An experimental and theoretical study of the reactions OIO+NO and OIO+OH

The kinetics of the reaction OIO+NO were studied by pulsed laser photolysis/time-resolved cavity ring-down spectroscopy, yielding k(235-320 K)=7.6(+4.0)(-3.1) x 10(-13) exp[(607+/-128)/T] cm3 molecule-1 s-1. Quantum calculations on the OIO+NO potential-energy surface show that the reactants form a weakly bound OIONO intermediate, which then dissociates to the products IO+NO2. Rice-Ramsberger-Kassel-Markus (RRKM) calculations on this surface are in good accord with the experimental result. The most stable potential product, IONO2, cannot form because of the significant rearrangement of OIONO that would be required. The reaction OIO+OH was then investigated by quantum calculations of the relevant stationary points on its potential-energy surface. The very stable HOIO2 molecule can form by direct recombination, but the bimolecular reaction channels to HO2+IO and HOI+O2 are closed because of significant energy barriers. RRKM calculations of the HOIO2 recombination rate coefficient yield krec,0=1.5x10(-27) (T/300 K)(-3.93) cm6 molecule-2 s-1, krec,infinity=5.5x10(-10) exp(46/T) cm3 molecule-1 s-1, and Fc=0.30. The rate coefficients of both reactions are fast enough around 290 K and 1 atm pressure for these reactions to play a potentially important role in the gas phase and aerosol chemistry in the marine boundary layer of the atmosphere.     

Infrared spectroscopy of copper-resveratrol complexes: a joint experimental and theoretical study

Infrared multiple-photon dissociation spectroscopy has been used to record vibrational spectra of charged copper-resveratrol complexes in the 3500-3700 cm(-1) and 1100-1900 cm(-1) regions. Minimum energy structures have been determined by density functional theory calculations using plane waves and pseudopotentials. In particular, the copper(I)-resveratrol complex presents a tetra-coordinated metal bound with two carbon atoms of the alkenyl moiety and two closest carbons of the adjoining resorcinol ring. For these geometries vibrational spectra have been calculated by using linear response theory. The good agreement between experimental and calculated IR spectra for the selected species confirms the overall reliability of the proposed geometries.     

Electronic delocalization in discotic liquid crystals: a joint experimental and theoretical study

Discotic liquid crystals emerge as very attractive materials for organic-based (opto)electronics as they allow efficient charge and energy transport along self-organized molecular columns. Here, angle-resolved photoelectron spectroscopy (ARUPS) is used to investigate the electronic structure and supramolecular organization of the discotic molecule, hexakis(hexylthio)diquinoxalino[2,3-a:2',3'-c]phenazine, deposited on graphite. The ARUPS data reveal significant changes in the electronic properties when going from disordered to columnar phases, the main feature being a decrease in ionization potential by 1.8 eV following the appearance of new electronic states at low binding energy. This evolution is rationalized by quantum-chemical calculations performed on model stacks containing from two to six molecules, which illustrate the formation of a quasi-band structure with Bloch-like orbitals delocalized over several molecules in the column. The ARUPS data also point to an energy dispersion of the upper pi-bands in the columns by some 1.1 eV, therefore highlighting the strongly delocalized nature of the pi-electrons along the discotic stacks.     

Molecular energetics of cytosine revisited: a joint computational and experimental study

A static bomb calorimeter has been used to measure the standard molar energy of combustion, in oxygen, at T = 298.15 K, of a commercial sample of cytosine. From this energy, the standard (p degrees = 0.1 MPa) molar enthalpy of formation in the crystalline state was derived as -(221.9 +/- 1.7) kJ.mol(-1). This value confirms one experimental value already published in the literature but differs from another literature value by 13.5 kJ.mol(-1). Using the present standard molar enthalpy of formation in the condensed phase and the enthalpy of sublimation due to Burkinshaw and Mortimer [J. Chem. Soc., Dalton Trans. 1984, 75], (155.0 +/- 3.0) kJ.mol(-1), results in a value for the gas-phase standard molar enthalpy of formation for cytosine of -66.9 kJ.mol(-1). A similar value, -65.1 kJ.mol(-1), has been estimated after G3MP2B3 calculations combined with the reaction of atomization on three different tautomers of cytosine. In agreement with experimental evidence, the hydroxy-amino tautomer is the most stable form of cytosine in the gas phase. The enthalpies of formation of the other two tautomers were also estimated as -60.7 kJ.mol(-1) and -57.2 kJ.mol(-1) for the oxo-amino and oxo-imino tautomers, respectively. The same composite approach was also used to compute other thermochemical data, which is difficult to be measured experimentally, such as C-H, N-H, and O-H bond dissociation enthalpies, gas-phase acidities, and ionization enthalpies.     

Vacancy-induced chemisorption of NO2 on carbon nanotubes: a combined theoretical and experimental study

The role of structural defects on the adsorption of NO2 on carbon nanotubes (CNTs) is analyzed here by means of both statical density functional theory calculations and Car-Parrinello molecular dynamics and further confirmed by X-ray photoelectron spectroscopy measurements. The interaction of a NO2 molecule with an active site produced by a single vacancy on the sidewall follows two possible reaction routes, leading to the formation of a C-N bond or to dissociation of NO2. Accounting for defective adsorption sites allows a better understanding of microscopic mechanisms involved in technological applications of CNTs, e.g., gas-sensing devices.     

Conformations in dioctyl substituted polyfluorene: a combined theoretical and experimental Raman scattering study

The structural properties of polyfluorenes (PF) are extremely sensitive to the choice of functionalizing side chains. Dioctyl substituted PF (PF8) adopts metastable structures that depend upon the thermal history and choice of solvents used in film forming conditions. We present a detailed study of the changes in the backbone and side chain morphology in PF8, induced by the various crystallographic phases, using Raman scattering techniques. The vibrational frequencies and intensities of fluorene oligomers are calculated using hybrid density-functional theory with a 3-21G(*) basis set. The alkyl side chains are modeled as limiting conformations: all anti, anti-gauche-gauche, and end gauche representations. The calculated vibrational spectra of single chain oligomers in conjunction with our experimental results demonstrate the beta phase, which is known to originate in regions of enhanced chain planarity as a direct consequence of the alkyl side chain conformation.     

Characterization of the interface dipole at the paraphenylenediamine-nickel interface: a joint theoretical and experimental study

In organic-based (opto)electronic devices, charge injection into conjugated materials is governed to a large extent by the metal-organic interface dipole. Controlling the injection of charges requires a better understanding of the fundamental origin of the interface dipole. In this context, photoelectron spectroscopies and density functional theory calculations are used to investigate the interaction between para-phenylenediamine (PPDA), an electron donor, and a polycrystalline nickel surface. The interface dipole formed upon chemisorption of one PPDA monolayer strongly modifies the work function of the nickel surface from 5.10 to 3.55 eV. The work function decrease of 1.55 eV is explained by the electron-donor character of PPDA and the modification of the electronic density at the metal surface. PPDA monolayers are composed of tilted molecules interacting via the nitrogen lone-pair and PPDA molecules chemisorbed parallel to the surface via their pi-electron density. Annealing the monolayer leads to dehydrogenation of PPDA activated by the nickel surface, as found for other amines.     

Vibrationally resolved photoelectron spectroscopy of BO- and BO2-: a joint experimental and theoretical study

We report a photoelectron spectroscopy and computational study of two simple boron oxide species: BO- and BO2-. Vibrationally resolved photoelectron spectra are obtained at several photon energies (355, 266, 193, and 157 nm) for the 10B isotopomers, 10BO- and 10BO2-. In the spectra of 10BO-, we observe transitions to the 2Sigma+ ground state and the 2Pi excited state of 10BO at an excitation energy of 2.96 eV. The electron affinity of 10BO is measured to be 2.510+/-0.015 eV. The vibrational frequencies of the ground states of 10BO- and 10BO and the 2Pi excited state are measured to be 1725+/-40, 1935+/-30, and 1320+/-40 cm-1, respectively. For 10BO2-, we observe transitions to the 2Pig ground state and two excited states of 10BO2, 2Piu, and 2Sigmau+, at excitation energies of 2.26 and 3.04 eV, respectively. The electron affinity of 10BO2 is measured to be 4.46+/-0.03 eV and the symmetrical stretching vibrational frequency of the 2Piu excited state of 10BO2 is measured to be 980+/-30 cm-1. Both density functional and ab initio calculations are performed to elucidate the electronic structure and chemical bonding of the two boron oxide molecules. Comparisons with the isoelectronic AlO- and AlO2- species and the closely related molecules CO, N2, CN-, and CO2 are also discussed.     

Joint experimental and theoretical study of vibrationally inelastic electron scattering on propane

Vibrational electron energy loss spectra were measured for propane at incident energies of 3, 6, 10, 15, 20, and 25 eV at scattering angles of 40 degrees, 55 degrees, 70 degrees, 85 degrees, and 100 degrees . The spectra are compared with the results of ab initio calculations using a recently developed two-channel discrete momentum representation method. Good agreement between theory and experiment was found for large scattering angles and energies above the resonant region.     

Photophysical properties of charged cyclometalated Ir(III) complexes: a joint theoretical and experimental study

The photophysical properties of a series of charged biscyclometalated [Ir(ppy)(2)(N^N)](1+) complexes, where ppyH is 2-phenylpyridine and N^N is 2,2'-bipyridine (bpy), 6-phenyl-2,2'-bipyridine (pbpy), and 6,6'-diphenyl-2,2'-bipyridine (dpbpy) for complexes 1, 2, and 3, respectively, have been investigated in detail. The photoluminescence performance in solution decreases from 1 to 3 upon attachment of phenyl groups to the ancillary ligand. The absorption spectra recorded over time suggest that complex 3 is less stable compared to complexes 1 and 2 likely due to a nucleophilic-assisted ancillary ligand-exchange reaction. To clarify this behavior, the temperature dependence of the experimental intrinsic deactivation rate constant, k(in) = 1/τ, has been investigated from 77 K to room temperature. Temperature-dependent studies show that nonemitting metal-centered (MC) states are accessible at room temperature for complex 3. The experimental results are interpreted with the help of theoretical calculations performed within the density functional theory (DFT) approach. Calculations suggest that attachment of a phenyl group to the ancillary ligand (2) promotes the temperature-independent deactivation pathways, whereas attachment of a second phenyl group (3) also makes the temperature-dependent ones accessible through population of nonradiative (3)MC excited states.     

Reference molecules for nonlinear optics: a joint experimental and theoretical investigation

Hyper-Rayleigh scattering (HRS) experiments and quantum chemical calculations are combined to investigate the second-order nonlinear optical responses of a series of reference molecules, namely, carbon tetrachloride, chloroform, trichloroacetonitrile, acetonitrile, and dichloromethane. The multipolar decomposition of the first hyperpolarizability tensor through the use of the spherical harmonics formalism is employed to highlight the impact of the symmetry of the molecular scatterers on their nonlinear optical responses. It is demonstrated that HRS is a technique of choice to probe the molecular symmetry of the compounds. Coupled-cluster calculations performed at the coupled-cluster level with singles, doubles, and perturbative triples in combination with highly extended basis sets and including environment effects by using the polarizable continuum model qualitatively reproduce the molecular first hyperpolarizabilities and depolarization ratios of the molecular scatterers.     

Mechanism of the antioxidant action of silybin and 2,3-dehydrosilybin flavonolignans: a joint experimental and theoretical study

Flavonolignans from silymarin, the standardized plant extract obtained from thistle, exhibit various antioxidant activities, which correlate with the other biological and therapeutic properties of that extract. To highlight the mode of action of flavonolignans as free radical scavengers and antioxidants, 10 flavonolignans, selectively methylated at different positions, were tested in vitro for their capacity to scavenge radicals (DPPH and superoxide) and to inhibit the lipid peroxidation induced on microsome membranes. The results are rationalized on the basis of (i) the oxidation potentials experimentally obtained by cyclic voltammetry and (ii) the theoretical redox properties obtained by quantum-chemical calculations (using a polarizable continuum model (PCM)-density functional theory (DFT) approach) of the ionization potentials and the O-H bond dissociation enthalpies (BDEs) of each OH group of the 10 compounds. We clearly establish the importance of the 3-OH and 20-OH groups as H donors, in the presence of the 2,3 double bond and the catechol moiety in the E-ring, respectively. For silybin derivatives (i.e., in the absence of the 2,3 double bond), secondary mechanisms (i.e., electron transfer (ET) mechanism and adduct formation with radicals) could become more important (or predominant) as the active sites for H atom transfer (HAT) mechanism are much less effective (high BDEs).     

Response of solid Ne upon photoexcitation of a NO impurity: a quantum dynamics study

The ultrafast geometrical rearrangement dynamics of NO doped cryogenic Ne matrices after femtosecond laser pulse excitation is studied using a quantum dynamical approach based on a multi-dimensional shell model, with the shell radii being the dynamical variables. The Ne-NO interaction being only weakly anisotropic allows the model to account for the main dynamical features of the rare gas solid. Employing quantum wave packet propagation within the time dependent Hartree approximation, both, the static deformation of the solid due to the impurity and the dynamical response after femtosecond excitation, are analysed. The photoinduced dynamics of the surrounding rare gas atoms is found to be a complex high-dimensional process. The approach allows to consider realistic time-dependent femtosecond pulses and the effect of the pulse duration is clearly shown. Finally, using the pulse parameters of previous experiments, pump-probe signals are calculated and found to be in good agreement with experimental results, allowing for a clear analysis of the ultrafast mechanism of the energy transfer into the solid.     

Molecular beam study of F2+I2

Crossed molecular beam techniques have been used to study the endoergic reaction between F₂ and I₂. Above a threshold energy of 4 kcal/mole the observed products are I₂F and F. At higher energies IF is also produced. Angular and velocity distributions indicate that the IF does not result from a four-center exchange reaction.     

Molecular beam study of the system Kr + HCl

Angular distributions of HCl scattered off Kr have been measured at collision energiesE ranging from 66.3 to 208.7 meV. The curves exhibit a clearly resolved damped rainbow structure. In addition time-of-flight distributions of scattered HCl were measured indicating significant energy transfer. To a part of these data (E=99 meV) the parameters of the M5 potential energy surface of Hutson and Howard [3] were fitted using the infinite order sudden approximation (IOSA) and quasi classical trajectories (QCT). A comparison of QCT and IOSA results suggests applicability of the IOSA to the present system atE=99 meV. We find a well depth of ?31.8 meV (compared to Hutson and Howard's value of ?26.5 meV) at the collinear Kr ... HCl conformation and strong indication for the existence of a secondary potential minimum at the collinear Kr ... ClH conformation. The analysis of the angular distributions based on a one channel scattering calculation leads to an isotropic potential similar to the full potential energy surface cut along the 90°-conformation rather than to the spherical average or spherical limit of the surface.     

The Si-B chromophore: A joint experimental and theoretical investigation

Silylboranes with aromatic substituents linked to boron and silicon exhibit an unexpected absorption band in the UV-Vis spectral region. When polar groups were introduced, a marked solvatochromic effect was observed in their fluorescence emission spectra, revealing a strong excited state dipole moment. Semi-empirical MNDO/d and AM1 calculations showed that, upon UV excitation, the polarity of the Si-B bond increased and the aromatic π-electrons migrated toward the Si-B bond, consistent with experimental observations.     

A joint experimental and theoretical study of the palladium-catalyzed electrophilic allylation of aldehydes

Palladium-catalyzed electrophilic allylation of aldehydes with allylstannanes has been proposed in the literature as a model reaction illustrating the potential of nucleophilic eta(1)-allyl palladium pincer complexes to promote new catalytic processes. This reaction was studied by a joint experimental and theoretical approach. It was shown that pincer palladium complexes featuring a S approximately P approximately S and a S approximately C approximately S tridentate ligand are efficient catalysts for this reaction. The full mechanism of this transformation was studied in detail by means of DFT calculations. Two pathways were explored: the commonly proposed mechanism involving eta(1)-allyl palladium intermediates and a Lewis acid promoted mechanism. Both of these mechanisms were compared to the direct transformation that was shown experimentally to occur under mild conditions. The mechanism involving an eta(1)-allyl palladium intermediate has been discarded on energetic grounds, the nucleophilic attack and the transmetalation step being more energetically demanding than the direct reaction between allyltin and the aldehyde. On the other hand, a mechanism where the palladium acts as a Lewis acid proved to be fully consistent with all experimental and theoretical results. This mechanism involves (L approximately X approximately L)Pd(+) species which activate the aldehyde moiety toward nucleophilic attack.