Ultrafast Proton Transfer and Contact Ion-Pair Formation in Formic Acid Clusters.

The ultrafast proton transfer dynamics of homogeneous formic acid clusters (FA)n, n < 10, are investigated with femtosecond time-resolved mass spectrometry. We monitor the proton transfer pathway following Rydberg state electronic relaxation and find that successful ion pair formation increases logarithmically with cluster size. Ab initio calculations demonstrate similar excitation/relaxation behavior for each cluster, revealing a contact ion pair forms between two molecules composing the cluster before finally a formate anion (HCOO-) is dissociated by the probe pulse. The sub-ps time scale for rearrangement and proton transfer increases almost linearly with cluster size, requiring ∼67 fs per additional formic acid molecule and ranging from 213 ± 51 fs for the trimer to 667 ± 116 fs for FA9. The near-linear trends measured for both rearrangement lifetime and ion pair formation suggest that proton transfer is unlikely in the formic acid dimer but becomes prominent in small clusters.

Limited Formation of CO3+ through Strong-Field Ionization and Coulomb Explosion of Formic Acid Clusters.

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 C3+ and O3+ and also increasing the KER values to several hundred electronvolts in magnitude for such ions. At a lower laser intensity (3.8 × 1014 W/cm2), we record an enhancement in the signal of the (FA)5(H2O)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 CO3+ is more likely to arise from the molecular case. Thus, the relative production of CO3+ is reduced in comparison to the highly charged ions upon clustering and is likely due to the higher ionization levels achieved, which facilitate dissociation.

Production of Metastable CO3+ through the Strong-Field Ionization and Coulomb Explosion of Formic Acid Dimer.

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 × 1015 W/cm2 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 CO3+, in agreement with results from a classical Molecular Dynamics simulation of fully ionized formic acid dimers. Potential energy curves for CO3+ are calculated using the multireference configuration interaction (MRCI+Q) method to confirm the existence of an excited metastable 2Σ state with a significant potential barrier with respect to dissociation. This combined experimental and theoretical effort reveals the existence of metastable CO3+ through direct observation for the first time.

Electrochemical Capture and Release of Carbon Dioxide Using a Disulfide-Thiocarbonate Redox Cycle.

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 electrochemical behavior of a sulfur-bound terminal thiocarbonate.

Single-Molecule Conductance through Hydrogen Bonds: The Role of Resonances.

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.

Solvent Effects on the Dynamic Polarizability and Raman Response of Molecule-Metal Oxide Hybrid Clusters.

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.

Pseudocarbynes: Charge-Stabilized Carbon Chains.

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.

A nickel phosphine complex as a fast and efficient hydrogen production catalyst.

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.

Catalytic hydrogen evolution by Fe(II) carbonyls featuring a dithiolate and a chelating phosphine.

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.

Probing Raman enhancement in a dopamine-Ti2O4 hybrid using stretched molecular geometries.

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.

Probing the Nature of Charge Transfer at Nano-Bio Interfaces: Peptides on Metal Oxide Nanoparticles.

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.

Chemically induced magnetism in atomically precise gold clusters.

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.

Vibrational signatures of Watson-Crick base pairing in adenine-thymine mimics.

The vibrational fingerprints of hydrogen-bonding associated with the adenine-thymine (A-T) Watson-Crick (WC) base pair have been identified in an infrared study of the A-T mimics 4-aminopyrimidine-1-methylthymine (4APM-1MT) and 4-aminopyrimidine-6-methyl-4-pyrimidinone (4APM-M4PMN) in the gas-phase. The IR vibrational spectra were measured via a double resonance scheme utilizing femtosecond multiphoton ionization. The changes in the molecular structure, anharmonic vibrational parameters, and the assignment of the observed vibrational spectra in the NH/CH stretch region were investigated by carrying out high-level theoretical calculations of the anharmonic spectra. The experimental observations and theoretical calculations indicate that the hydrogen bonds associated with WC base-pairing are relatively stronger than those associated with reverse WC (rWC) base pairing. This is manifested in a more pronounced red-shift of the H-bonded vibrational modes associated with the WC as compared with the rWC base-pairing. An analysis of the factors contributing to the anharmonicity of the vibrational modes associated with H-bonding reveals that the magnitude of the off-diagonal anharmonic coupling of the H-bonded -NH2 stretch and the -NH2 bend is much smaller in WC base-pairing than in the corresponding rWC base-pairing. The chemical and biological implications of these results, especially in the context of using vibrational spectroscopy as a tool for identifying the signatures of nucleotide base vibrations is addressed.

Simple and accurate correlation of experimental redox potentials and DFT-calculated HOMO/LUMO energies of polycyclic aromatic hydrocarbons.

The ability to accurately predict the oxidation and reduction potentials of molecules is very useful in various fields and applications. Quantum mechanical calculations can be used to access this information, yet sometimes the usefulness of these calculations can be limited because of the computational requirements for large systems. Methodologies that yield strong linear correlations between calculations and experimental data have been reported, however the balance between accuracy and computational cost is always a major issue. In this work, linear correlations (with an R(2) value of up to 0.9990) between DFT-calculated HOMO/LUMO energies and 70 redox potentials from a series of 51 polycyclic aromatic hydrocarbons (obtained from the literature) are presented. The results are compared to previously reported linear correlations that were obtained with a more expensive computational methodology based on a Born-Haber thermodynamic cycle. It is shown in this article that similar or better correlations can be obtained with a simple and cheaper calculation.

CO2 Preactivation in Photoinduced Reduction via Surface Functionalization of TiO2 Nanoparticles.

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.

4-Aminobenzimidazole-1-methylthymine: a model for investigating Hoogsteen base-pairing between adenine and thymine.

We report the infrared spectrum of the 4-aminobenzimidazole-1-methylthymine (4ABI:1MT) heterodimer, detected by femtosecond multiphoton ionization. Based on calculations of both the harmonic and the anharmonic frequencies, the observed vibrational spectrum is assigned to a structure that mimics the Hoogsteen base pairing of adenine and thymine. A notable observation made in the course of this study is that there is a significant imbalance in the observed strengths of the H-bonds. While the N···H-N bond reveals a large red shift of >700 cm(-1) for the NH stretch frequency, the N-H···O bond is characterized by only a 50 cm(-1) shift. The importance of this observation in the formation of Hoogsteen duplexes by thymine-based oligonucleotides is discussed.

Photoinduced kinetics of SERS in bioinorganic hybrid systems. a case study: dopamine-TiO2.

The reported observation of SERS on semiconductors has confirmed the feasibility of distinguishing the charge-transfer mechanism from the electromagnetic one responsible for the enhancement of the signal in metal nanoparticles. Experimental investigation of the well characterized dopamine-TiO(2) system revealed an unexpected dependence on coverage and size. We propose here a theoretical model applicable to SERS on semiconducting substrates that explains this remarkable behavior. The model is based on a competition mechanism arising from the formation of an electron gas in the conduction band of the semiconductor due to the photoexcitation of a charge-transfer complex. Taking into account the two competing effects, a linear increase in the Raman intensity arising from increasing coverage and a quenching effect due to the photon absorption by the electron gas, provides excellent agreement between our model and the experiment for 5 nm nanoparticles. Discrepancies for the case of 2 nm nanoparticles are attributed to quantum confinement, an effect that is investigated elsewhere.

Pseudorotation in pyrrolidine: rotational coherence spectroscopy and ab initio calculations of a large amplitude intramolecular motion.

Pseudorotation in the pyrrolidine molecule was studied by means of femtosecond degenerate four-wave mixing spectroscopy both in the gas cell at room temperature and under supersonic expansion. The experimental observations were reproduced by a fitted simulation based on a one-dimensional model for pseudorotation. Of the two conformers, axial and equatorial, the latter was found to be stabilized by about 29 +/- 10 cm(-1) relative to the former one. The barrier for pseudorotation was determined to be 220 +/- 20 cm(-1). In addition, quantum chemical calculations of the pseudorotational path of pyrrolidine were performed using the synchronous transit-guided quasi-Newton method at the MP2 and B3LYP levels of theory. Subsequent CCSD(T) calculations yield the energy preference of the equatorial conformer and the barrier for pseudorotation to be 17 and 284 cm(-1), respectively.

Characterization of weak NH-pi intermolecular interactions of ammonia with various substituted pi-systems.

Among the several weak intermolecular interactions pervading chemistry and biology, the NH-pi interaction is one of the most widely known. Nevertheless its weak nature makes it one of the most poorly understood and characterized interactions. The present study details the results obtained on gas-phase complexes of ammonia with various substituted pi systems using both laser vibrational spectroscopy and ab initio calculations. The spectroscopic measurements carried out by applying one-color resonant two-photon ionization (R2PI) and IR-vibrational predissociation spectroscopy in the region of the NH stretches yield the first experimental NH stretching shifts of ammonia upon its interaction with various kinds of pi-systems. The experiments were complemented by ab initio calculations and energy decompositions, carried out at the second-order Møller-Plesset (MP2) level of theory. The observed complexes show characteristic vibrational spectra which are very similar to the calculated ones, thereby allowing an in-depth analysis of the interaction forces and energies. The interaction energy of the conformers responsible for the observed vibrational spectra has the maximum contribution from dispersion energies. This implies that polarizabilities of the pi-electron systems play a very important role in governing the nature and geometry of the NH-pi interaction. The larger polarizability of ammonia as compared to water and the tendency to maximize the dispersion energy implies that the characteristics of the NH-pi interactions are markedly different from that of the corresponding OH-pi interactions.