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The dimers involving benzene or other substituted [pi] systems were investigated to examine the relevance of the magnitudes of their attractive and repulsive interaction energy components in governing the geometries of several [pi]-[pi] systems. The findings suggest that the presence of substituents on the aromatic ring, irrespective of their electron withdrawing or donating nature leads to an increase in the binding energy.

The structures of neutral and anionic gold-silver binary clusters and pure gold and silver clusters are investigated. It is shown that the calculated electronic properties and dissociation energy of both pure and binary clusters are in agreement with the available experimental data.

Research discusses the origin of the fast diffusion reactivity and the role of low activation barrier in the catalytic mechanism in the context of preorganization, short strong hydrogen bond, and the charge buffering/dissipation of the catalytic residues. Results show that the charge buffering by the catalytic residues reduces negative charge accumulation.

Research examines a new family of ionophores, [n]beltenes, which possess a cation-olefinic pi type interaction as opposed to [n]collarenes exhibiting cation-aromatic pi interaction. Data indicate that the [n]beltenes have binding affinities to metal cations and also show delicate ion selectivity properties.

The interactions of first-row hydrides with ethene and benzene are investigated with regard to olefinic vs aromatic pi-H interactions.

The authors review the publications on the weakly bound van der Waals complexes of pi-systems and their properties. The topics of interest include the theoretical methods, the salient results obtained from the ab initio calculations for different pi-complexes and the results obtained via empirical and semiempirical calculations for the larger pi-complexes.

This research focuses on a comparison of the electronic properties, vibrational frequencies, binding energies, and the geometries of the minimal energy conformers of the C6H6-AlX3 and the corresponding C2H4-AlX3 systems.

Research was conducted to predict the binding energies and geometries of benzene-BX3 and ethene-BX3 complexes using quantum mechanical ab initio calculations. Single point measurements at a much higher correlation level and larger basis sets were also performed. Results based on binding energies indicate that all the complexes were weakly bound van der Waals complexes except for the C2H4-BH3 complex. There was also a strong evidence of an unusual increase in the nucleophilicity of one of the benzene carbons in the lowest energy conformer of systems involving benzene.

A study was conducted to analyze the role of Lewis acid(AlCl3)-aromatic ring interactions in Friedel-Craft's reaction. High-level ab initio computations were carried out on two model systems. Findings showed unusual molecular orbital interactions during the ring interactions. Results also indicated that Lewis acid supports a critical role in the activation of the aromatic substrate prior to attack by the electrophile.

The nature of the interaction and saturation of hydrogen molecules on small titanium clusters is examined by using ab initio calculations. The peaks in the experimental mass and photoelectron spectra of hydrogen saturated titanium clusters are attributed to structures possessing hydrogen multicenter bonds, which are in turn ascribed to the origin of the broad shoulder in the vibrational spectra of hydrogen cycled Ti-doped NaAl[H.sub.4].

Cation-pi and the corresponding anion-pi interactions are investigated as binary complexes. Compared to cation-pi-anion ternary complexes, the magnitude of the cation-pi interaction in pi-cation-anion ternary complexes is markedly lower due to the charge transfer from the anion to the cation.

The role of the metal electrode influencing the structure and electronic properties of semiconducting carbon nanotubes near the interface at low bias is examined. The calculations at the density functional level of theory, including full geometry optimizations, indicates that bonds are formed between the metal atoms of the electrode and the carbon atoms of the nanotubes.

The edge-to-face interactions for either axially or facially substituted benzenes are investigated by using ab initio calculation. The predicted maximum energy difference between substituted and unsubstituted systems is ~ 0.7 kcal/mol.

The theoretical studies yield several insights into the interactions prevailing in aqueous clusters of monovalent alkali metal, ammonium, and hydronium cations. The second order Moller-Plesset (MPs) and density functional levels of theory are used for the calculations of the structures, thermodynamic energies and IR spectra of several plausible conformers.

The interaction of various anions with many representative pie systems is investigated. The results of this research can be effectively employed to generate crafted molecular systems for size-and molecule-selective processes.

The use of femtosecond time-resolved, which degenerates four-wave mixing rotationally resolved spectroscopy to obtain very accurate structural information on the symmetric top cyclohexane is reported. Since, in the chair conformation belongs to the point D3d, it is not possible to employ microwave spectroscopy, which is the most precise structural method in the gas phase for small- and medium-sized molecules.

The authors have investigated alpha-amino acids exhibited by cobalt(III) metal complex. They report that the stereospecific and regiospecific recognition of these acids has been rationalized from the structural characteristics via the use of density functional calculations.

The nature of the cation-Pi interaction was examined by carrying out high level ab initio calculations of metallic and organic cations with different classes of Pi systems. The calculations included a rigorous decomposition of the interaction energies indicating that the interaction of the Pi systems with the metal cations was characterized by contributions from electrostatic and induction energies.

The synthesis of calix[4]hydroquinone nanotube arrays self-assembled with infinitely long one-dimensional (1-D) short hydrogen bonds and aromatic-aromatic interactions is presented. The competition between H-bonding and displaced pi-pi stacking in the assembling process is described.

A new type of interaction between silane (SiH4) and borane (BH3) using high level ab initio calculations were investigated that might have a hitherto unknown role in the mechanism of chemical vapor deposition (CVD), which is employed in the fabrication of boron doped silicon semiconductor materials. It is also noted that the SiH4-BH3 complex would dissociate to form SiH3 and BH4 radicals.

We present a systematic theoretical investigation of the interaction of an organic molecule with gold and palladium electrodes. We show that the chemical nature of the electrode elicits significant geometrical changes in the molecule. These changes, which are characteristic of the electrode atomic species and the interface geometry, are shown to occur at distances as great as 10 Å from the interface, leading to a significant modification of the inherent electronic properties of the molecule. In certain interface geometries, the highest occupied molecular orbital (HOMO) of the palladium-contacted molecule exhibits enhanced charge delocalization at the center of the molecule, compared to gold. Also, the energy gap between the conductance peak of the lowest unoccupied molecular orbital (LUMO) and the Fermi level is smaller for the case of the palladium electrode, thereby giving rise to a higher current level at a given bias than the gold-contacted molecule. These results indicate that an optimal choice of the atomic species and contact geometry could lead to significantly enhanced conductance of molecular devices and could serve as a viable alternative to molecular derivatization. [ABSTRACT FROM AUTHOR]

Using the computer-aided molecular design approach, we recently reported the synthesis of calix[4]hydroquinone (CHQ) nanotube arrays self-assembled with infinitely long one-dimensional (1-D) short hydrogen bonds (H-bonds) and aromatic-aromatic interactions. Here, we assess various calculation methods employed for both the design of the CHQ nanotubes and the study of their assembly process. Our calculations include ab initio and density functional theories and first principles calculations using ultrasoft pseudopotential plane wave methods. The assembly phenomena predicted prior to the synthesis of the nanotubes and details of the refined structure and electronic properties obtained after the experimental characterization of the nanotube crystal are reported. For better characterization of intriguing 1-D short H-bonds and exemplary displaced π-π stacks, the X-ray structures have been further refined with samples grown in different solvent conditions. Since X-ray structures do not contain the positions of H atoms, it is necessary to analyze the system using quantum theoretical calculations. The competition between H-bonding and displaced π-π stacking in the assembling process has been clarified. The IR spectroscopic features and NMR chemical shifts of 1-D short H-bonds have been investigated both experimentally and theoretically. The dissection of the two most important interaction components leading to self-assembly processes would help design new functional materials and nanomaterials. [ABSTRACT FROM AUTHOR]

Femtosecond laser pulses are utilized to drive multiple ionization in gas-phase formic acid clusters (FA) n . Experimental measurements of the kinetic energy release (KER) of the ions through Coulomb explosion are studied using time-of-flight mass spectrometry and compared to the values recorded from molecules. Upon interacting with 200 fs linearly polarized laser pulses of 400 nm, formic acid clusters facilitate the formation of higher charge states than the formic acid dimer, reaching both C 3+ and O 3+ and also increasing the KER values to several hundred electronvolts in magnitude for such ions. At a lower laser intensity (3.8 × 10 14 W/cm 2 ), we record an enhancement in the signal of the (FA) 5 (H 2 O)H + cluster, which suggests that it has a higher stability, in agreement with previous studies. A molecular dynamics simulation of the Coulomb explosion shows that the highly charged atomic ions arise from larger clusters, whereas the production of CO 3+ is more likely to arise from the molecular case. Thus, the relative production of CO 3+ is reduced in comparison to the highly charged ions upon clustering and is likely due to the higher ionization levels achieved, which facilitate dissociation.

Femtosecond laser pulses are utilized to drive multiple ionization of formic acid dimers and the resulting ions are studied using time-of-flight mass spectrometry. The interaction of formic acid dimer with 200 fs linearly polarized laser pulses of 400 nm with intensities of up to 3.7 × 10 15 W/cm 2 produces a metastable carbon monoxide trication. Experimental kinetic energy release (KER) measurements of the ions are consistent with molecular dynamics simulations of the Coulomb explosion of a formic acid dimer and suggest that no significant movement occurs during ionization. KER values were recorded as high as 44 eV for CO 3+ , in agreement with results from a classical Molecular Dynamics simulation of fully ionized formic acid dimers. Potential energy curves for CO 3+ are calculated using the multireference configuration interaction (MRCI+Q) method to confirm the existence of an excited metastable 2 Σ state with a significant potential barrier with respect to dissociation. This combined experimental and theoretical effort reveals the existence of metastable CO 3+ through direct observation for the first time.

The single-molecule conductance of hydrogen-bonded and alkane systems are compared in this theoretical investigation. The results indicate that for short chains, the H-bonded molecules exhibit larger conductance than the alkanes. Although earlier experimental investigations attributed this observation to a large density of states (DOS) corresponding to an occupied molecular orbital below the Fermi energy, the current work indicates the presence of a Fano resonance in the transmission function in the vicinity of the Fermi energy. The inclusion of this observation is essential in understanding the behavior of these systems. We also address the characteristics of the H-bond for transport and provide an explanation for the presence of a turnover regime wherein the conductance of the alkanes becomes larger than the H-bonded systems. Incidentally, this feature cannot be explained using a simple DOS argument.

Carbyne is the long-sought linear allotrope of carbon. Despite many reports of solid carbyne, the evidence is unconvincing. A recent report of supposed carbyne shows gold clusters in transmission electron microscopy (TEM) images. In order to determine the effects of such clusters, we performed ab initio calculations of uncapped and capped linear carbon chains and their complexes with gold clusters. The results indicate that gold dramatically alters the electron densities of the C≡C bonds. The resulting charge-stabilization of the carbon chains leads to pseudocarbynes. These findings are corroborated in calculations of the structures of crystals containing isolated carbon chains and those intercalated with gold clusters. Calculated Raman spectra of these pseudocarbynes with gold clusters are in better agreement with experiment than calculated spectra of isolated carbon chains. The current work opens the way toward the design and development of a new class of metal-intercalated carbon compounds.

Here we report the electrocatalytic reduction of protons to hydrogen by a novel S2P2 coordinated nickel complex, [Ni(bdt)(dppf)] (bdt = 1,2-benzenedithiolate, dppf = 1,1'-bis(diphenylphosphino)ferrocene). The catalysis is fast and efficient with a turnover frequency of 1240 s(-1) and an overpotential of only 265 mV for half activity at low acid concentrations. Furthermore, catalysis is possible using a weak acid, and the complex is stable for at least 4 h in acidic solution. Calculations of the system carried out at the density functional level of theory (DFT) are consistent with a mechanism for catalysis in which both protonations take place at the nickel center.

Characterizing the nano-bio interface has been a long-standing endeavor in the quest for novel biosensors, biophotovoltaics, and biocompatible electronic devices. In this context, the present computational work on the interaction of two peptides, A6K (Ac-AAAAAAK-NH2) and A7 (Ac-AAAAAAA-NH2) with semiconducting TiO2 nanoparticles is an effort to understand the peptide-metal oxide nanointerface. These investigations were spurred by recent experimental observations that nanostructured semiconducting metal oxides templated with A6K peptides not only stabilize large proteins like photosystem-I (PS-I) but also exhibit enhanced charge-transfer characteristics. Our results indicate that α-helical structures of A6K are not only energetically more stabilized on TiO2 nanoparticles, but the resulting hybrids also exhibit enhanced electron transfer characteristics. This enhancement can be attributed to substantial changes in the electronic characteristics at the peptide-TiO2 interface. Apart from understanding the mechanism of electron transfer (ET) in peptide-stabilized PS-I on metal oxide nanoparticles, the current work also has implications in the development of novel solar cells and photocatalysts.

Hybrids consisting of a metal oxide nanoparticle and a molecule show strong enhancement of Raman modes due to an interfacial charge transfer process that induces the formation of midgap states, thereby reducing the effective gap compared to that of the nanoparticle and creating the posibility of an electronic resonance at energies substantially lower than the nanoparticles's band gap. We have developed a simple methodology to mimic the presence of the nanoparticle through a deformation of the bond involved in the chemical binding between the two entities forming the hybrid. The results provide a convincing interpretative frame to the enhancements observed in Raman spectra when all atoms are included. In addition, these enhancements can be correlated to a crossing of excited molecular orbitals that take part in the virtual excitation associated with the Raman process. We illustrate our method for the dopamine-Ti2O4 hybrid using the most acidic molecular O-H bond as the control parameter for the deformation.

Salicylate and salicylic acid derivatives act as electron donors via charge-transfer complexes when adsorbed on semiconducting surfaces. When photoexcited, charge is injected into the conduction band directly from their highest occupied molecular orbital (HOMO) without needing mediation by the lowest unoccupied molecular orbital (LUMO). In this study, we successfully induce the chemical participation of carbon dioxide in a charge transfer state using 3-aminosalicylic acid (3ASA). We determine the geometry of CO2 using a combination of ultraviolet-visible spectroscopy (UV-vis), surface enhanced Raman scattering (SERS), (13)C NMR, and electron paramagnetic resonance (EPR). We find CO2 binds on Ti sites in a carbonate form and discern via EPR a surface Ti-centered radical in the vicinity of CO2, suggesting successful charge transfer from the sensitizer to the neighboring site of CO2. This study opens the possibility of analyzing the structural and electronic properties of the anchoring sites for CO2 on semiconducting surfaces and proposes a set of tools and experiments to do so.