Chemical Physics Letters (v.381, #1-2)

The interface between water and an organic liquid has been exploited to prepare nanocrystals of CdS. The technique involves introducing an appropriate precursor of cadmium in the organic layer and the sulfiding reagent in the aqueous layer. The size distribution of the nanocrystals formed at the interface can be controlled by varying parameters such as the reactant concentration, temperature, viscosity of the medium, reaction time and the choice of the reagents. The CdS nanocrystals have been characterized by TEM, electronic absorption and emission spectroscopy. Nanocrystaline γ-Fe2O3 and ZnO can be prepared by taking cupferron complex of the metals in the organic layer and NaOH in the aqueous layer.

The electronic structure of metal/alkane thiol self-assembled monolayers/metal junctions for magnetoelectronics applications by Y.A. Ovchenkov; H. Geisler; J.M. Burst; S.N. Thornburg; C.A. Ventrice; Chunjuan Zhang; J. Redepenning; Y. Losovyj; Luis Rosa; P.A. Dowben; B. Doudin (7-13).
Long-chain alkane thiols use in metal to organic self-assembled monolayer to metal junctions may be limited by orientational disorder, and photoemission studies suggest that several molecular layers may be needed for the dielectric layer to be effective. Several alkane thiols were investigated in a range of junctions areas 10–102 μm2. Top layer contact deposition, activated with Pd clusters resulted in a high yield of junctions that were not electrically shorted and are stable over a wide temperature range. Zero-bias anomalies, observed at low temperatures, are attributed to a Coulomb blockade associated with the Pd clusters.

Atmospheric chemistry of C2F5CHO: mechanism of the C2F5C(O)O2  + HO2 reaction by M.P. Sulbaek Andersen; M.D. Hurley; T.J. Wallington; J.C. Ball; J.W. Martin; D.A. Ellis; S.A. Mabury (14-21).
Smog chamber/FTIR techniques were used to study the gas-phase reaction of C2F5C(O)O2 with HO2 radicals in 100–700 Torr of air, or O2, diluent at 296 K. The reaction proceeds by two pathways leading to formation of C2F5C(O)OH and O3 in a yield of 24 ± 4% and C2F5C(O)O radicals, OH radicals and O2 in a yield of 76 ± 4%. The gas phase reaction of C n F2n  + 1C(O)O2 with HO2 radicals offers a potential explanation for at least part of the observed environmental burden of fluorinated carboxylic acids, C n F2n  + 1C(O)OH. As part of this work an upper limit for the rate constant of reaction of Cl atoms with C2F5C(O)OH at 296 K was determined; k(Cl + C2F5C(O)OH) < 1 × 10−17 cm3 molecule−1 s−1.

On the structures of the methanol trimer and their cooperative effects by Marcos Mandado; Ana M Graña; Ricardo A Mosquera (22-29).
A previously non-described trimer with a C–H ⋯ O hydrogen bond has been characterized as minimum at several computational levels. This conformer allows a complete assignment of the experimental infrared cavity ringdown laser absorption spectrum (IR-CRLAS) in the O–H stretch region. B3LYP/6-311++G(d,p) charge densities of the methanol dimer and trimers analyzed with the atoms in molecules (AIM) theory indicate the hydrogens of the methyl group play a significant role on the stabilization of these clusters. On the contrary, their cooperative effects can be numerically reduced to those exhibited by the OH groups.

The optical Stark spectrum of the origin bands of the E   3 Π 0–X   3 Δ 1,A   3 Φ 2–X   3 Δ 1, and B   3 Π 0–X   3 Δ 1 electronic transitions of titanium monoxide, TiO, were analyzed to produce permanent electric dipole moments of 3.34(1), 3.2(4), 4.89(5) and 4.9(2) D for the X   3 Δ 1,E   3 Π 0,A   3 Φ 2 and B   3 Π 0 states, respectively. The observations are compared with a simple molecular orbital description for the low-lying states and electronic structure calculations.

Direct dynamics study of the photodissociation of triplet propanal at threshold by M.N.D.S Cordeiro; E Martı́nez-Núñez; A Fernández-Ramos; S.A Vázquez (37-44).
The near threshold photodissociation of propanal via the HCO + C2H5 channel on the triplet surface is studied by direct PM3-SRP classical trajectory calculations. The initial conditions for the trajectories are selected by either the quasi-classical or the Wigner distribution function. The HCO translational energy and rotational distributions obtained in this study are in good agreement with experiment. Even though the results obtained with the Wigner distribution showed a better agreement with the observed experimental HCO translational energy distribution, in general both sampling methods lead to similar product energy partitioning.

Calculated structures of [AuCAu]2+ and related systems by Pekka Pyykkö; Michael Patzschke; Jaak Suurpere (45-52).
Calculated structures are reported for the mass-spectroscopically observed CAu2 2+ and CAu3 +. Linear D∞h and D3h geometries are obtained, respectively. The calculated properties of other multiply-bonded species, notably PtCPt, PtCCCPt and AuCCAu are reported. The Au+ and Pt function in these systems as chemical analogues to O. Further ligands are not needed for such ‘autogenic isolobality’.

Molecular dissociation observed with an atomic wavepacket and parametric four-wave mixing by A.A. Senin; H.C. Tran; J. Gao; Z.H. Lu; C.J. Zhu; A.L. Oldenburg; J.R. Allen; J.G. Eden (53-59).
We report the observation of the dissociation of an electronically-excited molecule (Rb2) with an atomic wavepacket and time-delayed parametric four-wave mixing. The dynamics of the molecular dissociation transient are captured by the temporal histories of the relative number densities of the Rb product states, determined by reference to the amplitude and phase of the Fourier (spectral) components of the wavepacket. Clear experimental evidence of the appearance of atomic fragments (7s, 5d, 6p, 4d, and 5p) generated by dissociation is observed in the pressure-dependent temporal behavior of the amplitude of the 608 cm−1 (7s–5d) wavepacket spectral component.

Crossover from diffusion to annihilation limited phosphorescence in conjugated polymers by M. Reufer; F. Schindler; S. Patil; U. Scherf; J.M. Lupton (60-66).
Trace concentrations of metallic impurities present at the ppm level in conjugated polymers enable highly efficient radiative triplet decay under both optical and electrical excitation. We demonstrate that the phosphorescence yield is governed by both diffusion of triplets to metallic sites as well as non-radiative triplet decay. Both the triplet mobility and non-radiative decay are enhanced at elevated temperatures. As triplet emission occurs from a small number of emitting sites, the relative dominance of these two processes depends sensitively on triplet density. Two extreme regimes of triplet density are probed by optical and electrical excitation. Unusual thermochromic properties of triplet excitons are also discussed.

Effect of electron correlation on momentum properties of helium atom by B. Talukdar; A. Sarkar; S.N. Roy; P. Sarkar (67-73).
An explicitly r 12 dependent wavefunction for the ground state of helium is Fourier transformed and the resulting momentum-space wavefunction is used to study the momentum properties. In particular, results are obtained for the momentum density, moments of momentum density and Compton profile. It is demonstrated that the use of our simple minded wavefunction provides a straightforward analytic method to estimate the effect of electron–electron correlation on the momentum properties of helium atom and gives results which are in good agreement with those obtained by more detailed calculations.

Fabrication and characterization of hollow spherical boron nitride powders by Liqiang Xu; Yiya Peng; Zhaoyu Meng; Debao Wang; Wanqun Zhang; Yitai Qian (74-79).
Hollow spherical boron nitride powders with diameters ranging from 100 nm to a few micrometers were prepared by Co-pyrolysis of NH4BF4 and KBH4 with zinc powder at 600 °C The roles of each reactant played in this experiment and the possible formation mechanism was discussed.

An explicitly correlated wave function, expanded in a basis set of 100 Gaussian geminals, is fully optimized for the ground state of the helium atom. The wave function is used to obtain a systematic set of intracular and extracular electron-pair properties in both position- and momentum-spaces. Comparison of the position-space properties with nearly exact values obtained from Hylleraas-type wave functions shows that the Gaussian geminal values are rather accurate. This lends credence to our claim that the momentum-space properties reported here are essentially definitive. Comparison is made with recent results obtained from multi-configuration Hartree-Fock calculations, and from Monte Carlo computations.

Organization of single-walled nanotubes into macro-sized rectangularly shaped ribbons by M.L. Terranova; S. Orlanducci; E. Fazi; V. Sessa; S. Piccirillo; M. Rossi; D. Manno; A. Serra (86-93).
We present here a graphite-helped self-organization method for the production of macroscopic ribbons constituted by single-walled carbon nanotubes (SWCN) assembled in organized structures. The approach is based on the recondensation of SWCNs dispersed in aqueous solutions and the final deposits consist of rectangular-shaped ribbons long up to some mm and rather large (up to some tens of μm). Raman, RHEED, EDAX, SEM ad STM analysis has been used to gain information about compositional, morphological and structural features of such unusual nanotube-based material. The resulting structure could be rationalized as a three-dimensional matrix where randomly distributed nanoflakes of turbostratic carbon fill the spacing between adjacent nanotube ropes and encompass nanotube mats.

Structural calculation and properties of one-dimensional Pt materials by Li Hui; F. Pederiva; Wang Guanghou; Wang Baolin (94-101).
The structures of free-standing platinum nanowires are systematically investigated by using genetic algorithm simulation with an EAM potential. The magnetic properties are also studied. Several helical multi-shell cylindrical and pentagonal packing structures are observed. These multi-shell structures are composed of coaxial atomic shells with the three and four strand helical, centered pentagonal, and hexagonal, and parallel double chain-core curved surface epitaxy. Under the same growth sequence, the numbers of atomic strands in inner and outer shells exhibit even–odd coupling.

The symmetric hydrogen bond in potassium hydrogen maleate has been re-investigated by diffraction and computational methods. Single crystal neutron diffraction under varying temperature (30–295 K) and pressure (ambient-4 kbar) confirmed the crystallographically-constrained symmetric disposition of the hydrogen atom in the short, strong hydrogen bond for each of eight sets of (p,T) conditions. Plane-wave DFT calculations show the hydrogen atom still to occupy this symmetric position even when crystallographic symmetry is removed, in stark contrast to the results from isolated molecule ab initio calculations. These results demonstrate that the symmetric (or close to symmetric) nature of this hydrogen bond is intrinsic.

Ab initio study of hydride abstraction reaction in the Mg+–NH2CH3 complex by Wenyue Guo; Xiaoqing Lu; Songqing Hu; Shihe Yang (109-116).
The hydride abstraction reaction in the Mg+–NH2CH3 complex has been investigated by using the DFT and MP2 methods. Two local minima and two transition states have been found along the reaction pathway. Accompanied with IRC calculations, the reaction is revealed to be a three-step process, i.e., a hydrogen-migration process followed by a charge-transfer process and a nonreactive-dissociation process. On the basis of single-point calculations using CCSD(T) in conjunction with 6-311++G(d, pd) basis set with the geometries optimized at B3LYP/6-311++G** with zero point energy (ZPE) corrections, the reaction energies for all the steps are predicted.

Gaussian resolutions for equilibrium density matrices by Pavel Frantsuzov; Arnold Neumaier; Vladimir A. Mandelshtam (117-122).
The Gaussian wavepacket propagation method of Hellsing et al. [Chem. Phys. Lett. 122 (1985) 303] for the computation of equilibrium density matrices ρ ̂ T is revisited and modified. The variational principle applied to the ‘imaginary time’ Schrödinger equation provides the equations of motion for Gaussians in a resolution of ρ ̂ T , described by their width matrix, center and scale factor, all treated as dynamical variables. The method is computationally very inexpensive, has favorable scaling with the system size and, with the current implementation, is surprisingly accurate in a wide temperature range, even for cases involving quantum tunneling. Incorporation of symmetry constraints, such as reflection or particle statistics, is discussed as well.

The adiabatic electron affinity (EA) of sulfur hexafluoride (SF6) was determined using six different density functional or hybrid Hartree–Fock/density functional methods, including the recently developed KMLYP functional. With the extensively calibrated DZP++ basis set, the KMLYP functional predicts an EA for SF6 of 1.48 eV. This value is in better agreement with experiment (1.07 ± 0.01 eV) than the best previously established DFT method (BHLYP, 1.69 eV) for related systems. The other four functional give electron affinities of 2.66 (B3LYP), 2.83 (B3P86), 3.22 (BLYP), and 3.00 eV (BP86). However, the difference from the experimental value is still about twice a large as DFT is usually able to obtain.

A scheme for calculating thermally averaged observables for quantum dissipative systems is presented. The method is based on a wavefunction with equal amplitude and random phase composed of a complete set of states, which is then propagated in imaginary time β/2. Application to a Surrogate Hamiltonian simulation of a molecule subject to an ultrafast pulse coupled to a bath is studied. Compared to Boltzmann thermal averaging the method scales more favorably with an increase in the number of bath modes. A self-averaging phenomenon was identified which reduces the number of random sets required to converge the thermal average.

Ultrafast infrared heterodyne detected vibrational stimulated echoes with full phase information are used to obtain the vibrational correlation spectrum from a mixture of metal-carbonyl compounds. The linear absorption spectrum displays four peaks in the carbonyl stretching region. In the absence of knowledge of the molecules that make up the mixture, the absorption spectrum could arise from four molecules that each produces a single peak to one molecule with four peaks. In contrast, the correlation spectrum displays four peaks on the diagonal and off-diagonal peaks that make it straightforward to determine which peaks belong to a particular molecule.

Paracyclophanes, in which four or more benzene units are linked at the 1,4-positions by ethylene bridges, show annulene characteristics upon charging. It was proved that a Hückel-like rule of superaromaticity holds for molecular ions of these species. Here superaromaticity represents energetic stabilization due to cyclic motion of π-electrons along the macrocycle. If this type of paracyclophane molecule or any of its closed-shell molecular ions is super-antiaromatic, it will probably become superaromatic either by acquiring two more π-electrons or by losing two π-electrons.

The mixed oxide photocatalyst, TiO2–SiO2 has been observed to contain trapped conduction band electrons after annealing in vacuum. These electrons are consumed by adsorption of particular electrophilic molecules. It is found that O2 and an adsorbed organic molecule containing a Cl moiety can remove trapped electrons, whereas a similar organic molecule, not containing Cl, is ineffective in charge removal. The measurement of trapped electronic charge is made by observation of the intensity of the background IR absorption in the 4000 to 1400 cm−1 region, which originates from trapped electron excitation into a continuum of excited electronic states by IR photons.

We present a mechanism able to show intrinsic bistable behavior involving single Yb3+ ions embedded into bromide lattices, in which intrinsic optical bistability (IOB) has been observed. The mechanism is based on the experimentally found coupling between the Yb3+ ion and the totally symmetric local mode of vibration of the [YbBr6]3− coordination unit. The model reproduces the IOB observed in CsCdBr3:1% Yb3+ and allows to understand the experimentally found presence of the phenomenon in the other bromides, but its absence in Cs3Lu2Cl9:Yb3+.

Sulfur hexafluoride corona discharge decomposition: gas-phase ion chemistry of SOF x + (x=1–3) ions by Federico Pepi; Andreina Ricci; Marco Di Stefano; Marzio Rosi (168-176).
The structure and reactivity of gaseous SOF x + (x=1–3) cations obtained by SF6 corona discharge were studied by joint application of mass spectrometric techniques and computational methods. Consistent with theoretical calculations at the B3LYP and CCSD(T) level of theory, the structurally diagnostic collisionally activated dissociation (CAD) mass spectrometric results show that the SOF x + ions are characterized by the F x –SO connectivity in which the fluorine atoms are bonded to the sulfur. The reactivity of these ionic species with selected compounds was also investigated. The SOF2 + ion behaves in the gas phase as a SOF+ donor, whereas the SOF3 + and SOF+ ions are practically inert towards the molecules used.

An interpolation technique was developed for constructing ab initio potential energy surfaces (PES) around the equilibrium structure efficiently and accurately, using polar coordinates coupled with systematic generation of sample points on the hypersphere. Test calculations were performed for H2O and HCHO molecules, and interpolated PES and direct ab initio data were compared for randomly chosen 1000 points. Root mean square errors were found to be 4.8 × 10−5 (2.4 cm−1) for H2O and 1.7 × 10−4 (8.4 cm−1) for HCHO with respect to the maximum energy of 50 000 cm−1. The present approach was found to provide considerably more accurate PES than conventional methods.

Electronic properties of formaldehyde in water: a theoretical study by A. Amadei; M. D’Abramo; C. Zazza; M. Aschi (187-193).
In this Letter, we use the recently introduced perturbed matrix method (PMM) to study in detail the electronic properties of formaldehyde in water, as obtained by applying this method to Molecular Dynamics simulation data. Results show that PMM provides an accurate description at relatively low computational costs.

Reduction–boronation route to chromium boride (CrB) nanorods by Jianhua Ma; Yunle Gu; Liang Shi; Luyang Chen; Zeheng Yang; Yitai Qian (194-198).
Chromium boride (CrB) nanorods were synthesized via a reduction–boronation route at 650 °C in molten salt in an autoclave. The X-ray diffraction pattern of the product was indexed as the orthorhombic CrB. Transmission electron microscope images indicated that the sample consisted of single-crystalline nanorods with the diameter of 10–30 nm and a maximum length of 1.5 μm. The PL spectra showed the ultraviolet light emission at 340 nm. The thermogravimetric analysis showed that the product also had very good anti-oxidation properties below 630 °C. A possible growth mechanism was proposed.

Kinetic studies on reactions of NCO(X 2Πi) with alcohol molecules by Linsen Pei; Changjin Hu; Yunzhen Liu; Zhiqiang Zhang; Yang Chen; Congxiang Chen (199-204).
In this work, NCO radicals were produced by laser photolysis of CHBr3 at 266 nm followed by the reaction of the produced CH with NO in the photolysis reaction. The NCO radicals were then electronically excited from the ground state to the A2Σ+ state with a Nd:YAG laser pumped dye laser at 438.6 nm in the Q sub-band of A2Σ+(0 00  0) ← X 2Πi(0 01  0). The Laser induced fluorescence (LIF) intensities (taken as the area of the time-resolved LIF signal) as function of the delay time between the photolysis and probe lasers were measured. The reaction rate constants of NCO in ground state with alcohol molecules were measured by analyzing the time dependence of the LIF intensity. It is found that the reaction rate constant of NCO with alcohol molecule is larger than that with corresponding alkane molecule. The inductive effect of the OH group in alcohol molecule on the activation energy of the reaction NCO + alcohol is discussed.

The coplanar di-dodecahedral molecule C5N30 was studied by using the HF/6-31G* and the DFT B3LYP/6-31G* methods. Vibrational frequencies and the infrared spectrum were computed at the B3LYP/6-31G* level. The molecule has no imaginary vibrational frequency, it is predicted to be a relative minimum on its potential energy hypersurface. Ionization potential and heat of formation of the molecule are also predicted in this Letter.

The molecular distribution and diffusion of water confined in hydrophobic nanopores were studied by molecular dynamics simulations (MD). We found these water molecules have some unusual behavior than that of the bulk. The density profiles are extremely inhomogeneous and some vacuums and clusters occur in small-size pores at lower density. The diffusivity in nanopores is much lower than that of the bulk, and it decreases as the pore width decreases. The diffusivity in channel parallel direction (D x ) are 4–10 times larger than that in channel perpendicular direction (D y ,D z ) at higher temperatures.

Low energy electron attachment to CH3CN by W. Sailer; A. Pelc; P. Limão-Vieira; N.J. Mason; J. Limtrakul; P. Scheier; M. Probst; T.D. Märk (216-222).
Low energy electron attachment cross sections for acetonitrile (CH3CN) are reported in the energy range from about 0 up to 10 eV determined with an energy resolution of 140 meV. Electron attachment is shown to be a purely dissociative process with the production of the five anionic fragments: CH2CN, CHCN, CCN, CN and CH3 observed in two energy regions, the first between 1 and 4 eV, the second in excess of 6 eV. Quantum chemical and trajectory calculations have been carried out to complement the experimental results.

A size-extensive state-specific multi-reference many-body approach using incomplete model spaces by Dola Pahari; Sudip Chattopadhyay; Sanghamitra Das; Debashis Mukherjee (223-229).
We present a size-extensive and size-consistent state-specific multi-reference coupled-cluster approach based on an incomplete model space (IMS) and also discuss simplifications if the IMS is quasi-complete. Methods such as perturbation theory or CEPA-like schemes follow naturally as suitable approximants. Intermediate normalization for the wave-operator, Ω is abandoned in the formalism and suitable excitations, defined as open and quasi-open – which excite out of the IMS by their action on at least one of the model functions – are incorporated in Ω. The effective operator W, leading to the energy on diagonalization, is a closed connected operator.

Ab initio HF calculations of the ground state structural parameters, and the time dependent HF (TDHF) calculations of static nonlinear polarizabilities have been performed for a number of sesquifulvalene derivatives. The calculated NLO parameters show a good correlation with the hardness parameters. The nature of hetero-atoms and their positions can strongly influence the intramolecular charge transfer (ICT) interactions and the nonlinear polarizations of sesquifulvalene. Nonlinear polarizabilities in the twisted structures have been found to depend both on the energy barrier to twist and the transition energy corresponding to the twisted ICT (TICT) state characterized by the HOMO → LUMO transition.

Theoretical study on cation oscillation through the calix[4]arene-bis-crown-5 cavity by Kiyull Yang; Keum Duck Kang; Young Hee Park; In Sun Koo; Ikchoon Lee (239-243).
Density functional theory (B3LYP/6-31G*) and classical molecular dynamics simulations on the alkali metal cation oscillation/migration through the 1,3-alternate calix[4]arene cavity have been carried out to evaluate the barrier for the process. The estimated ΔG of 15.22 kcal/mol for the migration of K+ ion through the π-tube of the tetrakis(ethoxyethoxy)calix[4]arene (TEC) in methanol is comparable with the experimental value of 13.2 kcal/mol in dichloromethane–methanol mixture. The barrier for the oscillation of Cs+along the π-tube of 1,3-alternate calix[4]arene-bis-crown-5 (BC5) is much higher by about 20 kcal/mol (DFT) or 15 kcal/mol (MD) than that for the K+-oscillation.

Comment on ‘Thermodynamic cycles and the calculation of pK a’ [Chem. Phys. Lett. 367 (2003) 145] by Clarissa O. da Silva; Edilson C. da Silva; Marco A.C. Nascimento (244-245).
In a recent Letter, Pliego [Chem. Phys. Lett. 367 (2003) 145] has raised some questions about the methodology that we have employed for calculating pK a values in aqueous solutions. In this comment we show that the problem with Pliego’s analysis is the fact that he used Ben-Naim’s definition of ΔG sol for both the solute and the solvent, which implies that the concentration, for both components, should be equal to 1 M. For the solute, this is a reference state fully compatible with the quantum description, but for the solvent this choice is unphysical, as discussed in the Letter.

In the comment on my Letter about the use of the thermodynamic cycles to calculate pK a, da Silva and co-workers claim that my approach is incorrect due to an erroneous expression used for the chemical potential of water. In this reply, I have presented a rigorous statistical mechanics derivation of the chemical potential of liquid water, which shows that the expression for the chemical potential that I have used is correct. This result supports the previous conclusion that the thermodynamic cycle used by da Silva et al. leads to thermodynamically inconsistent equation for calculating the pK a.

A recent report by Wu [Chem. Phys. Lett. 370 (2003) 39] compared the calculated r e, ω e, and D e values of PbO, PbS, PbO, and PbS with experimental data. Various DFT and ab initio MP2 and QCISD methods were employed together with a large-core RECP for Pb and small-size basis sets. In this comment, we calculate the spectroscopic constants of PbO using a small-core RECP and large valence basis sets in order to meaningfully assess the performance of the methods. For PbO, spin–orbit effects should also be considered when comparing such scalar-relativistic results with experimental data.