Chemical Physics Letters (v.327, #3-4)

First-principles calculations for the structural, electronic and magnetic properties of clean and CO-adsorbed Ni{110} demonstrate that the spin-structure of the surface depends upon adsorption geometry in a highly localised fashion. The magnetic moments of top-layer atoms are enhanced on the clean surface, and attenuated by CO adsorption. Strikingly, however, the adsorbate-induced demagnetisation is limited primarily to those surface atoms directly bonded to the molecule. The adsorbate itself was found to be only weakly magnetised, in the opposite sense to the surface majority spin.

NMR study of antiferromagnetic black sodalite Na8(AlSiO4)6 by I. Heinmaa; S. Vija; E. Lippmaa (131-136).
27 Al and 29 Si NMR spectra and spin–lattice relaxation in heavily sodium-doped sodalite Na8(AlSiO4)6 are presented. The magnetic hyperfine shifts of 27 Al and 29 Si have Curie–Weiss-type temperature dependences with the same Weiss parameter Θ=−168±5 K. From an abrupt broadening of the resonance lines the transition temperature to the antiferromagnetic phase T N =54±2 K is determined. Above 200 K thermally activated ionic motion (activation energy, E A/k B=2000 K) is found in sodalite cages containing Na4 3+ ions which is much slower than the motion (E A/k B=750 K) in the cages containing Na3 3+ ions.

The dynamics and origin of NO photodesorbed from NO/Ag(111) by Tomas Vondrak; Daren J Burke; Stephen R Meech (137-142).
The photochemistry of the NO/Ag(111) system has been studied by time-of-flight, photon-induced desorption and temperature-programmed desorption measurements. It is shown that two species are photoactive for NO desorption, one of which had previously been thought photoinactive. The time-of-flight (TOF) data show that NO is desorbed from both sites by a photochemical mechanism. This allows the previously unassigned high-temperature desorption of NO detected in temperature-programmed desorption to be assigned to a new form of adsorbed NO. In addition, the temperature-programmed desorption measurements show that there are a range of inter-adsorbate interactions on the surface. Hence, the surface should not be thought of as comprising a range of non-interacting adsorbates.

Effect of solvent polarity on the aggregation of C60 by Sukhendu Nath; Haridas Pal; Avinash V. Sapre (143-148).
C60 forms aggregates in solution when the dielectric constant of the solvents exceeds some critical value viz. ⩾13. Further, for aggregation the C60 concentration has to exceed some critical value. The aggregation process is reversible and the aggregates exist in equilibrium with monomers. Picosecond laser flash photolysis experiments indicate that the excited (S1) aggregates undergo very fast relaxation without contributing towards the triplet formation.

A highly correlated conformational motion of a tube-like fully conjugated hydrocarbon by R. Herges; M. Deichmann; J. Grunenberg; G. Bucher (149-152).
By comparing the matrix infrared spectrum (Ar, 12 K) with theoretically calculated spectra (B3LYP/6-31G ), the structure of the tube-like hydrocarbon 1 has been determined to be a fast equilibrium of two D 2d symmetrical structures. The previously assumed higher symmetrical D 4h structure was found to be the transition state of this conformational interconversion. The conformational process involves the correlated and simultaneous pincer-like movement of the four relatively rigid anthracene units. Two pincers are always closed and two are open. Changing the state of one of the pincers reverses the state of the other three pincers. Upon suitable substitution, the molecular framework meets the requirements to be used as a logic gate.

Surface-enhanced hyper-Raman scattering (SEHRS) and surface-enhanced Raman scattering (SERS) have been employed to study phenazine adsorbed on the silver electrode surface at various negative potentials. It was observed that the scattering spectra of phenazine change drastically in frequencies and intensities with the change of applied potential, resulted from the electroreduction of phenazine (P) as the electrode potential moves negatively. The product of the electroreduction, 5,10-dihydrophenazine (PH2), as well as intermediate species, monoprotonated phenazine (PH+), phenazine semiquinone radical (PH) and protonated semiquinone radical (PH2 ⋅+) have been detected by SERS. One of the intermediate species, PH2 ⋅+ radical, was also detected by SEHRS.

The coordination compounds TlTp3−R with Tp3−R =hydridotris-(3-R-pyrazolyl-1-yl)-borate anion (R=tert-butyl, phenyl, 2-thienyl and p-tolyl) show a photoluminescence which originates from the metal-centered sp triplet of the Tl+ ion. In the solid state, the emission wavelength depends on the substituent R. Since the Tl+…Tl+ distance varies with R the emission energy seems to reflect the extent of metal–metal interaction in the excited state.

ESR study of monomer and triplet state dimer NO adsorbed on sulfated zirconia by Alexander Volodin; Daniele Biglino; Yoshiteru Itagaki; Masaru Shiotani; Anders Lund (165-170).
Triplet state NO dimers stabilized on the surface of sulfated zirconia (g=1.993, g =1.942, D=195 G, E≈0 ) have been observed by ESR. They are formed by adsorption of NO molecules on pair (acceptor) surface sites represented, most likely, by two adjacent (d=5.2 Å) low-coordinated Zr4+ ions. A new single line (Lorentz line with g=1.993 and line width 18 G) of NO monomers adsorbed on active sites of sulfated zirconia has been observed. The sites where this form is stabilized seem to be very strong electron acceptors responsible for the formation of radical cations after adsorption of aromatic molecules.

Optical heterodyne–Stark modulation spectroscopy by Ming Luo; Zhiyi Bi; Peipei Cai; Hua Zhuang; Yangqin Chen; Longsheng Ma (171-175).
A spectroscopic scheme, which combines the Stark modulation with the optical heterodyne detection technique, improves the detection sensitivity by a factor of about 30 over that of Stark modulation spectroscopy alone.

Measuring the AC susceptibility and registering the ESR spectra of the stable 1,1-diphenyl-2-picryl-hydrazyl (DPPH) radical at 77.4–125 K, we have studied the effect of the radical on the current properties of the intergranular contacts in the superconducting ceramics Bi1.8Pb0.3Sr1.9Ca2.8Cu3.4O y . Critical current in the ceramics is decreased by the magnetic field generated by the radical spin. The effect of the field on the critical current corresponds to the location of the adsorbed radical at a distance of 56 Å from the superconducting phase.

Charge-separation energy in films of π-conjugated organic molecules by I.G. Hill; A. Kahn; Z.G. Soos; R.A. Pascal, Jr (181-188).
We use inverse photoelectron spectroscopy (IPES) and ultraviolet photoelectron spectroscopy (UPS) to investigate unoccupied and occupied electronic states of five organic semiconductor materials: CuPc (copper phthalocyanine), PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride), α-6T (α-sexithiophene), α-NPD (N,N′-diphenyl-N,N′-bis(l-naphthyl)-l,l′ biphenyl-4,4′′ diamine), and Alq3 (tris(8-hydroxy-quinoline)aluminum). The transport gap, E t, is the difference between the highest occupied and lowest unoccupied molecular orbitals, measured via UPS and IPES. The charge separation energy, or exciton binding energy, is the difference between E t and the optical gap, E opt, measured via absorption. E t-E opt in these correlated materials ranges from 0.4 to1.4 eV.

Electric-field-induced broadening of spectral holes in zinc phthalocyanine by R. Purchase; M.J. Sellars; E. Krausz; N.B. Manson (189-196).
The influence of an externally applied electric field on spectral holes burnt into the Q band of zinc phthalocyanine doped into polymethylmethacrylate has been investigated. The changes in hole shape as a function of electric field have been monitored and a linear Stark effect has been observed. The broadening has a strong dependence on the direction of polarisation of the burning and reading laser relative to the applied electric field. This dependence on polarisation indicates that not only is there a dipole induced in the plane of the chromophore by the host, but also that there are strong interactions with the chromophore perpendicular to the molecular plane. The magnitude of the Stark broadening depends strongly on the presence or absence of solvent in the sample.

A crossed-beam study of the reaction C( 1 D)+H2(X   1 Σ + , v=0)  →  CH(X   2 Π , v′)+H( 2 S) by A. Bergeat; L. Cartechini; N. Balucani; G. Capozza; L.F. Phillips; P. Casavecchia; G.G. Volpi; L. Bonnet; J.-C. Rayez (197-202).
Product angular and time-of-flight distributions have been measured for the first time for the prototypical insertion reaction C( 1 D)+H2 using the crossed-beam technique with mass spectrometric detection at the collision energy of 1.86 kcal mol−1. Center-of-mass angular and kinetic energy distributions have been derived for CH(X   2 Π , v′=0)+H products and compared with those of statistical calculations based on phase-space theory.

A new kind of hydrogen bonding is proposed. It is a π-electron delocalization-assisted intramolecular dihydrogen bond. This bond is similar to the resonance-assisted hydrogen bonding investigated extensively by Gilli and co-workers (J. Am. Chem. Soc. 111 (1989) 1023). HF and MP2 calculations with 6-311++G(d,p), 6-311++G(2d,2p) and 6-311++G(3d,3p) basis sets have been performed in this study on boron molecules for which intramolecular dihydrogen bonds exist. The atoms-in-molecules theory of Bader is also applied to study these systems.

Enhanced density fluctuations in fluid systems with competing interactions by D Pini; Ge Jialin; A Parola; L Reatto (209-215).
The effects of competing interactions on the liquid-vapor transition in continuous systems have been studied by considering a simple hard-core model fluid with a two-Yukawa tail potential which is attractive at a short distance and repulsive at a long distance. We find that the size of the temperature-density region where the system experiences large density fluctuations without undergoing phase separation is greatly increased with respect to a fluid with a purely attractive tail potential. Moreover, in this region one observes a remarkable enhancement of the short-range correlations up to distances that can be more than one order of magnitude larger than the particle size. These effects are related with the occurrence of clouds of strongly correlated particles as a consequence of the competition between the attractive and the repulsive part of the interaction.

A new method is introduced which allows to form the exchange part of self-consistent field energy gradients in a way which scales linearly with molecular size for molecules with a non-vanishing HOMO–LUMO gap. This is not only important for Hartree–Fock but as well for the popular hybrid density functional theory methods. The method preserves the optimized integral-driven structure of conventional direct methods. Efficiency and scaling behavior of the new method are illustrated for DNA fragments.

Charge distribution in NaY zeolite from charge-transfer molecular dynamics by Evangelina Martı́nez Morales; Claudio M. Zicovich-Wilson; Jorge E. Sánchez Sánchez; Luis Javier Alvarez (224-229).
Charge-transfer molecular dynamics simulations of NaY zeolite in thermodynamical equilibrium in the microcanonical ensemble were carried out in order to obtain the charge distribution of all charge-transferring species. The interaction potentials used were parameterised ab initio calculations and take into account the Al–O and Si–O partially covalent bonds. The different kinds of O and Si atoms are characterised based on their charge distributions which are comparable with experimental 29 Si MAS NMR data.

An excited-state intramolecular proton transfer (ESIPT) process has been observed in the anionic species of 2-(2′-acetamidophenyl)benzimidazole (2-AMPBI). Dual fluorescence is observed in the case of monoanionic species. The presence of different monoanionic species has been characterized by means of steady-state and time-resolved fluorescene spectroscopy, as well as using AM1 semi-empirical quantum-mechanical calculations. The present study revealed that the presence of an acetyl group plays a vital role in the stability of different ionic and neutral species and in changes in their ground (S0) and excited singlet state (S1) acid–base properties with respect to 2-(2′-aminophenyl)benzimidazole (2-APBI).

Lattice-field calculations are performed on a Gaussian polymer chain confined to move within the region defined by two fused spheres. The results of the calculations are in accord with recent experimental measurements and computer simulations, and suggest that current theoretical understanding of polymer partitioning phenomena is inadequate when excluded volume interactions between the monomers are present. It is also shown that the notion of ground state dominance can fail even in the large monomer limit.

We present a sensitive analysis technique to explore the configurational space of non-stationary systems. With it, we discuss the pattern of large-scale molecular shape transitions observed during the computer-simulated unfolding of charged in vacuo lysozyme. Using a statistical treatment for the charge distribution among residues, we have estimated the value of the critical total charge that unleashes rapid unfolding in vacuo. Analyses of the molecular dynamics trajectories in a space of molecular shape descriptors indicate a transition between quasi-native folds and denatured conformers. At the critical charge, we find an intermediate regime of persistent partly unfolded structures. This behaviour is compatible with experimental observations of lysozyme ions in the gas phase. Our results indicate that there is a subtle balance between the unfolding tendency of the Coulomb repulsion and the tendency to implode associated with the in vacuo boundary conditions.