Chemical Physics Letters (v.331, #5-6)

Polarised Raman spectroscopy on a single class of single-wall nanotubes by nano surface-enhanced scattering by J Azoulay; A Débarre; A Richard; P Tchénio; S Bandow; S Iijima (347-353).
We report on the opportunity of performing polarised Raman spectroscopy on nanotubes by using surface-enhanced Raman scattering (SERS) mechanisms at the scale of a single hot site. In conjunction with the opportunity of selecting a single class of single wall nanotubes (SWNTs), it opens the way to fine spectroscopic studies of carbon nanotubes. Results obtained on a single class of nanotubes demonstrate first that polarised Raman spectroscopy is possible when a single hot site of a SERS substrate is selected and second that in this situation, unambiguous assignment of the modes is possible.

Femtosecond nearly degenerate four-wave mixing in C60 films between 0.55 and 0.70 μm by F.P. Strohkendl; R.J. Larsen; L.R. Dalton; Z.K. Kafafi (354-358).
We used femtosecond nearly degenerate four-wave mixing (nDFWM) to measure the instantaneous third-order nonlinear optical susceptibility function χ 1111[−(ωδω),ω,ω,−(ω+δω)] near the absorption edge of C60 films with absorption reaching up to 2.0×104   cm −1 . The frequency shift was set to ℏδω=0.07±0.01 eV, smaller than the width of the spectral feature studied but large enough to suppress non-instantaneous signal contributions. Our data are orders of magnitude below previous DFWM results but connect smoothly to data at longer wavelengths where absorption is low (<≈100 cm−1).

The in situ small-angle X-ray scattering (SAXS) of water adsorbed on pitch-based activated carbon fibers (ACF) of pore width w=0.8 and 1.1 nm were measured at 303 K and different relative pressures. The SAXS spectrum of ACF of w=1.1 nm in the direction of desorption did not overlap that in the adsorption direction, showing hysteresis. The density fluctuation data from Ornstein–Zernike (OZ) analysis for the SAXS profiles indicated that water molecules form great clusters in the course of adsorption, but desorption proceeds through uniform molecular evaporation.

Collisionally induced intramultiplet mixing of Sr(53 P J) metastable states by Ne, Kr and Xe atoms by C. Redondo; M.N. Sánchez Rayo; J.A. Fernández; D. Husain; F. Castaño (365-372).
An investigation is presented of the spin–orbit mixing within the Sr[5s5p(3 P 0,1,2)] manifold on collision with Ne, Kr and Xe at elevated temperatures following pulsed dye-laser generation of Sr(3 P 1). The population profiles of all three states were monitored individually by laser-induced fluorescence and fitted to a kinetic model involving six collisional rates connected by detailed balance. The model also includes mixing by Sr(1 S 0) itself. The resulting rate constants, together with those for Sr(1 S 0), Ar and He reported previously, are considered using a standard model employing the potential wells of the reactants and also a collision-complex model.

The activities of water splitting over NiO/NaTaO3 photocatalysts were improved by doping of lanthanides, La, Pr, Nd, Sm, Gd, Tb and Dy into NaTaO3. Lanthanum was the most effective as a dopant. The apparent quantum yield at 270 nm amounted to ≈50%. Moreover, the deactivation observed for a non-doped NiO/NaTaO3 photocatalyst was suppressed. SEM observation suggests that these effects on the photocatalytic properties are mainly due to the decrease in the particle size and the characteristic step structure created by the doping.

Femtosecond transient absorption spectroscopy of non-substituted photochromic spirocompounds by S.A. Antipin; A.N. Petrukhin; F.E. Gostev; V.S. Marevtsev; A.A. Titov; V.A. Barachevsky; Yu.P. Strokach; O.M. Sarkisov (378-386).
Primary processes in spironaphtopyran (SNP), spironaphtoaxazine (SNO) and spirophenantrooxazine (SPO) after excitation by the femtosecond light pulse with the carrier wavelength 308 nm were studied by the absorption `pump-probe' method with femtosecond time resolution. Probing was performed by means of the super-continuum light pulse with the carrier wavelength in the range 420–580 nm. Registration of the photoinduced absorption spectra dynamics allowed to observe time evolution of all intermediates. It appeared that photoinduced transformation of all three compounds has the same mechanism. Kinetic scheme is suggested and all rate constants entering this scheme are determined.

We report results on reflection microscopy and local spectroscopy of aggregates of pseudoisocyanine chloride prepared in thin film of polyvinylsulfate. The aggregates assemble into fiber-like structures of relatively high reflectivity of up to 20%. Locally, the fibers show a large variety of reflectance spectra from sharp peaks at the position of the absorption J-band to broad bands with further structures near their maxima. Polarization experiments confirm that transition dipoles responsible for the J-band are oriented along the fibers. A simple theoretical model taking into account polariton-like excitations and finite dimensions of the fibers is used to simulate the experimental results.

Coherent dynamics in ultrafast charge-transfer reaction of plastocyanin by Satoru Nakashima; Yutaka Nagasawa; Kazushige Seike; Tadashi Okada; Maki Sato; Takamitsu Kohzuma (396-402).
Using a blue copper protein, plastocyanin, we have studied protein dynamics associated with optical excitation and radiationless deactivation (τ=270 fs) of its charge-transfer state by ultrafast pump–probe spectroscopy. A coherent low frequency (33  cm −1) vibrational mode with its phase shifted from those of the other localized Raman modes was observed. The observed coherency of this low frequency mode appears via the excited-state deactivation, rather than arising merely from impulsive stimulated Raman. This mode most probably can be attributed to the delocalized mode including the protein skeletal motion.

EPR spectroscopy and hybrid density functional studies of dialkoxyphosphinyl-phenyl iminoxy radicals by Adrian R. Jaszewski; Zdzisław Siatecki; Julia Jezierska (403-412).
The dimethoxyphoshinyl-phenyl and 13 C enriched diisopropoxyphosphinyl-phenyl iminoxy radicals have been prepared by reaction of corresponding oximes with AgO. The magnetic properties of the radicals have been studied by a combined experimental and theoretical approach. The results of B3LYP and B1LYP computations are in excellent agreement with the EPR parameters determined on the basis of simulation of the experimental spectra. A difference observed in experimental hyperfine coupling with the iminoxy 13 C nucleus on going from Z to E isomer of the radical, 16.9 and 24.2 G, respectively, is discussed on the basis of calculated spin density distribution and natural bond orbitals analysis.

A new type of frequency-domain four-wave mixing (FD-4WM) spectroscopy is developed using an amplified Ti:sapphire laser as pump pulses and a Q-switched YLF laser locked to the Ti:sapphire laser as a probe pulse. This method enables us to directly obtain frequency information of the material, which is difficult to obtain by means of homodyne-detected time-domain four-wave mixing. We have measured the frequency-domain four-wave mixing spectrum for liquid samples and have shown that these spectra agree well with the results obtained by optically heterodyne-detected optical Kerr effect measurement. Furthermore, we have demonstrated the polarization-selected frequency-domain four-wave mixing. We also discuss the distortion of the frequency-domain four-wave mixing spectrum observed frequently even under low pump intensity.

Photofragmentation of the Ag+–furan complex in the gas-phase by Po-Hua Su; Chen-Sheng Yeh (420-424).
The Ag+–furan complex was generated using laser vaporization combined with a supersonic molecular beam. A photo-induced charge transfer with an exclusive furan cation formation was observed via the photodissociation technique. The photofragment spectrum of Ag+–furan was measured and the threshold of the photodissociative charge transfer determined. The upper limit on the Ag+–furan bond strength was derived to be 28.1 kcal/mol.

Spectroscopy and dynamics of selected rotational levels in the B ̃ 2A″ state of the vinoxy radical by Hidekazu Nagai; Robert T. Carter; J.Robert Huber (425-432).
The fluorescence lifetimes of single rotational states in the 81, 71 and 51 vibrational levels of the B ̃ 2A″ electronic state in jet-cooled vinoxy radicals have been investigated following coherent excitation. The emission decays were found to be exponential and independent of the rotational quantum number J, and the observed quantum beats were due exclusively to hyperfine interactions. The dynamics of two regimes of radiationless processes (internal conversion with and without curve crossing) were explored and the electronic structure of the B ̃ 2A″ state further characterised.

The electronic properties at the maleic anhydride/Si(1 0 0)-2×1 interface by T. Bitzer; T. Dittrich; T. Rada; N.V. Richardson (433-438).
The formation of electronic states in the fundamental gap at the maleic anhydride/Si(1 0 0)-2×1 interface has been studied with photoluminescence (PL) measurements and high resolution electron energy loss spectroscopy (HREELS). We observe that the room temperature adsorption of maleic anhydride on Si(1 0 0)-2×1, where the organic molecules are mainly in a di-σ coordination, results in a gap state density D it =1.8(2)×1012   eV −1   cm −2 which is slightly lower than determined for the clean Si(1 0 0)-2×1 surface. After heating maleic anhydride/Si(1 0 0)-2×1 to 1115 K, the gap state density has increased to 6.4(6)×1012   eV −1 cm −2 which we relate to the formation of silicon carbide structures on the silicon surface.

An empirical molecular model is used to explore the ground-state potential energy surface of a mixed crystal in which reversible, reproducible, photo-induced frequency jumps of single molecules were recently reported. In the model, which accounts for many observed features of terrylene in p-terphenyl, optical switching of the guest transition frequency corresponds to relaxation of host molecules in the second solvation shell.

Using the maximum likelihood method, a formalism is derived to analyze a series of biased Monte Carlo or molecular dynamics simulations. The formalism is applied to different examples, in particular the estimation of thermodynamic properties of molecular systems such as potentials of mean force and free energy differences. The formalism is shown to be a generalization of existing methods that are known to be efficient. For the derivation, it is assumed that the values of the distribution function of subsequent states that are analyzed are uncorrelated.

Monte Carlo simulations of zero electric field gradient liquid crystal mixtures by E.Elliott Burnell; Roberto Berardi; Raymond T. Syvitski; Claudio Zannoni (455-464).
We explore with Monte Carlo simulations the NMR finding that solutes in special nematic liquid crystal mixtures `feel' zero average electric field gradient F ZZ . The simulations involve solute–solvent mixtures of 1000 identical Gay–Berne ellipsoidal particles with added electric quadrupoles of varying strengths placed at particle centres. As long as all quadrupoles are sufficiently small, F ZZ calculated at the particle centres is equal for all particles. For larger quadrupole moments, interactions among quadrupoles alter the interparticle distribution and subsequently F ZZ depends on both solvent and solute quadrupoles; in general, the quadrupolar interactions produce a negative contribution to energy.

The magnetic interactions between organic π-radicals adsorbed on 1D and 2D α-boron-nitride (α-BN) crystals are investigated theoretically. As in the case of analogous systems with graphite surfaces [N. Tyutyulkov, F. Dietz, K. Müllen, Chem. Phys. 255 (2000) 223], an intramolecular exchange interaction between the π-electrons within a narrow half-filled band occurs, with the main component from the Coulomb (Hund) exchange. Compared to graphite the values of the effective Heisenberg exchange integral are smaller in the case of α-BN. The energy spectrum, and the magnetic and thermodynamic characteristics of Ising models consisting of methyl radicals adsorbed on 1D and 2D α-BN surfaces are discussed based on the analytically solved equations.

We report a single-valued potential energy surface for HO 4(2 A) from the double many-body expansion method. All n-body (n=2–4) energy terms are taken from published studies on the relevant fragments, with a five-body energy term of Gaussian form added to mimic the experimental activation energy for the OH(v=0)+O3 reaction. A detailed dynamics study of this reaction is also reported using classical trajectories. Good agreement with existing experimental data is obtained.

A theoretical and experimental study of the HPCl radical: the A ̃ 2 A X ̃ 2 A″ visible emission spectrum by M.J. Bramwell; D.M. Rogers; J.J.W. McDouall; J.C. Whitehead (483-488).
We report the first observation of the A ̃ 2 A X ̃ 2 A″ visible emission spectrum of HPCl. The spectrum shows a series of blue-degraded bands with two prominent progressions in the ground state bending vibration. Ab initio calculations are used to help to assign the observed spectrum. The experimentally derived constants are T0=21 305 cm−1, ω2=868 cm−1, ω 2 =622 cm−1 and ω 3 =529 cm−1 compared with calculated values of T0=23 518 cm−1, ω2=893 cm−1, ω 2 =670 cm−1 and ω 3 =524 cm−1. The bending progression results from a substantial change in the bond angle between the ground and electronically excited states from 96° to 118°.

By the application of zero-field magnetic resonance (ZFMR)/FID technique and fast-field sweeping around the level anti-crossing region with fixed rf frequency, we obtained the (minimum) energy gaps between the two crossing levels as 16.7±2.0 and 14.3±3.7 MHz for pentacene-h14 in p-terphenyl and pentacene-d14 in p-terphenyl, respectively, when B 0x. The minimum energy gaps occur at 14 mT for both systems. The hyperfine interaction contributes only partially to the observed energy gap.

Badger's rule revisited by Jerzy Cioslowski; Guanghua Liu; Ricardo A. Mosquera Castro (497-501).
Numerical experiments demonstrate that the accuracy of stretching force constants k e provided by Badger's rule is unlikely to be substantially improved either by modification of the functional dependence on the equilibrium bond length R e or the inclusion of bond parameters related to electron density. These results, based upon both the experimental and QCISD/6-311++G(3d2f, 3p2d) values of R e and k e, imply that most of the universal characteristics of the bond strength vs. bond length dependence are accounted for by Badger's rule, the more detailed features being unexplainable by first-order response properties such as electron density.

DFT–SQM force field for cobalt corrinoids by Tadeusz Andruniow; Marek Z. Zgierski; Pawel M. Kozlowski (502-508).
This Letter introduces a systematic development of a quantitative vibrational force field (FF) capable of modeling vibrational spectra of cobalt corrinoids (B12 derivatives). The scaled quantum mechanical (SQM) method is used to refine density functional theory (DFT) based force constants calculated at the B3LYP level of theory. The reliability of the DFT–SQM FF for cobalt corrinoids is tested on a six-coordinate methylcobalamin Im–[CoIII–corrin]–CH3 model. It is shown that the computed frequencies and Raman intensities permit detailed vibrational assignment. This analysis illustrates the utility of DFT–SQM FF in making detailed connections between the structure of cobalt corrinoids and their vibrational spectra.

New light on the Co–C bond activation in B12-dependent enzymes from density functional theory by Tadeusz Andruniow; Marek Z. Zgierski; Pawel M. Kozlowski (509-512).
Density functional theory (DFT) is applied to the calculation of activation of the Co–CR bond in models of vitamin B12, B–[CoIII(corrin)]–R. It is shown that there is a positive correlation of the bond lengths between the Co atom and the two axial ligands, B and R. The electron donation from axial ligands to the cobalt atom either by electron donating substituents or by a properly oriented external electric field caused by external electric charges is argued to be the main trigger for the activation of the Co–CR bond.

The pressure tensor profile, in the vicinity of the planar interface between isotropic liquid and nematic liquid crystal phases of rod-like molecules, is calculated using Onsager's density functional theory. Two different conventions for the transverse pressure, due respectively to Harasima and to Irving and Kirkwood, are shown to give distinguishable, but similar, forms. The equivalence of these definitions with those based on the grand potential density is clarified. The profiles, and the location of the surface of tension, depend dramatically on the orientation of the director relative to the interface. These physical differences are much larger than the ambiguities associated with the different pressure conventions.

On the charge factors of the simple overlap model for the ligand field in lanthanide coordination compounds by Rodrigo Q. Albuquerque; Gerd B. Rocha; Oscar L. Malta; Pierre Porcher (519-525).
In this Letter, we propose a semi-empirical procedure through which the charge factors, appearing in the simple overlap model (SOM) for the ligand field in lanthanide compounds, can be obtained. The idea is based on the concept of bond valence and bond strength introduced by Pauling in the 1920's. The charge factors thus obtained are used in the calculation of the so-called ligand field parameters, B q k 's, and, subsequently, in the prediction of the Stark levels of the 7 F J manifolds (J=1,2,3 and 4) of the Eu3+ ion in coordination compounds with mixed ligands. Comparison with experiment shows that the results are quite satisfactory.

Reaction mechanism of CO2 with Ca atom: A theoretical study by Der-Yan Hwang; Alexander M Mebel (526-532).
Ab initio G2M(MP2) and CCSD(T)/6-311 + G(3df)//MP2/6-31G(d) calculations show that the Ca + CO2 reaction proceeds by formation of a C2v-symmetric cyclic CaOCO molecule, ∼1 kcal/mol below the reactants, with the initial reaction step barrier of about 13 kcal/mol. Then, CaOCO dissociates to the CaO + CO products without exit barrier. The total reaction endothermicity is calculated as 35.4 and 40.4 kcal/mol at the G2M(MP2) and CCSD(T)/6-311 + G(3df)//QCISD/6-311G(d) levels, respectively. The reverse CaO + CO reaction is predicted to occur without barrier and can rapidly produce Ca + CO2. The reaction mechanisms of different alkali earth metals with carbon dioxide are compared.

A first study of diastatides: BeAt2, BAt2 and CAt2 by Edmond P.F Lee; Timothy G Wright (533-537).
Effective core potentials (ECPs) augmented with large, flexible valence basis sets (tested on HAt) have been used to obtain the geometries and harmonic vibrational frequencies of the diastatide compounds BeAt2,BAt2 and CAt2 for the first time. For CAt2, the ground electronic state is calculated to be the X ̃ 1 A 1 state, and the lowest singlet–triplet gap is calculated to be larger than that of CI2, which is against expectations based upon electronegativity.

Topological analysis of the electron localisation function (ELF) and the theory of atoms in molecules are applied to analyse the nature of bonds in MCCH (M=Li, Na, K) molecules. The substitution of hydrogen by alkali metal leads to a transfer of about one electron from M to C1C2H and all alkali metal acetylides may be described by the M+[C1C2H] formula. This means that the bonding between carbon and metal is mainly of ionic nature. One can explain the observed difference in properties of organosodium compounds by rather high electronegativity of sodium according to the Allred–Rochow scale.

Density functional study of the isomerisation of MOH (M=Be and Mg) by Sapan K. Jain; Chittaranjan Rout; R.C. Rastogi (547-552).
The equilibrium geometries, vibrational frequencies and isomerism of MOH (M=Be and Mg) have been investigated at the B3PW91 level using two basis sets, 6-31G* and 6-311 + G(2d,2p). The calculated geometries, vibrational frequencies and zero-point vibrational energies are in good agreement with the CASSCF level studies of Kong and Boyd. The isomerization products of BeOH and MgOH are structurally similar. The M–O bond length in HMO (M=Be and Mg) is predicted to be longer than that in the corresponding MOH. The predicted reaction energies for the reactions MOH → HMO are endothermic and in good agreement with the CASSCF level results. The predicted ESR hyperfine coupling constants for MgOH and the dissociation energies for both BeOH and MgOH compare quite well with experimental results.

DFT/B3-LYP(6-31++G**) calculations are used to provide information on closed N–H⋯O H -bonded complexes, containing two hydrogen bonds. The studied model systems with two H-bonds are the 2-pyridone dimer, 2-pyridone–H2O and 2-pyridone–CH3OH. For the sake of comparison, the open, singly H-bonded complex 2-pyridone–CH3OCH3 is also examined. In the closed H-bonded complexes, an H-bond cooperativity effect has been observed, but it depends on the linearity of the hydrogen bond N–H⋯O. For the 2-pyridone dimer, which has the perfect, linear geometry, the strongest cooperativity is observed, whereas in all the other closed H-bonded systems, the cooperativity is reduced by deformation from the linear H-bond geometry. These observations are illustrated by different correlations established between the H-bond angle and other characteristic H-bond parameters.

CEO/semiempirical calculations of UV–visible spectra in conjugated molecules by S. Tretiak; A. Saxena; R.L. Martin; A.R. Bishop (561-568).
The collective electronic oscillators (CEO) approach based on the TDHF approximation is combined with INDO/S, MNDO, AM1, and PM3 semiempirical Hamiltonians. This technique is applied to compute and analyze the electronic structure of acceptor-substituted oligomers and conjugated polymers. Calculated excited-state energies and oscillator strengths agree well with the experimental data and with each other. In particular, the results using the Hamiltonians parameterized for ground-state calculations such as AM1 and PM3 agree well with the INDO/S results. In addition, a two-dimensional analysis of the corresponding transition density matrices provides an efficient way for tracing the origin of various optical transitions by identifying the underlying changes in charge densities and bond-orders.

Index (569-577).