Metalloid-Organic Intermolecular Complexes with Charge State-Controlled Conformations.

Shape alterations of molecular systems, induced by their (electric) charging/discharging, could facilitate useful electronic and/or mechanical functions in molecular-scale devices and machines. The present study reports structures, stabilities, charge distributions, and IR spectra for a group of complexes of a main-group metalloid (boron) atom with hydrocarbon molecules. The considered systems include the smallest species demonstrating the basic principle of operation, as well as their size-extended analogues, generalizing it to larger counterparts based on such units. The system geometries vary considerably between neutral and ionic counterparts and exhibit two-three typical conformations related to twisting by up to about 90 degrees. The predicted structures correlate with specific infrared spectra, which can enable their experimental identification and transformation tracking. The above-mentioned characteristics suggest the potential utility of such systems for intermolecular switches, with the possible spectral monitoring of their functioning.

Main-Group Metal Complexes of Benzene: Predicted Features of Stabilization and Isomerization.

Complexes of metal atoms with organic molecules represent a broad variety of systems with many important applications, e.g., in metal-organic interfaces and organometallic chemistry. One class involves aromatic species like benzene (Bz). Here, such complexes with second-group metals are investigated systematically in terms of structure and shape, stability and isomerization, charge distribution and aromaticity, and polarity and IR spectra. Three groups of isomers are identified, varying from metastable to stable ones, in effect featuring "physisorption" or "chemisorption". In particular, the high polarity of binary complexes and nonadditive stabilization of ternary systems for some isomers are found. Also, the Bz component's shape alteration for different isomers and system sizes and related aromaticity variations are predicted to be considerable. Property evolution for the series of metal components is analyzed.

Shape Symmetrization and IR-Spectral Enhancement of Aluminum Clusters via Doping with a Carbon Core.

Composite nanosystems are a class of objects with interesting and potentially useful properties. Here we study mixed-composition species representing interfaces at the molecular level between such technologically relevant materials as carbon and aluminum. Specifically, core-shell C8@Aln (n = 16, 18) species and their isomers with the core and relaxed-shell attached outside are investigated at a DFT level in terms of structures and stabilities, charge distributions and polarities, and IR spectra and electron affinities. Among the interesting findings is the possibility of bringing the aluminum cluster into a more symmetric shape (thus making a convenient building block) via insertion of a suitable molecular-carbon skeleton. Another notable feature is the system-selective dependence of polarity on spin multiplicity, suggesting possible molecular-electronic applications. The IR spectra of the composite species are much brighter compared to those of the separated components and are highly focused for the core-shell isomers. A related aspect of interest is the apparent reflections of the system structural details in the IR spectra features (line intensities and separations) via related vibrations, facilitating an experimental analysis of the structure and detection of the species formation and transformation as well as potentially enabling the means of achieving desirable optical characteristics via a geometric design.

Highly Polar Insertion Complexes with Focused IR Spectra and Internal Field-Inhibited Isomerization.

Complexes of a polar molecule (benzene trioxide) and alkali halide diatoms are predicted to form stable conformers through not only a common attachment, but also trapping the molecule between the counterions. Two possible low- and no-barrier routes of formation of such an insertion complex are identified, and stability and other properties of this and other conformers are analyzed, including polarity and charge distribution. Calculated IR spectra indicate a bright feature specific for the insertion complex, facilitating its reliable experimental detection. Isomerization of the ion-pair-trapped molecule shows a nonobvious inhibition effect (through an increased potential energy barrier) compared to the free molecule due to the reduction of its polarity in the isomerization. Formation of a flatter isomer, trioxonine, is clearly "reported" by a sharp alteration of the IR spectrum, distinguishable also from its variation for the nonreactive relaxation of the insertion complex into an attached one.

Counterion-Trapped-Molecules: From High Polarity and Enriched IR Spectra to Induced Isomerization.

We report extensive computational studies of some novel intermolecular systems and their properties. Recombination of alkali-halide counterions separated by a noncovalently trapped hydrocarbon molecule is prevented by significant potential energy barriers, resulting in unusual metastable insertion complexes. These systems are extremely polar, while the inserted molecule is strongly counter-polarized, leading to significant cooperative nonadditivity effects. The compression and electric field produced by the counterions favours isomerization of the trapped molecule via a significant reduction of the barriers to bond rearrangement, in a field-induced mechanochemical process. The predicted IR intensity spectra clearly reflect (1) formation of the insertion complex, rather than simple attachment of alkali halide, and (2) isomerization of the trapped molecule, thus allowing experimental access to these events.

Ligand Impact on Monolayer Electrochromic Material Properties.

In this study, we present a range of efficient highly durable electrochromic materials that demonstrate excellent redox and lifetime stability, sufficient coloration contrast ratios, and the best-in-class electron-transfer constants. The materials were formed by anchoring as little as a monolayer of predefined iron complexes on a surface-enhanced conductive solid support. The thickness of the substrate was optimized to maximize the change in optical density. We demonstrate that even a slight change in molecular sterics and electronics results in materials with sufficiently different properties. Thus, minor changes in the ligand design give access to materials with a wide range of color variations, including green, purple, and brown. Moreover, ligand architecture dictates either orthogonal or parallel alignment of corresponding metal complexes on the surface due to mono- or bis-quaternization. We demonstrate that monoquaternization of the complexes during anchoring to the surface-bound template layer results in redshifts of the photoabsorption peak. The results of in-solution bis-methylation supported by density functional theory calculations show that the second quaternization may lead to an opposite blueshift (in comparison with monomethylated analogs), depending on the ligand electronics and the environmental change. It is shown that the variations of the photoabsorption peak position for different ligands upon attachment to the surface can be related to the calculated charge distribution and excitation-induced redistribution. Overall, the work demonstrates a well-defined method of electrochromic material color tuning via manipulation of sterics and electronics of terpyridine-based ligands.

Dipoles Inside of Dipoles: Insertion Complexes of Polar versus Nonpolar Molecules in Ion Pairs.

Highly polar molecular systems are in demand as a means of enabling many important practical applications based on light-matter interactions. In the present work, the insertion complexes of recently synthesized polar molecules trapped between alkali halide counterions are studied. For specific selected compositions, the M-molecule-X systems are predicted to be stable to dissociation into molecule + alkali halide. It is found that unlike their nonpolar molecule-based counterparts, the polar molecule complexes can be even more stable than their common dipole-dipole MX-molecule isomers. This makes them thermodynamically stable, highly polar species, with very large dipoles of about 20 D, and they could be used, for example, to develop efficient light sensors. Furthermore, due to the neutralization of the M-X charge transfer in the excited triplet state, such complexes represent unique spin-controlled dipole-switch molecular systems with the large dipole turned off and even inverted by the spin state for the nonpolar and polar molecule complexes, respectively. This potentially could allow various spintronic and optoelectronic applications. In addition, the IR intensity spectra are predicted to sensitively indicate the formation of both the M-molecule-X and MX-molecule isomers, thus facilitating their reliable detection and differentiation in experiments.

Computational cogitation of Cn@Al12 clusters.

A variety of novel Cn Al12 core-shell nanoclusters have been investigated using density functional calculations. A series of Cn cores (n=1-4) have been encapsulated by icosahedral Al12 , with characteristic physical properties (energetics and stabilities, ionisation energies, electron affinities) calculated for each cluster. Other isomers, with the Cn moiety bound externally to the Al12 shell, have also been studied. For both series, a peak in stability was found for n(C)=2, a characteristic that appears to be inextricably linked with the relaxation of the constituent parts upon dissociation. Analysis of trends for ionisation energies and electron affinities includes evaluation of contributions from the carbon and aluminium components, which highlights the effects of composition and morphology on cluster properties.

Communication: a density functional investigation of structure-property evolution in the tetrakis hexahedral C4Al14 nanocluster.

Nanoclusters are prime objects of study in modern nanotechnology and offer a variety of applications promoted by their properties tunable by size, shape, and composition. DFT calculations are employed to analyze structure, stability, and selected electronic properties of a core-shell C4Al14 species. With insertion of the carbon core, the original low-symmetry aluminum cluster is predicted to undergo a considerable reshaping and acquire a striking D4h tetrakis-hexahedral geometry, with proportions controlled by a near-degenerate spin state or charge. The system also becomes more stable to dissociation. Surprisingly, other properties such as ionisation energy and electron affinity do not change significantly, although still exhibit some interesting features including opposite variations for vertical and adiabatic values. The stability and property evolutions are analyzed in terms of contributions from reshaping of the shell and its further interaction with the core. The system thus has potential applications as a symmetric building unit and a molecular device for nano-electronics/spintronics.

A computational study of 'Al-kanes' and 'Al-kenes'.

Two novel series of 'Al-kanes' (CnAl2n+2) and 'Al-kenes' (CnAl2n) have been studied theoretically in order to shed light on their structure, stability and properties. Density functional calculations suggest that the structures tend to be dictated by the constituent aluminium atoms, rather than the carbon backbone. This is the net effect of the aluminiums attempting to adopt preferred close-packed structures. Calculated energetics suggest a special stability of clusters with n(C) = 2 and 4 in both series and plausible interpretations are suggested.

Structure and properties of small aurocarbons: a selective study.

Gold clusters are versatile catalysts, and adding nonmetal dopants can allow tuning of their electronic properties via both shape and composition alteration. In the present work, mixed clusters of carbon and gold atoms are studied in terms of structure, stability, and the correlation between the shape and electronic properties by using a density functional theory approach. Four series of isomers (hydrocarbon analogues, carbon chains and cycles on gold surface, and carbon cores encapsulated by gold atoms) are investigated, exhibiting variation of the relative stability with the system size. Calculated vertical ionization energies, vertical electron affinities, and HOMO-LUMO energy gaps of the mixed clusters show a considerable change relative to the values for the pure gold clusters, the properties generally altering more strongly for the gold-encapsulated-carbon isomers. Also discussed are the structure, stability, and properties of larger clusters with a few such encapsulated-carbon units, with pronounced effects due to aggregation.

H2 molecules encapsulated in extended Ben cluster cages: toward light-metal nanofoams for hydrogen storage.

Efficient storage of hydrogen is a bottleneck problem for hydrogen-based energy solutions. We demonstrate the feasibility of trapping a pair of hydrogen molecules in beryllium cluster cages. The systems are constructed by merging two smaller units with single molecules trapped, which are known to be stable in isolation. The resulting (H(2))(2)@Be(n) species can have hydrogen cores and beryllium shells of different shapes, and we report the calculated energy barriers for hydrogen exit from the cage. The relative stabilities are related to the molecular structure and charge distributions, and some initially counterintuitive features are explained. Aspects of the release of hydrogen from such structures, and of possible scaling up to larger extended systems of fused cages, are discussed in terms of hydrogen storage. The predicted capacity could potentially be sufficient for practical usage.

A small molecule in metal cluster cages: H2@Mgn (n = 8 to 10).

Core-shell isomers of small magnesium clusters "doped" with a hydrogen molecule are theoretically studied and predicted to be weakly stable or metastable for different sizes of the system, allowing a low-temperature release of hydrogen. Evolution of H(2) inside Mg(n) is followed from the molecular species to two H atoms. Among properties analyzed are equilibrium geometries, dissociation energies, charge distributions, vertical energies of electronic excitation, ionization and electron attachment, and vibrational frequencies.

Trapped-molecule charge-transfer complexes with huge dipoles: M-C(2)F(6)-X (M = Na to Cs, X = Cl to I).

Novel intermolecular complexes, with a small molecule sterically trapped between a pair of ions, are designed and characterized. The molecule is found to undergo minor shape distortions and to remain almost neutral, and the complexes to have very large dipole moments (up to approximately 40 D). The systems' stability, ion-molecule interaction and stored energy are investigated, and the vertical excitation, ionization and electron-attachment are described. Variations of geometry and of the above properties with the alkali-metal and halogen components are analysed as well.

Flat-structural motives in small alumino-carbon clusters C(n)Al(m) (n = 2-3, m = 2-8).

Small clusters consisting of a carbon diatom or triatom and several aluminum atoms are investigated ab initio, at an MP2 level of theory. The mainly ionic character of C-Al bonding predominantly leads to structures different from corresponding hydrocarbons (also if starting from analogous initial geometries), while still producing closed-shell ground states. It is found that in many cases stable geometries correspond to flat CAl(3) units. These include unique metal-framed dicarbon and tricarbon all-flat species with unusual planar tetra-coordination. Another frequent feature is a hyper-coordination of carbon atoms, supported by their high negative charges and critically examined via atom-in-molecule calculations. Also characterized are anionic states, electronic excitation and ionization, electron attachment and detachment, and charge distributions.

Induced super-halogen behavior of metal moieties in halogen-doped clusters: LinI(-) and AlnI(-), n = 13, 1, 2, 3.

A comparative density functional theory (DFT) study of a series of neutral and negative-ionic lithium and aluminum clusters doped with iodine atom is presented. The I atom is found to preserve the same position at Li13 with and without the negative charge and Li13 to vary its shape from prolate to oblate with changing spin state of Li13I. Both the Mulliken and natural charges are considered, the natural-charge separation between the metal and halogen moieties being generally much larger (except for Al13I-). In LinI-, the additional electron is strongly localized on the metal moiety starting from n = 1, even though the electron affinity of Lin is much smaller than that of I. Such a super-halogen behavior of Lin is induced by highly electronegative iodine making the two components charged in LinI and leading to a charge-dipole interaction with the additional electron. In AlnI-, similar factors result in Aln being more negative than I already for n = 3, even though the electron affinity of I is higher, the effect escalating for n = 13.

Metastable intermolecular charge-transfer complexes with a pentavalent carbon atom.

A novel type of metastable complexes of metal (such as Na) atoms with a super-fluorinated carbon molecule is investigated, with a carbon atom exhibiting a unique, pentavalent character. It is induced by the charge transfer from the alkali metal component, followed by a geometric compression of the ion pair system. Analysis of the electron-density distribution confirms the real chemical nature of the extra C-F bond. Structure and stability of the system are characterized ab initio, and a spectrum of electronic perturbations is considered. The ways of forming the systems are discussed, and the spectroscopic parameters are predicted, facilitating their detection in experiments.

Induced hyper-valence of carbon in metal:fluorocarbon complexes.

Charge-transfer in combination with geometric features induces a pentavalent state of the carbon atom in metastable super-fluorinated alkali-metal compounds, M:C2F7 (M = K to Cs).

Photoinduced charge transfer reaction at surfaces. III. (HF)2...Na n/LiF(001)+hnu(640 nm)-->HFF-Na n +/LiF(001)+H(g).

A sub-monolayer of atomic sodium was deposited on a LiF(001) surface at 40 K. The adsorbed sodium exists at the surface as single atoms and clusters. The surface was dosed with 1 L of HF, to form adsorbed (HF)2...Na(n) (n=1,2,3,...) complexes, which were then irradiated by 640 nm laser light, to induce charge-transfer reaction. The reaction-product atomic H(g) was observed leaving the surface by two-color Rydberg-atom time-of-flight (TOF) spectroscopy. The TOF spectrum of the desorbed H atoms contained two components; a "fast" component with a maximum at approximately 0.85 eV, and a "slow" component with a maximum at 0.45 eV. These two components were attributed to photoreaction on adsorbed single atoms and clusters of sodium, respectively. The fast component exhibited a structure (48+/-17 meV spacing) near the high-energy end of spectrum. This structure was attributed to vibration of NaFHF photoproduct residing on the surface. The cross section of the harpooning event in the Na...(HF)2 adsorbed complex was determined as (9.1+/-2.0)x10(-19) cm(2). To interpret the experimental vibrational structure and the relative energies of the fast and slow components of the TOF spectrum, high-level ab initio calculations were performed for reactants Na(n)...(HF)(m) (n,m=1,2) and reaction products Na(n)F(m)H(m-1). The calculated NaF-HF and Na-Na(HF)(2) bond dissociation energies indicated that photoexcitation of the precursor complexes led not only to ejection of H atoms, but also to dissociation of the Na(n)...(HF)(2) (n=1,2) species through cleavage of the NaF-HF and Na-Na(HF)(2) bonds.