lack of current is definitive evidence against aromaticity.

Side chain mediated electronic contact between a tetrahydro-4H-thiopyran-4-ylidene-appended polythiophene and CdTe quantum dots.

The properties of a mixed CdTe quantum dot/tetrahydro-4H-thiopyran-4-ylidene-functionalized polythiophene system are reported. This system was prepared by exposing trioctylphosphine (TOP)-capped CdTe quantum dots to the polythiophene in solution. Strong fluorescence emission quenching and shortening of the fluorescence emission lifetimes of both the polythiophene and the quantum dots occur when they are mixed, indicating the occurrence of photoinduced charge separation.

Persistence of paratropic ring currents in nonplanar, tub-shaped geometries of 1,3,5,7-cyclooctatetraene.

[reaction: see text] Surprisingly, the anti-aromatic paratropic ring current of the planar D(4)(h) transition state of 1,3,5,7-cyclooctatetraene (COT) survives almost all the way to the equilibrium tub-shaped D(2)(d) COT structure (plane-to-plane distance, d = 0.76 A); the current vanishes at ca. 80% of the geometric change and ca.

Molecular three-terminal devices: fabrication and measurements.

Incorporation of a third, gate electrode in the device geometry of molecular junctions necessary to identify the transport mechanism. At present, the most popular technique fabricate three-terminal molecular devices makes use of electromigration. Although it statistical process, we show that control over the gap resistance can be obtained.

Mixed self-assembled monolayers of semirigid tetrahydro-4H-thiopyran end-capped oligo(cyclohexylidenes).

Single-component and mixed self-assembled monolayers (SAMs) of one- and three-ring semirigid tetrahydro-4H-thiopyran end-capped oligo(cyclohexylidenes)-that is, thiopyran (1), 4-(4-cyclohexylidene-cyclohexylidene)tetrahydro-4H-thiopyran (2), and 4-(tetrahydro-4H-thiopyran-4-cyclohexylidene-4'-ylidene)tetrahydro-4H-thiopyran (3)--on Au(111) substrates have been prepared and studied by cyclic voltammetry (CV), atomic force microscopy (AFM), and scanning tunneling microscopy (STM). It was found that the shortest adsorbate 1 more readily forms a SAM than 2 or 3. Notwithstanding, the SAMs of 2 or 3 are thermodynamically more stable due to favorable intermolecular attractions.

The electronic structure of inorganic benzenes: valence bond and ring-current descriptions.

Valence bond (VB) theory and ring-current maps have been used to study the electronic structure of inorganic benzene analogues X(6)H(6) (X = C (1), Si (2)), X(6) (X = N (3), P (4)), X(3)Y(3)H(6) (X,Y = B,N (5), B,P (6), Al,N (7), Al,P (8)), and B(3)Y(3)H(3) (Y = O (9), S (10)). It is shown that the homonuclear compounds possess benzene-like character, with resonance between two Kekulé-like structures and induced diatropic ring currents. Heteronuclear compounds typically show localization of the lone pairs on the electronegative atoms; Kekulé-like structures do not contribute.

An ab initio valence bond study on cyclopenta-fused naphthalenes and fluoranthenes.

To probe the effect of external cyclopenta-fusion on a naphthalene core, ab initio valence bond (VB) calculations have been performed, using strictly atomic benzene p-orbitals and p-orbitals that are allowed to delocalize, on naphthalene (1), acenaphthylene (2), pyracylene (3), cyclopenta[b,c]acenaphthylene (4), fluoranthene (5), and cyclopenta[c,d]fluoranthene (6). For the related compounds 1-4 and 5,6 the total resonance energies (according to Pauling's definition) are similar. Partitioning of the total resonance energy in contributions from the possible 4n + 2 and 4n pi-electron conjugated circuits shows that only the 6pi-electron conjugated circuits (benzene-like) contribute to the resonance energy.

4-(Tetrahydro-4H-thiopyran-1-oxide-4-ylidene)-cyclohexanone oxime in the solid-state. A two-dimensional network of enantiomorphous chains interconnected by weak hydrogen bonds.

From a saturated C6H6 solution of racemic 4-(tetrahydro-4H-thiopyran-1-oxide-4-ylidene)-cyclohexanone oxime [1(1-R/1-S)] the co-crystal is crystallized. Single crystal X-ray analysis showed that (1)4.C6H6 (P1 space group) in the solid-state consists of enantiomorphous, non-covalent polymer-like chains that contain, in an alternating fashion, the crystallographically independent enantiomers 1-R and 1-R' or 1-S and 1-S'', respectively.

CP-arene oxides: the ultimate, active mutagenic forms of cyclopenta-fused polycyclic aromatic hydrocarbons (CP-PAHs).

The bacterial mutagenic response (Ames-assay, Salmonella typhimurium strain TA98+/-S9-mix) of a series of monocyclopenta-fused polycyclic aromatic hydrocarbons (CP-PAHs) identified in combustion exhausts, viz. cyclopenta[cd]pyrene (1), acephenanthrylene (2), aceanthrylene (3) and cyclopenta[hi]chrysene (4), is re-evaluated. The mutagenic effects are compared with those exerted by the corresponding partially hydrogenated derivatives, 3,4-dihydrocyclopenta[cd]pyrene (5), 4,5-dihydroacephenanthrylene (6), 1,2-dihydroaceanthrylene (7) and 4,5-dihydrocyclopenta[hi]chrysene (8).

A unified orbital model of delocalised and localised currents in monocycles, from annulenes to azabora-heterocycles.

Why are some (4n+2)pi systems aromatic, and some not? The ipsocentric approach to the calculation of the current density induced in a molecule by an external magnetic field predicts a four-electron diatropic (aromatic) ring current for (4n+2)pi carbocycles and a two-electron paratropic (antiaromatic) current for (4n)pi carbocycles. With the inclusion of an electronegativity parameter, an ipsocentric frontier-orbital model also predicts the transition from delocalised currents in carbocycles to nitrogen-localised currents in alternating azabora-heterocycles, which rationalises the differences in (magnetic) aromaticity between these isoelectronic pi-conjugated systems. Ab initio valence-bond calculations confirm the localisation predicted by the naive model, and coupled-Hartree-Fock calculations give current-density maps that exhibit the predicted delocalised-to-localised/carbocycle-heterocycle transition.

Cyclopenta[cd]fluoranthene and its precursors in combustion exhausts: a survey of their bacterial mutagenic activity.

Cyclopenta[cd]fluoranthene (1) and 3-ethynylfluoranthene (2) have both recently been identified in combustion exhausts. In this study, their mutagenic activities were compared to that of fluoranthene (3), one of the most abundant polycyclic aromatic hydrocarbons (PAHs) in combustion exhausts, in the Salmonella/microsome reversion assay (Ames assay) using S. typhimurium strain TA98.

Di-epoxides of the three isomeric dicyclopenta-fused pyrenes: ultimate mutagenic active agents.

To rationalize the high bacterial mutagenic response recently found for the (di-) cyclopenta-fused pyrene congeners, viz. cyclopenta[cd]-(1), dicyclopenta[cd,mn]-(2), dicyclopenta[cd,fg]-(3) and dicyclopenta[cd,jk]pyrene (4), in the presence of a metabolic activation mixture (S9-mix), their (di-)epoxides at the externally fused unsaturated five-membered rings were previously proposed as the ultimate mutagenic active forms. In this study, cyclopenta[cd]pyrene-3,4-epoxide (5) and the novel dicyclopenta[cd,mn]pyrene-1,2,4,5-di-epoxide (6), dicyclopenta[cd,fg]pyrene-5,6,7,8-di-epoxide (7) and dicyclopenta[cd,jk]pyrene-1,2,6,7-di-epoxide (8) were synthesised from 1 to 4, respectively, and subsequently assayed for bacterial mutagenicity in the standard microsomal/histidine reverse mutation assay (Ames-assay with Salmonella typhimurium strain TA98).

Which pi-clamped conjugated monocycles exhibit ring currents?

Quenching/survival of ring currents in pi-clamped conjugated monocycles is controlled by the match or mismatch in parity between the frontier orbitals of the central pi-conjugated 4n+2/4n monocycle and those of the clamps. Changes in ring current are not primarily caused by bond alternation or 'Mills-Nixon' effects; current and geometry changes on clamping are both consequences of electronic structure.