Chemical Physics Letters (v.663, #C)

Contents (iii-ix).

New mechanisms of macroion-induced disintegration of charged droplets by Styliani Consta; Myong In Oh; Anatoly Malevanets (1-12).
Display OmittedMolecular modeling has revealed that the presence of charged macromolecules (macroions) in liquid droplets dramatically changes the pathways of droplet fission. These mechanisms are not captured by the traditional theories such as ion-evaporation and charge-residue models. We review the general mechanisms by which macroions emerge from droplets and the factors that determine the droplet fission. These mechanisms include counter-intuitive “star” droplet formations and extrusion of linear macroions from droplets. These findings may play a direct role in determining macromolecule charge states in electrospray mass spectrometry experiments.
Keywords: Stability of charged droplets; Rayleigh limit; Dielectrics; Taylor cone; Electrospray ionization; Ejection of macroions; Charge residue model; Ion evaporation mechanism; Modeling of ejection mechanisms;

Display OmittedWe have derived an analytical formula which describes the field–dependent interface recombination velocity for the boundary of two materials characterized by different permittivities. The interface recombination of charge carriers has been considered in the presence of image force Schottky barrier. We suggest that this effect may play an important role in the loss of current for organic–inorganic hybrid heterojunctions. It has been proved that the presented method is a generalization of the Scott–Malliaras model of surface recombination at the organic/metal interface. We also discuss that this model is intuitively similar but not analogous to the Langevin mechanism of bulk recombination.
Keywords: Hybrid structures; Organic electronics; Recombination;

Theoretical study of the thermochemistry of chlorine oxyfluorides by Hernán R. Sánchez; Julián Del Pla (16-20).
Display OmittedThere is a lack of experimental thermochemical values for most chlorine oxyfluorides. Previous high level theoretical, CCSD(T), results showed uncommonly large errors in the standard heats of formation calculated through the atomization method. We propose that the differences are due to unusually large contributions to energy from higher excitations within the coupled cluster framework, and we tackle the problem by using a calculation scheme based on isodesmic reactions. Our suspicions are supported by results of static correlation diagnostics. Our final recommended values are in better agreement with the experimental data available. Other thermodynamic properties are also calculated.

Tunnel magnetoresistance of diamondoids by Yukihito Matsuura (21-26).
Display OmittedTunnel magnetoresistance (TMR) of diamondoids has been predicted by first principles density functional theory. Diamantane was used as a basic molecular proxy for diamondoids because hydrogen atoms in the apical position are easily substituted for a thiol group. The pristine diamantane exhibited a low TMR ratio of 7%, and boron-substitution considerably decreased the TMR ratio. Conversely, nitrogen-substitution enhanced the TMR ratio by up to 20%. Heteroatom-substitution changes the tunneling probabilities by varying the molecular bond lengths. Furthermore, when the spins of the electrodes are parallel, the heteroatoms resulted in transmittance probabilities at an energy range near the Fermi level. Consequently, heteroatom-substitution can control the TMR ratios of diamondoids very well.

Display OmittedIn this paper, we examined the structural, electronic and energetic data associated to the Mg-substitution in chlorophyll by three major toxic pollutants: Cd2+, Hg2+ and Pb2+. We evidenced a highly versatile bonding of the cations with the pheophytin ligand, with a strong out-of-plane distortion for Hg and Pb. The binding energies ranged from slightly stronger than Mg2+ in the case of Hg2+ to much smaller for Pb2+. Nevertheless, our various approaches of free cations solvation allowed us to evidence that Mg-substitution should be possible for all title elements.
Keywords: Chlorophyll; DFT; Solvation; Substitution;

A three-ions model of electrodiffusion kinetics in a nanochannel by Táňa Sebechlebská; Pavel Neogrády; Ivan Valent (33-39).
Display OmittedNanoscale electrodiffusion transport is involved in many electrochemical, technological and biological processes. Developments in computer power and numerical algorithms allow for solving full time-dependent Nernst-Planck and Poisson equations without simplifying approximations. We simulate spatio-temporal profiles of concentration and electric potential changes after a potential jump in a 10 nm channel with two cations (with opposite concentration gradients and different mobilities) and one anion (of uniform concentration). The temporal dynamics shows three exponential phases and damped oscillations of the electric potential. Despite the absence of surface charges in the studied model, an asymmetric current-voltage characteristic was observed.
Keywords: Ion transport; Electrodiffusion; Ion channel; PNP theory; Time-dependent Nernst-Planck-Poisson equations; Ion current rectification ICR;

Display OmittedWe study the ballistic conduction of a monolayer MoS2 subject to a spatially modulated magnetic field by using the Landauer-Buttiker formalism. The band structure depends sensitively on the field strength, and its change has profound influence on the electron conduction. The conductance is found to demonstrate multi-step behavior due to the discrete number of conduction channels. The sharp peak and rectangular structures of the conductance are stretched out as temperature increases, due to the thermal broadening of the derivative of the Fermi-Dirac distribution function. Finally, quantum behavior in the conductance of MoS2 can be observed at temperatures below 10 K.

A push-pull organic dye with a quinoidal thiophene linker: Photophysical properties and solvent effects by Clàudia Climent; Abel Carreras; Pere Alemany; David Casanova (45-50).
Display OmittedIn the present work we perform a computational study of the properties of a push-pull organic dye with a quinoidal thiophene unit as the conjugated linker between the electron donor and acceptor groups. We investigate the photophysical properties of the dye related to its potential use as a molecular sensitizer in dye-sensitized solar cells. We rationalize the solvation effects on the absorption band of the dye in protic and aprotic solvents, identifying the interaction of alcohol solvents with the amine in the donor group as the source for the blue shift of the absorption band with respect to aprotic solvents.

Kinetics of enzyme-mediated hydrolysis of lipid vesicles by Vladimir P. Zhdanov; Björn Agnarsson; Fredrik Höök (51-56).
Display OmittedMembrane enzymatic reactions can now be experimentally studied at the level of single sub-100 nm lipid vesicles. To interpret such experiments, we scrutinize theoretically various aspects of the hydrolysis of vesicles by single enzyme molecules and enzyme (e.g., PLA2) supplied with the constant rate. Using the mean-field kinetic model, we illustrate the shape of the corresponding kinetics and the dependence of the time scale of the reaction on the vesicle radius. Stochastic effects are illustrated as well. In addition, we discuss the likely mechanisms of the reaction-induced pore formation and bilayer rupture.
Keywords: Enzymatic catalysis; Lipid membranes; Mean-field kinetic equations; Monte Carlo simulations; Stochastic kinetics;

Bubble nucleation in a Lennard-Jones binary liquid mixture by Vladimir G. Baidakov; Sergey P. Protsenko; Vasiliy M. Bryukhanov (57-60).
Display OmittedWe report a molecular dynamics (MD) study of homogeneous bubble nucleation in a stretched Lennard-Jones binary mixture at a temperature close to the solvent triple point. The pressure of the limiting stretching p n corresponding to a fixed value of the nucleation rate has been determined. The values of p n achieved in MD simulation are lower than those calculated from classical nucleation theory (CNT). The discrepancy between the data of MD simulation and CNT may be connected with the neglect in the latter of the size dependence of the surface tension of critical bubbles.
Keywords: Molecular dynamics simulation; Bubble nucleation; Metastable state; Nucleation rate; Lennard-Jones liquid mixture;

Super-microporous solid base MgO-ZrO2 composite and their application in biodiesel production by Jiaojiao Su; Yongfeng Li; Huigang Wang; Xiaoliang Yan; Dahai Pan; Binbin Fan; Ruifeng Li (61-65).
Display OmittedThe super-microporous microcrystalline MgO-ZrO2 nanomaterials (pore size 1–2 nm) was prepared successfully via a facile one-pot evaporation-induced self-assembly (EISA) method and employed in the transesterification of soybean oil and methanol. X-ray diffraction, transmission electron microscope, temperature programmed desorption of CO2, and N2 adsorption porosimetry were employed to characterize the nanocomposites. Nitrogen sorption isotherms revealed that these materials had large surface areas of more than 200 m2/g. Moreover, the sample with a Mg/Zr molar ratio of 0.5 and calcined at 400 °C showed high biodiesel yield (around 99% at 150 °C).
Keywords: Super-microporous; MgO-ZrO2 microcrystalline; Large surface area; Transesterification;

Ab initio kinetics for isomerization reaction of normal-chain hexadiene isomers by Feiyu Yang; Fuquan Deng; Youshun Pan; Zemin Tian; Yingjia Zhang; Zuohua Huang (66-73).
Display OmittedThe ground-state potential energy surface (PES) of isomerization philosophy of ten normal-chain hexadiene isomers is computed by density functional methods using the geometries optimized at B3LYP/6-311++G (d, p) level of theory. These detailed reaction pathways are used to calculate the rate constants for the unimolecular isomerization reactions by transition state theory (TST) in the temperature range of 500–2500 K. Difference of rate constant between each hexadiene isomer is interpreted through the PES and Ḣ atom transfer, and only 2,4-hexadiene readily fulfills cis-cis to trans-trans conformation conversion. All the conversions are kinetically interpreted from the PES and ST geometry.
Keywords: Hexadienes; Isomerization; Transition state; PES; Rate constant;

Display OmittedRectifying properties of three cyano side group substituted arylethynylene thiolate molecules are theoretically investigated based on abinitio calculations. The current-voltage curves show that rectifying performance of the junctions correlates negatively with conjugation of the molecules, which reveals a much more pronounced rectification behavior for that with broken-conjugated dihydroanthracene core (sAH) than the counterpart with linearly-conjugated anthracene core (sAC) or cross-conjugated anthraquinone core (sAQ). It is demonstrated that the conjugation breaking induced localization of molecular orbital and asymmetric evolution of its spatial distribution under positive and negative biases play the key role for the rectification enhancement in sAH.
Keywords: Molecular diode; Rectifying property; Non-equilibrium Green’s function method;

Display OmittedAlthough advances in heteroatom incorporation into the single-layer graphene lattice resulted in films with large carrier densities, careful characterization by Raman spectroscopy is important for assessment of the material’s quality. We investigated theoretically I(D)/I(D′) Raman intensity ratios induced by B- and N- substitutional doping, demonstrated to be consistent with measurements. Calculated Fermi level shifts showed that for a moderate doping density results are comparable to electrolyte gating, while analytical analysis of the electron-defect scattering provided insight into changes of cross-sections. Effects of doping density on the D band intensity and broadening were quantified, and will assist in graphene characterization.

Display OmittedIn this contribution the influence of spatial confinement on the dipole moment of carbon monoxide and boron fluoride was investigated. The spatial confinement was modeled by applying the two-dimensional harmonic oscillator potential. The dipole moment calculations were performed for both molecules using their experimental geometries and also including the effect of geometry relaxation in the presence of confining potential. Our results demonstrate that the dipole moment is noticeably affected by the confinement. Moreover, it was found that the changes in the dipole moment, caused by the presence of harmonic oscillator potential, are substantially different for each of the studied molecules.
Keywords: Spatial confinement; Dipole moment; Carbon monoxide; Boron fluoride; Hirshfeld analysis;

Display OmittedDue to the drawbacks of conventional negative electron affinity (NEA) GaN photocathodes activated by Cs or Cs/O, a new-type NEA GaN photocathodes with heterojunction surface dispense with Cs activation are proposed. This structure can be obtained through the coverage of an ultrathin n-type Si-doped GaN cap layer on the p-type Mg-doped GaN emission layer. The influences of the cap layer on the photocathode are calculated using DFT. This study indicates that the n-type cap layer can promote the photoemission characteristics of GaN photocathode and demonstrates the probability of the preparation of a NEA GaN photocathode with an n-type cap layer.
Keywords: NEA GaN photocathodes; n-type GaN cap layer; first-principle;

Relativistic frozen core potential scheme with relaxation of core electrons by Yuya Nakajima; Junji Seino; Masao Hayami; Hiromi Nakai (97-103).
Display OmittedThis letter proposes a relaxation scheme for core electrons based on the frozen core potential method at the infinite-order Douglas–Kroll–Hess level, called FCP-CR. The core electrons are self-consistently relaxed using frozen molecular valence potentials after the valence SCF calculation is performed. The efficiency of FCP-CR is confirmed by calculations of gold clusters. Furthermore, FCP-CR reproduces the results of the all-electron method for the energies of coinage metal dimers and the core ionization energies and core level shifts of vinyl acetate and three tungsten complexes at the Hartree–Fock and/or symmetry-adapted cluster configuration interaction levels.

Display OmittedThe 2H2@C60 minimum structure of C 2 symmetry has been fully characterized at the density-fitting local second-order Møller–Plesset (DF-LMP2) level of theory. Its uncorrected and zero-point energy (ZPE) corrected complexation energies equal 1.9 and 6.2 kcal/mol, respectively, confirming the instability of the complex. This structure exhibits the largest intermolecular host-guest and guest-guest separations among all the complexes studied in this work. The calculated infrared spectrum of 2H2@C60 does not show any frequency shifts for the modes associated with radial or tangential displacements in C60 (except for one mode), but shows a weak red H―H vibrational frequency shift.
Keywords: Computational chemistry; Endohedral fullerene complexes; Geometry optimization; Vibrational frequencies; Accurate quantum-chemistry calculations;

Surface-enhanced infrared absorption studies of copper nanostructures formed by oblique-angle deposition by William A. Henry; Alexandru S. Biris; Fumiya Watanabe; Taylor E. Huntington; Aaron R. Owen; Adam S. Price; Jon W. Golden; Katherine M. Primm; Brandon M. Wilde; Morgan D. Sweere; Caleb N. Denton; Bakarie S. Branch; Thomas M. Banh; Mary E. Lindsey; Donald A. Perry (111-114).
Display OmittedIt is demonstrated that Cu nanostructures fashioned by oblique angle deposition (OAD) of gaseous Cu atoms onto CaF2 substrates have SEIRA enhancement factors that are close to par with Ag or Au nanostructures formed using vapor deposition at a normal deposition angle. Cu nanostructures grown at normal angle show about 5× in SEIRA enhancement of a p-nitrobenzoic acid (PNBA) monolayer and have a range of over 100 nm in particle size. Conversely, Cu nanostructures deposited by OAD show SEIRA enhancement of approximately 40× and are more uniform with approximate circular shapes averaging 25 nm and spacing less than 10 nm.
Keywords: SEIRA; SERS; Surface-enhanced; Copper; p-Nitrobenzoic acid;

Hydrolysis mechanism of anticancer drug lobaplatin in aqueous medium under neutral and acidic conditions: A DFT study by Venkata P. Reddy B.; Subhajit Mukherjee; Ishani Mitra; Sujay Mahata; Wolfgang Linert; Sankar Ch. Moi (115-122).
Display OmittedWe have studied the hydrolysis mechanism of lobaplatin in aqueous medium under neutral and acidic conditions using density functional theory combining with CPCM model. The stationary states located on potential energy surface were fully optimized and characterised. The rate limiting step in neutral conditions, ring opening reaction with an activation energy of 110.21 kJ mol−1. The completely hydrolysed complex is expected to be the reactive species towards the DNA purine bases. In acidic conditions, ligand detachment is the rate limiting step with an activation energy of 113.82 kJ mol−1. Consequently, monohydrated complex is expected to be the species reacting with DNA.
Keywords: Lobaplatin; Third generation anticancer drug; DFT; Aqueous medium; Hydrolysis mechanism;

Display OmittedBased on density functional theory, use of nitrogen-doped graphene nanoribbons as novel electrocatalysts for oxygen reduction reaction (ORR) was investigated. Adsorption of O2 was found to depend on the structure of nitrogen-doped graphene nanoribbons. Compared to nitrogen-doped armchair graphene nanoribbons, graphitic-type nitrogen-doped zigzag graphene nanoribbons could stably capture O2 and efficiently catalyze ORR. ORR on graphitic-type nitrogen-doped zigzag graphene nanoribbons exhibited a four-electron reduction process with the formation of second H2O molecule as the rate-determining step. These results would help to understand the origin and nature of ORR catalytic activity on nitrogen-doped carbon materials.
Keywords: Oxygen reduction reaction; Density functional theory; Electrocatalyst; Nitrogen-doped graphene nanoribbon;

Structural transition of (InSb)n clusters at n = 6–10 by Qi Liang Lu; Qi Quan Luo; Shou Guo Huang; Yi De Li (128-132).
A new structural growth pattern of (InSb)n clusters were observed.Display OmittedAn optimization strategy combining global semi-empirical quantum mechanical search with all-electron density functional theory was adopted to determine the lowest energy structure of (InSb)n clusters with n = 6–10. A new structural growth pattern of the clusters was observed. The lowest energy structures of (InSb)6 and (InSb)8 were different from that of previously reported results. Competition existed between core–shell and cage-like structures of (InSb)8. The structural transition of (InSb)n clusters occurred at size n = 8–9. For (InSb)9 and (InSb)10 clusters, core–shell structure were more energetically favorable than the cage. The corresponding electronic properties were investigated.

Display OmittedThe NbN C 3Π−X 3Δ (1–0) band is studied in the free-jet setup. The spin-orbit interaction between v = 1 of C 3Π1 and e 1Π states destroyed the symmetric splitting of C 3Π substates. Unusually high 685 cm 1 spin-orbit interaction matrix element between them pushed the 3Π1 substate below 3Π0 component. Our ab initio calculations estimate 3 Π 2 - 3 Π 0 separation 414 cm 1, which is in an excellent agreement with the observed value 404 cm 1, and places the 3 Π 1 state 244 cm−1 below 3 Π 0 state, in somewhat poor agreement with the experimental separation of 449 cm−1. This error is primarily due to the error in our calculated unperturbed 3 Π 1 - 1 Π energy separation.

Display OmittedEfficient spirobifluorene-based organic small molecule (RTh-Sp-CF3) was synthesized in a simple manner via Suzuki coupling reaction containing an alkyl bithiophene as donor and 3,5-bis (trifluoromethyl) benzene as acceptor unit. The spirobifluorene-based small molecule was utilized as an electron-donor materials with well-known electron-acceptor material, phenyl-C61-butyric acid methyl ester (PC61BM) in the solution-processed small molecule organic solar cells (SMOSCs). The incorporation of 3,5-bis (trifluoromethyl) benzene unit as electron-acceptor has significantly tuned the energy levels of small molecule and obtained the HOMO and LUMO energy levels of −5.35 eV and −3.92 eV, respectively. SMOSCs fabricated with RTh-Sp-CF3 accomplished an overall power conversion efficiency (PCE) of ∼2.12% with short circuit current (JSC) of ∼8.42 mA/cm2 and the open-circuit voltage (VOC) of ∼0.66 V. The reasonable JSC and VOC of devices might be attributed to the presence of strong electron-withdrawing fluorine units in RTh-Sp-CF3, which resulted from the improved absorption and electrochemical properties.
Keywords: 3,5-Bis (trifluoromethyl) benzene; Electrochemical properties; Spirobifluorene; Small molecule; Organic solar cells;

Structure and internal dynamics of n-propyl acetate studied by microwave spectroscopy and quantum chemistry by Lilian W. Sutikdja; Wolfgang Stahl; Vincent Sironneau; Ha Vinh Lam Nguyen; Isabelle Kleiner (145-149).
Display OmittedThe gas phase structure of n-propyl acetate was determined using molecular beam Fourier transform microwave spectroscopy from 2 to 40 GHz supplemented by quantum chemical calculations. The experimental spectrum revealed only one conformer with trans configuration and C1 symmetry. Torsional splittings occurred for each rotational transition due to the internal rotation of the acetyl methyl group with a barrier height of approximately 100 cm−1. The XIAM and BELGI-C1 codes were applied to reproduce the spectrum within the measurement accuracy. This investigation on n-propyl acetate has accomplished our studies on saturated linear aliphatic acetates CH3COOCnH2n+1 (n = 1–6).
Keywords: Rotational spectroscopy; Internal rotation; Large amplitude motion; Conformational landscape; Ab initio calculations;