Chemical Physics Letters (v.344, #5-6)

Rupture force of adsorbed self-assembled surfactant layers by O. Teschke; G. Ceotto; E.F. de Souza (429-433).
The tip applied force necessary to obtain tip/substrate contact, i.e., rupture force between adsorbed layers of self-assembled surfactant films and atomic force microscope (AFM) tips in water has been measured. A substantial contribution of this rupture force is due to the dielectric exchange force (DEF). The DEF model is in agreement with the observation that the surfactant layer rupture forces are smaller in the thickest layers, where the compactness of the adsorbed film results in the smallest values of the dielectric permittivity. Within experimental accuracy a dielectric permittivity value of ∼4 for bilayers and of ∼36 for monolayers is found.

We explore the possibility of controlling electronic properties along an inorganic nanotube (INT) through the influence of nanometer-scale features in the underlying substrate. We examined single multi-walled WS2 INTs using scanning tunneling microscopy (STM) in high vacuum. As long as the INTs lie flat on MoS2 (0 0 0 1) or graphite (0 0 0 1) surfaces, they appear semimetallic. However, when the INT is suspended above the surface due to crossing steps or other nanotubes, a band gap opens up. We discuss this observation in terms of either a potential drop under the INT, or a change in its electronic properties due to its distortion when it lies flat on a surface.

The redox behavior of structurally constrained bis(1,2-bis(9-methyl-1,10-phenanthrolin-2-yl)ethane)dicopper (I)/(1,2-bis(9-methyl-1,10-phenanthrolin-2-yl)ethane)copper(II) couple was examined in acetonitrile: gated ET was observed even for this dimeric Cu(I)/monomeric Cu(II) couple. A dicopper(II) intermediate with a high-energy tetrahedral coordination geometry was spectrophotometrically identified for the first time for copper(II)/(I)-polypyridine redox couples. A low-intensity but low-energy LMCT band observed for the metastable dicopper(II) species indicates that gated ET process involves participation of a superexchange preceded by the deformation of ground-state Cu(II)N4 (in trigonal bipyramidal or D2d symmetry) to Cu(II)N4 with a teterahedral geometry.

Si and SiO x nanostructures formed via thermal evaporation by Yong-jun Chen; Jian-bao Li; Jin-hui Dai (450-456).
Various Si and SiO x  (x=1  to 2) nanostructures were formed via a thermal evaporation method of heating pure silicon powder at 1373 K under Ar flow. An alkali-treated quartz glass plate coating with catalyst precursor of a Fe(NO3)3 aqueous solution was used as substrate. The product exhibited morphologies of fist-capped SiO x fibers (Si-core), tree-like SiO x nanofibers and tadpole-like SiO x nanofibers in different areas of the substrate. The different local temperature gradient, concentration of silicon vapor and silicon oxide vapor, and also the substrate surface condition were suggested to be responsible for the versatile morphologies of the products.

13CO exchange process between the hemoglobin irons observed with 13C NMR by Camille Loupiac; Serge Pin; Hervé Vezin; Bernard Alpert (457-462).
13C NMR signals of 13CO free and bound to hemoglobin (Hb13CO) were investigated at 9.4 T. Existence of 13CO exchange between α and β hemoglobin irons was proven by 13C NMR temperature line splitting and by the shift of the 13CO NMR signal, after mixing α13CO with deliganded β or β13CO with deliganded α. 13C NMR bandwidth of 13CO bound to hemoglobin was investigated for various HbCO ligandation levels: from low (16%) to full 13CO association, the kinetics of the 13CO exchange decreases. A model using the exchange kinetics can generate the sigmoidal shape of the hemoglobin ligandation curve.

Nanoparticles of palladium were synthesized by a `ship-in-a-bottle' technique in supercages of Y-zeolite. The ions of Pd2+ were introduced into the zeolite by ion-exchange between PdCl2 and NaY zeolite. They were then reduced by an electrochemical method and assembled to form nanoparticals by admission and release of CO. The electrode of palladium-loading zeolite film was characterized by cyclic voltammetry, and the adsorption of carbon monoxide on the electrode was studied using in situ electrochemical FTIR reflection spectroscopy. A novel phenomenon of enhanced IR absorption (EIRA) was observed for the first time. In comparison with CO adsorbed on a massive Pd electrode, the IR absorption of CO adsorbed on nanoparticles of Pd confined in supercages of Y-zeolite has been enhanced to about 51 times.

Dissociation pathways in low energy (0–2 eV) electron attachment to Cl2O by Wolfgang Sailer; Petra Tegeder; Michael Probst; Herwig Drexel; Verena Grill; Paul Scheier; Nigel J. Mason; Eugen Illenberger; Tilmann D. Märk (471-478).
Dissociative electron attachment (DA) to ClOCl is studied in a high resolution crossed beam experiment. Two complementary ion pairs, Cl/ClO and O/Cl2 , are observed. The Cl/ClO pair arises from a simple Cl–OCl bond cleavage with the electron sitting on either of the two fragments. The O/Cl2 pair is formed by a concerted reaction with the expulsion of O (or O) and formation of Cl2 (or Cl2 ). Ab initio calculations indicate that in low energy electron attachment an electronically excited state of the precursor anion ( ClOCl (2 B 2)) is involved.

S atoms have been generated through vacuum ultraviolet photolysis of carbonyl sulfide (OCS) in Ar matrices. Excitation spectra measured by monitoring the 1 S→1 D emission of S at 784 nm are assigned to Ar+S←ArS charge-transfer (CT) (135 nm, substitutional site (SS) and 159 nm, interstitial site (IS)) and 3 S←3 P Rydberg (160.8 nm) transitions. Empirically derived potential curves of Rg+S and Rg+O species predict the transition energies with reasonable accuracy. S atoms are found to be more photomobile than O atoms in Ar matrices.

This Letter reports in situ Fourier transform infrared (FTIR) spectroscopic data on thermal TiO2 films fabricated by heating titanium plates in air at 475, 700 and 800 °C. The films were studied in the dark and under UV-irradiation in aqueous 0.1  M NaClO 4 in the presence and absence of 0.1 M Na2(OOC)2 and at 10, 25 and 50 °C. The film fabricated at 800 °C showed a broad feature near 1580  cm −1 under UV-irradiation that was not observed in the dark, whilst the films fabricated at lower temperatures, 475 and 700 °C, showed no such feature. This feature appears to be associated with the accumulation of surface-mobile holes at the complex, porous film–electrolyte interface and the capacity of such holes to enhance the absorption cross-section of optical phonons characteristic of the rutile crystal form at and near the surface of the TiO2/electrolyte interface.

Mechanism of positronium–nitrobenzene complex formation in water by Debarshi Gangopadhyay; Bichitra Nandi Ganguly; Tapas Mukherjee; Binayak Dutta-Roy (495-500).
Rate constants for complex formation between the positronium (Ps)-atom and the weak acceptor (Ac) nitrobenzene in water have been measured by us at various temperatures with particular emphasis on the slopes of the Arrhenius plot on either side of the observed turn-over point. Taken together with the temperature dependence of the surface tension and the viscosity of water, our data enable us to discuss the implications of our experimental findings as to the tenability of a proposed underlying reaction mechanism.

Fluorescence lifetime distribution of single molecules undergoing Förster energy transfer by Minyung Lee; Jianyong Tang; Robin M. Hochstrasser (501-508).
The Förster energy transfer process of a donor–acceptor system was investigated at the single-molecule level as well as in bulk by confocal fluorescence lifetime microscopy. While the fluorescence decay of bulk tetramethylrhodamine ethyl ester (TMR) was nonexponential, the single TMR molecules exhibit single exponential decay in PMMA in the absence or presence of azulene. The lifetime distribution of single TMR molecules undergoing fluorescence energy transfer shows some deviation from that deduced from the Förster equation. Although the bulk data are well fitted by two exponentials, the measured distribution shows no physical basis for that assumption.

The electronic coupling between adjacent bases, belonging to the same and to complementary stacks of DNA, is calculated using an ab initio procedure at the HF level. The mutual geometry of pair-bases has been accurately selected from the available crystallographic data and the variance of the coupling with the geometry is discussed. Results are applied to the description of single step hole transfer between two guanines separated by a base bridge. The effects of interstrand jumps and bases sequence on the hole transfer mechanism are considered.

Large-scale ab initio multi-reference configuration interaction calculations are carried out for ground and excited states of chloromethanol ClCH2OH to investigate photofragmentation processes relevant to atmospheric chemistry. Five low-lying excited states (13 A″, 13 A , 11 A″, 21 A and 23 A ) in the energy range between 6.8 and 8.5 eV are found to be highly repulsive for C–Cl elongation leading to CH 2 OH(X 2 A ) and Cl(2 P). Photodissociation along the C–O bond leading to CH 2 Cl(X 2 B 2) and OH(X 2Π) has to overcome a small barrier of about 0.3 eV because the low-lying excited states 11 A″, 13 A and 13 A″ become repulsive only after the C–O bond is elongated by about 0.2 Å.

The trends for cation binding for several conformers of diglyme are predicted by mapping the topography of the molecular electrostatic potential (MESP) at the Hartree–Fock (HF) level. Different Li+–(diglyme) geometries derived by exploiting the MESP cooperative effects are used subsequently in ab initio computations. The binding energies for Li+ with diglyme have been calculated in mono-, bi- and tridentate coordinations by employing the HF, second-order Møller–Plesset (MP2) and the hybrid density functional methods. The calculated vibrational spectrum of Li+–(diglyme) also points to a gauche conformation of diglyme in the complex.

A density functional investigation on d0-Zr(IV) organometallic fragments by Paola Belanzoni; Marzio Rosi; Antonio Sgamellotti (536-542).
The density functional approach has been used to compare the geometry and the frontier orbitals of the [(COT)Zr]2+, [CpZr]3+, [Cp2Zr]2+ and [calix[4]–(O)4Zr] fragments. The investigation on the [(COT)Zr]2+ and [Cp2Zr]2+ complexes shows that, in spite of the same number of low-lying empty orbitals available for bonding with additional ligands, the symmetries and the spatial extensions of these orbitals are different, and this has important consequences on their chemical behavior.

Efficient calculation of canonical MP2 energies by P. Pulay; S. Saebo; K. Wolinski (543-552).
An efficient canonical second-order Møller–Plesset theory (MP2) procedure, based on the Saebo–Almlöf integral–direct transformation technique, coupled with efficient prescreening of the atomic orbital (AO) integrals, is described. For large molecules, a fraction of the AO integrals suffices to produce energies to microhartree accuracy. Calculations up to 1800 basis functions and 240 correlated electrons have been performed on a single processor computer for symmetrical molecules. Calculations with ∼1000 basis functions and ∼120 electrons can be performed routinely for molecules with no symmetry. However, scaling of the second half transformation is still steep. Several basis sets used in correlated calculations are compared for economy and performance.

Comparison of the bond order indices calculated for neutral molecules and for their ions obtained by removing one electron either from HOMO or from one of the near-to-HOMO occupied orbitals can be used to predict the primary bond cleavage processes taking place in an electron impact mass spectrometric experiment.

Saddle-node states in the spectra of HCO and DCO: a periodic orbit classification of vibrational levels by S. Stamatiadis; S.C. Farantos; Hans-Martin Keller; Reinhard Schinke (565-572).
The general structures of the vibrational states of HCO and its isotopomer DCO are analyzed in terms of periodic orbits (POs) and continuation/bifurcation diagrams. Both bound and resonance states are considered. It is shown that the members of the pure overtones are guided by POs, even in the continuum. In particular, it is demonstrated that the highly anharmonic states localized along the dissociation coordinate correspond to orbits originating from saddle-node bifurcations in the classical phase space.

Hydration and `inverse hydration' of platinum(II) complexes: an analysis using the density functionals PW91 and BLYP by Jacqeline Bergès; Jacqueline Caillet; Jacqueline Langlet; Jiřı́ Kozelka (573-577).
The interaction between two platinum complexes and water was investigated using the DFT functionals PW91 and BLYP. Previous MP2 and HF calculations have shown that when a H2O molecule approaches Pt with its O atom, the interaction is governed by electrostatics and the MP2 and HF interaction energy curves nearly coincide, whereas for the approach with H pointing towards Pt, there is a significant dispersion component portrayed only by MP2 calculations. Here we show that both PW91 and BLYP satisfactorily reproduce the MP2 curves, PW91 slightly exaggerating and BLYP slightly underestimating the dispersion-induced energy minima. This result is discussed in view of the different behavior of the two exchange functionals at large density gradients.

A general coupled cluster study of the N2 molecule by Jesper Wisborg Krogh; Jeppe Olsen (578-586).
The equilibrium distance, harmonic frequency and potential curve of the nitrogen molecule are investigated using the cc-pVDZ basis and various single- and multi-reference coupled cluster (CC) methods. Including single and double excitations from all determinants of the smallest active space that ensures correct dissociation, the CC method gives deviations from full configuration interaction (FCI) of 0.0001 Å for the equilibrium distance, 1  cm −1 for the frequency, and a non-parallelity error (NPE) of 0.0006 E h for the potential curve. Restricting the single and double excitations from the active space to those that are at most quadruple excited compared to the Hartree–Fock determinant, produces results that are very close to those obtained including all excitations up to quadruple excitations.

Generalized 6j symbols are defined in terms of orthonormalized Racah polynomials of a discrete variable and given explicitly as hypergeometric 4 F 3(1) series. They extend the recoupling coefficients of ordinary angular momentum algebra, including multiples of 1/4 as quantum numbers. A three-term recurrence relationship is exploited for extensive calculations and illustration of their properties. Their role is outlined as matrix elements for superpositions (or overlaps) both between alternative spherical and hyperspherical harmonics and between alternative Sturmian sets, an important case being that of four-dimensional harmonics of S 3, which apply to the momentum-space hydrogen atom orbitals.

3nj-symbols and harmonic superposition coefficients: an icosahedral abacus by Vincenzo Aquilanti; Cecilia Coletti (601-611).
Angular momentum recoupling coefficients of angular momentum theory and matrix elements for basis set transformation of hyperspherical harmonics enjoy properties and sum rules crucial for applications but complicated without the guidance of graphical techniques. These coefficients being related to Racah's polynomials, the graphs also apply to polynomials of the hypergeometric family, their q-analogues and their `elliptic' extensions. A useful `abacus' exploiting the connections with presentations of icosahedral and related symmetries is introduced. Particular and limiting cases, such as those of the semiclassical type, allow a unified view of properties of angular and hyperangular momentum algebra, including relationships among vector coupling coefficients and rotation matrix elements.

Towards intrinsic optical bistability of rare earth ion pairs in solids by O. Guillot-Noël; L. Binet; D. Gourier (612-618).
A mechanism for intrinsic optical bistability (IOB) is presented for atomic systems which can be decomposed into two weakly interacting subsystems. It is shown that optical bistability should occur when the interaction between the two subsystems fluctuates more rapidly than the characteristic time of the interaction. This approach provides a semi-quantitative explanation of IOB for asymmetric ytterbium pairs in CsCdBr3 matrix.

Electron hydration: interface shells by Yulia V. Novakovskaya; Nikolai F. Stepanov (619-624).
Interface water cluster anions (H2O) n (n ⩽ 12) composed of two to four fragments are simulated in the unrestricted Hartree–Fock approximation with the second order Moeller–Plesset perturbation theory corrections taken into account with the 6-31++G** basis set either augmented or not with the floating center of eight s functions. A linear dependence of the circumsphere radius involving central molecules of the anions on 1/n provides an estimate of the excess electron radius in condensed water (about 2.5 Å). Vertical detachment energies, approximated with linear dependences on n −1/3, are extrapolated to the values around 3.4 eV for bulk water.

Infrared spectra of UV-photolysed argon matrices doped with cyanoacetylene reveal the presence of HNCCC, a carbene (imine) isomer of the parent molecule. The identification of three stretching bands is based on deuterium and nitrogen-15 substitution experiments and on a B3LYP/6-311++G** density functional theory study. The detection of this energetically unfavourable isomer is facilitated by the high absolute intensity of its IR absorption.

13 C–1 H dipolar-assisted rotational resonance in magic-angle spinning NMR by K. Takegoshi; Shinji Nakamura; Takehiko Terao (631-637).
A new 13 C–13 C recoupling mechanism which occurs under magic-angle spinning (MAS) is presented. The mechanism can basically be attributed to rotational resonance (R2), but the conventional R2 condition is modified by a recoupled 13 C–1 H dipolar interaction. The 13 C–1 H recoupling is attained by 1 H rf irradiation fulfilling a rotary-resonance condition. The present method does not have the drawbacks associated with rf irradiation on 13 C and is applicable for band-selective recoupling between carbonyl/aromatic carbons and aliphatic carbons. The 13 C–13 C recoupling mechanism under 13 C–1 H recoupling is theoretically explained and is experimentally demonstrated using N-acetyl[1,2-13C] dl-valine and uniformly 13 C, 15 N-labeled glycylisoleucine.

The Letter presents a generalization of the kinetic model for radiative flows of reacting gas mixtures with strong vibrational and chemical non-equilibrium. The new model takes into account the vibration–electronic (VE) transitions and radiative transitions from the electronically excited states. A closed set of macroscopic equations taking into account the coupling of vibrational relaxation, chemical reactions and radiative transitions is derived. The peculiarities of non-equilibrium kinetics of CO in different conditions are discussed.