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In view of the inherent pseudocapacitance, rich redox pairs (Nb5+/Nb4+ and Nb4+/Nb3+), and high lithiation potential (1.0–3.0 V vs Li/Li+), Nb2O5 is considered a promising anode material. However, the inherent low electronic conductivity of Nb2O5 limits its lithium storage performance, and the rate performance after carbon modification is still unsatisfactory because the intrinsic conductivity of Nb2O5 has not been substantially improved. In this experiment, taking the improvement of the intrinsic electrical conductivity of Nb2O5 as the guiding ideology, we prepared F-doped Nb2O5@fluorocarbon composites (F–Nb2O5@FC) with a large number of oxygen vacancies by one-step annealing. As the anode electrode of lithium-ion batteries, the reversible specific capacity of F–Nb2O5@FC reaches 150 mA g−1 at 5 A g−1 after 1100 cycles, and the rate performance is particularly outstanding, with a capacity up to 130 mA g−1 at 16 A g−1, which is far superior to other Nb2O5@carbon-based anode electrodes. Compared with other single conductivity sources of Nb2O5@carbon-based composites, the electrical conductivity of F–Nb2O5@FC composites is greatly improved in many aspects, including the introduction of free electrons by F− doping, the generation of oxygen vacancies, and the provision of a three-dimensional conductive network by FC. Through analytical chemistry (work function, UV–Vis diffuse reflectance spectroscopy, and EIS) and theoretical calculations, it is proved that F–Nb2O5@FC has high electrical conductivity and realizes rapid electron transfer. [ABSTRACT FROM AUTHOR]

Supported metallic nanoparticles play a central role in catalysis. However, predictive modeling is particularly challenging due to the structural and dynamic complexity of the nanoparticle and its interface with the support, given that the sizes of interest are often well beyond those accessible via traditional ab initio methods. With recent advances in machine learning, it is now feasible to perform MD simulations with potentials retaining near-density-functional theory (DFT) accuracy, which can elucidate the growth and relaxation of supported metal nanoparticles, as well as reactions on those catalysts, at temperatures and time scales approaching those relevant to experiments. Furthermore, the surfaces of the support materials can also be modeled realistically through simulated annealing to include effects such as defects and amorphous structures. We study the adsorption of fluorine atoms on ceria and silica supported palladium nanoparticles using machine learning potential trained by DFT data using the DeePMD framework. We show defects on ceria and Pd/ceria interfaces are crucial for the initial adsorption of fluorine, while the interplay between Pd and ceria and the reverse oxygen migration from ceria to Pd control spillover of fluorine from Pd to ceria at later stages. In contrast, silica supports do not induce fluorine spillover from Pd particles. [ABSTRACT FROM AUTHOR]

The high-pressure properties of fluorine and chlorine are not yet well understood because both are highly reactive and volatile elements, which have made conducting diamond anvil cell and x-ray diffraction experiments a challenge. Here, we use ab initio methods to search for stable crystal structures of both elements at megabar pressures. We demonstrate how symmetry and geometric constraints can be combined to efficiently generate crystal structures that are composed of diatomic molecules. Our algorithm extends the symmetry driven structure search method [R. Domingos et al., Phys. Rev. B 98, 174107 (2018)] by adding constraints for the bond length and the number of atoms in a molecule while still maintaining generality. As a method of validation, we have tested our approach for dense hydrogen and reproduced the known molecular structures of Cmca-12 and Cmca-4. We apply our algorithm to study chlorine and fluorine in the pressure range of 10 GPa–4000 GPa while considering crystal structures with up to 40 atoms per unit cell. We predict chlorine to follow the same series of phase transformations as elemental iodine from Cmca to Immm to Fm 3 ¯ m, but at substantially higher pressures. We predict fluorine to transition from a C2/c to Cmca structure at 70 GPa, to a novel orthorhombic and metallic structure with P42/mmc symmetry at 2500 GPa, and finally to its cubic analog form with Pm 3 ¯ n symmetry at 3000 GPa. [ABSTRACT FROM AUTHOR]

We present a method to approximate post-Hartree-Fock correlation energies by using approximate natural orbitals obtained by the random phase approximation (RPA). We demonstrate the method by applying it to the helium atom, the hydrogen and fluorine molecule, and to diamond as an example of a periodic system. For these benchmark systems, we show that RPA natural orbitals converge the MP2 correlation energy rapidly. Additionally, we calculated full configuration interaction energies for He and H2, which are in excellent agreement with the literature and experimental values. We conclude that the proposed method may serve as a compromise to reach good approximations to correlation energies at moderate computational cost, and we expect the method to be especially useful for theoretical studies on surface chemistry by providing an efficient basis to correlated wave function based methods. [ABSTRACT FROM AUTHOR]

We have investigated the performance of a core-valence separated scheme within the coupled cluster (CC) hierarchy of methods CC singles (CCS), CC singles and approximate doubles (CC2), and CC singles and doubles (CCSD) in reproducing the K-edge x-ray absorption spectra of the low-Z elements carbon, oxygen, and fluorine in formaldehyde (CH2O), carbonyl fluoride (CF2O), formyl fluoride (CHFO), and formic acid (CHOOH). The analysis covers the entire frequency region from the first 1s → π* excitation to the core-ionization limit, encompassing the region of Rydberg transitions. Moreover, a simulation of the vibronic progressions in the 1s → π* bands of both carbon and oxygen in formaldehyde has been performed at the core-valence separated CCSD level, and the results are critically compared with highly resolved experimental data for this molecule. [ABSTRACT FROM AUTHOR]

We use MRSF-TDDFT and NEVPT2 methods to design singlet fission chromophores with the building blocks of cyclic (alkyl)(amino)carbenes (CAACs). CAAC dimers with C2, C4, and p-phenylene spacers are considered. The substitutions with trifluoromethyls and fluorine atoms at the α C position are investigated. The electronegative substituents enhance the π accepting capability of the α C while maintaining it as a quaternary C atom. The phenylene-connected dimers with the two substitutions are identified as promising candidates for singlet fission chromophores. The cylindrically symmetric C2 and C4 spacers allow for substantial structural reorganizations in the S0-to-S1 and S0-to-T1 excitations. Although the two substituted dimers with the C4 spacer satisfy (or very close to satisfy) the primary thermodynamics criterion for singlet fission, the significant structural reorganizations result in high barriers so that the fission is kinetically unfavorable. [ABSTRACT FROM AUTHOR]

The chemistry of fluorine in the interstellar medium is particularly simple, with only a few key species and important reactions. Of the latter, the rate of the reaction of C+ ions with HF is not well established but is one of the key reactions that sets the relative abundance of HF and the CF+ ion, the two fluorine-bearing species that have been observed in interstellar clouds. The C+ + HF → CF+ + H reaction proceeds through a deeply bound HCF+ well. In this work, statistical methods, namely, the statistical adiabatic channel method originally developed by Quack and Troe and the quantum statistical method of Manolopoulos and co-workers, are applied to compute the total cross section as a function of energy for this reaction. This reaction proceeds on the ground 12A′ potential energy surface (PES), and there are also two non-reactive PES's, 12A″ and 22A′, correlating with the C+(2P1/2,3/2) + HF reactants. Two sets of scattering calculations were carried out, namely, a single-surface calculation on the 12A′ PES and the one in which all three PES's and the spin-orbit splitting of C+ are included in the description of the entrance channel. In the latter, reactivity of the spin-orbit excited 2P3/2 level can be computed, and not just assumed to be zero, as in the single-state adiabatic approximation. [ABSTRACT FROM AUTHOR]

We investigate the effects of anion doping with fluorine impurities on the chemistry of the CeO2 (111) facet, using the results of DFT+U calculations. We consider three prototypical processes: the formation of oxygen vacancies, the adsorption of O2 and H2O molecules, and the re-oxidation of the surface with fragments of the two molecules. We find that the first two of these processes are not strongly affected, but that the presence of F lowers the energy gained in the re-oxidation of the surface in comparison to the healing of an oxygen vacancy, by 1.47 eV in the case of O2 (provided that the F is part of a cluster) and by 0.92 eV in the case of H2O. Based on these results, we suggest that F could enhance the redox chemistry of ceria by toggling between being in the surface and on the surface, effectively facilitating the release of lattice O by acting as a "place holder" for it. Finally, we find that the desorption of F as either 1/2F2 or HF is energetically unfavourable, suggesting that F doped ceria should be stable in the presence of O2 and H2O. [ABSTRACT FROM AUTHOR]

The effect of a fraction of Hartree-Fock exchange on the calculated spin-spin coupling constants involving fluorine through a hydrogen bond is analyzed in detail. Coupling constants calculated using wavefunction methods are revisited in order to get high-level calculations using the same basis set. Accurate MCSCF results are obtained using an additive approach. These constants and their contributions are used as a reference for density functional calculations. Within the density functional theory, the Hartree-Fock exchange functional is split in short- and long-range using a modified version of the Coulomb-attenuating method with the SLYP functional as well as with the original B3LYP. Results support the difficulties for calculating hydrogen bond coupling constants using density functional methods when fluorine nuclei are involved. Coupling constants are very sensitive to the Hartree-Fock exchange and it seems that, contrary to other properties, it is important to include this exchange for short-range interactions. Best functionals are tested in two different groups of complexes: those related with anionic clusters of type [F(HF)n]- and those formed by difluoroacetylene and either one or two hydrogen fluoride molecules. [ABSTRACT FROM AUTHOR]

The effect of fluorine substitution on the molecular structure, crystal packing, and n-type semiconducting properties of a set of poly(arylene-ethynylene) polymers based on alternating thiadiazole and phenyl units linked through ethynylene groups has been studied by means of Density Functional Theory. As a result, an enlargement in the interplanar distance between cofacial polymer chains, as well as a decrease of the electronic coupling and electron mobility is predicted. On the other hand, fluorination could facilitate electron injection into the material. A polymer containing both alkoxy pendant chains and fluorine atoms is proposed as a compromise solution between efficiency of electron injection and charge transport within the material. [ABSTRACT FROM AUTHOR]

Reaction rate constants and cross sections are computed for the radiative association of carbon cations (C+) and fluorine atoms (F) in their ground states. We consider reactions through the electronic transition 1¹Π → X¹Σ+ and rovibrational transitions on the X¹Σ+ and a³Π potentials. Semiclassical and classical methods are used for the direct contribution and Breit-Wigner theory for the resonance contribution. Quantum mechanical perturbation theory is used for comparison. A modified formulation of the classical method applicable to permanent dipoles of unequally charged reactants is implemented. The total rate constant is fitted to the Arrhenius-Kooij formula in five temperature intervals with a relative difference of <3%. The fit parameters will be added to the online database KIDA. For a temperature of 10-250 K, the rate constant is about 10-21 cm³ s-1, rising toward 10-16 cm³ s-1 for a temperature of 30 000 K. [ABSTRACT FROM AUTHOR]

Reaction rate constants and cross sections are computed for the radiative association of carbon cations (C+) and fluorine atoms (F) in their ground states. We consider reactions through the electronic transition 1¹Π → X¹Σ+ and rovibrational transitions on the X¹Σ+ and a³Π potentials. Semiclassical and classical methods are used for the direct contribution and Breit-Wigner theory for the resonance contribution. Quantum mechanical perturbation theory is used for comparison. A modified formulation of the classical method applicable to permanent dipoles of unequally charged reactants is implemented. The total rate constant is fitted to the Arrhenius-Kooij formula in five temperature intervals with a relative difference of <3%. The fit parameters will be added to the online database KIDA. For a temperature of 10-250 K, the rate constant is about 10-21 cm³ s-1, rising toward 10-16 cm³ s-1 for a temperature of 30 000 K. [ABSTRACT FROM AUTHOR]

The dependence of the negative-ion fractions on incident energy and angle is reported for 8.5-22.5 keV F- ions scattered from a Si(100) surface at a fixed scattering angle of 38°. The negativeion fraction increases monotonically with incident velocity for specular scattering. In particular, the variation of the fraction with incident angle is bell shaped for a given incident energy. We interpret this variation using the incident-velocity effect at short distances where the yield of negative ions depends on the number of initial neutrals. It strongly indicates that at short distances, a dynamical equilibrium population is never achieved. This nonadiabatic feature is supported by simple calculations using modified rate equations. [ABSTRACT FROM AUTHOR]

Absolute differential cross sections (DCSs) for electron interaction with BF3 molecules have been measured in the impact energy range of 1.5-200 eV and recorded over a scattering angle range of 15°-150°. These angular distributions have been normalized by reference to the elastic DCSs of the He atom and integrated by employing a modified phase shift analysis procedure to generate integral cross sections (ICSs) and momentum transfer cross sections (MTCSs). The calculations of DCSs and ICSs have been carried out using an independent atom model under the screening corrected additivity rule (IAM-SCAR). The present elastic DCSs have been found to agree well with the results of IAM-SCAR calculation above 20 eV, and also with a recent Schwinger multichannel calculation below 30 eV. Furthermore, in the comparison with the XF3 (X = B, C, N, and CH) molecules, the elastic DCSs reveal a similar angular distribution which are approximately equal in magnitude from 30 to 200 eV. This feature suggests that the elastic scattering is dominated virtually by the 3-outer fluorine atoms surrounding the XF3 molecules. The vibrational DCSs have also been obtained in the energy range of 1.5-15 eV and vibrational analysis based on the angular correlation theory has been carried out to explain the nature of the shape resonances. Limited experiments on vibrational inelastic scattering confirmed the existence of a shape resonance with a peak at 3.8 eV, which is also observed in the vibrational ICS. Finally, the estimated elastic ICSs, MTCSs, as well as total cross sections are compared with the previous cross section data available. [ABSTRACT FROM AUTHOR]

The earlier proposed multi-reference state-specific coupled-cluster theory with the complete active space reference [CASCC; Lyakh et al., J. Chem. Phys. 122, 024108 (2005)] suffered from a problem of energy discontinuities when the formal reference state was changing in the calculation of the potential energy curve (PEC). A simple remedy to the discontinuity problem is found and is presented in this work. It involves using natural complete active space self-consistent field active orbitals in the complete active space coupled-cluster calculations. The approach gives smooth PECs for different types of dissociation problems, as illustrated in the calculations of the dissociation of the single bond in the hydrogen fluorine molecule and of the symmetric double-bond dissociation in the water molecule. [ABSTRACT FROM AUTHOR]

We report the reaction dynamics of F atom with selected alkanes studied by crossed beam scattering with DC slice ion imaging. The target alkanes are propane, n-butane, and n-pentane. The product alkyl radicals are probed by 157 nm single photon ionization following reaction at a collision energy of ~10 kcal mol-1. The analyzed data are compared with the corresponding theoretical studies. Reduced translational energy distributions for each system show similar trends with little of the reaction exoergicity appearing in translation. However, the pentane reaction shows a somewhat smaller fraction of available energy in translation than the other two, suggesting greater energy channeled into pentyl internal degrees of freedom. The center-of-mass angular distributions all show backscattering as well as sharp forward scattering that decreases in relative intensity with the size of the molecule. Possible reasons for these trends are discussed. [ABSTRACT FROM AUTHOR]

Multiple fluorine atom substitution effect on photophysics of an aromatic chromophore has been investigated using phenol as the reference system. It has been noticed that the discrete vibronic structure of the S1 S0 absorption system of phenol vapor is completely washed out for pentafluorophenol (PFP), and the latter also shows very large Stokes shift in the fluorescence spectrum. For excitations beyond S1 origin, the emission yield of PFP is reduced sharply with increase in excess vibronic energy. However, in a collisional environment like liquid hydrocarbon, the underlying dynamical process that drives the non-radiative decay is hindered drastically. Electronic structure theory predicts a number of low-lying dark electronic states of πσ* character in the vicinity of the lowest valence ππ* state of this molecule. Tentatively, we have attributed the excitation energy dependent non-radiative decay of the molecule observed only in the gas phase to an interplay between the lowest ππ* and a nearby dissociative πσ* state. Measurements in different liquids reveal that some of the dark excited states light up with appreciable intensity only in protic liquids like methanol and water due to hydrogen bonding between solute and solvents. Electronic structure theory methods indeed predict that for PFP-(H2O)n clusters (n = 1-11), intensities of a number of πσ* states are enhanced with increase in cluster size. In contrast with emitting behavior of the molecule in the gas phase and solutions of nonpolar and polar aprotic liquids, the fluorescence is completely switched off in polar protic liquids. This behavior is a chemically significant manifestation of perfluoro effect, because a very opposite effect occurs in the case of unsubstituted phenol for which fluorescence yield undergoes a very large enhancement in protic liquids. Several dynamical mechanisms have been suggested to interpret the observed photophysical behavior. [ABSTRACT FROM AUTHOR]

Doubly charged ions HDO2+, H2O2+, and D2O2+ were prepared selectively to triplet or singlet excited states in collisions with F+ or H+ projectiles at 3 keV. Excitation energies of dications following two-body or three-body dissociation channels were measured and compared with recent calculations using ab initio multi-reference configuration interaction method [Gervais et al., J. Chem. Phys. 131, 024302 (2009)]. For HDO2+, preferential cleavage of O-H rather than O-D bond has been observed and the ratio between the populations of the fragmentation channels OD+_H+ and OH+_D+ were measured. The kinetic energy release has been measured and compared with previous experiments. [ABSTRACT FROM AUTHOR]

Momentum distributions associated with ortho-positronium (o-Ps) pick-off annihilation photon are often influenced by light elements, as, e.g., carbon, oxygen, and fluorine. This phenomenon, so-called element specificity of o-Ps pick-off annihilation, has been utilized for studying the elemental environment around the open spaces. To gain an insight into the element specificity of o-Ps pick-off annihilation, the chemical shift of oxygen 1s binding energy and the momentum distributions associated with o-Ps pick-off annihilation were systematically investigated for alkali-metal loaded SiO2 glasses by means of X-ray photoelectron spectroscopy and positron-age-momentum correlation spectroscopy, respectively. Alkali metals introduced into the open spaces surrounded by oxygen atoms cause charge transfer from alkali metals to oxygen atoms, leading to the lower chemical shift for the oxygen 1s binding energy. The momentum distribution of o-Ps localized into the open spaces is found to be closely correlated with the oxygen 1s chemical shift. This correlation with the deepest 1s energy level evidences that the element specificity of o-Ps originates from pick-off annihilation with orbital electrons, i.e., dominantly with oxygen 2p valence electrons and s electrons with lower probability. [ABSTRACT FROM AUTHOR]

A ¹H and 19F nuclear magnetic resonance study of [Mg(H2O)6](BF4)2 has confirmed the existence of two phase transitions at Tc1 ≈ 257 K and Tc2 ≈ 142 K, detected earlier by the DSC method. These transitions were reflected by changes in the temperature dependences of both proton and fluorine of second moments M2H and M2F and of spin-lattice relaxation times T1H and T1F. The study revealed anisotropic reorientations of whole [Mg(H2O)6]2+ cations, reorientations by 180° jumps of H2O ligands, and aniso- and isotropic reorientations of BF4- anions. The activation parameters for these motions were obtained. It was found that the phase transition at Tc1 is associated with the reorientation of the cation as a whole unit around the C3 axis and that at Tc2 with isotropic reorientation of the BF4- anions. The temperature dependence of the full width at half maximum value of the infrared band of ρt (H2O) mode (at ~596 cm-1) indicated that in phases I and II, all H2O ligands in [Mg(H2O)6]2+ perform fast reorientational motions (180° jumps) with a mean value of activation energy equal to ca 10 kJ mole-1, what is fully consistent with NMR results. The phase transition at Tc1 is associated with a sudden change of speed of fast (τR ≈ 10-12 s) reorientational motions of H2O ligands. Below Tc2 (in phase III), the reorientations of certain part of the H2O ligands significantly slow down, while others continue their fast reorientation with an activation energy of ca 2 kJ mole-1. This fast reorientation cannot be evidenced in NMR relaxation experiments. Splitting of certain IR bands connected with H2O ligands at the observed phase transitions suggests a reduction of the symmetry of the octahedral [Mg(H2O)6]2+ complex cation. [ABSTRACT FROM AUTHOR]

We report absolute elastic differential cross sections for electron interactions with the C4F6 isomers, hexafluoro-1,3-butadiene (1,3-C4F6), hexafluoro-2-butyne (2-C4F6), and hexafluorocyclobutene (c- C4F6). The incident electron energy range is 1.5-200 eV, and the scattered electron angular range for the differential measurements varies from 15° to 150°. In all cases the absolute scale of the differential cross section was set using the relative flow technique, with helium as the reference species. Atomic-like behaviour in these scattering systems is shown here for the first time, and is further investigated by comparing the elastic cross sections for the C4F6 isomers with other fluorinated molecules, such as SF6 and CnF6 (n = 2, 3, and 6). We note that for all the six-F containing molecules, the scattering process for electron energies above 30 eV is indistinguishable. Finally, we report results for calculations of elastic differential cross sections for electron scattering from each of these isomers, within an optical potential method and assuming a screened corrected independent atom representation. The level of agreement between these calculations and our measurements is found to be quite remarkable in all cases. [ABSTRACT FROM AUTHOR]

We present an analysis of the spin-rotation and absolute shielding constants of XF6 molecules (X = S, Se, Te, Mo, W) based on ab initio coupled cluster and four-component relativistic densityfunctional theory (DFT) calculations. The results show that the relativistic contributions to the spin- rotation and shielding constants are large both for the heavy elements as well as for the fluorine nuclei. In most cases, incorporating the computed relativistic corrections significantly improves the agreement between our results and the well-established experimental values for the isotropic spin-rotation constants and their anisotropic components. This suggests that also for the other molecules, for which accurate and reliable experimental data are not available, reliable values of spin-rotation and absolute shielding constants were determined combining ab initio and relativistic DFT calculations. For the heavy nuclei, the breakdown of the relationship between the spin- rotation constant and the paramagnetic contribution to the shielding constant, due to relativistic effects, causes a significant error in the total absolute shielding constants. [ABSTRACT FROM AUTHOR]

Near carbon K-edge X-ray absorption fine structure spectra of a series of fluorine-substituted ethenes and acetone have been studied using coupled cluster and density functional theory (DFT) polarization propagator methods, as well as the static-exchange (STEX) approach. With the complex polarization propagator (CPP) implemented in coupled cluster theory, relaxation effects following the excitation of core electrons are accounted for in terms of electron correlation, enabling a systematic convergence of these effects with respect to electron excitations in the cluster operator. Coupled cluster results have been used as benchmarks for the assessment of propagator methods in DFT as well as the state-specific static-exchange approach. Calculations on ethene and 1,1-difluoroethene illustrate the possibility of using nonrelativistic coupled cluster singles and doubles (CCSD) with additional effects of electron correlation and relativity added as scalar shifts in energetics. It has been demonstrated that CPP spectra obtained with coupled cluster singles and approximate doubles (CC2), CCSD, and DFT (with a Coulomb attenuated exchange-correlation functional) yield excellent predictions of chemical shifts for vinylfluoride, 1,1-difluoroethene, trifluoroethene, as well as good spectral features for acetone in the case of CCSD and DFT. Following this, CPP-DFT is considered to be a viable option for the calculation of X-ray absorption spectra of larger π-conjugated systems, and CC2 is deemed applicable for chemical shifts but not for studies of fine structure features. The CCSD method as well as the more approximate CC2 method are shown to yield spectral features relating to π*-resonances in good agreement with experiment, not only for the aforementioned molecules but also for ethene, cis-1,2-difluoroethene, and tetrafluoroethene. The STEX approach is shown to underestimate π*-peak separations due to spectral compressions, a characteristic which is inherent to this method. [ABSTRACT FROM AUTHOR]

Quasiclassical trajectory calculations have been carried out for the F+HCl reaction in three dimensions on a recent DHSN PES of the ground 12A′ electronic state [M. P. Deskevich, M. Y. Hayes, K. Takahashi, R. T. Skodje, and D. J. Nesbitt, J. Chem. Phys. 124, 224303 (2006)]. The effects of the collision energy and the reagent initial rotational excitation on the cross sections and product polarization are studied for the v = 0 and j <= 10 states of HCl over a wide collision energy range. It has been found that either the collision energy or the HCl rotational excitation increase remarkably reaction cross sections. The QCT-calculated integral cross sections are in good agreement with previous QM results. A detailed study on product polarization for the title reaction is also performed. The calculated results show that the product rotational angular momentum j′ is not only aligned, but also oriented along the direction perpendicular to the scattering plane. The orientation of the HF product rotational angular momentum vector j′ depends very sensitively on the collision energy and also affected by the reagent rotation. The theoretical findings and especially the roles of the collision energy and initial rotational momentum on the product polarization are discussed and reasonably explained by the HLH mass combination, the property of the PES, as well as the reactive mechanism. [ABSTRACT FROM AUTHOR]

We follow a suggestion by Lipoff and Herschbach [Mol. Phys. 108, 1133 (2010)] and compare dressed and bare adiabatic potentials to get insight regarding the low-energy dynamics (e.g., cold reaction) taking place in molecular systems. In this particular case, we are interested to study the effect of conical intersections (ci) on the interacting atoms. For this purpose, we consider vibrational dressed adiabatic and vibrational dressed diabatic potentials in the entrance channel of reactive systems. According to our study, the most one should expect, in case of F + H2, is a mild effect of the (1, 2) ci on its reactive/exchange process-an outcome also supported by experiment. This happens although the corresponding dressed and bare potential barriers (and the corresponding van der Waals potential wells) differ significantly from each other. [ABSTRACT FROM AUTHOR]

Fluorination has become a versatile route to tune the electronic and optical properties of organic conjugated materials. Herein we report a new phenomenon, excited-state switching by per-fluorination of para-oligophenylenes, placing a low intensity 11B2 state below the 11B1 state, giving rise to large Stokes shifts. The switching is attributed to the specific impact of fluorine on the delocalized and localized frontier orbitals as elucidated by quantum-chemical calculations. The sterical demands of the fluorine atom additionally diminish efficient conjugation along the chain, leading to hypsochromic shifts with respect to the unsubstituted counterparts and to a weak chain length dependence of the absorption and unstructured emission spectra and enhanced internal conversion. [ABSTRACT FROM AUTHOR]

By means of electric force microscopy, composition depth profiles were measured with nanometric resolution for a series of fluorinated networks. By mapping the dielectric permittivity along a line going from the surface to the bulk, we were able to experimentally access to the fluorine concentration profile. Obtained data show composition gradient lengths ranging from 30 nm to 80 nm in the near surface area for samples containing from 0.5 to 5 wt. % F, respectively. In contrast, no gradients of concentration were detected in bulk. This method has several advantages over other techniques because it allows profiling directly on a sectional cut of the sample. By combining the obtained results with x-ray photoelectron spectroscopy measurements, we were also able to quantify F/C ratio as a function of depth with nanoscale resolution. [ABSTRACT FROM AUTHOR]

A recently published alternative hierarchy of coupled-cluster approximations is reformulated as a perturbative correction. A single variant, a model for the total electronic energy based on the perfect quadruples model, is explored in detail. The computational scaling of the method developed is the same as canonical second order Mo\ller-Plesset perturbation theory (fifth order in the number of molecular orbitals), but its accuracy competes with the high-accuracy, high-cost standard CCSD(T), even when the latter is allowed to break spin-symmetry. The variation presented can be implemented without explicit calculation and storage of the most expensive energy contributions, thereby improving the range of systems which can be treated. The performance and scaling of the method are demonstrated with calculations on the water, fluorine, and oxirane molecules, and compared to the parent model. [ABSTRACT FROM AUTHOR]

We have studied the excitation and dissociation processes of the molecule W(CO)6 in collisions with low kinetic energy (3 keV) protons, monocharged fluorine, and chlorine ions using double charge transfer spectroscopy. By analyzing the kinetic energy loss of the projectile anions, we measured the excitation energy distribution of the produced transient dications W(CO)62+. By coincidence measurements between the anions and the stable or fragments of W(CO)62+, we determined the energy distribution for each dissociation channel. Based on the experimental data, the emission of the first CO was tentatively attributed to a nonstatistical direct dissociation process and the emission of the second or more CO ligands was attributed to the statistical dissociation processes. The dissociation energies for the successive breaking of the W-CO bond were estimated using a cascade model. The ratio between charge separation and evaporation (by the loss of CO+ and CO, respectively) channels was estimated to be 6% in the case of Cl+ impact. [ABSTRACT FROM AUTHOR]

We have developed the uncoupled version of multireference Mukherjee's coupled cluster method with connected triexcitations. The method has been implemented in ACES II program package. The agreement between the uncoupled and the standard version of Mukherjee's multireference coupled cluster method has been reported previously at the singles and doubles level by Das et al. [J. Mol. Struct.: THEOCHEM 79, 771 (2006); Chem. Phys. 349, 115 (2008)]. The aim of this article is to investigate this method further, in order to establish how its performance changes with the size of the basis set, size of the model space, multireference character of different molecules, and inclusion of connected triple excitations. Assessment of the new method has been performed on the singlet methylene, potential energy curve of fluorine molecule, and third b 1Σg+ electronic state of oxygen molecule. [ABSTRACT FROM AUTHOR]

The electronic structure of energetically low-lying excited singlet states of fluorobenzene molecules is investigated here. Increasing fluorine substitution alters the nature of the excited electronic states and the so-called perfluoro effect is observed for penta- and hexafluorobenzene. Detailed quantum chemistry calculations are carried out at the equation-of-motion coupled-cluster singles and doubles level of theory to establish the potential energy surfaces of the low-lying electronic states of mono-, di- (ortho- and meta-), and pentafluorobenzene molecules. A sequence of low-energy conical intersections among the electronic potential energy surfaces is established. It is found that increasing fluorine substitution lowers the energy of the πσ* electronic state and leads to conical intersections between the S1 and S2 electronic states of pentafluorobenzene. Existence of numerous conical intersections among the excited electronic states of these molecules forms the mechanistic details underlying their nonradiative internal conversion. In particular, the slow and biexponential fluorescence emission in pentafluorobenzene is attributed to the existence of low-lying S1-S2 conical intersections. The electronic structure data are analyzed in detail and the coupling mechanism among various electronic excited states of mono-, di-, and pentafluorobenzene molecules is established. [ABSTRACT FROM AUTHOR]

Nuclear dynamics in the coupled electronic states of mono-, di-(ortho and meta), and pentafluorobenzene molecules is investigated here. Attempts are specifically made to understand the complexity and broadening of the recorded gas phase electronic absorption spectra of these molecules. Justification is also provided for the low quantum yield of fluorescence emission with increasing number of fluorine substitutions. The nuclear dynamics is simulated from first principles both by time-independent and time-dependent quantum mechanical methods. Potential energy surfaces of the low-lying excited electronic states of these molecules constructed in Paper I [Mondal and Mahapatra, J. Chem. Phys. 133, 084304 (2010)] are employed for the purpose. Theoretical results presented in this paper are compared with the available experimental data and the agreement between the two is found to be excellent. While structured electronic absorption bands are observed for the S1 state of mono- and difluorobenzene molecules, the same for the pentafluorobenzene is broad and structureless. Occurrence of S1-S2 conical intersections in pentafluorobenzene leads to a nonradiative internal conversion of the S1 state in ∼165 fs and contributes to the broadening of the S1←S0 absorption band and a biexponential decay of its fluorescence emission. [ABSTRACT FROM AUTHOR]

The low-lying doublet and quartet electronic states of the species SeF correlating with the first dissociation channel are investigated theoretically at a high-level of electronic correlation treatment, namely, the complete active space self-consistent field/multireference single and double excitations configuration interaction (CASSCF/MRSDCI) using a quintuple-zeta quality basis set including a relativistic effective core potential for the selenium atom. Potential energy curves for (Λ+S) states and the corresponding spectroscopic properties are derived that allows for an unambiguous assignment of the only spectrum known experimentally as due to a spin-forbidden X 2Π-a 4∑- transition, and not a A 2Π-X 2Π transition as assumed so far. For the bound excited doublets, yet unknown experimentally, this study is the first theoretical characterization of their spectroscopic properties. Also the spin-orbit coupling constant function for the X 2Π state is derived as well as the spin-orbit coupling matrix element between the X 2Π and a 4∑- states. Dipole moment functions and vibrationally averaged dipole moments show SeF to be a very polar species. An overview of the lowest-lying spin-orbit (Ω) states completes this description. [ABSTRACT FROM AUTHOR]

The potential energy curve of the fluorine molecule in the ground electronic state 1Σg+ is determined and analyzed in the long-range region. The analysis is based on expressing the potential as the sum of the potential energy curve of the uncorrelated, but properly dissociating wave function and the correlation energy contribution. It is shown that, in the long-range region, the former becomes identical with the interaction between the quadrupoles of the fluorine atoms and the latter becomes the London dispersion interaction. The former is repulsive because of the coaxial quadrupole alignments in the 1Σg+ ground state and proportional to 1/R5. The latter is attractive and proportional to 1/R6. There moreover exists an additional repulsive force due to the loss of spin-orbit coupling upon the bond formation. As a result of these antagonistic interactions, the potential energy curve has a barrier at about 4 Å, with a value about +0.04 mhartree. The descent of the potential toward the minimum, when the atoms approach each other from infinity, begins therefore only at internuclear distances less than about twice the equilibrium distance and is then very steep. [ABSTRACT FROM AUTHOR]

Chemical sensing properties of phthalocyanine thin-film transistors have been investigated using nearly identical n- and p-channel devices. P-type copper phthalocyanine (CuPc) has been modified with fluorine groups to convert the charge carriers from holes to electrons. The sensor responses to the tight binding analyte dimethyl methylphosphonate (DMMP) and weak binding analyte methanol (MeOH) were compared in air and N2. The results suggest that the sensor response involves counterdoping of pre-adsorbed oxygen (O2). A linear dependence of chemical response to DMMP concentration was observed in both n- and p- type devices. For DMMP, there is a factor of 2.5 difference in the chemical sensitivity between n- and p-channel CuPc thin-film transistors, even though it has similar binding strength to n- and p-type CuPc molecules as indicated by the desorption times. The effect is attributed to the difference in the analyte perturbation of electron and hole trap energies in n- and p-type materials. [ABSTRACT FROM AUTHOR]

Ionenes are alkyl polymer chains in which hydrophobic groups are separated by ionic charges. They are useful for studying the properties of water as a solvent because they demonstrate a sufficiently complex combination of hydrophobicity, charge interactions, and specific-ion effects that some properties cannot be predicted by implicit-solvation theories. On the other hand, they are simple enough that their molecular structures can be varied and controlled in systematic experiments. In particular, implicit-solvent models predict that all such solutes will have negative enthalpies of dilution, whereas experiments show that enthalpies of dilution are positive for the chaotropic counterions. Here, we study ionenes that are short chains (six monomer units) in solutions of different counterions, with sodium as the coion by molecular dynamics simulations in explicit water. We explore the pair distributions of various atoms within the system at three different temperatures: T=278, 298, and 318 K. We find (i) that the molecular dynamics simulations are consistent with the experimental trends for the osmotic coefficients and enthalpies of dilution, (ii) that the fluorine-nitrogen and fluorine-carbon correlations decrease with decreasing temperature, (iii) while the opposite behavior is found for iodine ions, and (iv) that in the counterion-Na+ pair distributions, too, fluorine ions behave oppositely to iodine ions upon temperature increase. [ABSTRACT FROM AUTHOR]

The polarizabilities of the ground state of fluorine are determined by the box-based B-spline configuration-interaction method. The ground-state static and tensor polarizabilities are calculated as αd=3.49a03 and α2=0.303a03, respectively. The resulting excitation matrix elements are employed to compute the interatomic dispersion interactions for the HF and F2 molecules. [ABSTRACT FROM AUTHOR]

The diffusion process of fluorine (F) atoms on the Si(111)-(7×7) surface is investigated using high-temperature scanning tunneling microscopy. The kinetic parameters of F hopping agree well with those of the diffusing silicon (Si) atoms, which implies that of all reaction processes, the Si diffusion serves as the rate-determining one. Deposition of Si on the surface is found to enhance F hopping, which supports the above-mentioned observation. Theory reveals that the replacement of F adsorption sites by diffusing Si atoms is the key process in the diffusion mechanism. [ABSTRACT FROM AUTHOR]

We describe fully quantum, time-independent scattering calculations of the F+H2→HF+H reaction, concentrating on the HF product rotational distributions in v′=3. The calculations involved two new sets of ab initio potential energy surfaces, based on large basis set, multireference configuration-interaction calculations, which are further scaled to reproduce the experimental exoergicity of the reaction. In addition, the spin-orbit, Coriolis, and electrostatic couplings between the three quasidiabatic F+H2 electronic states are included. The calculated integral cross sections are compared with the results of molecular beam experiments. At low collision energies, a significant fraction of the reaction is due to Born–Oppenheimer forbidden, but energetically allowed reaction of F in its excited (2P1/2) spin-orbit state. As the collision energy increases, the Born–Oppenheimer allowed reaction of F in its ground (2P3/2) spin-orbit state rapidly dominates. Overall, the calculations agree reasonably well with the experiment, although there remains some disagreement with respect to the degree of rotational excitation of the HF(v′=3) products as well as with the energy dependence of the reactive cross sections at the lowest collision energies. [ABSTRACT FROM AUTHOR]

Ab initio and density functional theory calculations with aug-cc-pVDZ and aug-cc-pVTZ basis sets have been performed on the HCl–CH3SH dimer and HCl–(CH3SH)2 and (HCl)2–CH3SH trimers. Structures, energetics, and infrared frequencies are calculated. The results are discussed in terms of the cooperativity effect which is a characteristic of H-bonded systems and compared to oxygen-containing analogs of the same trimers, HCl–(CH3OH)2 and (HCl)2–CH3OH, which have been published recently. [ABSTRACT FROM AUTHOR]

A perturbatively truncated version of the reduced multireference coupled-cluster method with singles and doubles and noniterative triples RMR CCSD(T) is described. In the standard RMR CCSD method, the effect of all triples and quadruples that are singles or doubles relative to references spanning a chosen multireference (MR) model space is accounted for via the external corrections based on the MR CISD wave function. In the full version of RMR CCSD(T), the remaining triples are then handled via perturbative corrections as in the standard, single-reference (SR) CCSD(T) method. By using a perturbative threshold in the selection of MR CISD configuration space, we arive at the truncated version of RMR CCSD(T), in which the dimension of the MR CISD problem is significantly reduced, thus leaving more triples to be treated perturbatively. This significantly reduces the computational cost. We illustrate this approach on the F2 molecule, in which case the computational cost of the truncated version of RMR CCSD(T) is only about 10%–20% higher than that of the standard CCSD(T), while still eliminating the failure of CCSD(T) in the bond breaking region of geometries. To demonstrate the capabilities of the method, we have also used it to examine the structure and binding energy of transition metal complexes Ni(CO)n with n=1, 2, and 4. In particular, Ni(CO)2 is shown to be bent rather than linear, as implied by some earlier studies. The RMR CCSD(T) binding energy differs from the SR CCSD(T) one by 1–2 kcal/mol, while the energy barrier separating the linear and bent structures of Ni(CO)2 is smaller than 1 kcal/mol. [ABSTRACT FROM AUTHOR]

In this paper we present integral cross sections (in the 5–220 meV collision energy range) and rate constants (in the 100–300 K range of temperature) for the F+HD reaction leading to HF+D and DF+H. The exact quantum reactive scattering calculations were carried out using the hyperquantization algorithm on an improved potential energy surface which incorporates the effects of open shell and fine structure of the fluorine atom in the entrance channel. The results reproduce satisfactorily molecular beam scattering experiments as well as chemical kinetics data for both the HF and DF channels. In particular, the agreement of the rate coefficients and the vibrational branching ratios with experimental measurements is improved with respect to previous studies. At thermal and subthermal energies, the rates are greatly influenced by tunneling through the reaction barrier. Therefore exchange of deuterium is shown to be penalized with respect to exchange of hydrogen, and the isotopic branching exhibits a strong dependence on translational energy. Also, it is found that rotational excitation of the reactant HD molecule enhances the production of HF and decreases the reactivity at the D end, obtaining insight on the reaction stereodynamics. [ABSTRACT FROM AUTHOR]

The broad absorption of Cl3 radical was observed between 1150 and 1350 nm using cavity ring-down spectroscopy at 213–265 K and 50–200 Torr with He, N2, Ar, or SF6 diluents. The absorption intensity of Cl3 increased at lower temperature and higher pressure. SF6 was the most efficient diluent gas. The temperature dependent equilibrium constants for Cl3 formation from Cl+Cl2 were theoretically calculated at the MP4SDQ/6-311+G(d) level. Observed decay time profiles of Cl3 and the pressure dependence of Cl3 formation are explained by the equilibrium reaction and a decay reaction of Cl+Cl3. [ABSTRACT FROM AUTHOR]

The lifetime of the à 2Σu+ state of N3 has been determined from line broadening in the rotationally resolved laser-induced fluorescence (LIF) spectrum of the à 2Σu+–X 2Πg transition. N3 radicals, produced by fluorine atom abstraction from HN3, were probed with an intracavity doubled ring laser operating near 272 nm. Careful examination of the LIF spectrum indicates a significant Lorentzian component to the line shape due to a rapid predissociation in the à 2Σu+ state. The predissociation lifetime is found to be 132±21 ps for the 000 vibrational level and 64±10 ps for the 010 level. The short lifetime is consistent with the low intensity of the LIF signal especially for the 010-010 hot band. [ABSTRACT FROM AUTHOR]

Chemiluminescent exit channels of Mn+F2→MnF*+F were investigated using the molecular beam technique in a beam-gas configuration with an array detector. Two uncongested regions, corresponding to MnF transitions c 5Σ+(b)-a 5Σ+(b) and b 5Πi(int)-a 5Σ+(b) were fit for vibrational and rotational populations, which were used to develop a microscopic reaction mechanism for these chemiluminescent exit channels. In both MnF* electronic states, significant vibrational excitation but little rotational excitation was found. Significant vibrational excitation has been attributed to early energy release as Mn loses an electron at long range to the lowest unoccupied molecular orbital on F2. The incipient bond is strengthened as backbonding from the F2- to Mn+ increases the covalent character of the intermediate. Finally, no strict geometric constraints are placed on the exit channel and hence there is no significant repulsive energy release into product rotation. Our proposed mechanism exhibits dynamic control in that the course of the reaction is determined by both geometric factors and dynamic factors. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Potential surfaces, dipole moments, and polarizabilities are calculated by ab initio methods [unrestricted MP2(full)/6-311++G(2d,2p)] along the reaction paths of the F+CH4 and Cl+CH4 reaction systems. It is found that in general dipole moments and polarizabilities exhibit peaks near the transition state. In the case of X=F these peaks are on the products side and in the case of X=Cl they are on the reactants side indicating an early transition state in the case of fluorine and a late transition state in the case of chlorine. An analysis of the geometric changes along the reaction paths reveals a one-to-one correspondence between the peaks in the electric properties and peaks in the rate of change of certain internal geometric coordinates along the reaction path. Interaction with short infrared intense laser fields pulses leads to the possibility of interferences between the dipole and polarizability laser-molecule interactions as a function of laser phase. The larger dipole moment in the Cl+CH4 reaction can lead to the creation of deep wells (instead of energy barriers) and new strongly bound states in the transition state region. This suggests possible coherent control of the reaction path as a function of the absolute phase of the incident field, by significant modification of the potential surfaces along the reaction path and, in particular, in the transition state region. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Our ab initio all-electron Dirac-Fock and the corresponding nonrelativistic limit calculations performed at four U-F bond distances yield for octahedral UF6- the optimized U-F bond distance of 2.091 and 2.088 Å, respectively. We have also performed Dirac scattered wave calculations at the optimized U-F bond distances using the first-order pertubational procedure to obtain the Zeeman and hyperfine magnetic tensors for the octahedral anion UF6-. The calculated isotropic Zeeman tensor of Δg=-2.87 is in fairly good agreement with the value of Δg=-2.78±0.10 obtained in electron spin resonance experiments on the H3O+UF6- adduct and the unpaired electron-spin spends ∼2.5% of its time on the fluorine 2p3/2 spinors. The calculated relativistic transition energies of the near-IR and visible absorption bands are also in good agreement with the experimental results. The octahedral uranium hexafluoride anion has a simple crystal field f1 configuration; however, relativistic four-component wave functions are necessary to interpret correctly the available magnetic data, while a relativistic treatment taking into account double group symmetrized basis functions should suffice for the interpretation of the optical data. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Our ab initio all-electron Dirac-Fock and the corresponding nonrelativistic limit calculations performed at four U-F bond distances yield for octahedral UF6- the optimized U-F bond distance of 2.091 and 2.088 Å, respectively. We have also performed Dirac scattered wave calculations at the optimized U-F bond distances using the first-order pertubational procedure to obtain the Zeeman and hyperfine magnetic tensors for the octahedral anion UF6-. The calculated isotropic Zeeman tensor of Δg=-2.87 is in fairly good agreement with the value of Δg=-2.78±0.10 obtained in electron spin resonance experiments on the H3O+UF6- adduct and the unpaired electron-spin spends ∼2.5% of its time on the fluorine 2p3/2 spinors. The calculated relativistic transition energies of the near-IR and visible absorption bands are also in good agreement with the experimental results. The octahedral uranium hexafluoride anion has a simple crystal field f1 configuration; however, relativistic four-component wave functions are necessary to interpret correctly the available magnetic data, while a relativistic treatment taking into account double group symmetrized basis functions should suffice for the interpretation of the optical data. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Stereochemical dependences of vicinal fluorine-fluorine nuclear magnetic resonance coupling constants (3JFF) have been studied with the multiconfigurational self-consistent field in the restricted active space approach, with the second-order polarization propagator approximation (SOPPA), and with density functional theory. The SOPPA results show the best overall agreement with experimental couplings. The relationship with the dihedral angle between the coupled fluorines has been studied by Fourier analysis, the result is very different from that of proton-proton couplings. The Fourier coefficients do not resemble those of a typical Karplus equation. The four nonrelativistic contributions to the coupling constants of 1 ,2-difluoroethane configurations have been studied separately showing that up to six Fourier coefficients are required to reproduce the calculated values satisfactorily. Comparison with Fourier coefficients for matching hydrogen fluoride dimer configurations suggests that the higher order Fourier coefficients (Cn &ges; 3) originate mainly from through-space Fermi contact interaction. The through-space interaction is the main reason 3JFF do not follow the Karplus equation. [ABSTRACT FROM AUTHOR]

The spatial changing feature of the shapes and sizes of the system consisted of one hydrogen atom and one fluorine atom of forming a hydrogen fluoride molecule is investigated. We give formalism of the potential acting on an electron in a molecule and derive its concrete expression in Hartree–Fock self-consistent molecular orbital theory including configuration interaction. The program of calculating the potential acting on an electron in a molecule is programmed and compiled in the framework of the MELD program package. We formulate briefly the approach of the molecular intrinsic characteristic contour (MICC) which is defined in terms of the classical turning points of electronic motion. The MICC for a molecular system is intrinsic and can be calculated by means of an ab initio CI method. Then, the polarization and bonding features of the intrinsic characteristic contours of hydrogen and fluorine atoms forming a hydrogen fluoride molecule are presented and discussed from ab initio calculations. Furthermore, electron density distribution as an added dimension has been demonstrated on the changing MICC and thus the vivid polarization and bonding features for a chemical process have been shown. It seems that at the early stage (internuclear distance Ind=5.0–20.0 a.u.) the fluorine atom gives more enthusiastic with the sensitive and expanded polarization to welcome coupling with the hydrogen atom while the latter has little response even “shy” with shrinking a bit its size at the beginning of putting the two atoms into a system and it is only around the critical point, the contact point (Ind=4.73 a.u.), that both of them stretch their hands and arms to meet and then fuse together. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]