Chemical Physics Letters (v.348, #3-4)

On the role of electromagnetic boundary conditions in single molecule fluorescence lifetime studies of dyes embedded in thin films by R. Vallée; N. Tomczak; H. Gersen; E.M.H.P. van Dijk; M.F. Garcı́a-Parajó; G.J. Vancso; N.F. van Hulst (161-167).
Single molecule fluorescence lifetime studies are generally performed in thin polymer films, where the influence of the interface on the behaviour of fluorescing molecules is not negligible. In order to describe this influence, we investigate annealed films of different thickness. We show that the distribution of fluorescence lifetimes of the embedded dyes is shifted to lower values as the thickness of the film increases. We explain this shift by simple electromagnetic arguments related to the boundary conditions at the interfaces of the polymer film with air and glass, respectively. The conclusion is that extreme care must be taken in order to interpret single molecule data with respect to the true chemical nature of the phenomena.

Growth of n-alkane films on a single-crystal substrate by Z. Wu; S.N. Ehrlich; B. Matthies; K.W. Herwig; Pengcheng Dai; U.G. Volkmann; F.Y. Hansen; H. Taub (168-174).
The structure and growth mode of alkane films (n-CnH2n+2; n=4, 6, 7) adsorbed on a Ag(1 1 1) surface have been investigated by synchrotron X-ray scattering. New models are proposed for the butane (n=4) and hexane (n=6) monolayer and butane bilayer structures. Specular reflectivity scans reveal that growth of all films is preempted between two and three layers by nucleation of bulk particles oriented with a single bulk crystal plane parallel to the film. In the case of butane, the bulk particles also have a fixed azimuthal relationship with the film resulting in complete epitaxy.

Time evolution analysis of a 2D solid–gas equilibrium: a model system for molecular adsorption and diffusion by S Berner; M Brunner; L Ramoino; H Suzuki; H.-J Güntherodt; T.A Jung (175-181).
The adsorption of sub-phthalocyanine molecules on Ag(1 1 1) has been studied by means of scanning tunneling microscopy (STM). The molecules are observed in different two-dimensional (2D) phases of adsorption which coexist in thermodynamic equilibrium. In the condensed phase the molecules form well-ordered islands with a honeycomb pattern. In the gas phase single molecules can be discriminated in single scan lines by characteristic tip excursions which occur randomly. The energy barrier for surface diffusion as well as the condensation energy to form 2D islands is estimated and discussed.

Nanodisks of Au and Ag produced by laser ablation in liquid environment by A.V Simakin; V.V Voronov; G.A Shafeev; R Brayner; F Bozon-Verduraz (182-186).
Ablation of Au and Ag targets in water by a Cu vapor laser generates Au and Ag sols. The metal nanoparticles obtained after evaporation are disk-shaped (diameter in the 20–60 nm range, thickness of few nanometers). Their formation is observed at laser fluence between 10 and 20  J/cm 2 . Both aqueous sols are characterized by well-resolved plasmon bands around 400 nm (Ag) and 520 nm (Au).

Anomalous potential barrier of double-wall carbon nanotube by R. Saito; R. Matsuo; T. Kimura; G. Dresselhaus; M.S. Dresselhaus (187-193).
The stable structure of a double-wall carbon nanotube (DWNT) is calculated for various chirality pairs, (n,m) – (n,m), of inner and outer constituent layers. The stability of a double-wall nanotube is found not to depend on chirality, but rather on the diameter difference between inner and outer layers. However, the potential barrier for the relative displacement of the inner and outer nanotube layers is found to depend significantly on the chirality difference of the pair. Mechanical motions like a bolt–nut pair or discrete rotations can be expected for special pairs of chiralities in double-wall nanotubes, and these special motions will be important for nano-technology.

Binding energy of C60 + revisited – What is the problem? by S. Matt; O. Echt; P. Scheier; T.D. Märk (194-202).
The binding energy of C60 + (or C60) has been the subject of numerous experimental reports. Published values range from E a=2–13 eV for C2 loss from C60 +. One reason for this enormous scatter is the fact that the transition state of metastable C60 has unusual properties which, if not taken into account in the data analysis, lead to erroneous binding energies. We critically examine and re-analyze previously published reports within a common framework (with an A-factor of 5×1019   s −1 or, equivalently, a Gspann factor of 33.8). The resulting binding energies of 17 previously reported gas-phase studies agree surprisingly well with each other, and their average, E a =10.0±0.2  eV , agrees with theory.

We report on surface-enhanced Raman scattering (SERS) from bundles of size-selected single-wall carbon nanotubes (SWCNTs) deposited on ordered arrays of silver- and gold-coated silica spheres. Selective enhancement of the Raman-active tangential and radial modes of SWCNTs is exhibited in backscattering at surface plasmon resonance conditions. Enhancement at plasmon wavelength and sub-wavelength is shown for various periodic pitches of the substrates used.

Degenerate Four Wave Mixing on FeI atomic vapours during thermal decomposition of Fe(CO)5: saturation and absorption effects by L. De Dominicis; M. Di Fino; R. Fantoni; S. Martelli; O. Bomatı̀ Miguel; S. Veintemillas Verdagüer (209-216).
Thermal decomposition of Fe(CO)5 vapours diluted in a buffer gas (Ar) was performed in a static optical cell heated up to 673 K. The nascent FeI formation in the gas phase was monitored by recording the Degenerate Four Wave Mixing (DFWM) spectrum of the a 5D j y 5D0 j atomic transitions around 302 nm. Line-splitting phenomena, occurring upon strong saturation conditions at high laser power, were investigated on the DFWM lineshape. The role played by the absorption of exciting and signal beams in altering the relative lines intensities, predicted assuming an optically thin medium, is discussed and modelled in order to account for the experimental results.

On the fate of laser-produced NH2 in a constrained pulsed expansion of trimethylamine alane and NH3 by Alexander Demchuk; John J Cahill; Steven Simpson; Brent Koplitz (217-222).
The effects of both 193 nm radiation and NH3 on an expansion of trimethylamine alane (TMAA) have been studied. In neat TMAA, 193 nm radiation induces small but significant clustering. When NH3 is introduced in the absence of 193 nm photons, no visible reaction occurs. However, when NH3 is introduced along with 193 nm photons, the H3Al:N(CH3)3 parent molecule is completely replaced by H3AlNH2. Moreover, the clustering channels observed with neat TMAA disappear. The apparent stability or inertness of the R3MNH2 (R=H, CH3 or C2H5; M=Ga or Al) species in a variety of metal nitride reactive environments is discussed.

An accurate isotopically invariant potential function of the hydrogen sulphide molecule by Vladimir G. Tyuterev; Sergei A. Tashkun; David W. Schwenke (223-234).
The potential function of the electronic ground state of the hydrogen sulphide molecule is obtained by the simultaneous fit of a very large sample of high-resolution vibration–rotation data. The use of an extensive set of more than 12 000 experimental rovibrational transitions, corresponding to 4175 rovibrational states with J max=15, Ka  max =15 for bands of 7 isotopomers ( H 2 32 S,H 2 33 S,H 2 34 S,D 2 32 S,D 2 34 S , HD 32 S , and HD 34 S ), allows improved calculations of `local-mode states' and highly excited bending-rotational states. The root-mean-square (RMS) deviation of the rovibrational fit is 0.05  cm −1 . For the entire set of all 73 band centres of all isotopic species for which experimental data exists, the RMS deviation of the results of calculations with a single isotopic invariant potential function is 0.03   cm −1 .

The 1Bu-type singlet state of β-carotene as a precursor of the radical cation found in chloroform solution by sub-picosecond time-resolved absorption spectroscopy by Jian-Ping Zhang; Ritsuko Fujii; Yasushi Koyama; Ferdy S. Rondonuwu; Yasutaka Watanabe; Alan Mortensen; Leif H. Skibsted (235-241).
Generation of the radical cation from 1Bu-type states (1Bu + and/or 1Bu ) of all-trans-β-carotene in chloroform was demonstrated by sub-picosecond time-resolved absorption spectroscopy. In the visible region, ground-state absorption bleaching completely recovered in n-hexane but not in chloroform. In near-infrared, broad transient absorptions from 1Bu + and 1Bu states appeared and decayed within 1 ps in both solvents, while weak transient absorption ascribable to the cation appeared and stayed until 40 ps after excitation in chloroform, but not in n-hexane. In chloroform, direct transformation from the 1Bu-type singlet state to the radical cation had a time constant 0.14±0.03 ps.

Convective oscillation in binary and ternary polymer solutions with the Soret effect by Shinichi Sakurai; Yu-chang Wang; Takahiro Kushiro; Takeshi Nambu; Shunji Nomura (242-248).
We report experimental observations of a novel kind of convective oscillation in binary and ternary polymer solutions confined in a very thin layer (0.1 mm). This convective oscillation was considered to be driven by the Soret effect, which has been recently predicted by theory for a fluid mixture with a very small value of the Lewis number, namely, having much smaller mass diffusivity as compared to thermal diffusivity. For binary polymer solutions of polystyrene (PS) in dioctyl phthalate (DOP) and for ternary solutions of PS/PB/DOP (polybutadiene (PB)), the magnitude of convective flow was oscillating with time until reaching a stationary state.

An ESR study on structures of a series of silylnitrenes by Yoshiteru Itagaki; Takashi Iseoka; Toshiyuki Iida; Joji Ohshita; Masaru Shiotani; Atsutaka Kunai (249-254).
Structures of a series of silylnitrenes formed from silyl azides were investigated by means of ESR. γ-Irradiation and photo-illumination of all the silyl azides resulted in the formation of triplet states even for those having two or three Si–N3 groups in a molecule. ESR spectra of the silylnitrenes exhibited a part of the fine structure at around 820 mT. All the silyl azides studied gave nearly identical D-values (ca. 1.5  cm −1 ) and much larger than those in phenylnitrenes. The results suggested that electron spins are localized in the nitrogen p-orbitals to a large extent and was interpreted in terms of a mono-silane linkage of nitrene, –Si–N:, i.e., interrupting spin delocalization.

Fluorescence decay times of the nucleosides: adenosine, guanosine, cytidine and thymidine, and of the corresponding nucleotides, were determined using the technique of fluorescence up-conversion with femtosecond time resolution. The excited-state lifetimes of these nucleic acid molecules all fall in the sub-picosecond time scale, confirming the presence of an ultrafast internal conversion channel for both the nucleotides and the nucleosides; the nucleotides lifetimes are longer than those of the nucleosides by up to 20%. The ultrafast internal conversion is biologically relevant to the stability of DNA, and our results support the sub-picosecond repopulation of the ground state, consistent with transient absorption studies on the femtosecond time scale.

Proton magnetization enhancement of solvents with hyperpolarized xenon in very low-magnetic fields by S Appelt; F.W Haesing; S Baer-Lang; N.J Shah; B Blümich (263-269).
The polarization transfer between hyperpolarized xenon (Xe) and the protons (H) of different solutes at very low magnetic fields ranging from 0.001 to 0.014 T is reported. Due to Xe–H cross-relaxation (Spin Polarized Induced Nuclear Overhauser Effect (SPINOE)) the observed proton NMR signal of liquid ethanol, butanol and toluene is enhanced at room temperature by a factor of 1000 in a field of B=0.001 T. Smaller proton enhancement factors between 10 and 100 are measured for ethanol/water mixtures and for sunflower oil. Using high-field Xe NMR, the phase transitions of melting hyperpolarized Xe ice frozen onto the surface of ethanol ice is investigated.

The vibrational spectra of aniline(H2O) n + (n=2–6) in the 3 μm region have been measured. Cluster cations were generated by near resonant two-photon ionization of neutral clusters, and magic numbers were observed at n=4 and 5.The infrared spectra were investigated with the aid of the B3LYP/6-31++G** method. It was found that several isomers coexisted. A linear type cluster is the main component for the small clusters. A ring type cluster in which the NH2 group of aniline and water molecules form a ring appears in the spectrum of the clusters of n=3, and it becomes the main component in the clusters of n⩾4.

Photoinduced electron transfer in phenothiazine and pyrene based dyades studied by picosecond time-gated Raman spectroscopy by S Schneider; J Kurzawa; A Stockmann; R Engl; J Daub; P Matousek; M Towrie (277-284).
Picosecond time-resolved resonance Raman (TR3) spectroscopy was applied to investigate the photoinduced electron transfer in dyades employing phenothiazine as electron donor and pyrene as electron acceptor. Because the vibrational spectra of the pyrene excited state and the pyrene radical anion exhibit pronounced differences, the occurrence of photoinduced electron transfer can be proven beyond doubt for those derivatives, for which estimates yield ΔG et<0.

The interaction of HCl with the (0 0 0 1) face of hexagonal ice studied theoretically via Car–Parrinello molecular dynamics by Yves A. Mantz; Franz M. Geiger; Luisa T. Molina; Mario J. Molina; Bernhardt L. Trout (285-292).
The depletion of stratospheric ozone at high latitudes is caused by gas-phase catalytic cycles involving active chlorine compounds produced through heterogeneous chemical reactions involving HCl. In this Letter, results aimed at understanding the activation of chlorine on ice cloud particle surfaces are presented. We focus specifically on HCl interacting with the ice surface prior to reaction. We conclude that, in regions with a low surface density of dangling OH groups, HCl may be either ionically or molecularly adsorbed, but in regions with a high surface density of dangling OH groups, HCl is ionically adsorbed.

Low-power decoupling in fast magic-angle spinning NMR by Matthias Ernst; Ago Samoson; Beat H Meier (293-302).
We investigate the possibility of a low-power rf-irradiation approach to heteronuclear spin decoupling in solid-state NMR under high-frequency magic-angle sample spinning. Decoupling is achieved by applying an rf-field with an amplitude corresponding to a precession frequency much lower than the spinning frequency. This leads to a reversal of the averaging processes compared to normal high-power continuous-wave decoupling. Such an approach becomes increasingly interesting with increasing MAS spinning frequency. In rigid solids, low-power decoupling becomes competitive above about 40 kHz MAS frequency.

Theoretical study of the reaction of beryllium oxide with methane by Der-Yan Hwang; Alexander M Mebel (303-310).
Ab initio G2M(MP2) calculations show the BeO+CH4 reaction to proceed by barrier-less formation of the CH4BeO complex bound by 20.7 kcal/mol followed by isomerization to a CH3BeOH molecule (87.8 kcal/mol below BeO+CH4). CH3BeOH can dissociate without an exit barrier to BeOH+CH3 (0.5 kcal/mol below the reactants) or rearrange through a high barrier (25.7 kcal/mol above BeO+CH4) to a weakly bound CH3OHBe complex. Direct hydrogen abstraction from methane leading to BeOH and CH3 may be also feasible. The calculations demonstrate BeO as a useful catalyst at the initial stage of the conversion of methane to important organic compounds.

The overall atomic mobility of AOT reverse micelles in CCl4 as a function of the molar ratio W=[D2O]/[AOT] has been studied with an elastic neutron scattering experiment. The measured intensity suggests that, for small contents of water, the average reverse micelles mobility increases as a function of the hydration. A marked departure of the estimated mean square displacements from low hydration conditions has been revealed above W=1. Such a hydration-dependent dynamical activation is in agreement with previous NMR and dielectric measurements. The dependence of the atomic mean square displacements as a function of W has been explained by taking into account for the existence of two different dynamical contributions from more mobile completely-hydrated and less mobile partially-hydrated headgroups.

We present a simple closed-form equation for calculating vibrational polarisabilities of diatomic molecules. The equation enables one to estimate the effect of anharmonicity without numerically solving the Schrödinger equation.

Studies of the effects of hydrogen bonding on monolayer structures of C18H37X (X=OH, SH) on HOPG by Shuxia Yin; Chen Wang; Qingmin Xu; Shengbin Lei; Lijun Wan; Chunli Bai (321-328).
Based on Scanning tunneling microscopy (STM) observations of 1-C18H37X (X=OH, SH) adsorbed on HOPG, the hydrogen bonded complexes were studied using density functional theory (DFT) and molecular mechanics (MM). At B3LYP/6-31g and B3LYP/6-311g** levels, molecular dimers connected by hydrogen bond were obtained. The models of 1-C18H37X monolayer were optimized by consistent valence force field (CVFF), whose structures and interaction energies agree well with DFT calculations. It is noticed that due to the stronger hydrogen bond interactions between C18H37OH molecules, C18H37OH adsorbed on HOPG shows only one kind of molecular arrangement, while C18H37SH exhibits two kinds of packing induced by alkyl–alkyl and adsorbate–substrate interactions.

Ab initio molecular orbital calculations at the G2(MP2,SVP) level have been employed to explore a large part of the [H3,C,N,O] •+ potential energy surface. Ionized aminohydroxycarbene, NH2–C+• –OH, 1 , is found to correspond to the global minimum of the surface. The other stable species are also unconventional structures: ion–neutral complexes OC⋯NH3 +• , 2 , and CO⋯NH3 +• , 2 , and the distonic ion, H3N+CO, 3 . The more classical structures [HCONH2] •+, 4 , and [HC(OH)NH] •+, 5 are higher in energy. The heat of formation of the five radical cations have been determined using their atomization energies. The various isomerization reactions connecting 15 as well as their dissociation by H or CO losses have been theoretically investigated and compared with the available experimental data.

We propose a simple, efficient bridge correction of the one-dimensional reference interaction site model (1D-RISM) theory. By combining the modified RISM method with the Kirkwood–Buff theory, the partial molar volume (PMV) is calculated for the 20 amino acids and for oligopeptides of glutamic acids in extended and α-helix conformations. The bridge correction drastically improves agreement between the calculated values and the experimental data.

A microwave study of the hetero-chiral dimer of butan-2-ol by Adrian K. King; Brian J. Howard (343-349).
The first observation of a rotationally resolved spectrum for a dimeric complex of chiral molecules is reported. The complex in question is a hydrogen-bonded dimer of butan-2-ol and is shown to be hetero-chiral in composition. The rotational constants and components of the dipole moment agree well with the predictions of an ab initio study, the initial results of which are also reported. The ability of chiral species to form diastereoisomeric hetero- and homo-chiral complexes is thus given strong support.

The transition state (TS) for the Menshutkin reaction H3N+CH3Cl→H3NCH3 ++Cl in aqueous solution was located on the free energy surface (FES) by the free energy gradient (FEG) method. The solute–solvent system was described by a hybrid quantum mechanical and molecular mechanical (QM/MM) method. The reaction path in water was found to deviate largely from that in the gas phase. It was concluded that, in such a reaction including charge separation, TS structure optimization on an FES is inevitable for obtaining valid information about a TS in solution.