Chemical Physics Letters (v.320, #3-4)

Improving the McClelland inequality for total π-electron energy by Jack H. Koolen; Vincent Moulton; Ivan Gutman (213-216).
The McClelland inequality, E⩽ 2mn , estimates the total π-electron energy (E) of a conjugated hydrocarbon by means of the number of carbon atoms (n) and the number of carbon–carbon bonds (m). We now show that in the general case 2m/n+ (n−1)(2m−4m2/n2) is a better (n,m)-type upper bound for E. An even better estimate, 4m/n+ (n−2)(2m−8m2/n2) , holds for alternant hydrocarbons.

Partial electron yield spectrum of N2: doubly excited states at the K-shell threshold by M Neeb; A Kivimäki; B Kempgens; H.M Köppe; K Maier; A.M Bradshaw; N Kosugi (217-221).
The partial electron yield spectrum of N2 has been measured at a kinetic energy corresponding to the Auger decay of doubly excited core–hole states. The spectrum reveals previously unresolved double excitations just below and above the K-shell ionization threshold. Their excitation energies and dissociative nature agree with calculated potential energy curves of the doubly excited states.

Charge decomposition analysis of the chemisorption bond by Maite Garcı́a Hernández; Ariana Beste; Gernot Frenking; Francesc Illas (222-228).
Charge decomposition analysis (CDA) has been applied to study the interaction of CO and NH3 on several metal surfaces. For NH3 chemisorption, CDA predicts a large donation from the NH3 lone pair to the surface and a vanishing contribution for the backdonation. However, for CO chemisorption, CDA predicts the σ-donation to be larger than the π-backdonation, contrary to other methods. CDA overestimates the extent of the σ-donation because of its large overlap with the metal orbitals compared with the overlap of the π orbitals. The same situation is found in Ni(CO)4. Therefore, the CDA method should only be applied to the investigation trends.

The aggregation and syneresis of silica particles during hydrogel polymerisation has been observed in situ for the first time with near-Å resolution using a new approach based on the combined fluorescence anisotropy decay of solvated and bound dye molecules. Primary particles of mean hydrodynamic diameter ∼1.5 nm are found to be present within 20 min of mixing sodium silicate solution and sulphuric acid. Clustering then occurs during siloxane polymerisation to produce after ∼30 h secondary particles with a mean diameter up to ∼4.5 nm at a growth rate which depends on silicate concentration and time to microgelation, t g. Subsequent condensation to ∼4 nm diameter occurs within 1 week as particle syneresis dominates. The effects on particle growth of adding D2O and inorganic salts are demonstrated.

The molecular parameters of carbon suboxide, C3O2, have been determined in large-scale ab initio calculations using the coupled-cluster method, CCSD(T), and basis sets of double- through quadruple-zeta quality. The potential energy function for the large-amplitude CCC bending motion (the ν 7 mode) was determined to be strongly anharmonic. The equilibrium structure of the molecule was found to be bent, with a barrier to linearity of only 18 cm−1. The rotation-bending energy levels were then calculated using a semirigid-bender Hamiltonian. The vibrational energy levels and effective rotational constants determined for various ν 7 states were found to be in good agreement with the experimental data.

A parametric study is performed to test the validity of some of the most common models of gas-surface scattering used in particle simulations of rarefied flows. The simplified models are compared with results of semiclassical molecular dynamics trajectory calculations for inelastic scattering of Xe atoms on GaSe. The simple models are shown to be inadequate to accommodate all the features of the real dynamics. Sample calculations of oxygen heterogeneous recombination on silica are also provided to emphasise the importance of vibrationally state-resolved models.

Deuterium NMR studies of the alignment by a magnetic field of the director in the smectic A phase of 4-octyl-4′-cyanobiphenyl have shown this to be a complex process. To explore the influence of the interaction between the smectic A phase and the surface of its container on the pathways available for director alignment we have investigated a droplet of this phase suspended in glycerol. The field induced alignment of the director in the droplet is found to be dramatically different, both quantitatively and qualitatively, to that found for the smectic A phase confined to a glass tube.

Dual asymptotic behavior in geminate diffusion-influenced reaction by Kyril M Solntsev; Noam Agmon (262-268).
The kinetics of excited-state proton transfer to solvent from the strong photoacid 5-cyano-2-naphthol exhibits two different power-law asymptotic tails for acid and base, arising from competition between reversible and irreversible geminate reprotonation. We show that the data are in quantitative agreement with a recently developed theory for this diffusion-influenced reaction.

Multi-exponential relaxation and concomitant relaxation allowed multiple quantum coherence among methyl protons in paramagnetic proteins arising from cross-correlation between Curie spin relaxation (CSR) and dipolar relaxation (DD) are investigated. Numerical simulations and semi-quantitative multi-quantum experiments in the cyanide-inhibited paramagnetic protein horseradish peroxidase underscore the presence of CSR×DD cross-terms. The magnitude of the resulting two-spin order depends on the distance and relative orientation of the methyl axis to the paramagnetic centre which can be exploited to derive structural constraints.

Metropolis Monte Carlo (MC) Brownian dynamics simulations are performed to determine the anisotropy of the electrical polarizability Δα of a model DNA fragment in aqueous salt solutions. A 64 base-pair fragment of the double-stranded DNA is modeled as an impenetrable cylinder and placed in a spherical MC cell. Taking into consideration the contribution of coions to the electroneutrality condition, at every simulation step we obtain a list of counterions which constitute the net charge that compensates the polyion charge. According to the fluctuation–dissipation theorem, polarizability is calculated from fluctuations of the dipole moment created in the distribution of the net charge. Electric properties of polyelectrolyte solutions are reproduced: Δα decreases with the addition of salt yielding a steady value comparable to the experiment.

The mass resolved electronic spectrum of cold tryptophan molecules has been obtained using a novel desorption method as a vaporization source coupled with a supersonic expansion. This desorption device is characterized by its simplicity, stability suitable for spectroscopic studies and by a very low yield of fragmentation products. The unique performance of the desorption source is demonstrated by the possibility to measure hole-burning spectra of tryptophan by resonance enhanced two-photon ionization which confirms strongly the presence of only a small number of stable conformers for this species in its ground state.

Zeeman spectroscopy of Nd3+ ions in CsCdCl3 crystal by Samar Jana; R.K. Mukherjee (289-294).
A transverse Zeeman study of the high-concentration Nd3+-doped CsCdCl3 crystal at 25 K in an external magnetic field of 7.8 T applied both parallel and perpendicular to the c axis, is presented here. Observed splittings of the transitions 4 I 9/2→4 G 5/2 and 4 I 9/2→2 G 7/2 of the 4f3 electronic configuration of Nd3+ ions are investigated in detail. From the observed splittings for Bc, the parallel splitting factors s are determined for the levels in these multiplets. But for Bc no resolvable splitting is found at any level in these two multiplets.

Electron localization–delocalization in mixed-valence iron dimers by S.M. Ostrovsky; R. Werner; K. Nag; W. Haase (295-302).
Two mixed-valence iron dimers, [L 1 Fe 2 (μ-OAc)2](ClO4) and [L 1 Fe 2 (μ-OBz)(OBz)(H2O)](ClO4), were investigated. The temperature dependence of the effective magnetic moments and the position and shape of the intervalence absorption bands were calculated using a Hamiltonian taking into account isotropic exchange interaction, double exchange, Zeeman interaction, zero-field splitting for the ground state and vibronic coupling with the PKS out-of-phase mode. The similarities and differences of the investigated compounds were explained on the basis of analysis of the adiabatic potentials. The behaviour of both systems is determined by a strong competition between two main processes: double exchange interaction and vibronic coupling with PKS out-of-phase mode. Degree of delocalization of the itinerant extra electron was calculated at different values of temperature.

The [17.0]  2 Π 1/2←X   2 Π 1/2 system of AlCa by Jacqueline C. Fabbi; Jon D. Langenberg; Michael D. Morse (303-306).
Laser-induced fluorescence spectroscopy has been used to study supersonically cooled AlCa. This study investigates under higher resolution (0.007 cm-1) a single band previously studied and tentatively assigned as the (0-0) vibrational transition of the [17.0]  2 Δ 3/2(?)←X   2 Π 1/2 system of AlCa. The resolution of the rotational structure in the present study enabled a definite assignment as a 2 Π 1/2←2 Π 1/2 transition. Analysis of the spectrum gives B 0′=0.096685(19) cm-1, (p+2q)′=−0.013078(370) cm−1, and B 0″=0.105518(20) cm-1. These convert to ground and excited state bond lengths of r 0″=3.14942(30) and r 0′=3.29014(32) Å, respectively.

Inversion of stationary point levels in a flat transition structure region on account of vibrational energy by Ikchoon Lee; Chang Kon Kim; Hong Guang Li; Bon-Su Lee; Hai Whang Lee (307-310).
Characterization of the stationary points in a flat transition structure region of the potential energy surface for the HCOCl+Cl reaction has been carried out at the MP2/6-311+G∗∗ level. Two stationary points are found: an intermediate with CS symmetry and all positive eigenvalues in the Hessian and a transition state with C1 symmetry and only one negative eigenvalue in the Hessian. The MP2/6-311+G∗∗ electronic energies of these two only differ by δΔE(=ΔE TS−ΔE Int)=0.01 kcal mol−1. The zero-point and thermal energy corrections to the electronic energy lead to an inversion of levels: δΔE ZPE=−0.06 and δΔH=−0.59 kcal mol−1, which becomes normal when the −TΔS term is added to obtain δΔG=+0.90 kcal mol−1 (+0.80 kcal mol−1 at the G2(+)MP2 level). This inversion arises solely from one less vibrational mode in the TS on the flat transition structure region.

Detection of the transient molecule HOPO in the gas phase by infrared laser spectroscopy by Iain S. Bell; Imtiaz K. Ahmad; Peter A. Hamilton; Paul B. Davies (311-315).
The transient molecule HOPO has been detected in the gas phase for the first time using high resolution infrared laser absorption spectroscopy. The molecule is formed in the reaction between the products of a microwave discharge in hydrogen and oxygen, and white phosphorus vapour. Many absorption lines were detected in the region of the predicted P–O (ν 4) and PO (ν 2) stretching modes of cis-HOPO around 850 and 1250 cm−1 respectively. About 170 rotational lines of the 850 cm−1 spectrum have been assigned to an a-type transition of an asymmetric rotor. The derived rotational constants are compatible with the values for cis-HOPO produced by a density functional theory calculation.

The biologically important compound hydrated disodium adenosine 5′-triphosphate (ATP) was studied focusing on the 23 Na nucleus (spin quantum number I=3/2) as probe in various solid-state NMR techniques including magic-angle spinning (MAS), cross-polarization MAS (CPMAS) and multi-quantum MAS (MQMAS) at 4.7 and 9.4 T. MQMAS experiments enabled the resolution of all the four crystallographically different 23 Na sites in the unit cell in the MQMAS spectrum at 4.7 T, but not at 9.4 T. The four sites were successfully assigned and the principal elements of the quadrupolar tensors and isotropic chemical shifts of the 23 Na nuclei at each lattice site determined by analyzing and computer simulating the above experimental spectra.

Raman spectral changes of neat CO2 across the ridge of density fluctuation in supercritical region by Hideyuki Nakayama; Ken-ichi Saitow; Maki Sakashita; Kikujiro Ishii; Keiko Nishikawa (323-327).
Density dependence of Raman spectra for carbon dioxide was studied along the isotherm in the temperature region 0.96⩽T/T c⩽1.06 and the density region 0<ρ/ρ c<2. Spectrum in the small-wavenumber region below 100 cm−1 and that in the region of the 2ν 2 intramolecular band, i.e. one of the Fermi dyad, were carefully measured and analyzed. The results are interpreted by assuming the presence of the gas- and liquid-like regions in the supercritical region separated by the ridge of density fluctuation.

Six-dimensional classical dynamics simulations are carried out to study the dissociative adsorption of H2 on a Pd(111) surface. The 6D potential energy surface (PES) used in the simulations is constructed by interpolation of ab initio results. The high accuracy of the PES construction is ascertained by comparison with a set of ab initio data which are not used in the interpolation. We obtain a non monotonic variation of the dissociative adsorption probability with normal incident energy as found experimentally. Our results provide further support to the importance of dynamical steering.

According to the properties of the interactions between the thermal hysteresis proteins (THPs) and an ice crystal surface in the THP solution, we present a kinetic theory of the adsorption of thermal hysteresis proteins on the ice crystal surface. The thermal hysteresis activities of the THP solutions are given. The cooperative properties in the adsorption process of the THPs on the ice crystal surface are discussed.

Spatial concentration and temperature profiles of the CH radical in a hot-filament chemical vapor deposition reactor are measured by cavity ring-down spectroscopy. The CH concentration is found to be on the order of 1011 molecules/cm3. The spatial distribution indicates that CH formation primarily occurs at or very near the filament. At a distance of 2 mm from the filament the [H]/[H2] ratio is determined to be 0.011±0.003.

The bonding properties of CO molecules adsorbed at a stepped MgO surface have been studied by means of cluster models and gradient-corrected DFT calculations. At a step site CO interacts with both a Mg4c and a Mg5c ion forming a relatively strong bond, 0.2 eV, but with very small shift of the C–O stretching frequency, +1 cm−1. The results allow us to assign the feature observed at 2148 cm−1 in the IR spectrum to CO at step sites. The other features of the spectrum at 2200, 2159 and 2152 cm−1 are due to CO adsorbed at corner, edge, and terrace sites, respectively, as found in other studies. The results for the step suggest that for some surface defects the direct relationship between strength of the surface electric field and frequency shift is no longer valid because of inhomogeneities in the field.

We report the results of classical, grand canonical Monte Carlo simulations of the physisorption of H2 in finite-diameter `ropes' of parallel single-walled carbon nanotubes. The strong dependence of the gravimetric adsorption on the diameter of the rope is correlated with computed values of the specific surface area. The grooves on the external surfaces of the ropes are shown to provide a high binding-energy adsorption site, comparable in strength to those in the endohedral pores. Our results suggest that delamination of nanotube ropes should increase the gravimetric storage capacity.

We have studied the dynamics of electrons in single-wall carbon nanotube (bucky paper) samples using femtosecond time-resolved photoemission. The lifetime of electrons excited to the π bands is found to decrease continuously from 130 fs at 0.2 eV down to less than 20 fs at energies above 1.5 eV with respect to the Fermi level. This should lead to a significant lifetime-induced broadening of the characteristic van Hove singularities in the nanotube density of states.

Synthesis and characterization of single-walled nanotubes produced with Ce/Ni as catalysts by Bingbing Liu; T Wågberg; Eva Olsson; Ruseng Yang; Hongdong Li; Shulin Zhang; Haibin Yang; Guangtian Zou; B Sundqvist (365-372).
SWNTs were synthesized in high yield in the presence of Ce/Ni as catalyst. Raman and SEM results show that a fraction of the tubes are chiral and have small diameters (the snallest one is 0.79 nm), indicating that Ce plays an important role in the formation. Acid treatment is effective in eliminating impurities but changes the morphology and the electric resistance of mats with increasing treatment time. The resistance of as-grown and short time acid-treated mats follow a 2D VRH behaviour while mats treated for a long time follow a 3D model. The results indicate that hopping conduction occurs on bundles or on tubes and that there are intrinsic links between tubes in as-grown mats.

Forbidden 21 A g –11 A g fluorescence of 1,8-diphenyloctatetraene has been recorded in n-tetradecane and aromatic crystal hosts (naphthalene, bibenzyl, p-quaterphenyl) in the temperature interval from 5 to 50 K and under the He gas pressure up to 200 bar. Both the pressure and the temperature-induced shifts of lines have opposite signs in Shpolskii and aromatic matrices. The pressure and temperature shifts as well as the increase of S1–S0 one-photon transition probability in aromatic crystals were rationalised in terms of the change in S2–S1 splitting that is smaller in aromatic hosts as compared to n-alkanes.