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Understanding the chemistry of carbon dioxide is key to affecting changes in atmospheric concentrations. One area of intense interest is CO2 capture in chemically reversible cycles relevant to carbon capture technologies. Most CO2 capture methods involve thermal cycles in which a nucleophilic agent captures CO2 from impure gas streams (e.g., flue gas), followed by a thermal process in which pure CO2 is released. Several reviews have detailed progress in these approaches. A less explored strategy uses electrochemical cycles to capture CO2 and release it in pure form. These cycles typically rely on electrochemical generation of nucleophiles that attack CO2 at the electrophilic carbon atom, forming a CO2 adduct. Then, CO2 is released in pure form via a subsequent electrochemical step. In this Perspective, we describe electrochemical cycles for CO2 capture and release, emphasizing electrogenerated nucleophiles. We also discuss some advantages and disadvantages inherent in this general approach.

We describe a new electrochemical cycle that enables capture and release of carbon dioxide. The capture agent is benzylthiolate (RS–), generated electrochemically by reduction of benzyldisulfide (RSSR). Reaction of RS– with CO2 produces a terminal, sulfur-bound monothiocarbonate, RSCO2–, which acts as the CO2 carrier species, much the same as a carbamate serves as the CO2 carrier for amine-based capture strategies. Oxidation of the thiocarbonate releases CO2 and regenerates RSSR. The newly reported S-benzylthiocarbonate (IUPAC name benzylsulfanylformate) is characterized by 1H and 13C NMR, FTIR, and electrochemical analysis. The capture–release cycle is studied in the ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMP TFSI) and dimethylformamide. Quantum chemical calculations give a binding energy of CO2 to benzyl thiolate of −66.3 kJ mol–1, consistent with the experimental observation of formation of a stable CO2 adduct. The data described here represent the first report of...

Practical conversion of waste heat into electricity via thermoelectrochemical cells requires high Seebeck coefficient (α) to increase cycle efficiency. The complexation of Cu2+ species with dissolved multidentate ligands, such as ethylenediaminetetraacetic acid, and the control of dimerization equilibria with bridging ligands, such as 1,6-diaminohexane or 1,2-diaminoethane, dramatically improve, by up to ∼185%, the magnitude of the α of Cu/Cu2+ thermoelectrochemical cells. This results in the highest α for any Cu/Cu2+ redox system yet reported. The coefficient α is directly proportional to the change in entropy (ΔS). It was experimentally measured and correlated with ΔS obtained from quantum-chemical methods. This offers a deeper insight about a molecule-based interpretation of the macroscopic response. The agreement between the theoretically estimated and experimentally observed α is remarkable. Hence, we believe that this synergistic approach allows us to systematically scan different systems to obtain efficient thermoelectrochemical cells with enhanced Seebeck coefficient.

We explore a connection between the static molecular polarizability and the molecular conductance that arises naturally in the description of electrified molecular interfaces and that has recently been explored experimentally. We have tested this idea by using measured conductance of few different experimental design motifs for molecular junctions and relating them to the molecular polarizability. Our results show that for a family of structurally connected molecules the conductance decreases as the molecular polarizability increases. Within the limitations of our model, this striking result is consistent with the physically intuitive picture that a molecule in a junction behaves as a dielectric that is polarized by the applied bias, hence creating an interfacial barrier that hinders tunneling. The use of the polarizability as a descriptor of molecular conductance offers significant conceptual and practical advantages over a picture based on molecular orbitals. To further illustrate the plausibility of th...

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.

Nanoparticles coated with different kinds of molecules are currently designed and synthesized for several important applications, including catalysis, solar cells, and biomedical uses. A crucial molecular design variable is whether the nanoparticle exhibits plasmonic activity, e.g., the case of nanoparticles made of coinage metals, where no band gap is present, or if it rather behaves as a nano-semiconductor with a band gap, e.g., metal oxide nanoparticles. In this tutorial, we will discuss the literature for both plasmonic and non-plasmonic materials and our own recent theoretical and experimental work in two different showcases. First, we will present the example of using gold nanoparticles to monitor molecular sensing activity to follow changes in antibody/antigen binding through changes of the surface plasmon resonance (SPR) response. Second, we will discuss the case of surface-enhanced Raman resonance (SERS) in hybrid systems molecule-TiO2 nanoparticles and clusters, where the important physical quantity is the Raman signal to monitor the formation of chemical bonds and interfacial electron transfer processes.

The dopamine-TiO2 system shows a specific spectroscopic response, surface enhanced Raman scattering (SERS), whose mechanism is not fully understood. In this study, the goal is to reveal the key role of the molecule–nanoparticle interface in the electronic structure by means of ab initio modeling. The dopamine adsorption energy on anatase surfaces is computed and related to changes in the electronic structure. Two features are observed: the appearance of a state in the material band gap, and charge transfer between molecule and surface upon electronic excitation. The analysis of the energetics of the systems would point to a selective adsorption of dopamine on the (001) and (100) terminations, with much less affinity for the (101) plane.

Interfacial charge transfer has been an area of intense interest because of its relevance in molecular electronics, dye-sensitized solar cells, surface-enhanced Raman scattering (SERS), and photocatalysis. Although the chemical natures of both the contact and the linker have been shown to play important roles in determining the properties of hybrid dye/molecule–metal oxide complexes, little is known about the nature of the charge-transfer pathways. In this work, we explore in detail the idea that Raman enhancement and charge transfer are intimately related. To this end, we analyze the vibrational modes of molecules exhibiting the maximum enhancement of the Raman activities when they are adsorbed on semiconducting metal oxide nanoparticles. Our analysis of the potential energy distributions of these modes in the hybrid complexes indicates the significant involvement of bending and torsional modes of atoms deep within the metal oxide nanoparticle. Whereas the individual contribution of each of these oxide b...

Comparative theoretical and experimental investigations are reported into chemically induced magnetism in atomically-precise, ligand-stabilized gold clusters Au25 , Au38 and Au55 . The results indicate that [Au25 (PPh3 )10 (SC12 H25 )5 Cl2 ](2+) and Au38 (SC12 H25 )24 are diamagnetic, Au25 (SC2 H4 Ph)18 is paramagnetic, and Au55 (PPh3 )12 Cl6 , is ferromagnetic at room temperature. Understanding the magnetic properties resulting from quantum size effects in such atomically precise gold clusters could lead to new fundamental discoveries and applications.

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.

Currently, there is considerable interest in the properties of semiconducting metal oxide nanoparticle substrates because of their utility in surface-enhanced Raman scattering, dye-sensitized solar cells, and photocatalysis. While the enhancement of Raman activities of molecules adsorbed on these nanoparticles is due to a large increase in the polarizability, because of charge transfer from the molecule to the semiconducting nanoparticle, little is known about the factors responsible for modulating the polarizability, particularly the influence of the solvent. Consequently, we have carried out Monte Carlo simulations of several hybrids to study the solvent effect on the dynamic polarizabilities and electronic spectra. Our results indicate that the presence of the solvent induces a shift and an increase in the polarization response that is dependent on the identity of the hybrid. The observed enhancement can be attributed to both the resonant character of the excitation and the participation of the solvent in the charge redistribution. The methodology employed in this work could be very valuable in both identifying and developing metal oxides as novel molecular sensors.

The transition from diamagnetic to ferromagnetic behavior is one of the most dramatic quantum size-dependent effects in noble metals. The origin of the ferromagnetic behavior of Au nanoparticles is associated with a spin symmetry breaking at the Fermi level, which leads to a quasi-degeneracy of states whose magnetic properties differ markedly from the diamagnetic behavior encountered in bulk Au. We have performed quantum-chemical density functional theory-based calculations of the electronic, optical, and magnetic properties of Au clusters to clarify several aspects of the behavior of nanogold. In some cases, we found a remarkable stability of the localized magnetic moments of the clusters that support a dual domain model for Au nanoparticles to explain their magnetic properties, that is, a diamagnetic core and localized surface moments. This magnetic transition influences the optical response of the nanoparticles, quenching the intensity of the absorption associated to the plasmon resonance. We found tha...

Semiconducting oxide nanoparticles have proven to be excellent in detecting extremely low-concentrations of molecules through surface-enhanced Raman scattering (SERS) effects. While the enhancement of the Raman activities arises from a large increase in polarizability due to charge transfer from the molecule to the semiconducting nanoparticle, little is known about how the oxide composition, nanoparticle size, solvent, or pH affects the observed Raman activities. In the current study, we examine these effects by carrying out extensive computational investigations of semiconducting TiO2, SnO2 and Fe2O3 nanoparticles and their complexes with both catechol and dopamine. An increase in the size of the oxide cluster or a decrease in the pH of the system under observation leads to enhanced Raman activities; the variation of the activities in different solvents is very much dependent on the nature of the vibrational modes. The marked increase in the Raman activities of molecules adsorbed on SnO2 or Fe2O3 over th...

The role of quantum confinement, size, and solvent effects on the surface enhanced Raman spectra of biologically important molecules absorbed on semiconducting titanium dioxide (TiO2) nanoparticles is investigated using density functional calculations. The results obtained for both the gas phase and solvated systems indicate significant changes in the electronic structure and the Raman spectra of molecules like formic acid and dopamine, when they are adsorbed on small TiO2 nanoparticles. A number of distinctive features that are determined by the formation of a charge-transfer complex at the nanoparticle-molecule interface can be noted in the Raman spectra. Both the spectra and the electronic properties are strongly size dependent and are also sensitive to the presence of the solvent and the nature of adsorbate interaction. Although these calculations reinforce recent experimental findings on the role of quantum confinement, they also pose new questions about the extension of collective effects and the effect of pH and other environmental variables. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010

The interaction of an organic molecule with metal electrodes possessing various work functions and atomic species is investigated by ab initio calculations. The results indicate that the chemical nature of the atomic species of the electrode and interface geometry elicit significant changes in both the physical structure and electronic properties at distances as great as 1 nm from the interface. Also, the metal electrodes having similar work functions contacting the same molecule give rise to different transmission profiles, thereby leading to widely varying I-V characteristics.

Presented herein are the first-principle calculations of the transport and other pertinent electronic properties of metal contacted semiconducting carbon nanotubes (CNTs). The investigation is focused on elucidating access resistance as a function of the work function difference and the chemical nature of the metal atomic species. Our results show that, for simple end-contact geometries, the Fermi level position within the gap differs between palladium-contacted CNTs and gold-contacted CNTs. This is interesting since both of these metals possess similar work functions. The role of the metal-CNT coupling is examined in light of the resulting - behavior of the system.

Cation-pi and the corresponding anion-pi interactions have in general been investigated as binary complexes despite their association with counterions. However, a recent study of the ammonia channel highlights the important but overlooked role of anions in cation-pi interactions. In an effort to examine the structural and energetic consequences of the presence of counterions, we have carried out detailed ab initio calculations on some model cation-pi-anion ternary complexes and evaluated the nonpair potential terms, three-body contributions, and attractive and repulsive energy components of the interaction energy. The presence of the anion in the vicinity of the pi system leads to a large redistribution of electron density and hence leads to an inductive stabilization. The resulting electronic and geometrical changes have important consequences in both chemical and biological systems. Compared to cation-pi-anion ternary complexes, the magnitude of the cation-pi interaction in pi-cation-anion ternary complexes is markedly lower because of charge transfer from the anion to the cation.

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.

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.

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 Angstrom 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.

IR-UV double resonance spectroscopy and ab initio calculations were employed to investigate the structures and vibrations of the aromatic amino acid, L-phenylalanine–(H2O)n clusters formed in a supersonic free jet. Our results indicate that up to three water molecules are preferentially bound to both the carbonyl oxygen and the carboxyl hydrogen of L-phenylalanine (L-Phe) in a bridged hydrogen-bonded conformation. As the number of water molecules is increased, the bridge becomes longer. Two isomers are found for L-Phe–(H2O)1, and both of them form a cyclic hydrogen-bond between the carboxyl group and the water molecule. In L-Phe–(H2O)2, only one isomer was identified, in which two water molecules form extended cyclic hydrogen bonds with the carboxyl group. In the calculated structure of L-Phe–(H2O)3 the bridge of water molecules becomes larger and exhibits an extended hydrogen-bond to the π-system. Finally, in isolated L-Phe, the D conformer was found to be the most stable conformer by the experiment and by the ab initio calculation., The authors would like to acknowledge the support from the Grant-in-Aids for Scientific Research (18205003) by the Ministry of Education, Science, Sports, and Culture, Japan and from the French-German CNRS-DFG binational project. T. E. gratefully acknowledges financial support from Satake foundation.

In earlier studies, the interactions of isolated ionic species with various solvents were investigated using ab initio calculations. The ionic species investigated included cations (proton, hydronium, ammonium, and metal cations) and anions (single electron, hydroxide, and halide anions). However in the present study, we investigate the interactions of these ionic species with the solvent in the presence of other competing ionic species. We also elaborate on how the information obtained from these extensive studies have been employed in designing and synthesizing various kinds of novel ionophores and receptors.

The equilibrium structures and binding energies of the benzene complexes of p-benzoquinones (PBQ) and its negatively charged anionic species (PBQ- and PBQ2-) have been investigated theoretically using second-order Møller-Plesset calculations. While neutral p-benzoquinone-benzene clusters (PBQ-Bz) prefer to have a parallel displaced geometry (P-c), CH...pi interactions (T-shaped geometries) prevail in the di-anionic PBQ-benzene (PBQ2- -Bz) complexes (T-e2-). Studies on dianionic p-benzoquinone-benzene clusters showed that two nonbonded intermolecular interactions compete in the most stable conformation. One is H-bonding interaction (C-H...O type) between carbonyl oxygen of p-benzoquinone and one of the hydrogen atoms of benzene, and the other is a pi-H interaction between pi-electron cloud of PBQ2- and another hydrogen atom of benzene. Blueshifted H-bonds were observed in T-shaped clusters. The changes in the geometrical preference of PBQ-Bz complex upon addition of electrons would be useful in designing optimized molecular mechanical devices based on the edge-to-face and face-to-face aromatic interactions.

This study details the interactions prevailing in aqueous clusters of monovalent alkali metal, ammonium, and hydronium cations. The calculations involve a detailed evaluation of the structures, thermodynamic energies, andIR spectra of several plausible conformers of M + .(H 2 O) 1 - 6 (M = Li, Na, K, Rb, Cs, NH 4 , H 3 O) clusters at the second-order Miller-Plesset (MP2) and density functional levels of theory. A detailed decomposition of the interaction energies has been carried out for complexes involving one or two water molecules using symmetry adapted perturbation theory. Some of the salient insights on the structures include the emergence of the second solvent shell even before the realization of the maximal coordination number of the cation. This effect was more pronounced in clusters involving the larger cations. The quantitative estimates of various components of the interaction energy indicate the predominance of electrostatic energies in the binding of these cations to water molecules. Interestingly, for all the hydrated alkali metal cation complexes, the contribution of electrostatic energy is almost the same as the total attractive interaction energy, whereas the sum of the induction and dispersion energies are almost canceled out by exchange-repulsion energy. However, the contribution of dispersion energies slowly starts increasing as the size of the cation increases and is quite substantial in case of the Cs + complexes. In the organic cations, the dispersion energies become significant, though not comparable to the electrostatic energies. In addition to the evaluation of the harmonic frequencies of -OH stretching mode of all the structures, the anharmonic frequencies were evaluated for the smaller clusters. As the size of the cation and the size of the water cluster increases, the red shifts associated with the -OH stretching mode progressively become larger for the alkali metal cation containing complexes. For the organic cation (NH 4 +, H 3 O + ) containing complexes, an opposite trend is observed. Compared to the isolated water monomer, the ratio of the infrared intensities of the asymmetric and symmetric -OH stretching modes is very small. However, this ratio progressively becomes larger as the size of the cation increases.

The nature of the anion−π interaction has been investigated by carrying out high level ab initio calculations of the complexes of halide (F-, Cl-, and Br-), linear organic (CN-, NC-), and trigonal planar organic (NO3- and CO32-) anions with different kinds of π systems, viz. olefinic (tetrafluoroethene), aromatic (hexafluorobenzene), and heteroaromatic (1,3,5-triazine). In an effort to comprehend the underlying basis of this interaction, we have also carried out a rigorous decomposition of the interaction energies using the symmetry adapted perturbational theory (SAPT) method. Contrary to our expectations, the results indicate that the magnitudes of total interation energies of anion−π and cation−π interactions are similar. In contrast to cation−π interactions, anion−π interactions are, however, marked by substantial contributions from dispersion energies. As in the case of cation−π interactions, the role of anions also have a marked influence on the nature and magnitude of the anion−π interaction with in...

Owing to the utility of redox phenomena of silver in many chemical systems, it is important to understand the coordination chemistry of Ag+ ion and hence the hydration structure. The lowest-energy conformations of Ag+(H2O)1–6 are sensitive to the calculation method employed. The coordination number (Nc) of Ag+(H2O)n is predicted to be 2 for n=2–6 at the density functional theory level, while the Nc for n=3–5 is 3, and that for n=6 is 4 at the second-order Moller–Plesset perturbation level. Further accurate analysis based on coupled-cluster singles and doubles theory with perturbative corrections for triple excitations agrees with the MP2 results except that Nc of 4 is also as competitive as Nc of 3 for n=5. To identify the correct Nc, it would be useful to facilitate the IR experimental characterization. We thus provide the OH spectra for various possible structures. It is interesting to note that the hydration chemistry of Ag+ ion is somewhat different from that of alkali metal ions.

The structures of pure gold and silver clusters (Auk, Agk, k = 1−13) and neutral and anionic gold−silver binary clusters (AumAgn, 2 ≤ k = m + n ≤ 7) have been investigated by using density functional theory (DFT) with generalized gradient approximation (GGA) and high level ab initio calculations including coupled cluster theory with relativistic ab initio pseudopotentials. Pure Auk clusters favor 2-D planar configurations, while pure Agk clusters favor 3-D structures. In the case of Au, the valence orbital energies of 5d are close to that of 6s. This allows the hybridization of 6s and 5d orbitals in favor of planar structures of Auk clusters. Even 1-D linear structures show reasonable stability as local minima (or as global minima in a few small anionic clusters). This explains the ductility of gold. On the other hand, the Ag-4d orbital has a much lower energy than the 5s. This prevents hybridization, and so the coordination number (Nc) of Ag in Agk tends to be large in s-like spherical 3-D coordination i...

The nature of the cation−π interaction has been examined by carrying out high level ab initio calculations of both metallic (Li+,Na+,K+, and Ag+) and organic (NH4+, C(NH2)3+, and N(CH3)4+) cations with different classes of π systems, viz. alkenes (ethene), arenes (benzene), and heteroarenes (pyrrole). The calculations, which include a rigorous decomposition of the interaction energies, indicate that the interaction of these π systems with the metal cations is characterized by contributions from both electrostatic and induction energies, with the contribution of the latter being dominant. Though the contributions of dispersion energies are negligible in the cation−π complexes involving Li+ and Na+, they assume significant proportions in the complexes involving K+ and Ag+. In the π complexes of the organic cations, the repulsive exchange contributions are much larger than the attractive electrostatic contributions in the π complexes of organic cations, and thus, the contributions of both induction and dispe...

We present the results obtained from spectroscopic investigations and quantum chemical calculations of the interaction of anisole (methoxybenzene) with small water clusters. The experiments have been carried out using resonant two-photon ionization (R2PI) and IR-UV double-resonance vibrational spectroscopy (IR/R2PI) in the region of the OH stretches. Apart from the vibrational spectra of the water moiety in the clusters, their intermolecular vibrations in the electronically excited S1 state are identified by IR/R2PI hole burning spectroscopy and assigned according to the vibrations calculated for the S1 state and compared with the vibrations calculated for the S0 state. The calculations for the S0 state were carried out at the second order Moller-Plesset level of theory using both the 6-31+G* and aug-cc-pVDZ basis sets and for the S1 state at the configuration interaction singles (CIS) level with the 6-31+G* basis set. In the electronic ground state (S0), the interaction of a water monomer to anisole is m...

[formula: see text] A new molecular system, 2,11-dithio[4,4]metametaquinocyclophane containing a quinone moiety, was designed and synthesized. As the quinone moiety can readily be converted into an aromatic pi-system (hydroquinone) upon reduction, the nanomechanical molecular cyclophane system exhibits a large flapping motion like a molecular flipper from the electrochemical redox process. The conformational changes upon reduction and oxidation are caused by changes of nonbonding interaction forces (devoid of bond formation/breaking) from the edge-to-face to face-to-face aromatic interactions and vice versa, respectively.

The interactions of the first-row hydrides (NH 3 , H 2 O, HF) with ethene have been investigated by carrying out calculations, at the second order Moller–Plesset (MP2) level of theory using both the 6-31+G ∗ and aug- cc -pVDZ basis sets. Unlike previous investigations of these systems, the geometries and vibrational frequencies in the present study were obtained by carrying out explicit counterpoise corrected optimizations. In an effort to understand the nature of the H⋯π interactions prevalent in these complexes, the interaction energies were decomposed into individual energy components using the symmetry adapted perturbation theory. Given the goals of the present investigation, the geometries, vibrational frequencies and interaction energy components of the water dimer have also been evaluated. While the interaction energy of the conventional H-bond is dominated by electrostatic interactions, electrostatic, dispersive and inductive interactions are important in the description of the π H-bond. An important distinction between conventional H-bonded complexes and the π H-bonded complexes is that the inductive interaction gets magnified at the MP2 level. Thus, the inclusion of electron correlation is an important prerequisite both for the magnification of the inductive interaction and to obtain an accurate estimate of the dispersion energies. It is observed that changes in various geometrical and vibrational parameters of these π H-bonded complexes can be correlated to the magnitude of either the individual or a combination of various interaction energy components.

We have investigated a new type of interaction between silane (SiH4) and borane (BH3) using high level ab initio calculations. The SiH4−BH3 complex is found to be extremely stable with the formation of a bridged hydrogen bond between SiH4 and BH3. Hence, it 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. In an attempt to unravel this role and the underlying reasons responsible for the stability of this complex, we have carried out a detailed analysis based on the structure and molecular orbitals. The results indicate that the binding strength and electronic character of the bridged hydrogen bond in the SiH4−BH3 complex is between those observed in double hydrogen bridged B2H6 and mono-hydrogen bridged anion B2H7-. In contrast to B2H6 and B2H7-, it should be noted that the complex is stabilized by direct strong electrostatic interaction between the positively charged Si atom and the ...

The nature of the interaction of paramagnetic atoms A (=4N,4P,3O,3S) with π systems and C60 in the A⋅⋅⋅C6H6 complexes and endohedral fullerenes (A@C60) has been investigated employing second-order Moller–Plesset perturbation level of theory and density functional theory calculations. The coupled cluster theory with single and double excitations, and with perturbed triplet excitations have also been carried out for the A⋅⋅⋅C6H6 complexes. The calculated geometries indicate that the paramagnetic N and P atoms lie on the C6 axis of benzene in the A⋅⋅⋅C6H6 complex and at the center of the C60 cage in A@C60. On the other hand, the O and S atoms are slightly shifted towards the C–C bond of benzene in the A⋅⋅⋅C6H6 complex. A comparison of the calculated binding energies (BEs) of these paramagnetic complexes and the corresponding rare gas complexes like He⋅⋅⋅C6H6 and He@C60 indicate that the BEs of the former are much larger than those of the latter. For both the rare gas and paramagnetic atom complexes dispersiv...

During the past decade, there has been much debate about the enormous catalytic rate enhancement observed in enzymatic reactions involving carbanion intermediates. Our recent theoretical study has demonstrated the importance of the short strong hydrogen bond (SSHB) in the enzymatic reactions. Nevertheless, other recent theoretical studies espouse the role of preorganization over that of the SSHB. To achieve a consensus on this issue and to find the truth, a more clarified explanation must be given. To this end, we have carried out an elaborate analysis of these enzymatic reactions. We here clarify that the catalytic mechanism needs to be explained with three important factors, viz., SSHB, preorganization, and charge buffering/dissipation. Since the charge buffering role is different from the commonly used concepts of the SSHB and preorganization (unless these definitions are expanded), we stress that the charge buffering role of the catalytic residues is an important ingredient of the enzymatic reaction in reducing the level of accumulation of the negative charge on the substrate during the reaction process. This charge reduction is critical to the lowering of activation barriers and the stabilization of intermediates.

The possible utilization of [n]beltenes as a new family of ionophores, which exhibit a cation-olefinic pi type of interaction in contrast to the cation-aromatic pi type of interaction exhibited by [n]collarenes, has been investigated using both ab initio calculations and molecular dynamic simulations. Like [n]collarenes, n ethene groups are linked by -CH(2)- linkages in the [n]beltenes. Our calculations indicate that these [n]beltenes exhibit strong binding affinities and high selectivity for alkali metal cations ([5]beltene to Li(+), [6]beltene to Na(+), [7]beltene to Na(+) and K(+), [8]beltene to K(+) and Rb(+), and [9]beltene to Cs(+) and Rb(+)). Compared to [n]collarenes, [n]beltenes are expected to have a finer ion selectivity because their cavity sizes can be varied with integral number n, while that of the former can be varied with an even number n. Suitable substituents could be employed to enhance both the binding and specificity of various sizes of [n]beltenes to different cations, as well as to increase the solubility.

Atomically precise nanoclusters, with sizes ranging from about ten to a few hundred atoms, are a new class of nanocatalysts that have been gaining prominence in the past decade as seen by increasing number of publications including review articles. This is especially true in the case of atomically precise gold nanocatalysts. In this chapter, we review this field starting with an introduction followed by highlighting synthesis and catalytic applications of the atomically precise gold clusters, an overview of their electronic and magnetic structure, and opportunities for correlation of the electronic and magnetic structure of the clusters with their catalytic activity. Especially new insights into the magnetic properties of these atomically precise catalysts are presented paving the way toward potentially chemically turn-on and tune-in their magnetism and thereby providing an opportunity not only to tailor-make atomically precise nanomagnetic clusters but also to investigate magnetic structure dependent catalysis. In general, we believe that correlation of the atomic structure of these atomically precise nanoclusters with their catalytic properties will provide significant guidelines for the future design of catalysts, with atomic precision, for any specific chemical reaction.

Two pentacoordinate mononuclear iron carbonyls of the form (bdt)Fe(CO)P2 [bdt = benzene-1,2-dithiolate; P2 = 1,1'-diphenylphosphinoferrocene (1) or methyl-2-{bis(diphenylphosphinomethyl)amino}acetate (2)] were prepared as functional, biomimetic models for the distal iron (Fe(d)) of the active site of [FeFe]-hydrogenase. X-ray crystal structures of the complexes reveal that, despite similar ν(CO) stretching band frequencies, the two complexes have different coordination geometries. In X-ray crystal structures, the iron center of 1 is in a distorted trigonal bipyramidal arrangement, and that of 2 is in a distorted square pyramidal geometry. Electrochemical investigation shows that both complexes catalyze electrochemical proton reduction from acetic acid at mild overpotential, 0.17 and 0.38 V for 1 and 2, respectively. Although coordinatively unsaturated, the complexes display only weak, reversible binding affinity toward CO (1 bar). However, ligand centered protonation by the strong acid, HBF4·OEt2, triggers quantitative CO uptake by 1 to form a dicarbonyl analogue [1(H)-CO](+) that can be reversibly converted back to 1 by deprotonation using NEt3. Both crystallographically determined distances within the bdt ligand and density functional theory calculations suggest that the iron centers in both 1 and 2 are partially reduced at the expense of partial oxidation of the bdt ligand. Ligand protonation interrupts this extensive electronic delocalization between the Fe and bdt making 1(H)(+) susceptible to external CO binding.

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.

The van der Waals complexes benzene-argon (BAr), fluorobenzene-argon (FAr), p-difluorobenzene-argon (DAr) are investigated at the second-order Moller–Plesset (MP2) level of theory using the 6-31+G(d), cc-pVDZ, aug-cc-pVTZ, and [7s4p2d1f/4s3p1d/3s1p] basis sets. Geometries, binding energies, harmonic vibrational frequencies, and density distribution are calculated where basis set superposition errors are corrected with the counterpoise method. Binding energies turn out to be almost identical (MP2/[7s4p2d1f/4s3p1d/3s1p]: 408, 409, 408 cm−1) for BAr, FAr, and DAr. Vibrationally corrected binding energies (357, 351, 364 cm−1) agree well with experimental values (340, 344, and 339 cm−1). Symmetry adapted perturbation theory (SAPT) is used to decompose binding energies and to examine the influence of attractive and repulsive components. Fluorine substituents lead to a contraction of the π density of the benzene ring, thus reducing the destabilizing exchange-repulsion and exchange-induction effects. At the same ...

The nature and origin of the pi-H interaction in both the ethene (olefinic) and benzene (aromatic) complexes of the first-row hydrides (BH(3), CH(4), NH(3), H(2)O, and HF) has been investigated by carrying out high level ab initio calculations. The results indicate that the strength of the pi-H interaction is enhanced as one progresses from CH(4) to HF. Unlike conventional H-bonds, this enhancement cannot be simply explained by the increase in electrostatic interactions or the electronegativity of the atom bound to the pi H-bonded proton. The contributions of each of the attractive (electrostatic, inductive, dispersive) and repulsive exchange components of the total binding energy are important. Thus, the inductive energy is highly correlated to the olefinic pi-H interaction as we progress from CH(3) to HF. On the other hand, both electrostatic and inductive energies are important in the description of the aromatic pi-H interaction. In either case, the contribution of dispersion energies is vital to obtain an accurate estimate of the binding energy. We also elaborate on the correlation of various interaction energy components with changes in geometries and vibrational frequencies. The red-shift of the nu(Y-H) mode is highly correlated to the inductive interaction. The dramatic increase in the exchange repulsion energies of these pi complexes as we progress from CH(4) to HF can be correlated to the blue-shift of the highly IR active out-of-plane bending mode of the pi system.

The optimal structures, binding energies, and harmonic vibrational frequencies of clusters containing a substituted benzene molecule microsolvated by four water molecules, termed as π-(water tetramer) clusters (π: p-difluorobenzene, fluorobenzene, benzene, toluene) have been evaluated at the second order perturbation level of theory (MP2) using both the 6-31+G* and aug-cc-pVDZ basis sets. In sharp contrast to the complexes of smaller water clusters with these π systems, wherein the water subcluster is most strongly bound to toluene, the water tetramer is most strongly bound to fluorobenzene. This exceptionally high binding energy results from both a π⋅⋅⋅OH H-bond and a competing σ F⋅⋅⋅OH bond between the water tetramer moiety and the aromatic molecule. The magnitudes of the many-body energy terms and their contribution to the binding energies of these π-(water tetramer) systems indicates that the contributions of three- and higher-order terms are much smaller when compared to the neutral water clusters. The two-body terms associated with the π- and σ-type of interaction indicates that in both the fluorobenzene and p-difluorobenzene complexes, the increase in the size of the water cluster enhances the π-H-bonding interaction and weakens the σ F⋅⋅⋅H interaction. This observation is in consonance with the calculated and experimentally observed redshifts of the OH vibrational frequencies. Thus, with an increase in the size of a water cluster bound to the fluorinated π system, there is a lowering of the redshift induced by the σ F⋅⋅⋅H interaction and an increase in the redshift due to the π-H interaction. The calculated redshift of the π H-bonded OH mode is very much dependent on the basis set, with larger basis sets yielding shifts which are in better agreement with the experimentally determined shifts.

The results of a combined experimental and theoretical investigation of the molecular structure of p-cyclohexylaniline (pCHA) in the electronic ground and the first electronically excited state are reported. The experimental investigations are performed for the crystalline phase by X-ray diffraction for the first time and related to former gas phase results obtained by time-resolved rotational laser spectroscopy. The theoretical results, from new ab initio calculations at the MP2/6-31+G(d) and CIS/6-31+G(d) level of theory for the electronic ground and excited state, respectively, give an adequate description of the rotational constants as obtained by the gas phase experiments. Thus, a detailed comparison of the ab initio structure for the ground state with the X-ray structure is performed in order to ascertain differences in the molecular geometry between the gas and crystalline phase. In particular, the size of the aromatic and cyclohexyl ring, their mutual orientation, and the conformation of the NH2 g...