Chemical Physics Letters (v.374, #3-4)

An improved dimethyl sulfoxide force field for molecular dynamics simulations by Patrice Bordat; Javier Sacristan; Dirk Reith; Séverine Girard; Alice Glättli; Florian Müller-Plathe (201-205).
A united-atom molecular simulation force field for liquid dimethyl sulfoxide has been found to produce unacceptably inaccurate densities when used with a reaction-field or Ewald treatment of the electrostatic interactions. The force field is mildly reparameterized leading to a smaller dipole moment and slightly larger methyl groups. In addition to being compatible with the more sophisticated treatment of Coulombic interactions, the new force field also results in a significantly better description of the diffusion coefficient, the shear viscosity and the dielectric constant. Other liquid properties remain at the satisfactory quality of the Liu et al. model.

Ab initio study of absorption and emission spectra of PM567 by P. Acebal; S. Blaya; L. Carretero (206-214).
The visible-light absorption and emission excitation energies of PM567 dye have been studied with different ab initio methods (TDHF, CIS, MP2, MP3 and CISD). The results show that its visible-light spectra are mainly due to a transition between the HOMO to the LUMO, with the CISD size consistency corrections values being the most accurate respect to the experimental data. In order to minimize the computational cost of the highly correlated methods, the range of orbital energies that must be included in correlation correction to carry out a complete description of each method was also studied. This range was fixed to 34 eV for the occupied and virtual orbital energies in the case of MP3 and CISD methods.

Highly efficient and substrate independent CsF/Yb/Ag cathodes for organic light-emitting devices by M.Y. Chan; S.L. Lai; M.K. Fung; C.S. Lee; S.T. Lee (215-221).
The application of a cathode system comprising of an ultrathin CsF layer and an overlaid ytterbium (Yb) metal in organic electroluminescent devices is reported. This cathode shows dramatically reduced operating voltage and improved current and power efficiencies, both in small molecule or polymer based devices. The improvement is attributed to the dissociation of CsF upon Yb deposition to liberate low-work function Cs which reduces the electron injection barrier at the cathode/organic interface. Thermodynamically, the CsF dissociation is a substrate-independent process, and should be applicable for most organic based devices. Devices with this bilayer cathode system also show good operational stability.

Growth behavior of carbon nanotubes on multilayered metal catalyst film in chemical vapor deposition by H. Cui; G. Eres; J.Y. Howe; A. Puretkzy; M. Varela; D.B. Geohegan; D.H. Lowndes (222-228).
The temperature and time dependences of carbon nanotube (CNT) growth by chemical vapor deposition are studied using a multilayered Al/Fe/Mo catalyst on silicon substrates. Within the 600–1100 °C temperature range of these studies, narrower temperature ranges were determined for the growth of distinct types of aligned multi-walled CNTs and single-walled CNTs by using high-resolution transmission electron microscopy and Raman spectroscopy. At 900 °C, in contrast to earlier work, double-walled CNTs are found more abundant than single-walled CNTs. Defects also are found to accumulate faster than the ordered graphitic structure if the growth of CNTs is extended to long durations.

Ab initio path integral study of isotope effect of hydronium ion by Motoyuki Shiga; Masanori Tachikawa (229-234).
The differences in molecular fluctuation of protonated water composed of hydrogen isotopes are elucidated by ab initio path integral molecular dynamics simulation, in which ‘on-the-fly’ calculation of potential energy surface is done accurately with the MP2 electron correlation to sample quantum nuclear configurations. It turned out that, at the room temperature 298 K, H3O+ is quite flexible in which pyramidal structure is inverted frequently with multi-dimensional molecular distortion, while the flexibility of D3O+ is much more restrained. Our analysis shows that anharmonicity in full-dimensional rovibrational motion is essential in this isotope effect.

Fast beam studies of I2 and I2  · Ar photodissociation by Alexandra A. Hoops; Jason R. Gascooke; Ann Elise Faulhaber; Kathryn E. Kautzman; Daniel M. Neumark (235-242).
The photodissociation dynamics of bare I2 and I2  · Ar at 413 and 390 nm have been investigated using a fast beam instrument coupled with a new photofragment coincidence imaging detector. Results from the application of this technique to the dissociation of I2 and I2  · Ar yielded the dissociation energy of I2 (D 0(I2 )=1.012±0.008 eV) and I2 –Ar binding energy (D 0(I2 –Ar)=45±8 meV). The experiments show that at these wavelengths, I2  · Ar undergoes three-body dissociation to I  + I* + Ar, with very low momentum in the Ar atom and unequal momentum partitioning between the two I atoms.

Quasi-classical trajectory calculations on a fast analytic potential energy surface for the C(1 D)+H 2 reaction by Luis Bañares; F.J. Aoiz; Saulo A. Vázquez; Tak-San Ho; Herschel Rabitz (243-251).
Quasi-classical trajectory (QCT) calculations have been carried out on a new implementation of the first singlet state a ̃ 1 A potential energy surface (PES) of the C(1 D)+H 2 system based on a set of 1748 ab initio points previously reported. The implementation is performed by using the Reproducing Kernel Hilbert Space (RKHS) interpolation method, which allows the fast evaluation of the PES values and, particularly, their gradients analytically. Although there is a general good correspondence between the present surface and the previous version, the new PES is free of spurious small scale features and permits a faster evaluation of the PES and its gradients. The QCT results on the present PES are in general good agreement with those obtain on the previous PES.

Characterization of cross-linked natural rubbers by proton longitudinal and rotating-frame relaxometry by H. Chaumette; D. Grandclaude; J.-C. Boubel; D. Canet (252-256).
A series of rubbers has been investigated in the 1–100 kHz range by proton relaxometry (measurement of the nuclear relaxation rate as a function of the amplitude of the magnetic field along which nuclear magnetization is aligned: the static field B 0 for the longitudinal rate R 1, the radio-frequency field B 1 for the rotating-frame rate R 1ρ ). Data can be interpreted according to R i =a i +b i /ν α i , where ν 0,1=γB 0,1/2π (γ being the gyromagnetic ratio), the index i characterizing the sample as well as the nature of the relaxation parameter (longitudinal or rotating-frame). a i and b i are related to residual dipolar couplings and thus afford the possibility of material discrimination.

Spectroscopic studies of the role of silyl radicals in photolysis of polysilanes by D. Azinović; D. Bravo-Zhivotovskii; M. Bendikov; Y. Apeloig; B. Tumanskii; S. Vepřek (257-263).
Absorption, ESR and photoluminescence of hexakis(pentamethyldisilyl)disilane [(Me3SiMe2Si)3Si]2 1 were studied. Two branched polysilyl radicals were observed by ESR spectroscopy under UV irradiation of 1. The smaller ‘star’ radical (Me3SiMe2Si)3Si 2 has half-lifetime 1 s whereas the bigger one (Me3SiMe2Si)3Si(Me3SiMe2Si)2Si 3 has a half-lifetime about 50 min. During irradiation the reaction 1  →  2  +  3 followed by 2  +  3  → products (P) occur. The behavior of the 300 nm absorption of radical 3 during the irradiation was found to be similar to the radical 3 appearance measured by ESR. The silane (Me3SiMe2Si)3SiH was found as main product (∼50% yield) of the photo-cleavage of the disilane 1.

Effects of N ion energy on titanium nitride films deposited by ion assisted filtered cathodic vacuum arc by G.Q. Yu; B.K. Tay; S.P. Lau; K. Prasad; L.K. Pan; J.W. Chai; D. Lai (264-270).
Titanium nitride (TiN) films were synthesized at ambient temperature on (1 0 0) silicon substrates by a filtered vacuum cathodic arc source and simultaneous bombardment from nitrogen ion beam. The effects of N ion energy on the film morphology, composition, microstructure, resistivity and mechanical properties were systematically investigated. With increasing N ion energy, the film was initially composed of many small spiky grains separated by deep boundaries, and then of many domed grains isolated by shallow boundaries, and finally of far smaller and much more spiky grains with the deepest boundaries. The chemical composition did not vary much with the ion energy. Increasing N ion energy also resulted in the change of the film phase composition and texture, where the competition between the strain energy and the stopping energy determined the film orientation. As the ion energy increased, the film resistivity increased while the film hardness decreased. The films possessed adhesion property independent of N ion energy and superior to that of the films deposited by magnetron sputtering. Some explanation will be given in detail.

Hydrogen bonding of DNA base pairs by consistent charge equilibration method combined with universal force field by Tetsuji Ogawa; Noriyuki Kurita; Hideo Sekino; Osamu Kitao; Shigenori Tanaka (271-278).
By combining the consistent charge equilibration (CQEq) method with universal force field (UFF), we developed the CQEq with UFF (CUFF) method and confirmed its efficiency for investigating the hydrogen bonding of DNA base pairs. By using the new parameters together with reduced van der Waals radii for hydrogen atoms, the CUFF method was capable of yielding hydrogen-bonding lengths and energies comparable with those by correlated ab initio MP2 calculations.

Photo-assisted growth and polymerization of C60 ‘nano’whiskers by M. Tachibana; K. Kobayashi; T. Uchida; K. Kojima; M. Tanimura; K. Miyazawa (279-285).
C60 nanowhiskers are grown by a liquid–liquid interfacial precipitation method at 21 °C. It is found that the growth of the nanowhiskers is promoted under illumination even with weak room light (fluorescent light). The maximum of their lengths exceeds 1 mm where the diameter is about 250 nm. From Raman scattering measurements, it is shown that C60 molecules in the pristine nanowhiskers are bonded by weak van der Waals forces. The nanowhiskers is easily polymerized under irradiation with laser light. The cross section for this photo-polymerization is much large compared with that for intrinsic C60 bulk crystals.

Molecular interactions of TATB clusters by Szczepan Roszak; Richard H. Gee; Krishnan Balasubramanian; Laurence E. Fried (286-296).
Electronic structure calculations of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) clusters are presented. The calculated gas phase structures of TATB are close to the experimental crystal structures. Two isomers of the TATB dimer are considered. One resembles the structures found experimentally for the A–B sheet of the molecular crystal. The other, a stacked ring (‘C’) configuration, yields an eclipsed structure, while the crystal data suggest two rings displaced relative to each other. Electron correlation effects are of greater importance for the stacked C-dimer than the planar AB-dimer. Furthermore, many-body contributions to the total interaction energy are found to be of limited importance.

Potential curve of the 41Σ+ state of NaK by polarisation labelling spectroscopy by W. Jastrzebski; R. Nadyak; P. Kowalczyk (297-301).
The 41Σ+ state of NaK molecule has been observed for the first time and studied by polarisation labelling spectroscopy technique. A total of 1567 rovibronic levels in this state were measured with an accuracy better than 0.1 cm−1. The inverted perturbation approach method was used to construct the potential energy curve of the 41Σ+ state. The irregular shape of the potential confirms earlier theoretical predictions.

Nonresonant enhancement factors in excess of 1011 for surface enhanced Raman spectroscopy were recorded for rhodamine 6G dye on surface plasmon resonance wavelength tunable gold films. Spectra were obtained from submonolayer samples, corresponding to the detection of at most 180 attograms of analyte.

Non-adiabatic effects in the photodissociation of bromoacetyl chloride by Marie-Christine Bacchus-Montabonel; Nathalie Vaeck; Benjamin Lasorne; Michèle Desouter-Lecomte (307-313).
The competitive photodissociation of bromoacetyl chloride has been investigated by means of ab initio methods. Quantum dynamics in full dimensionality is prohibitive for such a system and therefore a reduced dimensionality method based on constrained Hamiltonians has been used. A one-dimensional (1-D) non-adiabatic wave packet treatment in the CS optimized geometry (trans Cl and Br) on the first excited states leads to encouraging results when compared to experimental data. The slow relaxation of the torsion angle is assessed by a 2-D dynamics in the subspace including the CO bond length.

Photoluminescence spectra and ferromagnetic properties of GaMnN nanowires by F.L. Deepak; P.V. Vanitha; A. Govindaraj; C.N.R. Rao (314-318).
Mn-doped GaN nanowires have been prepared by reacting a mixture of acetylacetonates with NH3 at 950 °C in the presence of multi-walled (MWNTs) and single-walled (SWNTs) carbon nanotubes, the nanowires prepared with SWNTs being considerably smaller in diameter. GaMnN nanowires with 1%, 3% and 5% Mn so obtained have been characterized by X-ray diffraction, EDAX analysis and photoluminescence (PL) spectra. The GaMnN nanowires are all ferromagnetic even at 300 K, exhibiting magnetic hysteresis. The PL spectra of the GaMnN nanowires prepared with SWNTs show a large blue-shift of the Mn2+ emission.

The growth of CdS nanoparticles during aging at ambient temperature is investigated with respect to the type of stabilizer, the concentration of the colloidal solution, and the molar ratio of sulfide to cadmium. The UV/Vis absorption spectra recorded within a period of seven days are analyzed by Karhunen–Loeve decomposition. It revealed a single process with first-order decomposition kinetics for each of the measured time series, and the respective rate constants are calculated. An explanation describing possible mechanisms for the observed ripening of the colloids is given.

Impulsive stimulated Raman scattering of molecular vibrations using nonlinear pulse shaping by R.A. Bartels; S. Backus; M.M. Murnane; H.C. Kapteyn (326-333).
We demonstrate the excitation of a molecular vibrational coherence via impulsive stimulated Raman scattering (ISRS) by using a pump pulse longer than the vibrational period of the excited mode. By making use of the reshaping of the pump pulse during nonlinear propagation, a Raman vibrational coherence can be excited via ISRS with a pump pulse incapable of such excitation at the input of the medium.

Dissociative photoionization dynamics in ethane studied by velocity map imaging by Wen Li; Lionel Poisson; Darcy S. Peterka; Musahid Ahmed; Robert R. Lucchese; Arthur G. Suits (334-340).
We have studied the dissociative ionization of ethane from 12.0 to 12.8 eV using velocity map imaging with synchrotron radiation. We obtained translational energy release distributions and angular distributions for the C2H4 + product. The translational energy distributions are consistent with a ∼0.6 eV barrier to H2 elimination in the ethane cation, while the angular distributions are characterized by a limiting anisotropy parameter of β=−0.3. Theoretical calculations predict a lower anisotropy; possible reasons for the discrepancy are considered.

SERS is observed for thiophene-2-carboxylic acid (TCA) adsorbed on Ag-sols. The enhancement is maximum when TCA concentration, in Ag-sol, is 5 × 10−3 M. The SER spectra of TCA are different at different pH values, which suggest that the adsorption geometry is pH dependent. This is further supported by the absorption spectra at different pH values. It is inferred that the chemisorption takes place through both the S-atom and COOH group for pH ⩽ 7 and through the π-electronic system for pH=10.5. Quick desulphurization at pH=2 and stable perpendicular and parallel surface bonding for pH=7 and 10.5, respectively, is revealed.

Novel bismuth nanotube arrays synthesized by solvothermal method by Xin-yuan Liu; Jing-hui Zeng; Shu-yuan Zhang; Rong-bo Zheng; Xian-ming Liu; Yi-tai Qian (348-352).
Novel bismuth nanotube arrays have been successfully synthesized by a solvothermal method through the reduction of bismuth oxide (Bi2O3) by ethylene glycol (EG). The productions were characterized by XRD, TEM and HRTEM. The diameter of the nanotubes is about 3–6 nm and length is up to 500 nm. The possible growth mechanism of Bi nanotubes was discussed.

Pulse radiolysis study of radical cations of polysilanes by Tomoyo Kawaguchi; Shu Seki; Kazumasa Okamoto; Akinori Saeki; Yoichi Yoshida; Seiichi Tagawa (353-357).
Molar extinction coefficients of radical cations are quantitatively discussed for a variety of substituted polysilanes by nano-second pulse radiolysis. Polysilane solution in benzene under saturated oxygen exhibits a strong absorption band ascribed to polysilane radical cation in the transient spectra. The transient species react with N,N,N ,N -tetramethyl-p-phenylene-diamine (TMPD) to produce TMPD radical cations. Using the molar extinction coefficient of TMPD radical cation, the molar extinction coefficients for the radical cations of polysilanes are found to increase in the range 3.8 × 104–15.0 × 104 M−1 cm−1 with an increase in the segment length of the polymers. This is the first report on the extinction coefficient and oscillator strength of polysilane radical cations.

Self-assembled array of boron oxide nanowires on Mg surface by Renzhi Ma; Yoshio Bando (358-361).
A vitreous layer of boron oxide was formed on Mg ribbons by infrared (IR) irradiation of B + B2O3 powders (10:1 in molar ratio). The surface layer exhibits an evolution from the honeycomb pattern to self-assembled array of boron oxide nanowires depending on the irradiation time. The boron oxide nanowires, approximate 40–100 nm in diameter and 1–2 μm in length, are of amorphous nature. The nanowires are likely to be grown in a self-assembled feature during the cooling process of molten B2O3 and oxidized boron.

Ultrafast heterodyne detected infrared multidimensional vibrational stimulated echo studies of hydrogen bond dynamics by John B. Asbury; Tobias Steinel; C. Stromberg; K.J. Gaffney; I.R. Piletic; Alexi Goun; M.D. Fayer (362-371).
Multidimensional vibrational stimulated echo correlation spectra with full phase information are presented for the broad hydroxyl stretch band of methanol-OD oligomers in CCl4 using ultrashort (<50 fs) infrared pulses. Hydrogen bond breaking permits data to be acquired for times much greater than the vibrational lifetime. The data indicates that vibrational relaxation leads to preferential hydrogen bond breaking for oligomers on the red side of the spectrum. An off diagonal peak in the correlation spectrum that appears at long time (>1 ps) shows that there is frequency correlation between the initially excited hydroxyl stretch and the frequency shifted hydroxyl stretch formed by hydrogen bond breaking.

Comparison of experimental and calculated 1H NMR chemical shifts of geometric photoisomers of azo dyes by Katrina M. Tait; John A. Parkinson; Anita C. Jones; Warren J. Ebenezer; Simon P. Bates (372-380).
Quantum chemical calculations based on Density Functional Theory have been used to predict 1H NMR chemical shifts of the cis and trans isomers of three model azo-dye compounds. Calculated absolute chemical shift values, and differences between isomers, were in good agreement with experimentally assigned 1H NMR data. Simulated NMR spectra based on calculated chemical shifts proved useful in identifying differences between predicted and experimental data. The technique could prove valuable in assisting assignment of NMR spectra of more complex dyes of commercial interest, and preliminary investigations on larger systems indicate this is feasible.

Two-photon polymerization of gratings by interference of a femtosecond laser pulse by Hengchang Guo; Hongbing Jiang; Le Luo; Chengyin Wu; Hongcang Guo; Xi Wang; Hong Yang; Qihuang Gong; Feipeng Wu; Tao Wang; Mengquan Shi (381-384).
Two-photon polymerization, initiated by 2,7-bis[[4-(dimethylamino)-phenyl] methylene]-cycloheptanone and 1,1,2,2-bis(o-chlorophenyl)-4,4,5,5-tetraphenyl-bisimidazole, was employed to fabricate refractive index-modulated holographic gratings by the interference of a femtosecond laser pulse at wavelength of 810 nm. The diffraction efficiency of the grating was up to 57% and the refractive index modulation reached 4.3×10−3 after thermal fixation with the film thickness of 40 μm. And dependence of the diffraction efficiency and refractive modulation on laser pulse energy was also investigated.

The hybrid density functional B3LYP method has been used to study the mechanism of disilane formation from silylenes. The main findings can be summarized as follows: (a) Lewis acid–base complexes between silylenes and halocarbons only play a limited role in silylene insertions, and therefore the acid–base complex mechanism proposed by West et al. does not apply to disilane formation. (b) Regardless of whether transient or stable, all silylenes follow the energetically favorable general reaction pathway: (I) Y2Si:+ClCR3  → TS1 → Y2ClSi–CR3. (II) Y2Si:+Y2ClSi–CR3  → TS2 → Y2ClSi–SiY2CR3.

A new potential energy surface for He–H2CO by Martyn D. Wheeler; Andrew M. Ellis (392-399).
The intermolecular potential energy surface (PES) for He–H2CO has been calculated using the CCSD(T) method. A basis set extrapolation procedure was employed to estimate the complete basis set limit. For comparison, analogous calculations were also carried out on He–CO. Three minima were identified for He–H2CO, all arising from planar structures of the complex. The global minimum corresponds to an approximately T-shaped arrangement of the helium atom with respect to the CO bond. A well depth of 59.5 cm−1 was obtained for the global minimum, more than double that reported in an earlier ab initio study of He–H2CO. The implications of this new PES for spectroscopic studies of He–H2CO and for understanding He + H2CO scattering processes is discussed.

Carbon-encapsulated Fe3C nanoparticles with diameter of about 10 nm were synthesized on a large scale via co-carbonization of durene and ferrocene at 540 °C under autogeneous pressure. TEM and HREM examinations confirmed that the carbon-encapsulated Fe3C nanoparticles were uniformly dispersed in carbon matrix and the Fe3C nanoparticles were surrounded by several carbon layers with well ordered arrangement. The formation mechanism of carbon-encapsulated Fe3C nanoparticles was discussed briefly. This novel and simple approach constituted a more practical method to prepare encapsulated nanoparticles than the present ones.

High-energy structures of azafullerene C48N12 by M. Riad Manaa; David W. Sprehn; Heather A. Ichord (405-409).
We report optimized geometries and harmonic vibrational frequencies of new N–N linked, fullerene-analog structures of C48N12 with 6N2, 4N3, and 2N6 subunits, using B3LYP/6-31G* level of theory. These are high-energy structures with relative energy of 112, 195, and 269 kcal/mol, respectively. The N–N bonds in these isomers are in the range of 1.48–1.56 Å, indicative of weak, van der Waals type interactions rather than the N–N covalent bonds usually found in polynitrogen.

Organic-assisted growth of bismuth telluride nanocrystals by Yuan Deng; Ce-Wen Nan; Guo-Dan Wei; Lin Guo; Yuan-hua Lin (410-415).
Bi2Te3 nanocrystals with various shapes such as sheet, rag, sheet-rod and rod shapes were synthesized via a solvothermal process based on the reaction between BiCl3 and Te in N,N-dimethylformamide at 100–180 °C at the present of organic addition and/or reducing agent. The products were characterized by X-ray diffraction, X-ray fluorescence (XRF) analysis, inductively coupled plasma-atomic emission spectroscopy (ICP-AES) and transmission electron microscopy (TEM). KBH4 was used as a reducing agent. Organic additive plays a key role in the formation of the lamellar structures. A possible formation mechanism is proposed.