Electronically driven magnetic transition and dimensionality crossover in Li4 clusters

The existence of a new metastable phase of Li₄ with spin triplet in the shape of a distorted tetrahedron is proposed. It is 0.4 eV above the spin-singlet planar ground state of the tetramer. The transition from the singlet to the triplet configuration does not occur as soon as the cluster assumes a three-dimensional geometry, but waits till the dihedral angle is 110°. Such a dimensionality crossover accompanied by a change in spin multiplicity should be observable.     

How π-electron delocalisation reflects replacement of H+ with Li+ in variously substituted malonaldehydes

Correlations were studied between the properties of Bader's ring and bond critical points calculated for variously substituted malonaldehyde (containing intramolecular H-bond) and its π-electron delocalisation expressed by aromaticity indices HOMA and NICSs. It was observed that π-electron delocalisation of the system strongly depends on the substituent and its position. Replacement of the H⁺ with Li⁺ in malonaldehyde allowed us to study the role of unoccupied 2p orbital in π-electron delocalisation. In the case of lithium system the aromaticity is increased as compared with the malonaldehyde itself and moreover with the malonaldehyde anion. This proves that the unoccupied 2p orbital of Li⁺ may play a significant role in delocalisation of π-electrons due to its low orbital energy. In the case of lithium system the substituent effect is negligible, which resembles the situation in benzene derivatives. Dedicated to Prof. Karl Jug on the occasion of his 65th birthday anniversary.     

Graph-theoretical analysis of the bonding topology in polyhedral organic cations

It is shown that in several cases where planar delocalisation in organic cations would result in the formation of an anti-aromatic system, polyhedral delocalisation is the form of bonding actually preferred. This explains, for instance, why organic cations in such cases adopt cage-like structures. A full graph-theoretical analysis, similar to one previously published¹² for polyhedral boranes, carboranes and metal clusters, indicates that the nido structure for (CH)₅⁺ may readily be accounted for. Moreover, in the case of the dication (CH)₆²⁺ the fact that its energy minimum also corresponds to a nido structure is explained. In fact, no closo- or arachno-type structures appear to be possible for organic cations. A number of structural predictions concerning these species are given in the conclusion.     

Darstellung und eigenschaften von heterocyclischen silanen des typs Si4X MIT X = B,N, O

Five-membered heterocycles Si₄Ph₈X with X = BNMe₂, NMe, NEt and O were prepared and characterised with ¹H NMR, UV, far IR and Raman spectra. The problem of electron delocalisation in these ring systems containing an electron excess (N) or an electron deficit (B), respectively, is discussed. These measurements suggest that there is no delocalisation.     

The structure of arylosazones

The u.v./visible spectra of arylosazones have been examined. These show that in non-chelated arylosazones the π-electrons in both arylhydrazone groups are highly delocalised, compared to those of simple arylhydrazones, but that there is very little, if any, electronic interaction between the two 10π-electron systems. Both chromophores contribute separately to the absorption spectrum. In chelated arylosazones only one of the arylhydrazone groups has this enhanced electron delocalisation, and an explanation for this is proposed. The delocalisation causes an increase in the NN bond order and a reduction in the CN bond order. The delocalised structures proposed for both chelated and non-chelated arylosazones enable a number of the known properties of sugar phenylosazones to be now satisfactorily explained.     

Theoretical studies on carbon and silicon clusters: comparison of the structures and stabilities of neutral and ionic forms

Optimized molecular geometries and electronic structures are determined for neutral, positively charged, and negatively charged carbon and silicon clusters containing up to ten atoms. The effects of polarization functions and electron correlation are included in these claculations. Carbon clusters have linear or monocyclic ground state geometries whereas silicon clusters containing five or more atoms have three-dimensional ground state structures. Neutral C₄, C₆ and C₈ all have linear and monocyclic isomers of comparable stability whereas the ionic forms appear to be generally more stable as linear geometrical arrangements. In the case of neutral and positively charged carbon clusters, the odd-numbered clusters are significantly more stable than the adjacent even-numbered clusters whereas the opposite order of stability occurs for the negative ions. This is due to the large values of the electron affinities of the linear forms of even-numbered clusters such as C₄ and C₆. The relative stabilities of silicon clusters does not change with the charge state of the clusters.     

Ultrafast excited-state dynamics of strongly coupled porphyrin/core-substituted-naphthalenediimide dyads

The photophysics and excited-state dynamics of two dyads consisting of either a free-base or a zinc-tetraphenylporphyrin linked through a rigid bridge to a core-substituted naphthalenediimide (NDI) have been investigated by femtosecond-resolved spectroscopy. The absorption and fluorescence spectra differ substantially from those of the individual units, pointing to a substantial coupling and to a delocalisation of the excitation over the whole molecule, as confirmed by quantum chemistry calculations. A strong dependence of their excited-state dynamics on the solvent polarity has been observed. In toluene, the fluorescence quantum yield of the dyads is of the order of a few percent and the main decay channel of the emitting state is proposed as intersystem-crossing to the triplet state. However, in a medium polarity solvent like dichloromethane, the emitting state undergoes charge separation from the porphyrin to the NDI unit within 1-3 ps, and the ensuing charge-separated state recombines in about 10-20 ps. This solvent dependence can be explained by the weak driving force for charge separation in polar solvents and the large electronic coupling between the porphyrin and NDI moieties, making charge separation a solvent-controlled adiabatic process.     

Photoluminescence Intensity Fluctuations and Electric‐Field‐Induced Photoluminescence Quenching in Individual Nanoclusters of Poly(phenylenevinylene)

Substantial fluctuations of the fluorescence intensity have been detected for single clusters of poly(phenylenevinylene) containing more than 75 polymer chains or 30 000 monomer units. To the best of our knowledge, this is the first time such fluctuations (which resemble the “blinking” effect in single‐molecule fluorescence) have been reported for such a large molecular ensemble containing several macromolecules. Together with the distinct jumps, smooth fluctuations of the fluorescence intensity, with characteristic times from milliseconds to seconds, were observed. This fact distinguishes the fluorescence behaviour of the polymer clusters from that of other multichromophoric systems such as the single chains of conjugated polymers reported in the literature. The consecutive or simultaneous switching of one or several emitting sites from the “on” to “off” state does not explain the character of the fluctuations observed. We suggest that the quenching of the light‐emitting exciton by a long‐lived species, such as, for example, polarons, plays an important role in these unusual fluctuations. Electric field induced fluorescence quenching differs significantly for different clusters. It is proposed that this fluorescence was mainly quenched by polarons injected from the electrodes in the presence of an electric field. The specific behaviour of each cluster is explained by suggesting a different position of the clusters with respect to the electrodes.     

Threshold photoionization and density functional theory studies of the niobium carbide clusters Nb3C(n) (n = 1-4) and Nb4C(n) (n = 1-6)

We have used photoionization efficiency spectroscopy to determine ionization potentials (IP) of the niobium-carbide clusters, Nb3C(n) (n = 1-4) and Nb4C(n) (n = 1-6). The Nb3C2 and Nb4C4 clusters exhibit the lowest IPs for the two series, respectively. For clusters containing up to four carbon atoms, excellent agreement is found with relative IPs calculated using density functional theory. The lowest energy isomers are mostly consistent with the development of a 2 x 2 x 2 face-centered cubic structure of Nb4C4. However, for Nb3C4 a low-lying isomer containing a molecular C2 unit is assigned to the experimental IP rather than the depleted 2 x 2 x 2 nanocrystal isomer. For Nb4C5 and Nb4C6, interpretation is less straightforward, but results indicate isomers containing molecular C2 units are the lowest in energy, suggesting that carbon-carbon bonding is preferred when the number of carbon atoms exceeds the number of metal atoms. A double IP onset is observed for Nb4C3, which is attributed to ionization from the both the lowest energy singlet state and a meta-stable triplet state. This work further supports the notion that IPs can be used as a reliable validation for the geometries of metal-carbide clusters calculated by theory.     

Novel structures and energy spectra of hydroxylated (SiO2)8-based clusters: searching for the magic (SiO2)8O2H3- cluster

The prominent (SiO(2))(8)O(2)H(3) (-) mass peak resulting from the laser ablation of hydroxylated silica, attributed to magic cluster formation, is investigated employing global optimization with a dedicated interatomic potential and density functional calculations. The low-energy spectra of cluster isomers are calculated for the closed shell clusters: (SiO(2))(8)OH(-) and (SiO(2))(8)O(2)H(3) (-) giving the likely global minima in each case. Based upon our calculated cluster structures and energetics, and further on the known experimental details, it is proposed that the abundant formation of (SiO(2))(8)O(2)H(3) (-) clusters is largely dependent on the high stability of the (SiO(2))(8)OH(-) ground state cluster. Both the (SiO(2))(8)O(2)H(3) (-) and (SiO(2))(8)OH(-) ground state clusters are found to exhibit cagelike structures with the latter containing a particularly unusual tetrahedrally four-coordinated oxygen center not observed before in either bulk silica or silica clusters. The bare ground state (SiO(2))(8)O(2-) cluster ion core is also found to have four tetrahedrally symmetric Si==O terminations making it a possible candidate, when combined with suitable cations, for extended cluster-based structures/materials.     

Synthesis and X-ray structure of a novel 1,2,4-trithiolane

Photolysis of 1,2-dithiole-3-thione derivative 1 results in the formation of the 1,2,4-trithiolane derivative 2 , the structure of which was established by single crystal X -ray analysis. Interesting bond delocalisation is observed in the molecular structure. A mechanism involving dimerisation of 1 followed by loss of sulfur is proposed.     

Band structure of nonclassical polymers

It is shown that fully conjugated alternant nonclassical polymers are comparatively stable systems as a result of the considerable delocalisation energy.The energy characteristics of the polymers are strongly determined by the electron spin distribution of the degenerate nonbonding molecular orbitals; the full spin configuration,S>0, is favoured. The spin densities depend on the electron correlation and alternate; this corresponds to a ferrimagnetic state of the polymer at 0°K.     

Self-limiting aggregation leads to long-lived metastable clusters in colloidal solutions

The existence of a metastable state with limited Coulomb-blocked aggregation at the onset of instability in a colloidal solution is proposed and demonstrated both experimentally and theoretically (through Monte Carlo simulations). Such a stable state of small clusters of metallic colloids happens to be extremely important for techniques such as surface-enhanced Raman scattering (SERS), which profits explicitly from collective plasmon resonances in these clusters to boost Raman signals of specific analytes. In fact, SERS provides a unique tool to understand, monitor, and study the onset of aggregation in colloidal silver/gold and to prove the existence of the proposed state at the boundary of colloid coalescence.     

The synthesis and first full structural elucidation of a benzotriazole azo dye

Although azo dyes containing benzotriazole are of interest as substrates for surface enhanced resonance Raman scattering, SERRS, little is known of their molecular structure due to their poor crystal growth properties. We recently synthesised a highly crystalline dihydroquinoline via an unusual condensation reaction and we report herein that the azo‐benzotriazole dye subsequently formed was sufficiently crystalline to allow structural elucidation using synchroton radiation. It was found that this benzotriazole dye exists as the unexpected 6‐isomer and the structure of the dihydroquinoline moiety changes markedly on coupling due to increased delocalisation.     

Reconciling Electrostatic and n→π* Orbital Contributions in Carbonyl Interactions

Interactions between carbonyl groups are prevalent in protein structures. Earlier investigations identified dominant electrostatic dipolar interactions, while others implicated lone pair n→π* orbital delocalisation. Here these observations are reconciled. A combined experimental and computational approach confirmed the dominance of electrostatic interactions in a new series of synthetic molecular balances, while also highlighting the distance-dependent observation of inductive polarisation manifested by n→π* orbital delocalisation. Computational fiSAPT energy decomposition and natural bonding orbital analyses correlated with experimental data to reveal the contexts in which short-range inductive polarisation augment electrostatic dipolar interactions. Thus, we provide a framework for reconciling the context dependency of the dominance of electrostatic interactions and the occurrence of n→π* orbital delocalisation in C=O⋅⋅⋅C=O interactions.     

Reconciling Electrostatic and n→π* Orbital Contributions in Carbonyl Interactions

Interactions between carbonyl groups are prevalent in protein structures. Earlier investigations identified dominant electrostatic dipolar interactions, while others implicated lone pair n→π* orbital delocalisation. Here these observations are reconciled. A combined experimental and computational approach confirmed the dominance of electrostatic interactions in a new series of synthetic molecular balances, while also highlighting the distance‐dependent observation of inductive polarisation manifested by n→π* orbital delocalisation. Computational fiSAPT energy decomposition and natural bonding orbital analyses correlated with experimental data to reveal the contexts in which short‐range inductive polarisation augment electrostatic dipolar interactions. Thus, we provide a framework for reconciling the context dependency of the dominance of electrostatic interactions and the occurrence of n→π* orbital delocalisation in C=O???C=O interactions.     

Direct visualisation, by aberration-corrected electron microscopy, of the crystallisation of bimetallic nanoparticle catalysts

Using a scanning transmission electron microscope, corrected for aberration, ultra-high resolution images of the internal structure of nanoparticle clusters, as well as the number of atoms present per cluster, may be directly determined: it is shown that individual bimetallic clusters of Ru10Pt2 have a "molecular" structure and that when they coalesce into larger entities containing ca. 200 atoms they adopt the regular crystalline, and facetted, state of a bulk metal.     

Confocal laser microspectroscopic Rabi-flopping study of an iron oxide emitter surface used for Rydberg matter generation

Rydberg matter (RM) is a novel metal-like material in the form of electronically excited clusters of atoms (e.g. K and H) or molecules (e.g. H(2)). It is used as the inverted laser medium for IR in the RM laser. RM has recently been formed in its lowest state, which is proposed to be metallic hydrogen [Energy and Fuels 19 (2005) 2235]. An emitter material (K-doped iron oxide catalyst) that forms RM is studied by a specialized spectroscopic method, needed to detect the Rydberg states on the emitter surface. The spectroscopic method is phase-delay Rabi-flopping; it gives spectra from the time delay due to the periodic motion of the optical nutation vector. The formation of Rydberg species in the form of complexes K*-M (M a general small molecule) and (K-M)* is studied. So-called avoided transitions in K(+) ions are detected, of the same type as observed as transitions in the RM laser by stimulated emission. The formation and detection of Rydberg complexes containing H and H(2) is of great interest for metallic hydrogen production. Complexes with M=CH(2), H(2)O (or OH), CHO, H(2) and M'H are observed. Avoided transitions in RM clusters K(N)(*) are also identified. The identification of H containing Rydberg complexes on the surface indicates that metallic hydrogen is formed by the same cluster desorption route as other RM clusters.     

Solution and Solid State Structure of Highly Delocalised 5- Triphenylphosphoranylidenecyclopen Tene-3,4-Diones

The fully assigned ¹³C NMR spectra of the cyclic ylides 4 and 5 show that the C-1-C-2 bond is very strongly polarised consistent with substantial delocalisation of the ylide negative charge on to C-2. The X-ray structure of 5 confirms the occurrence of extensive delocalisation involving a significant contribution from up to six separate resonance forms.