Chemical Physics Letters (v.538, #C)

Contents (iii-ix).

Observation and assignment of the 2 1 ∏ g state of Rb 2 by Xiaomin Han; Xinhe Wang; Tao Wang; Wei Xiong; Xingcan Dai (1-4).
Display Omitted► We observed the 2 1 ∏ g state of Rb2 using infrared double resonance method. ► The absolute vibrational number is assigned. ► The molecular constants and the RKR potential curve are obtained.The 2 1 ∏ g state of Rb 2 has been observed using the infrared infrared double resonance method. The low part of the potential curve of the state is characterized for the first time. The absolute vibrational numbers are determined with isotope effect. The molecular constants of this state are obtained from a Dunham fit and the potential curve of the state is constructed using the Rydberg–Klein–Rees method.

Variations in the structures and binding energies of binary complexes with HBO by Janet E. Del Bene; Ibon Alkorta; Goar Sanchez-Sanz; José Elguero (5-9).
Binary complexes of HBO illustrating various bonding motifs.Display Omitted► Complexes of HBO with A may exhibit three types of structures. ► Dipole moment of A and interaction type influence structures and binding energies. ► HBO may act as a weak proton-donor acid or a relatively strong base. ► HBO forms parallel complexes with some A.Ab initio MP2/aug’-cc-pVTZ calculations have been carried out to determine the structures and binding energies of binary complexes formed by HBO with a series of small molecules A. Three different types of structures have been identified, which depend on the nature of A. In one structure A:HBO, HBO acts as a weak proton donor. In the second HBO:A, HBO is a relatively strong base. The third type of complex A||HBO has HBO and A in an approximately parallel arrangement. The dipole moment of A influences both the type of complex formed and its binding energy.

What is the mechanism of the OSO ring formation in sulfur tetroxide (SO4(C2v)) molecule? by Moein Goodarzi; Morteza Vahedpour; Mohammad Solimannejad (10-13).
Display Omitted► The mechanism of the OSO ring formation in SO4(C2v) molecule has been investigated, theoretically. ► There are no favorable paths in the SO2  + O2(3 ∑ g - ), SO2  + O2(1Δg) and O(3P) + SO3 reactions. ► The O(1D) + SO3 reaction is suitable for the OSO ring formation on the singlet PES.The mechanism of SO2  + O2 and O + SO3(D3h) reactions have been investigated at the MP2/6-31 + G(d) and CCSD(T)/cc-pV(Q + d)Z//MP2 levels on the triplet and singlet PESs. Although, no stable collision complexes have been found for the SO2  + O2(3 ∑ g - ), O(3P) + SO3(D3h) and O(1D) + SO3(D3h) reactions, 1IN(O2S–O2) has been considered on the singlet PES for the SO2  + O2(1Δg) reaction. The results show that there are no favorable paths for the OSO ring formation of SO4(C2v) in the atmospheric reactions of the SO2  + O2(3 ∑ g - ), SO2  + O2(1Δg) and O(3P) + SO3(D3h) while, the O(1D) + SO3(D3h) reaction can be suitable for the SO4(C2v) formation on the singlet PES.

Homo- and heterochiral dimers (PHFX)2, X = Cl, CN, CH3, NC: To what extent do they differ? by Janet E. Del Bene; Goar Sanchez-Sanz; Ibon Alkorta; José Elguero (14-18).
The C2 and Ci dimers of (PHFCl)2 with Cl–P⋯P–Cl linearDisplay Omitted► Optically active molecules PHFX give rise to homo- and heterochiral dimers (PHFX)2. ► Properties of (PHFX)2 depend primarily on X and Z in the linear Z–P⋯P–Z alignment. ► Binding energies of corresponding C2 and Ci dimers differ by only 1–3 kJ/mol. ► P–P distances in Ci isomers tend to be shorter than in corresponding C2 isomers. ► 1pJ(P–P) coupling constants fail to distinguish between corresponding isomers.Ab initio MP2/aug’-cc-pVTZ calculations have been performed to determine if intermolecular P–P distances, Z–P–P angles, binding energies, 31P chemical shieldings, or EOM–CCSD spin–spin coupling constants can differentiate between corresponding C2 (homochiral) and Ci (heterochiral) dimers (PHFX)2, X = Cl, CN, CH3, NC. With one exception, Ci isomers have shorter P–P distances than corresponding C2 isomers. Neither binding energies, Z–P–P angles, chemical shieldings, nor spin–spin coupling constants 1pJ(P–P) exhibit patterns which distinguish between corresponding C2 and Ci isomers. 1pJ(P–P) values correlate linearly with P–P distances, so that experimental values of 1pJ(P–P) could be used to extract intermolecular P–P distances.

Oxidation processes of chromium dimer and trimer cations in an ion trap by Tomonori Ito; Kazuhiro Egashira; Koichi Tsukiyama; Akira Terasaki (19-23).
Display Omitted► Reaction of Cr 2 + and Cr 3 + with O2 molecules in an ion trap. ► Measurements of reaction kinetics. ► Identification of reaction rates, products, and pathways.Oxidation processes of chromium cluster cations, Cr 2 + and Cr 3 + , are investigated by kinetics measurements under multiple-collision conditions by using a temperature-controlled ion trap. Both of the reactants commonly generate Cr+ and CrO+ as final products. However, the major reaction pathway to CrO+ presents a sharp contrast; Cr 2 + produces it directly by a single O2 collision, whereas Cr 3 + does via an intermediate Cr2O+. Comparison of the kinetics of the intermediate with that of cold Cr2O+ generated in the cluster source reveals that the intermediate Cr2O+ is thermalized promptly with the buffer gas filling the trap.

Dimerization of HCN in the gas phase: A theoretical mechanistic study by Min Kyoung Yim; Joong Chul Choe (24-28).
Display Omitted► Theoretical (CBS-QB3/APNO) reaction pathways for the dimerization of HCN. ► Thermal formation of iminoacetonitrile from 2HCN is not efficient. ► Proton catalysis significantly lowers the dimerization barriers.Potential energy surfaces for the formation of covalently bound HCN dimers from two molecules of HCN or HNC were determined from CBS-QB3/APNO calculations. Several novel pathways, with and without the aid of protons, were found for the formation of iminoacetonitrile (HN=CHCN), an intermediate in adenine synthesis from HCN by oligomerization. Covalent C–C or C–N bonds between the two monomers were formed after rearrangement of bimolecular complex intermediates. Energetic and kinetic analyses suggest that the proton-catalyzed dimerizations substantially lower reaction barriers but cannot occur efficiently under interstellar conditions.

Display Omitted► Shielding of 129Xe in H-bonded-like A–H⋯Xe dimers depends on A–H acidity. ► Isotropic 129Xe shielding follows its tensor anisotropy. ► The highest shielding tensor component is parallel to H⋯Xe. ► Additional, strong 129Xe deshielding is predicted for protonated Xe.H-bonding-like interactions between Xe and twelve A–H acids is examined by analysis of calculated A–H⋯Xe geometry and the 129Xe shielding. It was shown that δ(129Xe) is proportional to the 129Xe shielding anisotropy in most cases, generally increases with the A–H proton deshielding and depends on the A–H acidity (PA) on two ways: for stronger and weaker acids δ(129Xe) increases and decreases with PA, respectively. The highest contribution to the δ(129Xe) tensor variations comes from its principal components perpendicular to the H⋯Xe bond. These components are also sensitive on side interactions in non-linear dimers.

On the relation between hydrogen bonds, tetrahedral order and molecular mobility in model water by Rodolfo Guillermo Pereyra; Aleida J. Bermúdez di Lorenzo; David C. Malaspina; Marcelo A. Carignano (35-38).
Display Omitted► H-bond dynamics and tetrahedral order for SPC/E, TIP4P-Ew, TIP5P-Ew and Six-Site water. ► Explicit lone pairs sites favor tetrahedral order and slowdown dynamics at low T. ► Difference between models with and without lone pairs is very important at low T.We studied by molecular dynamics simulations the relation existing between the lifetime of hydrogen bonds, the tetrahedral order and the diffusion coefficient of model water. We tested four different models: SPC/E, TIP4P-Ew, TIP5P-Ew and Six-Site, these last two having sites explicitly resembling the water lone pairs. While all the models perform reasonably well at ambient conditions, their behavior is significantly different for temperatures below 270 K. The models with explicit lone-pairs have a longer hydrogen bond lifetime, a better tetrahedral order and a smaller diffusion coefficient than the models without them.

Display Omitted► The triplet–triplet annihilation in mixed Langmuir–Blodgett films with coumarin dye. ► Thermal activation process of TTA. ► Time and temperature dependence of magnetic field effects on delayed fluorescence. ► Dominance into process of TTA of contribution of ordered clusters or randomly oriented molecules clusters.The triplet–triplet annihilation (TTA) was studied in mixed Langmuir–Blodgett films of 7-decyloxy-3 (4′-ethoxyphenyl)-coumarin molecules and palmitic acid. The thermal activation of the TTA process is observed due to the inhomogeneous broadening of triplet energy levels. Modulation of the TTA rate constant in the external magnetic field for the multilayer films shows the dependence that is characteristic of crystals. Only negative magnetic effect is observed for the monolayers. Time dependence of the magnetic effect in conditions of high rate constant of triplet excitons migration is connected with the dominating contribution of ordered clusters or randomly oriented molecule clusters into the TTA process.

Quasi-free electron energy in near critical point helium by Yevgeniy Lushtak; Samantha B. Dannenberg; C.M. Evans; G.L. Findley (46-49).
Display Omitted► Field enhanced photoemission (FEP) is used to experimentally determine V 0(ρ) in He. ► V 0(ρ) in He from low density to the gas/liquid He (I)/liquid He (II) triple point is obtained. ► Critical point effect in V 0(ρ) in He is presented for the first time. ► Local Wigner–Seitz approach, developed for attractive fluids, is shown to model V 0(ρ) in He.We present for the first time the quasi-free electron energy V 0(ρ) in helium from low density to the density of the triple point liquid (gaseous helium/liquid helium I/liquid helium II triple point), on a noncritical isotherm and on an isotherm near the critical temperature. These data were obtained using field enhanced photoemission, a new experimental technique for the characterization of dense fluids. A novel critical point effect in helium is presented and is accurately fitted to the local Wigner–Seitz model. This model, which was developed for attractive fluids, is thereby shown to be applicable to repulsive fluids as well.

Display Omitted► We model properties of hydrocarbon + alcohol mixtures. ► The experimental data are reproduced well even with rough assumptions. ► Assuming only dimerization reproduces the system compressibility.A model assuming that equilibrium reaction of formation of non-compressible dimers dominates in and governs the acoustic properties of liquid mixtures of alcohols in inert hydrocarbon solvents was developed. It was found to describe satisfactorily the observed compressibility dependences on composition of the title systems, in particular those containing long-chained components. This attempt allows to estimate the dimerization equilibrium constants from the compressibility data.

Solvent effects on reactivity properties of dicarba-closo-dodecarboranes isomers by G.M.A. Junqueira; L.C. Rocha; V.T. Cotta; E.T. César (54-59).
Display Omitted► We study reactivity properties of dicarba-closo-dodecarboranes isomers. ► We calculated reactivity indexes in gas phase and in solution. ► We used explicit molecules (S-MC/QM) and continuum model (PCM) to include the solvent in calculations.In this Letter, a discussion on global reactivity indexes – electronic chemical potential (μ), chemical hardness (η) and electrophilicity (ω) – for dicarba-closo-dodecarboranes isomers, in gas phase and aqueous solution is reported. A sequential Monte Carlo/quantum mechanics methodology (S-MC/QM) was used in simulations. The results obtained showed the solvent must be considered on reactivity properties of the studied compounds. Generally, the solvated isomers become softer (η decreases) and the μ increases comparing to gas phase values, indicating the charge transfer from solvent to the solute.

Car–Parrinello molecular dynamics simulations of Na+ solvation in water, methanol and ethanol by Yongping Zeng; Junmei Hu; Yu Yuan; Xiaobin Zhang; Shengui Ju (60-66).
Structure and dynamics properties of Na+ in water, methanol and ethanol.Display Omitted► CPMD is used to investigate the properties of Na+ in solutions. ► The structure properties are in reasonable agreement with experiment. ► Reorientational correlations and translational dynamics in these systems is affected in a small degree by molecular size.Car–Parrinello molecular dynamics simulations have been performed on Na+ in water, methanol and ethanol. The structure of the first solvation shell around the Na+ is in a good agreement with the experimental data. It has been found the Na+ has a coordination number (5.13) in water greater than that in methanol and ethanol. However, the decay of orientational correlations for solvation shell methanol and ethanol molecules shows slower relaxation compared to that of bulk. Translational dynamics of Na+ in these systems is affected in a small degree by molecular size. It produces a significant difference on their relative relevance bands.

Theoretical study of 3,3′ substitution of 9,9,9′,9′-tetramethyl-fluorene-dimers by Cristina A. Barboza; Ramiro Arratia-Pérez; Desmond MacLeod Carey (67-71).
Display Omitted► We study the substitution with electron donor and attracting groups in fluorene dimers. ► Optical properties can be tuned by the substitution of the fluorene backbone. ► Electronic excitations are assigned mainly to π → π transitions. ► Linear correlations were found among E HOMO, E HOMO–LUMO and μ versus σ p.The effect of the 3,3′ substitution in 9,9,9′,9′-tetramethyl-fluorene-dimers with electron donor and withdrawing groups was analyzed. Ground state potential energy surfaces were obtained at DFT level using B3LYP/6-31+G(d,p). All studied dimers are nonplanar at their electronic ground states. The electronic transitions were investigated through the time-dependent-DFT method at their optimized ground states. The chemical potential (μ) as well as the HOMO and LUMO eigenvalues were plotted against the Hammet parameters, showing a good linear correlation, giving us insights about the modulation of the electronic properties, e.g. HOMO–LUMO gap, by means of the functionalization of fluorene dimers at strategical positions.

Mass-production of highly-crystalline few-layer graphene sheets by arc discharge in various H2–inert gas mixtures by Yani Chen; Hongbin Zhao; Leimei Sheng; Liming Yu; Kang An; Jiaqiang Xu; Yoshinori Ando; Xinluo Zhao (72-76).
Display Omitted► Gram-scale graphene sheets were produced by using hydrogen arc discharge method. ► Few-layered graphene sheets have high crystalline structure. ► Arc-discharged graphene sheets were used as anodes of lithium-ion batteries. ► The graphene sheets anodes show excellent cyclic performance.Large-scale production of graphene sheets has been achieved by direct current arc discharge evaporation of pure graphite electrodes in various H2–inert gas mixtures. The as-prepared few-layer graphene sheets have high purity, high crystallinity and high oxidation resistance temperature. Their electrochemical characteristics have been evaluated in coin-type cells versus metallic lithium. The first cell discharge capacity reached 1332 mA h g−1 at a current density of 50 mA g−1. After 350 cycles, the discharge capacity still remained at 323 mA h g−1. Graphene sheets produced by this method should be a promising candidate for the electrode material of lithium-ion batteries.

Display Omitted► Porous channel for effective dye adsorption by N+ ion-beam treatment. ► Effectively tailored surface properties of TiO2 film. ► Retardation of charge recombination in dye sensitized solar cells. ► Enhanced conversion efficiency of DSSCs by N+ ion-beam treatment.The photovoltaic performances and charge recombination of the dye sensitized solar cells (DSSCs) using N-modified TiO2 photoelectrodes were investigated under different ion-beam conditions. N-modified TiO2 films exhibited significantly enhanced BET surface area of 92.19 m2/g with the pore volume of 0.283 cm3/g compared to the surface area of 56.65 m2/g with a pore volume of 0.141 cm3/g of pristine TiO2 film. The interstitial and substitutional affixations of nitrogen atom inside the TiO2 lattice were confirmed by XPS and Raman spectroscopy. A 26% improvement in conversion efficiency was achieved by using N+ ion-beam treated TiO2 photoelectrode.

Temperature dependence of average relaxation times of PnAMA for mechanical α′ and α relaxations. The α relaxations are fitted by a single corresponding VFT equation, while the α′ relaxations are fitted by two VFT equations, indicating the presence of the dynamic crossover of α′ relaxation.Display Omitted►Relaxation behavior of poly(n-alkyl methacrylates) was investigated by the high precision shear mechanical spectroscopy. ► The dynamic crossover of the sub-Rouse modes above glass transition in poly(n-alkyl methacrylates) was observed and compared. ► The crossover relaxation time is essentially independent of side chain length and constant. ► The crossover is supposed to be the distinct variation of intermolecular coupling and cooperativity at T B.The dynamic crossover of the sub-Rouse modes (a transition from one Vogel–Fulcher–Tamman law to another in relaxation map) above glass transition in poly(n-alkyl methacrylates) (PnAMA) was observed and compared for the first time. The α and α′ relaxations become better resolved from each other with increasing side chain length, while the crossover relaxation time is 10−0.5 ± 0.5  s, essentially independent of side chain length of the PnAMA, indicating the universal presence of the sub-Rouse modes in PnAMA. Analysis of the mechanical spectra shows that the crossover originates from change of intermolecular coupling of the sub-Rouse modes when temperature crosses T B.

Display Omitted► We study the reaction efficiency of diffusion-controlled processes on surfaces. ► We consider finite, aperiodic planar lattices and supramolecular architectures. ► We consider the irreversible reaction A + B → C on a finite, discretized d  = 2 surface. ► A lattice-statistical model is developed to quantify the reaction efficiency. ► Aperiodic or regular surfaces better optimize surficial reactions than Ω  = 0 hosts.Supramolecular architectures provide a reproducible template on which surficial processes can be studied. We consider the irreversible reaction A + B → C where B is a stationary reaction center and A is a coreactant diffusing on a finite, discretized d  = 2 dimensional surface of a supramolecular assembly. A lattice-statistical model is developed to quantify how the reaction efficiency changes when the template is planar, Euler characteristic Ω  = 0, or wrapped on the surface of a d  = 3 host, Ω  = 2. We find that for aperiodic or regular surfaces of finite spatial extent, dispersed Ω  = 2 assemblies better optimize surficial reactions than Ω  = 0 planar hosts.

Thermodynamics of benzene adsorption on oxidized carbon nanotubes – experimental and simulation studies by Marek Wiśniewski; Sylwester Furmaniak; Piotr Kowalczyk; Karolina M. Werengowska; Gerhard Rychlicki (93-98).
Display Omitted► The thermodynamics of benzene adsorption on carbon nanotubes is studied. ► The series of systematic modified nanotubes is analyzed. ► The simple model of surface heterogeneity is proposed. ► The results of computer simulations reproduce observed experimentally regularities.We studied the thermodynamics of benzene adsorption on a series of oxidized closed-ended multiwalled carbon nanotubes at 298 K. Combined experimental and simulation results showed the significant effect of the surface heterogeneity on the benzene adsorption enthalpy and entropy at low surface coverages. For oxidized carbon nanotubes and low benzene uptakes, the differential entropy of the adsorbed phase is close to the differential entropy of solid benzene. Therefore, benzene molecules interacting with surface heterogeneities are ordered in quasi-solid structures at 298 K. At higher surface coverages, the ordering and packing of benzene molecules is liquid-like.

Strain sensing mechanism of the fabricated ZnO nanowire-polymer composite strain sensors by Wenjie Mai; Zhiwen Liang; Long Zhang; Xiang Yu; Pengyi Liu; Hanming Zhu; Xiang Cai; Shaozao Tan (99-101).
Display Omitted► A facile method was developed to fabricate ZnO nanowire-polymer composite strain sensors. ► The response to strain was attributed to Schottky barrier height changes. ► Thermionic emission–diffusion was the dominant process in this kind of devices.The nature of contacts between ZnO semiconductor nanowires and metal pads has attracted great research interests since its impact on enhancing sensitivity of the sensors. Herein, ZnO nanowire arrays were synthesized by a scalable wet chemical method and ZnO nanowire-polymer composite strain sensors were fabricated by utilizing Schottky contact. The electrical transport response to strain was attributed to piezoelectric-effect-induced Schottky barrier height changes at the metal–semiconductor junction under various strained states. When the strain increased from 0% to 0.12%, the average barrier height change increased to 26 meV, inducing sensitive current changes in these strain sensors.

Photoelectrochemical responses of silicon nanowire arrays for light detection by Shao-long Wu; Ting Zhang; Rui-ting Zheng; Guo-an Cheng (102-107).
Display Omitted► A light detector based on radial heterostructures of 1D semiconductor nanostructures is proposed. ► The detector performs high collection efficiency of photogenerated carriers. ► Photoelectrochemical responses of Si nanowires with different morphologies are investigated. ► The photoresponsivity of the black sample is 73 times larger than that of the planar silicon at 0.2 V.This Letter presents a novel low-cost light detector based on radial silicon nanowire heterostructures. Photoelectrochemical responses of silicon nanowire arrays with different morphologies, prepared by a metal assisted chemical etching method, have been systematically investigated. The photoresponsivity of the black silicon nanowire arrays is 73 times larger than that of the planar silicon at 0.2 V bias under white light illumination. The relationship between the photoresponsivity and nanowire morphology and doping level is discussed. Our results demonstrate that the heterostructures of the nanowire arrays with an optimized morphology are promising for light detection and other optoelectronic devices.

N-SWCNTs production by aerosol-assisted CVD method by Antal A. Koós; Frank Dillon; Rebecca J. Nicholls; Lyubov Bulusheva; Nicole Grobert (108-111).
Display Omitted► We were able to produce N-SWCNTs with a nitrogen content of ca. 1 at.%. ► Yields were higher than 10 mg/h. ► We used a cheap and scalable method which does not require catalyst preparation step.The insertion of dopant atoms into the carbon nanotube framework allows one to tailor their electronic properties, reactivity and structure. We investigated the effect of the reaction parameters on the yield and quality of N-doped single-walled carbon nanotubes (N-SWCNTs) produced via aerosol chemical vapour deposition. In this context, aerosols of ethanol/benzylamine mixtures in conjunction with ferrocene were pyrolysed at temperatures between 950 and 1100 °C. As a result, we were able to produce N-SWCNTs with a nitrogen content of ca. 1 at.%, diameters ranging between 0.9 and 1.8 nm, and production rates of more than 10 mg/h.

Display Omitted► Interaction between graphene and nickel surface with various orientations is examined. ► The Rosei and other hollow structures give rise to global and local energy minima. ► Interaction energy is almost constant for incommensurate structures. ► A few degrees rotation from Rosei structure diminishes energy barrier for translation.The interaction energy between a rigid defect-free graphene sheet and a fixed Ni(1 1 1) surface with respect to rotational and translational displacements is examined using an empirical bond order potential. The Rosei (hollow site) structure, and others in which metal atoms are adjacent to hollow sites in graphene, give rise to global and local energy minima, respectively, whereas the interaction energy is almost constant for incommensurate structures. Only a few degrees rotation from the Rosei structure substantially diminishes the energy barrier for translational displacement.

Display Omitted► Enthalpy–entropy compensation (EEC) has a definite physical sense. ► Here, we review EEC from a new standpoint using the notion of correlation. ► We rationalize EEC in terms of hidden, but physically real factors.Enthalpy–entropy compensation (EEC) has a definite physical sense. Here, we review EEC from a new standpoint, using the notion of correlation. The latter has two basic meanings: (a) ‘A’ is correlated to ‘B’ means ‘A’ results from ‘B’ or vice versa; (b) this same means there is some real, but hidden ‘C’ in connection to both ‘A’ and ‘B’). In accordance with the interpretation (b), we try rationalizing EEC in terms of hidden, but physically real factors.

Display Omitted► We propose a double-hybrid density functional without fitting to accurate data. ► This new functional is shown to perform well for self-interaction problems. ► This new functional is shown to perform well for noncovalent interactions.Without the use of any empirical fitting to experimental or high-level ab initio data, we present a double-hybrid density functional approximation for the exchange–correlation energy, combining the exact Hartree–Fock exchange and second-order Møller–Plesset (MP2) correlation with the Perdew–Burke–Ernzerhof (PBE) functional. This functional, denoted as PBE0-2, is shown to be accurate for a wide range of applications, when compared with other functionals and the ab initio MP2 method. The qualitative failures of conventional density functional approximations, such as self-interaction error and noncovalent interaction error, are significantly reduced by PBE0-2.