Chemical Physics Letters (v.507, #4-6)

Contents (iii-vi).

Semiconductor nanostructure-based photoelectrochemical water splitting: A brief review by Yongjing Lin; Guangbi Yuan; Rui Liu; Sa Zhou; Stafford W. Sheehan; Dunwei Wang (209-215).
Display Omitted► We summarize research on using nanoscale semiconductors for solar water splitting. ► Covers an area that has not been reviewed previously in a concise fashion. ► A comprehensive comparison is provided in the form of a table for the first time.Recent efforts on solar water splitting by nanoscale semiconductor materials is reviewed. We show that innovations in materials’ morphologies can improve charge transport and thereby increase overall power conversion efficiencies. Nanostructures of varying complexities, from one-dimensional nanotubes, nanowires, and nanorods, to two-dimensional films and nanonets, and three-dimensional porous structures have been reported to exhibit superior performance. We also summarize recent successes in advancing the field by heterogeneous nanostructures, which make it possible to achieve combined functionalities not observed with single-component materials.

Absorption spectroscopy probes the first excited state (12B1) of phenyl. The singly occupied molecular orbital of this state is shown.Display Omitted► Rotationally resolved spectra are reported for the 1 2 B 1 - X 2 A 1 transition of phenyl. ► Rotational constants are determined for three vibronic states. ► The excited state lifetime is dependent on vibrational excitation. ► Relaxation occurs by rapid internal conversion to the ground state.Absorption spectra for the 1 2 B 1 - X ˜ 2 A 1 transition of the phenyl radical have been recorded using the technique of cavity ring-down spectroscopy. The radicals were produced by an electric discharge through Ar/C6H5Br mixtures and cooled by supersonic expansion. Partially resolved rotational structure was observed for the 0 0 0 , 9 0 1 , and 10 0 1 bands, and excited state rotational constants were derived from these data. For the 9 0 1 band, homogeneous line broadening was indicative of an excited state lifetime of 96 ps.

The role of aromatic π-bridges in push–pull-chromophores on the transparency-hyperpolarizability tradeoff by B. Christopher Rinderspacher; Jan W. Andzelm; Adam M. Rawlett; Joseph M. Dougherty; MyVan Baranoski; Matthew C. Davis (221-225).
Display Omitted► Choice of aromatic bridge coupled with the substitution pattern on the ring allows targeting specific spectral features (absorption or transparency windows). ► Asymmetric bridges (e.g., azulene) produce large hyperpolarizabilities. ► Zwitter-ionic mesomers rationalize hyperpolarizability and spectral trends.We present electronic structure calculations of the ultraviolet/visible (UV–Vis) spectra as well as hyperpolarizabilities (HP) of push–pull chromophores based on a donor-aromatic bridge-acceptor framework. As expected, both the color of the chromophores and the static hyperpolarizability are sensitive functions of modifications of the conjugated framework. Effects can be rationalized by determining zwitter-ionic mesomers of the bridge that can be stabilized by the donor or acceptor. The choice of bridge coupled with the substitution pattern on the ring allows targeting specific spectral features (absorption or transparency windows). Exceptionally large hyperpolarizabilities were found for the cyclopentadieno[1,2]-benz[4,5-b]cycloheptatriene bridge.

Free radicals produced from thermally-irradiated polyethylene polymers: An ion attachment mass spectrometric study by Yuki Kitahara; Masamichi Tsukagoshi; Seiji Takahashi; Toshihiro Fujii (226-228).
Display Omitted► Many free radicals are formed in the pyrolysis of polyethylene polymers. ► Gas-phase radical products are detected by Li+ ion attachment mass spectrometry. ► Mass Spectra show many radicals, such as C n H2 n +1 (n  = 5–12) and C n H2 n −1 (n  = 5–11).Many gas-phase hydrocarbon free radical species produced during thermal irradiation of polyethylene polymers have been studied in detail by means of Li+ ion attachment mass spectrometry (Li+IAMS). Spectra of the thermal decomposition products, detected by means of the Li+ ion attachment technique, clearly show that radicals, such as C n H2 n +1 (n  = 5–12) and C n H2 n −1 (n  = 5–11), were the predominant species produced. Many of these species have been identified for the first time by mass spectrometry. Our results present direct evidence that free radicals are formed in the pyrolysis environment and are detectable with mass spectrometric techniques.

Structural studies of the water pentamer by Frank Ramírez; C.Z. Hadad; Doris Guerra; Jorge David; Albeiro Restrepo (229-233).
Display Omitted►Exhaustive exploration of the water pentamer PES. ► Dipole–dipole interactions stabilize the clusters. ► 12 Structural patterns. ► 5 Motifs within 3 kcal/mol of the most stable. ► Topological analysis of the electron densities.A computational study of the water pentamer gas phase conformational space is reported in this Letter. Forty-four stationary points distributed among 12 structural patterns were located at the MP2/6–311++G(d,p) level. At least 5 geometrical motifs (25 structures) are predicted within 3 kcal/mol of the most stable conformation at the CCSD(T)/aug–cc–pVTZ//MP2/6–311++G(d,p) level. We show evidence that dipole–dipole interactions are at play in stabilizing the clusters. Electron densities and their Laplacians at the hydrogen bond critical points were found to be linearly correlated with relative energies for all clusters. Logarithmic relationships were found for the [ r eq , ρ ( r c ) ] and [ r eq , ∇ 2 ρ ( r c ) ] pairs in all hydrogen bonds.

Display Omitted► First time observed the inversion in fluorescence quenching of carmoisine by HV in methanol. ► HV shows inversion while MV shows saturation in quenching. Mode of quenching is static. ► HV forms larger aggregates but MV primarily forms smaller aggregates like dimers. ► Larger aggregates of HV are less efficient quenchers. In acetinitrile, no inversion is seen. ► Fluorescence quenching by viologens can be used to monitor their self-aggregation.Fluorescence quenching is an important technique having chemical and biochemical applications and it follows the well-known Stern–Volmer theory. It is interesting to explore anomalously behaving systems, especially when two structurally related quenchers are used. We have observed for the first time, a unique dependence of fluorescence quenching of an azo-dye, carmoisine, on diheptyl viologen (HV) concentration in neat methanol. It shows recovery of fluorescence at higher quencher concentrations. Initially, quenching increases followed by a maximum and at still higher concentrations fluorescence is recovered. But structurally related methyl viologen (MV) instead shows only saturation with the same dye. Finally, fluorescence quenching so reported here has been found to follow static mechanism.

The role of acetone dipole moment in acetone–water mixture by Rodolfo Guillermo Pereyra; Maria Lila Asar; Marcelo A. Carignano (240-243).
Display Omitted►Acetone/water solutions. ► Model acetone. ► Polarizability. ► Dipole moment. ► Molecular dynamics.We present a molecular dynamics simulation study of acetone–water solutions. We focus in the dependency of the excess enthalpy with the acetone molar fraction. We found that by gradually increasing the acetone dipole moment as the systems gets diluted, the simulations capture the correct behavior for the excess enthalpy as a function of acetone molar fraction and temperature. Our results suggest that, in order to reproduce the experimental data for the excess enthalpy, it is necessary to use force fields that include many body terms to account for the polarization of the acetone molecule.

Assessing the solvation mechanism of C60(OH)24 in aqueous solution by Cleiton Maciel; Eudes E. Fileti; Roberto Rivelino (244-247).
Because of the large molecular surface area and volume of C60 and C60(OH)24, their hydration entropies are rather similar. However, different from C60 the hydration of C60(OH)24 is guided by an enthalpy-driven process.Display Omitted► Fullerenol aqueous solution. ► Solvation of C60(OH)24 under ambient conditions. ► Molecular dynamics simulation combined with thermodynamic integration technique. ► The solvation free energy of this fullerenol is an extremely negative quantity.Using molecular dynamics simulations, combined with the thermodynamic integration algorithm, we examine the hydration mechanism of C60(OH)24 under ambient conditions. We analyze its structural features, dynamics, and hydration free energy. Our results have been compared with a pristine fullerene aqueous system. Despite the number of hydroxyl groups in the fullerenol, its hydration entropy is rather similar to that calculated for C60. On the other hand, we have calculated a dramatically negative free energy of about −354 kJ/mol for the fullerenol, whereas pure fullerene presents a positive value of about 59 kJ/mol. On this basis, our study indicates that the hydration of C60(OH)24 is guided by an enthalpy-driven process.

Effect of ethylene diamine (EDA) on the photoluminescence (PL) behavior of various sized thioglycolic acid (TLA) capped CdTe QDs were examined. The PL intensity as well as decay time of thioglycolic acid capped CdTe quantum dots (QDs) greatly enhanced by the interaction with EDA. The photoluminescence spectra, AFM image and decay studies have demonstrated that EDA has a great influence on photophysical properties of CdTe QDs. The observed effect of EDA on PL properties of CdTe QDs gives insight in the importance of the interaction between the QDs surface and ethylene diamine.Display Omitted► Optical properties of ethylene diamine modified CdTe quantum dots were studied. ► Ethylene diamine treatment enhanced the luminescence intensity. ► Luminescence enhancement is much more pronounced in case of smaller sized QDs.Thioglycolic acid (TGA) capped CdTe quantum dots (QDs) have been synthesized in aqueous media. The effect of diamine on luminescence property of TGA capped CdTe QDs has been investigated and it has been found that the diamine significantly affects the optical properties of CdTe QDs. Luminescence efficiency as well as lifetime of CdTe QDs have seen to be enhanced notably up to certain concentration of the ethylene diamine added and the observation is much more pronounced in case of smaller size QDs. The results indicate that the local surface-trap states of CdTe QDs reduced effectively by the EDA treatment.

Formation of nano-pores in nano-crystalline diamond films by Armin Kriele; Oliver A. Williams; Marco Wolfer; Jakob J. Hees; Waldemar Smirnov; Christoph E. Nebel (253-259).
Display Omitted► CVD growth conditions determines morphology and chemical composition of NCD films. ► CVD-nano-crystalline diamond can achieve stiffness close to single crystal diamond. ► Nano-pores can be created in NCD films etching the non-diamond phase. ► Porous nano-crystalline diamond films can be used as molecular filters. ► Thin NCD films are extreme robust and flexible.Various nano-pores in nano-crystalline diamond (NCD) thin films have been fabricated and characterized. Therefore in this work two aspects of NCD thin films synthesized by microwave assisted chemical-vapour-deposition (MWCVD) have been investigated. Firstly, the influence of CVD-growth conditions on the film morphology and chemical grain boundary composition and their impact on the mechanical properties. Second, the formation of nano-pores by selective etching of the non-diamond phase. Freestanding NCD membranes were fabricated and bulged to calculate the Young’s modulus which can reach surprisingly high values (1100 GPa) close to single crystal diamond. The presence of nano-pores was verified by electrochemical experiments where ions have been used to detect the porosity.

Structural and electronic properties of neutral and charged Ca8C12 metal carbides by Gang Chen; Qi Peng; Yoshiyuki Kawazoe (260-264).
The charge density distribution of the highest occupied molecular orbitals for the ground states of neutral, anionic, and cationic clusters shows the bonding between calcium and dicarbon.Display Omitted► Low-lying structures for Ca8C12 and Ca 8 C 12 + clusters prefer cage-like structures. ► A compact structure is prevailing for Ca 8 C 12 - cluster. ► The hollow cage-like structures are magnetic. ► The donation and back donation of valence electrons account for the Ca―C bonding. ► The net charge on Ca ion in the low-lying structures is around +1.4e.This Letter reports our detailed first-principles study on neutral and charged Ca8C12 clusters. Except a compact structure owning geometrical characters of the CaC2 bulk, all the other low-lying structures within 0.2 eV in relative total energy are hollow configurations. Most of the low-lying structures are calculated to be magnetic. The ground states are a D 3d flat structure, a compact structure, and a D 3d hollow cage-like structure for Ca8C12, Ca 8 C 12 - , and Ca 8 C 12 + clusters, respectively. The net charge on Ca ion, except the dangling one of the compact structure, is calculated to be around +1.4e.

Display Omitted► Simple practical approximations for diffusion-reaction systems in 2D are provided. ► The deviations for irreversible reactions are less than 1.6% at all times. ► The approximations are generalized for more complicated systems.We introduce simple practical approximations for various diffusion-influenced reactions in two dimensions, which are found to be superior to previously known approximations at long times. The approximations for the irreversible reactions with the Smoluchowski boundary condition are found to be in excellent agreement with the numerically exact results at all times. We generalize the long-time approximations to be applied to more complicated geminate diffusion–reaction systems such as the excited-state reversible binding and transfer reactions.

Reaction operators for spin-selective chemical reactions of radical pairs by J.A. Jones; Kiminori Maeda; P.J. Hore (269-273).
Display Omitted► Spin-selective reactions of radical pairs modelled. ► Conventional (Haberkorn) and quantum measurement approaches compared. ► Quantum measurement form is limiting case of Haberkorn model in multi-site models. ► Two models predict twofold difference in rate of singlet–triplet dephasing. ► Expected to be difficult to distinguish two approaches experimentally.Spin-selective reactions of radical pairs have traditionally been modelled theoretically by adding phenomenological rate equations to the quantum mechanical equation of motion of the radical pair spin density matrix. More recently an alternative set of rate expressions, based on a quantum measurement approach, has been suggested. Here we show how these two reaction operators can be seen as limiting cases of a more general reaction scheme.

Author Index (274-278).