Chemical Physics Letters (v.345, #1-2)

Surface orientation in ionic liquids by George Law; Philip R Watson (1-4).
New surface tension data supports models of surface orientations previously derived from direct recoil spectrometry (DRS) data for the cations of 1-alkyl-3-methylimidazolium ionic liquids. For shorter-chain hexafluorophosphate compounds the favored average orientation appears to be one where the cation is oriented vertically with the N atoms uppermost, but the alkyl substituents do not protrude out of the surface. For longer chains or for compounds containing the smaller tetrafluoroborate anion the surface tension and DRS data are consistent with a shift to an average orientation embodying a rotation that moves the methyl group towards the surface.

Synthetic strategies for Y-junction carbon nanotubes by F.L Deepak; A Govindaraj; C.N.R Rao (5-10).
Y-junction carbon nanotubes can be produced in relatively large quantities by the vapor phase pyrolysis of a mixture of cobaltocene or ferrocene with thiophene in a hydrogen atmosphere. Pyrolysis of Ni- and Fe-phthalocyanines or Fe(CO)5 with thiophene also yields Y-junction nanotubes. Good yields of Y-junction nanotubes were obtained by pyrolyzing thiophene over an Ni(Fe)/SiO2 catalyst as well. Many of the nanotubes show multiple Y-junctions. Of the various methods employed by us, the pyrolysis of thiophene with metallocenes or over a Ni(Fe)/SiO2 catalyst yields the best results and the latter is a less expensive route. The availability of large quantities of Y-junctions by the procedures employed here should render them useful for exploitation in nanoelectronics.

Characterization of single-wall carbon nanotubes produced by CCVD method by J.-F Colomer; J.-M Benoit; C Stephan; S Lefrant; G Van Tendeloo; J B.Nagy (11-17).
Carbon single-wall nanotubes (SWNTs) can be produced by the catalytic chemical vapor deposition (CCVD) method. They are synthesized by catalytic decomposition of methane at 1000 °C on 2.5 wt% Co/MgO catalyst. SWNT samples have been characterized by transmission electron microscopy (TEM) and Raman spectroscopy. Using these two techniques, a comparison between the SWNTs produced by CCVD and synthesized by electric arc discharge has been made. Finally, we give conclusions about the diameter distribution and the electronic structure of SWNTs produced by the CCVD method.

Adsorption and capillarity of nitrogen in aggregated multi-walled carbon nanotubes by Quan-Hong Yang; Peng-Xiang Hou; Shuo Bai; Mao-Zhang Wang; Hui-Ming Cheng (18-24).
Pores in aggregated multi-walled carbon nanotubes (MWNTs) can be mainly divided into inner hollow cavities of smaller diameter (narrowly distributed, mainly 3.0–4.0 nm) and aggregated pores (widely distributed, 20–40 nm), formed by the interaction of isolated MWNTs. The two types of pores shall, respectively, determine nitrogen cryogenic capillarity process under different pressures. It is worth to note that ultra-strong nitrogen capillarity in the aggregated pores (590  mg N 2/g ) contributes to the 78.5% of the total adsorption amount (up to 750 mg/g near to the ambient pressure), showing that the aggregated pores of the MWNTs are much more important than their inner cavities for adsorption and capillarity in some cases.

Determination of the acidic sites of purified single-walled carbon nanotubes by acid–base titration by H Hu; P Bhowmik; B Zhao; M.A Hamon; M.E Itkis; R.C Haddon (25-28).
We report the measurement of the acidic sites in three different samples of commercially available full-length purified single-walled carbon nanotubes (SWNTs) – as obtained from CarboLex (CLI), Carbon Solutions (CSI) and Tubes@Rice (TAR) – by simple acid–base titration methods. Titration of the purified SWNTs with NaOH and NaHCO3 solutions was used to determine the total percentage of acidic sites and carboxylic acid groups, respectively. The total percentage of acidic sites in full length purified SWNTs from TAR, CLI and CSI are about 1–3%.

Solid–liquid–solid (SLS) growth of coaxial nanocables: silicon carbide sheathed with silicon oxide by Y.J Xing; Q.L Hang; H.F Yan; H.Y Pan; J Xu; D.P Yu; Z.H Xi; Z.Q Xue; S.Q Feng (29-32).
Coaxial silicon carbide–silicon oxide nanocables on silicon substrates were synthesized from the ternary system of Si–Ni–C at 950°C under Ar/H2 atmosphere. The nanocables consist of a hexagonal crystalline SiC core and a surface layer of amorphous silicon oxide, which have an average diameter of ∼50 nm and a length of several tens of microns. The microstructure and composition of the nanocables were characterized using high-resolution transmission electron microscope (HREM), and electron energy loss spectroscopy (EELS), and the growth mechanism of the nanocables was explained under the framework of a solid–liquid–solid (SLS) mechanism.

The intramolecular photoinduced energy and electron transfer in a fullerene–oligothiophene–fullerene triad with nine thiophene units (C60–9T–C60) has been investigated with sub-10 fs pump–probe spectroscopy in solvents of different polarity. Photoexcitation of the oligothiophene moiety results in an ultrafast singlet-energy transfer reaction to create the fullerene singlet-excited state with a time constant of 95 fs, irrespective of the polarity of the medium. In a polar solvent, the ultrafast singlet-energy transfer is followed by intramolecular electron transfer from the oligothiophene to the fullerene with a time constant of 10 ps, resulting in a charge-separated state (CSS) with a lifetime of 80 ps.

Controlled growth of carbon nanotubes in diameter and shape using template-synthesis method by Zhi-hao Yuan; Hua Huang; Liang Liu; Shou-shan Fan (39-43).
A template-synthesis technique based on self-ordered hexagonal nanopore alumina templates was applied for synthesizing highly ordered carbon nanotube arrays via thermal decomposition of ethylene with iron as catalyst electrodeposited into the bottom of the pores. It was found that the resultant carbon nanotubes in diameter and shape exactly reflect the pore of the template used. This shows that the diameter of nanotubes can be easily controlled and adjusted through the pore sizes of the templates, and the nanotubes with various shapes can be grown by applying templates with the same pore shapes as those of the tubes. This controllable synthesis method will be of great fundamental and practical interests.

14N/15N kinetic isotope effect in the association reaction O(3 P)+NO+Ar→NO 2+Ar by Hironobu Umemoto; Kunikazu Tanaka; Shigeki Oguro; Ryoji Ozeki; Masashi Ueda (44-50).
The termolecular rate constants for the O(3 P)+14 NO+Ar→14 NO 2+Ar and O(3 P)+15 NO+Ar→15 NO 2+Ar reactions were determined under room temperature bulk conditions. O(3P) was produced by the pulsed photodissociation of SO2 at 210.4 nm and was monitored by two-photon laser-induced fluorescence at 225.7 nm. The rate constants for 14NO and 15NO were determined to be 5.4±0.2 and 6.1±0.3×10−32   cm 6   s −1 , respectively. The error limits are twice the standard errors (S.E.). This isotope effect is opposite to that expected from a statistical model but is similar to that observed in the O3 formation reactions from O(3P) and O2.

A photodiode-array (PDA) UV–VIS detector for liquid chromatography is applied to time-resolved reaction yield detected magnetic resonance (RYDMR) measurements. The results derived from the yields of cage and escape products in the photoreaction of 2-methyl-1, 4-naphtnoquinone in a sodium dodecylsulfate micelle are found to be identical with those derived from the yield of escaping semiquinone radical detected by transient optical absorption. This implies practical linearity between the yields of escaping radicals and escape products. High sensitivity of the PDA detector enables application of escape product yields for kinetic analysis by reducing microwave-induced perturbation.

VUV photoionization of (CH3I) n (n=1–4) molecules by Jun Chen; Linsen Pei; Jinian Shu; Congxiang Chen; Xingxiao Ma; Liusi Shen; Yunwu Zhang (57-64).
The photoionization efficiency (PIE) spectra of the ions produced by the VUV synchrotron radiation photoionization of (CH3I) n (n=1–4) molecules in the photon energy range of 8–35 eV have been measured by using time-of-flight (TOF) mass spectrometer with supersonic cooling techniques. The appearance potentials (APs) of all the observed fragment ions have been determined from the PIE curves. Based on these data, the bond energies (D 0), standard formation enthalpies (Δ f H 0 °) and proton affinities (PA0) of the related fragments were obtained. Several autoionization structures of CH3I in the PIE spectrum of CH3I+ were assigned.

In this Letter, we report the unusual behavior of zeolites with LTA structure (a prototype of synthetic zeolite) under high external pressure. We found that these materials undergo pressure-induced amorphization. Interestingly, the pressure-induced amorphization is reversible for some of these materials. Apparently, the amorphization occurs via the weakening of the double four membered rings in zeolite lattice. We found that the structural memory of the glasses and the stability of the LTA zeolites depend strongly on the nature of the charge balancing cations present in the framework.

Velocity map imaging of femtosecond photodynamics in CF3I by W.G Roeterdink; M.H.M Janssen (72-80).
The femtosecond photodynamics of CF3I were studied employing the velocity mapping ion imaging technique. The angular and velocity distributions of I+ fragments were obtained as a function of the time delay between a femtosecond pump pulse at 264 nm and a probe pulse at 396 nm. The images reveal a dependence of the three-dimensional recoil distributions on the pump–probe delay time and the mutual polarization direction of pump and probe pulses. The ion velocity images provide information on the competition of different multi-photon pathways producing the ionic fragments.

The dynamics of intramolecular charge transfer (CT) in a family of conjugated aromatic electron donor/acceptor (D/A) systems has been studied by femtosecond pump-probe spectroscopy. Although the compounds have very similar structures we observed vastly different CT dynamics, ranging from a few 100 fs to the nanosecond time range. These results show that specific modifications in the substitution scheme can lead to alterations of the relevant D/A orbitals and thus to drastic changes in the excited state electronic coupling. As a result the CT process switches from the ultrafast adiabatic to the slow nonadiabatic regime.

Rotationally resolved A 3Σ u –X 3Σ g electronic transition of NC5N by H Linnartz; O Vaizert; P Cias; L Grüter; J.P Maier (89-92).
The rotationally resolved A 3Σ u –X 3Σ g electronic spectrum of the NC5N radical has been observed in the gas phase by cavity ring down spectroscopy in a supersonic plasma. The origin band is at ν00=22 832.7(1)  cm −1 and a rotational analysis gives constants B 0″=0.02799(4) and B0 =0.02778(3)  cm −1 . These are compared to the B e values available from structures predicted by density functional theory and show that the molecule has a linear and centro-symmetric NCCCCCN structure.

Ultrasonic absorption spectra have been measured in the frequency range from 40 kHz to 2 GHz for solutions of d-fructose in water and in two water–ethanol mixtures as well. It is found that a low frequency relaxation with relaxation time around 1 μs exists in the aqueous solution. The amplitude of that relaxation term is considerably smaller in the aqueous-ethanolic systems, in conformity with a reduction of the content of the α-d-fructopyranose tautomer. Our measurements confirm the assignment of the low frequency acoustical relaxation to a 2 C 5⇌5 C 2 ring inversion of α-fructopyranose in solution.

The growth and structure of anionic micelles sodium dodecyl trioxyethylene sulfate (SDES) in the presence of bivalent counterion Ca2+ were investigated by means of rheological methods and the technique of freeze-fracture transmission electron microscopy (FF-TEM). It was observed that wormlike micelles could be formed in SDES/CaCl2 aqueous micellar solutions, according to the measurements of zero-shear viscosity η 0, complex viscosity |η *|, dynamic moduli (storage modulus G and loss modulus G″), and the application of Cox–Merz rule and Cole–Cole plot. The structure was of character of a non-linear viscoelastic fluid and departed from the simple Maxwell model.

This Letter addresses process relaxation dynamics occurring in colloidal zirconia (ZrO2) nanosols under steady-state and pulsed laser excitation; the temperature dependence of the photoluminescence is also reported. The laser-induced emission emanating from these nanosols is thermally quenched with activation energy E tq =0.048±0.008  eV , identical to the activation energy (E tq =0.050±0.005  eV ) of thermal quenching of the photoluminescence under steady-state excitation. It is also identical to the energy (E ads =0.046±0.005  eV ) of thermal quenching of the photostimulated adsorption of oxygen on ZrO2 micron-size particles. A pathway to summarize the events is proposed.

Theoretical study of the CuRu+H2 molecular interaction by F Colmenares; A Ramı́rez-Solı́s; O Novaro (111-117).
The results of Hartree–Fock (with relativistic effective core potentials) followed by variational and second-order multireference perturbational configuration interaction (CIPSI) calculations are reported for the nine lowest-lying electronic states of the CuRu molecule and the eight low-lying states of the CuRuH2 molecule belonging to the C2v symmetry group which evolve from the H2 side-on interaction with the ruthenium atom. The effect of the copper atom on the ability of the ruthenium atom toward the capture and dissociation of the H2 molecule is analyzed through comparison with the results previously reported for the simpler systems RuH2 and RuH2 +. For all the reaction channels investigated here, the copper atom depresses the ability of ruthenium to dissociate the H2 molecule.

The NO+MO/HF theory has been previously proposed to determine the nuclear and electronic wave functions in the ground state without the Born–Oppenheimer approximation. In this study, we apply the configuration interaction method with single particle excitation operators to the NO+MO/HF wave function. This method, named NO+MO/CIS method, gives not only the electronic excited state but also the vibrational excited state. Numerical applications of the NO+MO/CIS method to H2, D2, T2, and H3 + molecules are performed and confirm its accuracy and feasibility.

Inverse isotope effects in the superconductivity of acenes: a theoretical study by Takashi Kato; Kazunari Yoshizawa; Tokio Yamabe (125-131).
Electron–phonon coupling constants are calculated for organic molecular crystals of anthracene, tetracene, and pentacene and their full deuterides. We find that the low-frequency C–C–C in-plane modes of 200–400  cm −1 and the C–C stretching modes of 1400–1600  cm −1 play an important role in the electron–phonon coupling in these acenes. Inverse isotope effects for hydrogen–deuterium substitution are predicted in the superconductivity of acenes. Such inverse isotope effects can be explained by electron–phonon couplings in the C–C stretching modes, which significantly increase by deuterium substitution.

Influence of crystal-field perturbations on the room-temperature magnetic anisotropy of lanthanide complexes by Vladimir S. Mironov; Yury G. Galyametdinov; Arnout Ceulemans; Christiane Görller-Walrand; Koen Binnemans (132-140).
The room-temperature magnetic susceptibility anisotropy of eight-coordinate lanthanide ions was modelled numerically for polyhedra resulting from distortions of the regular cube to a tetragonal prism, antiprism, and dodecahedron. Our aim is to illustrate how the magnitude and sign of the room-temperature magnetic anisotropy can be related to the shape of the coordination polyhedron and to estimate its maximum value. Tb(III), Dy(III), and Tm(III) ions are found to have the largest values of the magnetic anisotropy in all coordination polyhedra. These results are helpful to rationalize the orientational behavior of lanthanide-containing liquid crystals in an external magnetic field.

A study of energy transfer processes in zinc–porphyrin films using Monte Carlo simulation of fluorescence decay by Mikalai M Yatskou; Harry Donker; Rob B.M Koehorst; Arie van Hoek; Tjeerd J Schaafsma (141-150).
Energy transfer in thin films of zinc tetra(-octylphenyl)-porphyrin (ZnTOPP) doped with the copper (CuTOPP) or free-base (H2TOPP) analogues were investigated by time-resolved fluorescence and Monte Carlo simulations, applied to a single domain model of parallel porphyrin stacks. Rate constants for intra- and inter-stack energy transfer are (0.8–1.1)×1012 and (71–91)×109   s −1 , respectively. The fluorescence lifetimes for ZnTOPP and H2TOPP films are (1.80–1.88)×10−9 and (6.8–7.3)×10−9 s. The rate constant for H2TOPP to ZnTOPP back transfer is (8.8–9.4)×106   s −1 . The results agree with those of a previous analytical analysis .

A non-perturbative theory of spectroscopy with pulsed laser excitation is presented with approximations that make calculation with long pulses as accessible as for short pulses. We apply it to continuum Raman scattering with a strong field and show that: (a) for a pulse shorter than the molecular dissociation time the range of excitation frequencies giving fluorescence-like emissions is greater than in a weak field, and (b) for a pulse that is longer than the molecular dissociation time, Raman-like emissions are dominant, with vibrational progressions over a wider range of excitation frequencies than in a weak field.

Electronic decoherence for electron transfer in blue copper proteins by Daren M. Lockwood; Yuen-Kit Cheng; Peter J. Rossky (159-165).
We present a molecular dynamics investigation of the electronic decoherence rate for electron transfer (ET) in a solvated protein molecule. We find that decoherence occurs on an ultrafast time scale of 2.4 fs, considerably faster than fluctuations in the electronic coupling. Both protein and solvent dynamics play important roles. Solvent alone would give rise to a decoherence time of 3.0 fs, as compared to 4.1 fs from the protein matrix alone. This implies that both solvation and protein dynamics can strongly affect both the rate and mechanism of ET.

It is shown that the large electron spin polarization created in the photoexcited triplet species can be transferred to the nuclear spins even in a polycrystalline sample by utilizing a partial area of the extremely broad ESR powder spectrum for cross polarization (CP). The proton polarization 3160 times as large as the thermal equilibrium value was obtained in 0.018 mol% pentacene-doped naphthalene at 100 K in a field of 0.319 T. The influence of the proton spin–lattice relaxation and the sample thickness on the attained polarization is discussed.

The electronic spectrum of N-methylacetamide in aqueous solution: a sequential Monte Carlo/quantum mechanical study by Willian R Rocha; Katia J De Almeida; K Coutinho; Sylvio Canuto (171-178).
Sequential Monte Carlo/quantum mechanical (S-MC/QM) calculations are performed to study the solvent effects on the electronic transitions of N-methylacetamide (NMA) in aqueous solution. Full quantum mechanical INDO/CIS calculations are performed in the super-molecular clusters generated by Monte Carlo (MC) simulation. The largest calculation involves the ensemble average of 75 quantum mechanical results obtained with the NMA solute surrounded by 150 water solvent molecules. After extrapolation to the bulk limit we find that the n →  π * transition suffers a blue shift of 1755  cm −1 upon solvation and the π  →  π * transition undergoes a red shift of 1180  cm −1 , in good agreement with the experimental findings.

Vibrational analysis from linear response theory by Francesco Filippone; Michele Parrinello (179-182).
We present a density functional theory (DFT)-based ab initio method for calculating selected portions of the vibrational spectrum, circumventing the need to evaluate the full Hessian. Our method is based on a combination of variational density functional perturbation theory and the Lanczos diagonalization method. The method is best suited to evaluating the extremal portion of the vibrational spectrum, but it can be adapted to target preselected regions of the spectrum.

A relationship between electronic structure effective parameters and T c in monolayered cuprate superconductors by I.de P.R. Moreira; D. Muñoz; F. Illas; C. de Graaf; M.A. Garcia-Bach (183-188).
For a broad class of high-T c superconductor cuprates with nearly independent Cu–O planes, the values of the parameters defining the tJ model Hamiltonian are obtained from accurate ab initio electronic structure calculations on embedded cluster models. From the calculated t and J values a clear-cut correlation between T c and these values emerges. Based on the ideas of resonating-valence-bond (RVB) theory, a possible interpretation of the origin of this correlation is discussed. The present work stresses the adequacy of the tJ model and strongly supports the idea of a pairing mechanism in cuprates mediated by magnetic interactions.

Ab initio and density functional study of the 5-pentacyclo[6.2.1.13,6.02,7.04,10]dodecyl cation. A symmetrical μ-hydride bridged carbocation by José Walkimar de M. Carneiro; Carlton A. Taft; Carlos H.T. de Paula e Silva; José Glauco R. Tostes; Peter R. Seidl; Paulo Sérgio da S. Pinto; Valentim Emı́lio U. Costa; João Alifantes (189-194).
MP2/6-31g(d,p) and B3LYP/6-31g(d,p) calculations for the pentacyclo[6.2.1.13,6.02,7.04,10]dodecyl cation reveal two minima on the potential energy surface. The most stable minimum is the μ-hydride bridged cation 2 . The second minimum is the two-electron three-center bonded structure 3 . At MP2/6-31g(d,p) 2 is only 0.2 kcal/mol more stable than 3 , but at B3LYP/6-31g(d,p) this energy difference increases to 3.3 kcal/mol. The energy difference between 2 and 3 is only 3.8 kcal/mol. Solvent effect does not affect these numbers significantly. This low energy barrier may account for the product distribution observed on solvolysis of pentacyclic derivatives.

Molecular dynamics calculations have been performed based upon a fluctuating-charge model TIP4P-FQ over a wide range of state points from ambient to sub- and supercritical conditions in order to investigate dielectric properties of water. The TIP4P-FQ could successfully reproduce the experimental dielectric constant. The dielectric constant is dominated not only by the intermolecular orientational correlation but also by the magnitude of each molecular dipole moment. Interesting behavior of these two contributions is reported as a function of temperature and density. Averaged number of hydrogen bonds has also been found to have a strong correlation with the dielectric constant.

Explicit calculation of Coulomb correlation in bond orbitals by Padeleimon Karafiloglou (201-206).
The Coulomb correlation is calculated explicitly in bond orbitals, such as the natural bond orbitals (NBOs). The dependence of the correlation from the weights of NBO structures is investigated in an analytical way. It is stressed that when the electrons of a molecular system are distributed in NBOs, they show significantly smaller correlations than when they were distributed in atomic orbital positions. The NBO-set which corresponds to the main bonding scheme of the molecule, exhibits the minimum Coulomb correlation.

A bidimensional vibrational Hamiltonian expressed in terms of internal coordinates has been developed to describe the symmetric stretch and the inversion of asymmetric pyramidal ammonia-like molecules. This work is an extension of the approach we have recently presented on NH3 and on some of its C3v isotopomers [J. Chem. Phys. 115 (2001) 1243]. A new set of internal coordinates, which reflects the symmetry of the problem, has been defined and used to obtain a new bidimensional kinetic energy operator. Inversion levels for NH2D and NHD2 have been calculated variationally using an ab initio CCSD(T) bidimensional potential energy surface (PES). A good agreement with experimental values has been obtained.