Chemical Physics Letters (v.491, #1-3)

Contents (iii-viii).

Graphical representation of the assignment of 20 OH-stretch fundamental energies to local OH-bonds. Color range from orange to blue represents an energy range roughly 3150 to 3800 cm−1.We describe a full-dimensional, flexible potential energy surface for arbitrary numbers of water monomers built from ab initio 2- and 3-body potentials. These potentials are each permutationally invariant fits to roughly 30 000 electronic energies. Tests of these potentials are made against direct high-level ab initio results for the water dimer and trimer. An application to the hexamer (prism isomer) is made along with a review of recent ab initio calculations of harmonic frequencies. One-mode quantum calculations of the OH-stretch fundamentals are done with these potentials and their accuracy is assessed by comparisons with coupled MULTIMODE calculations for the water dimer and trimer. Local-mode calculations of OH-stretch fundamentals are presented for the hexamer and decamer, where it appears that the bulk limit is being approached. A new local-monomer model is presented and tested for the water dimer and trimer.

Two-bond 19F–15N spin-spin coupling constants across F–H···N hydrogen bonds have been calculated to investigate the changes of 2hJ F–N values with the hybridization of nitrogen atom in different H···N distances.The effects of substituent and hybridization of nitrogen atom on hydrogen bonding in the F–H···NCX, F–H···N(H)=CX, and F–H···N(H)2–CX complexes have theoretically been studied by MP2 and DFT methods with aug-cc-pVDZ and 6-311++G∗∗ basis sets. With respect to the hybridization of nitrogen, sp3-hybridized nitrogen forms the strongest bond, followed by sp2 and then sp. In equilibrium structures, the trend in the two-bond 19F–15N spin–spin coupling constants ( 2h J F–N) is sp < sp3  < sp2. The results of atoms in molecules (AIM) and natural bond orbital (NBO) analyses are in meaningful relationships with other characteristics of hydrogen bonds, especially with the 2h J F–N values.

High resolution FTIR investigation of 12C2H2 in the FIR spectral range using synchrotron radiation by B. Amyay; M. Herman; A. Fayt; L. Fusina; A. Predoi-Cross (17-19).
FIR spectrum of 12C2H2 simulated from a global model with a single transition dipole moment, of relevance to remote sensing.FIR spectra of C2H2 have been recorded at 0.00096 cm−1 spectral resolution using the Canadian Light Source synchrotron facility. The analysis allowed us to assign 731 new vibration–rotation lines from 48 bands in 12C2H2, 38 of which are reported for the first time. Two additional bands are assigned to 13CH12CH. The measured line positions and calculated spectra can be made available to help in the remote sensing of acetylene in the terahertz spectral range.

Kekulé structure ≡ Excited state.Resonance structures are presented for four small carbon nanotubes (CNTs) in the context of quantum chemical concepts. The results show that there might exist Kekulé structures, which are apparent excited electronic states and may mislead a chemist as being contributed to the CNT ground state.

UVA-initiated oxidation of gaseous mercury over a titania catalyst. In principle this system has the potential to scrub out toxic mercury fumes from an emission source. Mercury is trapped as the non-volatile oxide HgO.Hg0(g) is known to undergo photo-catalytic oxidation by UVA-irradiated TiO2 surfaces. One micrometre layers of TiO2 on quartz glass were irradiated within the 240–800 nm range. Gaseous mercury was measured by mass spectrometry single ion monitoring. The surface configuration and elemental characterization of TiO2 layer was evaluated using scanning electron microcopy with energy dispersive spectroscopy. The LH adsorption constant of was found to be K Hg  = (5.1 ± 2.4) × 10−14  cm3 and an apparent surface deposition rate of k  = (7.4 ± 2.5) × 1014  min−1  cm−2 under experimental conditions. Water did not affect the rate constant. We show TiO2 could be employed to reduce mercury concentrations in gas streams, even at very high Hg0 concentrations.

Absorption and corrected excitation spectra of pure 3-fluoropyridine vapor at 10 Torr and 30 °C.Emission and excitation spectra of 2- and 3-fluoropyridine (2- and 3-FP, respectively) vapors have been measured under the different conditions. It is shown that 3-FP vapor emits weak fluorescence from the S1(n, π) states. The fluorescence quantum yields are evaluated to be approximately 10−3 and 10−4 for 2- and 3-FP vapors, respectively, following the excitation into the S1 origin. The fluorescence yields decrease significantly with increasing excitation energy. The decrease of the yield is more significant for 2-FP than for 3-FP vapor. The data include vibrational analyses of the hot bands in the excitation spectra using the DFT calculation at the B3LYP/6-311++G∗∗ level.

Some stable structures containing planar double, triple, quadruple Si-ring are theoretically predicted. Three of them are shown below.Possible planar double, triple, and quadruple lithiosilicon ring structures have been theoretically predicted at the B3LYP/6-311+G (2d, p) level. The structures containing planar double silicon ring as well as triple and quadruple ring could be stable at this level. Meanwhile, it was also demonstrated that Li stabilization was still effective when extending to planar polycyclic silicon rings.

Time evolution of individual aggregates in micellized systems. We measure the corresponding shorter and longer relaxation times.We study a model for amphiphilic aggregation on a square lattice through Monte Carlo simulations. We consider the temporal evolution of the aggregates of an equilibrium micellar solution after a sudden change in temperature. The amphiphiles are represented by chains of five interconnected sites on the lattice, one site accounting for the hydrophilic part of the molecule while the others are related to its hydrophobic part. The remaining sites of the lattice are filled with water molecules, one molecule per site. Our simulations are performed in the NVT ensemble, and when the temperature is changed, we follow in time all the aggregates of the system until the new equilibrium state is reached. For each aggregate size we measured two well-separated relaxation times, a shorter one that is related to the quick exchange of amphiphiles with aggregates and a longer one, which is associated with the formation and disintegration of the micelles. We have seen that for all the aggregate sizes the longer relaxation time is around ten times the shorter one. We also show that the relaxation time to equilibrium of the mean size aggregate is longer in the cooling than in the heating process.

High vibrational levels, up to ν  = 22, are excited in I2–Xe complex isolated in solid krypton, using fs-CARS and resonant Raman techniques.High vibrational states, up to ν  = 22, are excited and investigated on the ground electronic state of a 1:1 I2–Xe complex isolated in solid Kr using femtosecond CARS technique and spontaneous resonant Raman measurements. The results show that this system is a promising candidate for investigations of coherent control of bimolecular reactions by using vibrational wavepackets on the ground electronic state.

Charge transfer transitions in cuprates by Sven Larsson (49-53).
Metal–metal electron transitions can explain the optical conductivity spectrum of cuprates.Absorption spectra of cuprates are discussed. Persistent photo-induced conductivity occurs in the visible spectrum (∼2 eV) and is commonly assigned to ligand–metal (LM) charge transfer (CT) transitions. However, LM CT is site local and cannot possibly generate persistent charges. The assignment in this Letter is ‘metal to adjacent metal’ (MM) CT transitions, while the absorption at hν  > 3 eV is still assigned to mainly LM CT. Only MM CT, defining the Mott–Hubbard gap, is exclusively polarized in the CuO2 plane, as found experimentally. Since MM CT is strongly affected by the local electric field, doping transfers spectral weight to the IR region.

The decrease in water accessible surface area – the blue region – upon plate association is the driving force of the process in both water and aqueous salt solution, because it leads to a gain in translational entropy for water molecules and ions. Low-charge density ions cause a weakening of hydrophobic interaction due to preferential binding on plate surfaces.The decrease in water accessible surface area drives the association of two large hydrophobic plates in both water and aqueous salt solutions: the contact configuration of the two plates possesses the minimum Gibbs energy in all cases. High charge density ions, having strong electrostatic attractions with waters, render more costly the process of cavity creation, as determined by classic scaled particle theory calculations, and strengthen pairwise hydrophobic interaction. Low-charge density ions, being preferentially bound to plate surfaces due to dispersion interactions, have to be removed to arrive at the contact configuration and weaken pairwise hydrophobic interaction.

Studies on adsorption of carnosine on silver nanoparticles by SERS by S. Thomas; N. Biswas; V.V. Malkar; T. Mukherjee; S. Kapoor (59-64).
SERS studies suggest that the interaction of carnosine is primarily through the carboxylate group with the imidazole ring in an upright position with respect to the silver surface and the alanine moiety assuming a parallel orientation with the surface where NH2 group is close to the silver surface.The surface-enhanced Raman scattering (SERS) studies of l-carnosine was carried out in aqueous silver sol at pH ∼ 9 and compared with the normal Raman spectrum of the molecule. The experimentally observed Raman bands were assigned based on the results of DFT calculations. Significant changes in the relative intensity are seen in the SERS spectrum when compared to the normal Raman spectrum. The studies suggest that the interaction of carnosine is primarily through the carboxylate group with the imidazole ring in an upright position with respect to the silver surface and the alanine moiety assuming a parallel orientation with the surface where NH2 group is close to the silver surface.

Reduced vibronic coupling density allows us to easily understand the local picture of vibronic coupling.Reduced vibronic coupling density (RVCD) and reduced atomic vibronic coupling density (RAVCD) are introduced in order to discuss the origin of vibronic (electron–vibration) couplings in a molecule. As an example, the RVCDs and RAVCDs of bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) cation are presented. The strongest vibronic coupling of the C=C stretching mode originates from large electron-density difference on the C=C bond.

Intactness and spatial proximity of acid–base groups in bifunctional SBA-15 as revealed by solid-state NMR by Wanling Shen; Wujun Xu; Qiang Gao; Jun Xu; Hailu Zhang; Anmin Zheng; Yao Xu; Feng Deng (72-74).
The acid and base groups coexist peacefully and in spatial proximity with suitable distance on the surface of the SBA-15 support.The intactness and spatial proximity of acid and base groups in bifunctional mesoporous SBA-15 has been studied by various NMR techniques. The NMR results show that Brønsted acid and Lewis base groups coexist peacefully on the surface of the same support and maintain their acidity and basicity, respectively. Meanwhile, the acid groups and the base groups are in close proximity with suitable distance.

Adenine enhances the fluorescence intensity of histidine-stabilized CdS quantum dots.Cadmium sulfide particles have been synthesized in the aqueous medium using the amino acid histidine as a stabilizing agent. These particles demonstrate the phenomenon of size quantization effect. The fluorescence of histidine-stabilized CdS was found to be enhanced and quenched by the addition of DNA bases adenine and guanine, respectively. The fluorescence enhancement of CdS in the presence of adenine has been explained on the basis of interaction between the quantum dot stabilizer and the amino group of adenine. Quenching of CdS fluorescence by guanine occurs due to interaction of the substrate with the quantum dot surface.

Role of the microenvironment on the fluorescent properties of a spirooxazine by Maria Rosaria di Nunzio; Pier Luigi Gentili; Aldo Romani; Gianna Favaro (80-85).
A spirooxazine exhibits dual fluorescence from π,π∗ and TICT excited states in microcrystalline phase and in PMMA polymer film.Emission properties of a Reversacol spirooxazine have been investigated in microcrystalline solid phase and in poly(methyl methacrylate), PMMA, films. Emission spectra and lifetimes of the fluorescent states have been determined using steady state spectrofluorimetry and time-correlated single-photon counting techniques, respectively. Dual fluorescence, previously detected in solution, was also observed in microcrystalline phase and in PMMA. Fluorescence relaxation kinetics were analyzed by non-linear least-squares method and Maximum Entropy Method (MEM). Inclusion in matrix had the effect of slowing down the decay rate and extending the lifetime distribution over a larger time-interval with respect to solution.

Many-body energy decomposition of hydrogen-bonded glycine clusters in gas-phase by Puspitapallab Chaudhuri; Sylvio Canuto (86-90).
Hydrogen-bonded cluster of three glycine molecules.A detailed analysis of the many-body contribution to the interaction energies of the gas-phase hydrogen-bonded glycine clusters, (Gly) N , N =  1–4 is presented. The energetics of the hydrogen-bonded dimer, trimer and tetramer complexes have been analyzed using density-functional theory. The magnitude of the two- through four-body energy terms have been calculated and compared. The relaxation energy and the two-body energy terms are the principal contributors to the total binding energy. Four-body contribution is negligible. However, the three-body contribution is found to be sizable and the formation of the cyclic glycine trimer presents geometric strains that make it less favorable.

A statistical approach to energy loss during gas–liquid collisions by Daniel M. Packwood; Leon F. Phillips (91-95).
By a statistical treatment of a simple model for atom + liquid surface collisions, general results for the energy loss and trapping probability of the incoming atom are derived, and a new graphical method of obtaining the potential well depth at the liquid surface is presented.A novel theoretical approach to collisional energy exchange during a gas–liquid collision is outlined for the case of a light, fast incoming atom. The method differs from the usual ballistic models of energy exchange in that the energy loss is statistically inferred from the eventual response of the liquid surface to the collision, rather than by directly considering the mechanics of energy transfer into the surface. With this approach, we obtain simple, accurate formulas for the average collisional energy loss and trapping probability of the incoming atom. We also present a graphical method for determining the well depth of the gas–liquid potential.

Thermal reaction rate constants are directly obtained from dynamics of the quantum trajectory ensemble in imaginary and real time.Reaction rate constants can be directly obtained from evolution of the flux operator eigenvectors under the Boltzmann and Hamiltonian operators. This is achieved by evolving the quantum trajectory ensemble, representing a wavefunction, in imaginary time seamlessly switching to the real-time dynamics. Quantum–mechanical effects are incorporated through the quantum potential dependent on the trajectory momenta or on the derivatives of the wavefunction amplitude. For practicality the quantum potential and wavefunction nodes are described using linear basis, which is exact for Gaussian wavefunctions. For the Eckart barrier approximate rate constants show significant improvement over the parabolic barrier rate constants.

Improved virtual orbital-complete active space configuration interaction analytical gradient theory has been used to investigate geometrical parameters, vibrational frequencies, and excitation energies of m-benzyne and phenol.Improved virtual orbital-complete active space configuration interaction (IVO-CASCI) analytical gradient theory has been used to investigate geometrical parameters, vibrational frequencies and excitation energies of m-benzyne and phenol using different reference spaces and basis sets. Compared to the results obtained via CASSCF treatments, it has been found that the IVO-CASCI gradient method provides very satisfactory results with low computational cost. IVO-CASCI values compare reasonably well with the earlier established theoretical results and experimental values whenever available.