Chemical Physics Letters (v.458, #4-6)

Contents (iii-xi).

Results from resonant UV second harmonic generation 21 (SHG) experiments that are best modeled by surface depletion of hydroxide and establish at most a weak 22 surface adsorption.Many physical phenomena are affected by the intrinsic acidity/basicity of the free liquid water surface, yet it remains an active and controversial subject. Macroscopic bubble and droplet experiments have been interpreted to indicate an air–water interface covered with hydroxide, whereas recent molecular-scale studies produce the opposite conclusion, viz. that hydroxide is repelled from the interface while hydronium is strongly adsorbed. Here we report results from resonant UV second harmonic generation (SHG) experiments that are best modeled by surface depletion of hydroxide and establish at most a weak surface adsorption. This finding is consistent with our earlier SHG measurements indicating surface enhancement of hydrated protons, as well as with other molecular-scale experiments and simulations, but stands in stark contrast to the results from macroscopic studies. The acidity, or basicity, of aqueous surfaces could strongly influence heterogeneous atmospheric chemical processes, such as aerosol reactions and gas uptake.

Shedding light on dark plasmons in gold nanorings by Feng Hao; Elin M. Larsson; Tamer A. Ali; Duncan S. Sutherland; Peter Nordlander (262-266).
Extinction spectra of a gold nanoring as a function of angle of incidence.We present an experimental and theoretical analysis of the optical properties of gold nanorings of different sizes and cross-sections. We show that for light polarized parallel to the ring, the optical spectrum can depend sensitively on the angle of incidence. For normal incidence, the spectrum is characterized by two dipolar ring resonances. As the angle of incidence becomes more oblique, several previously dark multipolar ring resonances appear in the spectra. We show that the appearance of the multipolar resonances is a consequence of retardation and can be understood in simple general terms.

Hyperfine structure of the Cs 2 3 3 Σ g + state.The Cs 2 3 3 Σ g + state, which was observed previously by two-photon excitation [D. Li, F. Xie, Li Li, S. Magnier, V.B. Sovkov, V.S. Ivanov, Chem. Phys. Lett. 441 (2007) 39], has been observed by infrared–infrared (IR–IR) double resonance spectroscopy. One hundred and seventy IR–IR double resonance lines have been assigned to transitions into the 3 3 Σ g + v  = 2–12 levels. Hyperfine structure of this state has been resolved in the excitation spectra. Molecular constants and the potential energy curve of this state are reported in this Letter.

Gaussian-2 theory as a linear system by Holmann V. Brand (272-275).
A method for accurately calculating atomization energies, electron affinities, ionization potentials, and proton affinities.We present a procedure for scaling the additive energy terms in the Gaussian-2 (G2) theory by solving a linear system of equations A x  =  b using matrix algebra. Our procedure reduces the average absolute deviation (AAD) from 1.20 kcal/mol of the original G2 theory to 0.94 kcal/mol with respect to the G2/91 test set of 125 energies of first-and second-row compounds.

Effect of ring strain on disulfide electron attachment by Élise Dumont; Pierre-François Loos; Xavier Assfeld (276-280).
Electron attachment on disulfide linkages is found to be strongly favored by ring strain.The possibility of excess electron binding of a series of (bi)cyclic, ring-constrained disulfides, in gas phase was investigated by ab initio MP2/6-31+G∗∗ calculations. It is shown that ring strain favors electron attachment, as neutral compounds are very sensitive to angular and dihedral compressions: cyclic disulfides will preferentially undergo a spontaneous electron capture compared to linear analogs, with superior positive values for adiabatic electron affinity. Cyclisation effect is progressively switched off for higher-member rings, but remains important for disulfides grafted on molecular bridges. Its structural consequences are analysed, with different behaviors for neutral and radical anionic moities.

Accelerated computation of extremely ‘slow’ molecular diffusivity in nanopores by Ryo Nagumo; Hiromitsu Takaba; Shin-ichi Nakao (281-284).
This Letter reports a simple approach to estimating very ‘slow’ molecular diffusivities of CH4 and CF4 through an LTA-type zeolite.We propose a statistical-mechanical methodology to estimate ‘slow’ diffusivity from conventional molecular dynamics (MD) calculations. In this method, the temperature dependence of the Helmholtz free energy profiles is predicted from one MD simulation at a particular temperature. These profiles are used to estimate the diffusivity, combined with transition state theory. We apply this methodology to molecular diffusion in a zeolite nanopore. Our methodology promises the efficient estimation of molecular ‘slow’ diffusivity in nanopores, e.g., values less than 10−14  m2/s.

Theoretical study of classical acetylcholinesterase inhibitors by Érica C.M. Nascimento; João B.L. Martins; Maria L. dos Santos; R. Gargano (285-289).
Molecular electrostatic potential map for ACheI molecules.Semi-empirical, RHF and DFT calculations were carried out to study well known acetylcholinesterase inhibitors, i.e., tacrine, donepezil, galantamine, physostigmine, and tacrine dimer (bis-tacrine). Electronic and structural parameters were used in order to correlate the acetylcholinesterase inhibition activity with their molecular structure. The optimized geometries of these drugs were analyzed by multivariate PCA statistical method. Frontier orbital energies (HOMO and LUMO), the (HOMO–LUMO) gap and the distance between more acidic hydrogen species were used to determine principal components. The PCA results indicated that these drugs were ordered into three groups according to the first principal component: galantamine/physostigmine, donepezil/tacrine dimer and tacrine.

On the Heisenberg behaviour of magnetic coupling in the manganese dimer by I. Negodaev; C. de Graaf; R. Caballol (290-294).
Strict Heisenberg behaviour in the magnetic coupling of the manganese dimer.The magnetic coupling in the manganese dimer has been investigated with multiconfigurational computational schemes. Whereas the perturbational approaches present severe deviations to the expected Heisenberg behaviour, variational techniques based on difference dedicated configuration interaction (DDCI) perfectly fit a Heisenberg spectrum corresponding to antiferromagnetic coupling. This shows that the study of weak magnetic coupling as in Mn2 is at (or beyond) the limit of applicability of perturbational techniques. Potential energy curves are obtained for the six low-lying Σ states by adding the DDCI energy difference to the reference potential energy surface calculated with coupled cluster techniques for the monoconfigurational 11 Σ g + state.

At high energies near the total dissociation threshold, the number of states W(E) reaches a constant value because in the case of even higher energies there are no more bound states to accumulate the rising energy. When W(E) tends to a constant value, its derivative, i.e., the density of states ρ(E), goes to zero.The analytical method based on the Airy function approximation and applicable for the calculation of vibrational densities of states of polyatomic molecules has been developed. The result is a smooth function accurate over a wide energy range from the zero-point energy to the total dissociation energy of the entire molecule.

Forces between air-bubbles in electrolyte solution by E.R.A. Lima; M. Boström; B.E. Sernelius; D. Horinek; R.R. Netz; E.C. Biscaia; W. Kunz; F.W. Tavares (299-302).
Using potentials from molecular simulation, we calculate the double layer force between two air-bubbles in salt solutions considering different dielectric functions. The result is compared with the attractive van der Waals force and discussed in terms of ion specific bubble coalescence.Ion specificity plays a key role in solution chemistry and many biological processes. However, the classical DLVO theory has not been able to explain the experimentally observed ion specific forces acting between air-bubbles in electrolyte solutions. We resolve this problem by using a generalized Poisson–Boltzmann equation. We demonstrate that inclusion of both short-range potentials obtained from simulation (acting between ions and the air–water interface) and the spatial variation of the local dielectric constant near the air–water interface may be essential to obtain correct results.

Femtosecond pump–probe 2D optical Kerr effect spectroscopy of molecular hydrogen crystals by Falk Königsmann; Mizuho Fushitani; Nina Owschimikow; David T. Anderson; Nikolaus Schwentner (303-307).
The time and wavelength dependence of femtosecond laser induced birefringence in hydrogen crystals display coherent phonon and roton dynamics.Femtosecond pump–probe 2D (wavelength and delay) optical Kerr effect (OKE) spectroscopy is used to measure laser induced birefringence in parahydrogen and normal hydrogen crystals at 4.2 K. At the probe center wavelength the OKE signal reveals underdamped sinusoidal modulations at frequencies corresponding to the Raman-active transverse optical phonon mode of solid parahydrogen (36.8 cm−1) and normal hydrogen (38.5 cm−1), respectively. For parahydrogen crystals, at small shifts away from the probe center wavelength the OKE signal displays high frequency oscillations (355 cm−1) which correspond to J  = 2 rotons with a 94 fs time period.

While there is a strong correlation between the C site of CO2 and the O site of acetone, no similar correlation have be seen in the case of 2-propanol.The Helmholtz free energy of solvation (Δ slv A) of 2-propanol and acetone in supercritical CO2 were calculated by expanded-ensemble method. For both solute molecules, Δ slv A is well represented by the polynomial equation of the third power of density and the density value correspond to the maximum stability decreases with increasing temperature. In all states under study acetone is more stable than 2-propanol. At low densities the solvation of acetone is more exothermic than 2-propanol while at high densities it come with less entropic penalty. The solvent reorganization energy is more favorable in 2-propanol case, although the role of solute–solvent interactions is dominant in both cases.

Comparison between theoretical predictions and experimental data for the mixed butane–heptane liquid–vapour phase diagram. The continuum integral equation theory, implemented using a square-well potential, is used to model the intermolecular structure of the pure alkanes; the mixture results are predictions and do not rely on fits to any mixture data.In this work we introduce our continuum integral equation methodology to the study of alkane mixtures. The theory is applied to predict liquid–vapour coexistence properties in the mixed butane–heptane system using a model parameterization based only on the pure fluids. These predictions are in strong agreement with corresponding experimental values available for the mixture, including temperature–composition phase envelopes, and temperature–pressure phase diagrams spanning a wide region of the system’s overall coexistence regime. The alkanes are modelled as square-well chains, marking here our first application of the theory to chain mixtures with attractive interactions.

Temperature dependence of photoluminescence efficiency in doped and blended organic thin films by Yichun Luo; Hany Aziz; Richard Klenkler; Gu Xu; Zoran D. Popovic (319-322).
PL efficiency in the fluorescent dye-doped Alq3 layers demonstrates only weak temperature dependence, which is in contrast to the dramatic decrease of PL efficiency observed upon increasing temperature for undoped Alq3.The temperature dependence of photoluminescence (PL) efficiency in fluorescent dye-doped tris(8-hydroxyquinoline)aluminum (Alq3) layers and blended Alq3 layers was investigated. Results show that the PL efficiency in the fluorescent dye-doped Alq3 layers has only weak temperature dependence, which is in contrast to the dramatic decrease of PL efficiency observed upon increasing temperature for undoped Alq3. This phenomenon is attributed to the narrower energy bandgap of the dopant, as compared to that of Alq3, which leads to the ‘trapping’ of mobile excitons on dopant molecules, hence impeding their migration by thermally activated diffusion, thereby decreasing their encounters with randomly distributed non-radiative recombination (i.e. quenching) sites.

Evidence for enhanced dipolar interactions between Pt centers in binuclear phosphorescent complexes by Xiangfei Qi; P.I. Djurovich; N.C. Giebink; M.E. Thompson; S.R. Forrest (323-328).
Transient studies are used to examine the radiative decay dynamics in a series of phosphorescent platinum binuclear complexes. The complexes studied consist of square planar (2-(4′,6′-difluorophenyl)pyridinato-N,C2′)Pt units bridged by either pyrazole or thiopyridine ligands whose radiative lifetimes are a function of the Pt–Pt spacings. A three-level zero-field splitting model is used to describe the lifetime dynamics of these complexes.Transient studies are used to examine the radiative decay dynamics in a series of phosphorescent platinum binuclear complexes. The complexes studied consist of square planar (2-(4′,6′-difluorophenyl)pyridinato-N,C2′)Pt units bridged by either pyrazole or thiopyridine ligands. We observe an increase in radiative lifetime as temperature is reduced from 300 K to 4 K when the binuclear complexes, named 1, 2 and 3 with Pt–Pt spacings 3.19 Å, 3.05 Å, and 2.83 Å, respectively, are doped into a p-bis(triphenylsilyly)benzene wide energy gap host. The lifetimes for 1, 2 and 3 are τ  = 6.3 ± 0.1 μs, τ  = 2.3 ± 0.1 μs, and τ  = 2.0 ± 0.1 μs at T  = 295 K, respectively. At T  = 4 K, those values increase to τ  = 8.6 ± 0.1 μs, τ  = 14.4 ± 0.1 μs, and τ  = 17.0 ± 0.1 μs, suggesting that the neighboring heavy metal centers in compounds 2 and 3 have significant orbital overlap. A three-level zero-field splitting model yields the lowest triplet energy splittings of 28 ± 3 cm−1, 142 ± 9 cm−1, and 113 ± 10 cm−1 for compounds 1, 2 and 3, respectively.

Solvation effects in SN2-type oxidative addition of MeI to PtMe2(NH3)2 was theoretically investigated with RISM–SCF. Though the reaction may not occur in gas phase because of the high barrier due to dissociation of iodide anion from methyl group, the reaction easily occurs in nitromethane because the iodide anion is stabilized by nitromethane solvent.Solvation effects in SN2-type oxidative addition of MeI to PtMe2(NH3)2 was theoretically investigated with RISM–SCF, which provides both of quantum chemical and statistical mechanical information on solvation system. Though the reaction is very difficult to occur in gas phase because of the high barrier which is induced by dissociation of iodide anion from methyl group, the reaction easily occurs in nitromethane because iodide anion is stabilized by nitromethane solvent. This stabilization mainly arises from the solvation free energy of platinum center and iodide anion. The solvation structure obtained from RISM–SCF provides the reason why this reaction easily occurs in nitromethane.

Large coercive field in magnetic-field oriented ε-Fe2O3 nanorods by Shunsuke Sakurai; Jun-ichi Shimoyama; Kazuhito Hashimoto; Shin-ichi Ohkoshi (333-336).
Oriented ε-Fe2O3 nanorods achieved a magnetic coercive field of 23.4 kOe, the largest value among metal oxide-based magnets.We report the fabrication and magnetic coercive field of oriented ε-Fe2O3 nanorods. Oriented ε-Fe2O3 nanorods embedded in SiO2 matrix were obtained by a sol–gel method under an applied external magnetic field. The longitudinal axis, which corresponds to a-axis of the crystal structure of ε-Fe2O3 nanorods, was oriented parallel to the direction of external magnetic field. The sample achieved a magnetic coercive field along the longitudinal axis of 23.4 kOe, which is the largest value among metal oxide-based magnetic materials. The shape of the magnetic hysteresis loop is well understood by considering the magnetization rotation process and the uniaxial magnetic anisotropy.

Precise determination of the first hyperpolarizability of a fluorescent triindole derivative with dicyanovinyl groups by the deconvolution method by Shigeru Ikeda; Hironobu Kumagai; Hideo Ooi; Koji Konishi; Hidetaka Hiyoshi; Tatsuo Wada (337-340).
The spectrum composed of the hyper-Rayleigh scattering (HRS) light and fluorescence, (a), and true HRS light and fluorescence spectra obtained by deconvolutions, (b).The hyper-Rayleigh scattering (HRS) method was employed to investigate the first hyperpolarizability (β) of a triindole derivative with dicyanovinyl groups (TDCV). Because TDCV showed fluorescence due to two-photon absorption, we tried to precisely determine the β value of TDCV by observations of spectra composed of the HRS light and fluorescence and deconvolutions of the HRS light and fluorescence components with a slit function. Moreover, we tried to determine the depolarization ratio of the HRS light from TDCV by polarization measurement. As results, we have found that the β value of TDCV is (319 ± 13) × 10−30  esu at 1070.0 nm and TDCV can be classified as a purely octopolar molecule with the C 3 h symmetry.

Partial geometry of interstitially NH-doped TiO2 (a) and its computed density of states (b).We employed DFT calculations to study the geometric and electronic structures of N- and NH-doped anatase TiO2 systems. With N dopant concentrations of 2.1% and 1.0%, the substitutional NH-doping resulted in bandgap narrowings of 0.12 and 0.13 eV, respectively, very close to that resulted from substitutional N-doping (0.12 and 0.11 eV). For interstitial N- and NH-doping with a nitrogen concentration of 2.1%, only NH-doping resulted in a bandgap narrowing of 0.07 eV. For both substitutional and interstitial NH-doping configurations, the crystalline structure adjacent to the NH dopants was heavily distorted, which might enhance the photocatalytic efficiency.

Synthesis of single-wall carbon nanotubes grown from size-controlled Rh/Pd nanoparticles by catalyst-supported chemical vapor deposition by Keita Kobayashi; Ryo Kitaura; Yoko Kumai; Yasutomo Goto; Shinji Inagaki; Hisanori Shinohara (346-350).
Single-wall carbon nanotubes have been synthesized from size-controlled Rh/Pd nanoparticles embedded in FSM-16 by catalyst-supported chemical vapor deposition.Single-wall carbon nanotubes (SWNTs) with a diameter range of 0.8–1.5 nm have been synthesized by using size-controlled Rh/Pd nanoparticles by catalyst-supported chemical vapor deposition (CCVD) at 1073 K. Nanoparticles of Rh/Pd with a diameter of ∼3 nm have been prepared within a regular hexagonal arrayed one-dimensional channel of mesoporous silica, FSM-16. The SWNTs obtained by the CCVD using these catalysts have smaller diameter distribution compared to that of the catalyst nanoparticles, indicating that the diameter distribution of synthesized SWNTs has little relevance to the diameter of Rh/Pd catalyst nanoparticles.

With increasing noise intensity, the SNR maximum increases and reaches a largest value, and then decreases, till lower than the SNR maximum for D=  0, representing the reaction rate oscillations can be enhanced by external noise and, furthermore, can be most highly enhanced by optimal external noise. This implies the reaction rate oscillations can selectively exploit the external noise to reach a best oscillatory performance.In this Letter, we study the effect of external noise (EN) on the intrinsic stochastic reaction rate oscillations (RRO) in CO oxidation on nm-sized Pd particles. It is found the optimal RRO can be enhanced by appropriate EN and, moreover, there is an optimal EN by which the optimal RRO can be most highly enhanced. This result shows EN may play a constructive role in the intrinsic stochastic reaction oscillations via the interplay with internal noise, and the oscillations can selectively employ the EN to reach a best oscillatory performance. Therefore, EN should be paid high attention in the study of the kinetics in CO oxidation on nm-sized Pd particles.

A deep metastable eutectic iron–aluminate by Frans J.M. Rietmeijer; Aurora Pun; Joseph A. Nuth (355-358).
An amorphous deep metastable eutectic compound, 1.8FeO · 0.6Fe2O3  · Al2O3, produced n a laboratory vapor phase condensation experiment.A small number of amorphous nanograins with a silica-free, iron–alumina composition was formed during laboratory condensation of a low-silica Al–Fe–SiO–H2–O2 vapor. Very rapid cooling of the vapor created conditions that were conducive to the formation of deep metastable eutectic compounds with a specific composition. High Fe2+/Fe3+ ratios during condensation are reflected in the 1.8FeO · 0.6Fe2O3  · Al2O3 composition of this amorphous deep metastable eutectic compound.

Protein, mRNA and ncRNA numbers as a function of time according to Monte Carlo simulations.The interplay of messenger RNA (mRNA), protein, produced via translation of this RNA, and nonprotein coding RNA (ncRNA) may include regulation of the ncRNA production by protein and (i) ncRNA–mRNA association or (ii) ncRNA–protein association resulting in degradation of the corresponding complex. The kinetic models, describing these two scenarios and taking into account that the association of ncRNA with a target occurs after ncRNA conversion from the initial form to the final form (e.g., from a long RNA to microRNA), are found to predict oscillations provided that the rate of ncRNA formation increases with increasing protein population.

We present a model for blinking statistics of single lipase molecules, showing either a stretched exponential decay or t −1/2 power-law decay.In this study of blinking behavior of lipase B from Candida antarctica (CALB) molecules, we consider a dynamic diffusion-controlled reactions model with two dark states and one light state in line with the well-known Michaelis–Menten enzymatic kinetics to explain the observed non-exponential blinking statistics. This model could lead to a stretched exponential decay or, t−1/2 power-law decay, depending on kinetic parameters, followed by a long-time single exponential decay.

The features of the NMR signals from intermolecular multiple-quantum coherences between spin 1/2 and quadrupolar nuclei were investigated theoretically and experimentally.An NMR pulse sequence was used to study the signal features from intermolecular multiple-quantum coherences between spin 1/2 and quadrupolar nuclei in solution NMR for the first time. General theoretical expressions were derived from the raising and lowering operator formalism. Theoretical predictions for the relative signal intensities and the optimal pulse flip angles are in excellent agreement with experimental results. Both the magic angle gradient and 2D experiments validate that the signals are indeed from intermolecular heteronuclear multiple-quantum coherences.

Brewster angle cavity ring-down spectroscopy, surface absorption cross section of HNO3 in the 290–330 nm region, monolayer versus multilayer adsorption.We have measured absorption cross sections of surface-adsorbed HNO3 in the 290–330 nm region with Brewster angle cavity ring-down spectroscopy. The 295 K absorption cross sections for HNO3 adsorbed on fused silica surfaces are at least two orders of magnitude larger than those in the gas phase in the wavelength region studied. Our work extends the application of Brewster angle cavity ring-down spectroscopy to the UV region, and further demonstrates the capabilities of this technique. Our results can account for the field-observed large differences between nitric acid photolysis rates on the surface and that of the gas phase.

Aromaticity in terms of ring critical point properties by A. Mohajeri; A. Ashrafi (378-383).
The use of topological electron density properties in the ring critical points of the aromatic systems has been investigated as a way of quantifying aromaticity indicators.The properties derived from topological analysis of electron density at the ring critical point, RCP, have been used to model six different aromaticity indices. The intercorrelations between various aromaticity indices and eight properties extracted from the RCPs of 43 aromatic rings have been investigated. It was found that individual RCP properties cannot be considered as reliable quantitative characteristics of π-electron delocalization in heterocyclic compounds. We showed that it is possible to describe aromaticity indices in terms of principal components derived from density based properties estimated at RCP of aromatic rings. The resulted models yield correlation coefficients greater than 0.9 for five of total six aromaticity indices studied here.

We present a methodology that enables fast anharmonic VSCF/VCI calculations for large molecules using a variational-perturbative selection scheme.We adapt a variation–perturbation method to perform vibrational configuration interaction (VCI) calculations on large molecular systems. Starting from a self-consistent vibrational wave function, we use the vibrational configuration interaction with perturbation selected interactions (VCIPSI) algorithm to select an active VCI space iteratively. We then extend this approach to use a reduced-coupling description of potential energy surfaces. The accuracy of the method is tested on methane and benzoic acid molecules. The significant reduction in the size of the VCI basis obtained using the VCIPSI scheme does not affect the quality of the computed vibrational frequencies and reduces computational time dramatically.

Heat of solution of helium in n-octane is positive, but low helium polarizability produces a surface free energy barrier, so Q∗ is negative.Values of the Onsager heat of transport Q∗ measured at the n-octane liquid–vapour interface are consistent with the results of molecular dynamics calculations by Simon et al. [J.-M. Simon, S. Kjelstrup, D. Bedeaux, B. Hafskjold, J. Phys. Chem. B 108 (2004) 7186.] The measured value of Q∗ is independent of gas pressure but is affected by variations in the size of the vapour gap over which the temperature gradient is applied. Measurements with added helium indicate that Q∗ is negative for helium at the surface of n-octane, even though the enthalpy of solution is positive. The helium results can be understood on the basis of an existing model.

Improved resolution is obtained for 1H solid-state NMR spectra by removing residual broadening due to interactions with neighboring 14N nuclei.Residual dipolar coupling between quadrupolar and other nuclei under MAS has not usually been thought to be important in high field NMR spectroscopy. We show that coupling to 14N broadens 1H lineshapes significantly even at 11.7 T, and that we can decouple 14N from 1H during 1H homonuclear decoupling to successfully improve 1H resolution. The method used for decoupling is the application of evenly spaced pulses to the quadrupolar nucleus.

Upper bounds for the Zagreb indices and the spectral radius of triangle- and quadrangle-free connected graphs are reported. Moreover, we determine the graphs for which the bounds are attained.The first ( M 1 ) and second ( M 2 ) Zagreb indices and the spectral radius ( ρ ) are topological indices. This Letter presents upper bounds for these indices of triangle- and quadrangle-free connected graphs, in terms of the number of vertices (n), number of edges (m), and the radius (r). These bounds are: M 1 ( G ) ⩽ n ( n + 1 - r ) , M 2 ( G ) ⩽ m ( n + 1 - r ) , ρ ⩽ n + 1 - r , and we determine the graphs for which the bounds are attained.

Author Index (399-405).