Collisional enhancement of the I(52P12) emission at 1.3μm by parent molecules CF3I,C2F5I,i-C3F7I and n-C3F7I

The magnetic dipole transition I(5²P_{$\text{1}\text{2}$}-5²P_{$\text{3}\text{2}$}) at 1.3152 μm is shown to be enhanced by collisions of the metastable iodine atoms with the parent RI molecules CF₃I, C₂F₅I, i-C₃F₇I and n-C₃F₇I. The enhancing mechanism is exciplex emission of the RI·I(5²P_{$\text{1}\text{2}$}) molecule at 1.3 μm, with different rates for each iodide. The influence of this effect on the measurement of the quantum yields to I(5²P_{$\text{1}\text{2}$}) and of the respective reaction rates by infrared fluorescence is discussed.     

Enrichment of 13C by IRMPD of CBr2F2

The CO₂-laser-induced infrared multiple photon decomposition of natural CBr₂F₂ in the presence of oxygen has been examined as a function of pulse number (30–1500), reactant pressures (CBr₂F₂, 10–150 Torr and O₂, 5–90 Torr), laser line [9P(8)–9P(32)], and laser fluence (1–3 J cm^–2) to optimize irradiation conditions for ¹³C-enrichment. CF₂O was the main carbon containing product and afterwards was converted into CO₂ via hydrolysis. A small amount of C₂Br₂F₄ was detected only under extreme conditions, for example, at high laser fluences or wavenumbers close to an absorption band. The ¹³C-atom fraction of the final product CO₂ was found to be 20–80%, depending on experimental conditions. The two-stage IRMPD process proposed previously has been examined in further detail in the present study. First, CBr₂F₂ containing about 30% of ¹³C was prepared in the ¹³C-selective IRMPD of natural CHClF₂ in the presence of Br₂. The second-stage IRMPD of the CBr₂F₂ in the presence of oxygen under selected conditions resulted in the high enrichment of ¹³C beyond 90%.     

Manifestation of resonance dipole-dipole interaction in spectra of low-temperature molecular mixtures of C2F6 in CF4 and CF4 in C2F6

We have measured IR absorption spectra of solutions C₂F₆ in CF₄ (T = 178 K) and CF₄ in C₂F₆ (T = 173 K) in the overtone range of the spectrum. We have studied how the resonance dipole-dipole interaction affects the formation of contours of bands that correspond to transitions to states involving the vibrations ν₁₀(C₂F₆) and ν₃(CF₄), which are strong in the dipole absorption. For the system C₂F₆ in liquid CF₄, the state ν₂ + ν₁₀(C₂F₆) resonantly interacts with the state ν₂(C₂F₆) + ν₃(CF₄), and, for the system CF₄ in liquid C₂F₆, the state ν₁ + ν₃(CF₄) resonantly interacts with the state ν₁(CF₄) + ν₁₀(C₂F₆). The contours of the bands ν₂ + ν₁₀ (C₂F₆) in the spectrum of the mixture with CF₄ and of the bands ν₁ + ν₃(CF₄) in the spectrum of the mixture with C₂F₆ have been calculated.     

Chemical reactions following the IRMPD of C2F3Cl

C₂F₃Cl is photolyzed with a TEA-CO₂ laser at 1050.44 cm^–1 with focussed fluences up to 280 J/cm². The stable products in the IRMPD of C₂F₃Cl are determined for up to 10 Torr of C₂F₃Cl being photolyzed both neat and with added O₂. C₂F₄ and trans-C₂F₂Cl₂ are found to occur in the greatest yield though C₃F₅Cl, C₃F₄Cl₂, C₄F₇Cl, and C₂F₃Cl₃ also appear to be primary products. When O₂ is present F₂CO, FClCO, and CF₂ClCOF are the exclusive products. The formation of these products are for the most part consistent with a carbene formation dissociation mechanism for C₂F₃Cl IRMPD. C₂F₃Cl₃ may best be explained by another mechanism competitive with carbene formation. Many products attributed to secondary photolysis mechanisms are observed for long photolysis times.This work was performed at Department of Chemistry and chemical Engineering, Michigan Technological University, Houghton, MI 49931, USA     

A density functional theory study of CF3CH2I adsorption and reaction on Ag(111)

The adsorption and reaction behaviors of CF₃CH₂I on Ag(111) were systematically studied by density functional theory (DFT) calculations. Physical adsorption of CF₃CH₂I on Ag(111) occurs due to the weak interactions between surface Ag atoms and iodine atom of CF₃CH₂I; while strong chemisorption occurs for CF₃CH₂ fragment on Ag(111). Electronic analysis indicates that the singly occupied molecular orbital (SOMO) of CF₃CH₂ strongly interacts with the surface Ag atoms. It is very interesting to find that the most stable structures of CF₃CH₂ on Ag(111) locate at the top site, instead of the hollow sites. This might be attributed to the facts that CF₃CH₂ adsorbed at the top site will maximize the sp³-type hybridization, and the possible weak interaction between the fluorine lone pair electrons of p orbitals for CF₃CH₂ and surface Ag(111) occurs, which is supported by the charge density difference (CDD) analysis with a low isosurface value. We propose that the charge density difference (CDD) analysis with a high or low isosurface value can be widely applied to analyze the strong or weak electronic interactions upon adsorption. Transition state calculations suggested that the energy barrier of CF bond rupture for CF₃CH₂I on Ag(111) (1.44 eV) is much higher than that of CI bond breakage for CF₃CH₂I (0.43 eV); and the activation energy of the CF bond dissociation for CF₃CH₂(a) is 0.67 eV.     

Product state distributions and rotational polarization for the crossed beam reactions of Ca with F2 and NF3

Product state distributions of the CaF products from the thermal crossed beam reactions Ca + F₂ and Ca + NF₃ have been measured using laser induced fluorescence (LIF) techniques. We obtain information about the rotational and vibrational distributions by generating synthetic band profiles and comparing them with those observed. The inverted vibrational distributions indicate direct reaction mechanisms. For Ca + F₂ the fractionf′ of energy is nearly half going into product translation and the remainder being devided nearly equally between product rotation and vibration. For Ca + NF₃ the largest fraction of the available reaction exoergicity goes into vibrational excitation of the newly formed CaF products. In addition, we have probed the rotational polarization of CaF product molecules. This gives direct information on the role of angular momentum alignment in reactive scattering.     

两种烷基碘化物分子C—I键解离的从头计算

分别采用多组态自洽场方法和二阶多组态准简并微扰论方法,计算了烷基碘化物分子CF₃I和C₂H₂F₃I沿C—I键的绝热势能曲线和垂直激发能. 结果发现,这两种分子的低激发态均为排斥态;基态的解离能分别为2.473eV和2.835eV,其中前者与实验结果符合较好. 关键词： 烷基碘化物分子 解离能 势能曲线     

Unwanted combustion enhancement by C6F12O fire suppressant

Several agents are under consideration to replace CF₃Br for use in suppressing fires in aircraft cargo bays. In a Federal Aviation Administration (FAA) performance test simulating the explosion of an aerosol can, however, the replacements, when added at sub-inerting concentrations, have all been found to create higher pressure rise than with no agent, hence failing the test. Thermodynamic equilibrium calculations as well as perfectly-stirred reactor (PSR) simulations with detailed reaction kinetics, are performed for one of these agents, C₆F₁₂O (Novec 1230), to understand the reasons for the unexpected enhanced combustion rather than suppression. The high pressure rise with added agent is shown to depend on the amount of agent, and can only occur if a large fraction of the available oxidizer in the chamber is consumed, corresponding to stoichiometric proportions of fuel, oxygen, and agent. A kinetic model for the reaction of C₆F₁₂O in hydrocarbon–air flames has been developed. Stirred-reactor simulations predict that at higher agent loadings, the inhibition effectiveness of C₆F₁₂O is relatively insensitive to the overall stoichiometry, and the marginal inhibitory effect of the agent is greatly reduced, so that the mixture remains flammable over a wide range of conditions corresponding to those of the FAA test. The present findings are consistent with and support the earlier analyses for C₂HF₅ and CF₃Br, which were also evaluated in the FAA test.     

Reactions of excited I*(2P1/2) and unexcited I(2P3/2) iodine atoms in perfluoroalkyl radicals

A method of investigating reactions of excited and unexcited atoms is discussed. It is based on pulsed photolysis of molecules with simultaneous passage of laser radiation through the working medium. The method proposed is used to investigate the reactions that accompany the photolysis of the molecules RI(CF₃I, n-C₃F₇I, i-C₃F₇I). The rate constants of the recombination of iodine atoms into I₂ in the presence of RI molecules are calculated for the atoms I(²P_3/2) and I*(²P_1/2), as are the recombination constants of the radicals R into R₂ and with the atoms I*(²P_1/2) and I(²P_3/2) into the RI molecule. It is shown that the I(²P_3/2) atoms are much more active in the recombination into Ia and RI than the I*(²P_1/2) atoms. The role of the investigated reactions in the kinetics of a photodissociation iodine laser (PDIL) is discussed. The results are compared with the published data.     

Geometric shielding corrections for total cross section calculations of electron scattering by CH4, C2H6, C2H3F3, C2H4, C2F4, C2Cl4 and C2Cl2F2 from 30–5000?eV

To quantify the changes in the geometric shielding effect in a molecule as the incident electron energy varies, an empirical fraction, which represents the total cross section contributions of shielded atoms in a molecule at different energies, is presented. Using this empirical fraction, the total cross sections for electron scattering by CH₄, C₂H₆, C₂H₃F₃, C₂H₄, C₂F₄, C₂Cl₄ and C₂Cl₂F₂ are calculated over a wide energy range from 30 to 5000 eV by the additivity rule model at the Hartree-Fock level. The quantitative total cross sections are compared with those obtained by experiment and other theories where available. Good agreement is attained above 100 eV.     

Quantum mechanical studies on the singlet and triplet potential energy surface of the reactions CF3O2 + I,CF3O + OI and CF3 + OIO

The singlet and triplet potential energy surfaces for the reactions CF₃O₂ + I (1), CF₃O + OI (2) and CF₃ + OIO (3) are investigated using ab initio quantum mechanical methods. Four important isomeric energy minima were found, three on the singlet surface, CF₃OOI, CF₃OIO and CF₃IO₂ and one on the triplet surface ³CF₃OIO. CF₂O + FOI are shown to be the most probable products for all reactions, CF₃O +I and CF₃O + O(³P) are possible for reactions (2) and (3) while the reaction pathway leading to CF₃O +OI is also possible for reaction (3).     

Reactive scattering of alkali dimers K2 + I2, CH2I2, CHI3, CBr4

Angular distribution measurements of KX reactive scattering of a potassium dimer K₂ beam by I₂ and by a series of halomethane molecules are reported. The K₂ + I₂ reactive scattering is similar to that previously observed for K₂ + Br₂. The predominant reaction path yields K + KI + I with the K and KI product recoiling in the forward direction. However, the forward peak of the KI differential cross section is lower than that for K from K₂ + I₂ and is broader than that observed for KBr from K₂ + Br₂. This is attributed to slow dissociation of the I ₂ ^- ion formed in the electron jump mechanism previously proposed for K₂ + Br₂. In the halomethane reactions, both alkali atoms of the K₂ dimer become bound alkali halide molecules in all reactive collisions, despite the direct dynamics of the corresponding supersonic K atom reactions. Thus, these reactions provide compelling evidence for a second electron jump mechanism, previously proposed for the reactions of K₂ dimers with polyhalide molecules. The differential cross sections for the K₂ dimer plus halomethane reactions indicate an osculating collision complex with a lifetime at least comparable to its rotational period, perhaps much longer. This reaction complex is identified with the doubly ionic state formed by the second electron jump transition.     

Structural and electrical properties of high-k HfO2 films modified by CHF3 and C4F8/O2 plasmas

As-deposited HfO₂ films were modified by CHF₃, C₄F₈, and mixed C₄F₈/O₂ plasmas in a dual-frequency capacitively coupled plasma chamber driven by radio frequency generators of 60 MHz as the high frequency (HF) source and 2 MHz as the low frequency source (60/2 MHz). The influences of various surface plasma treatments under CHF₃, C₄F₈, and C₄F₈/O₂ were investigated in order to understand the chemical and structural changes in thin-film systems, as well as their influence on the electrical properties. Fluorine atoms were incorporated into the HfO₂ films by either CHF₃ or C₄F₈ plasma treatment; meanwhile, the C/F films were formed on the surface of the HfO₂ films. The formation of C/F layers decreased the k value of the gate stacks because of its low dielectric constant. However, the addition of O₂ gas in the discharge gases suppressed the formation of C/F layers. After thermal annealing, tetragonal HfO₂ phase was investigated in both samples treated with CHF₃ and C₄F₈ plasmas. However, the samples treated with O-rich plasmas showed monoclinic phase, which indicated that the addition of O plasmas could influence the Hf/O ratio of the HfO₂ films. The mechanism of the t-HfO₂ formation was attributed to oxygen insufficiency generated by the incorporation of F atoms. The capacitors treated with C₄F₈/O₂ plasmas displayed the highest k value, which ascribed that the C/F layers were suppressed and the tetragonal phase of HfO₂ was formed. Good electrical properties, especially on the hysteresis voltage and frequency dispersion, were obtained because the bulk traps were passivated by the incorporation of F atoms. However, the H-related traps were generated during the CHF₃ plasma treatments, which caused the performance degradation. All the treated samples showed lower leakage current density than the as-deposited HfO₂ films at negative bias due to the reduced trap-assisted tunneling by the incorporation of F to block the electrons transferring from metal electrode to the trap level.     

PEO-based composite lithium polymer electrolyte, PEO-BaTiO3-Li(C2F5SO2)2N

Polyethylene oxide (PEO) based polymer electrolytes with BaTiO₃ as filler and Li(C₂F₅SO₂)₂N as salt have been examined in lithium polymer batteries. The aluminum disolution potential in PEO-Li(C₂F₅SO₂)₂N was estimated to be 4.1 V vs. Li/Li⁺ at 80 °C, which was compared to that of 3.8 V vs. Li/Li⁺ in PEO-Li(CF₃SO₂)₂N. The electrical conductivity of the system was measured as a function of O/Li ratio. The highest conductivity was observed in O/Li=8. The conductivity was 1.65×10⁻³ S/cm at 80 °C and 1.5×10⁻⁵ S/cm at 25 °C. The interfacial resistance of Li/polymer electrolyte/Li annealed at 80 °C for 15 days was lower than 100 Ωcm². Paper presented at the 8th EuroConference on Ionics, Carvoeiro, Algarve, Portugal, Sept. 16 – 22, 2001.     

The chemistry of C2 perfluoroalkyl iodide on the Cu(1 1 1) single crystal surface

The thermal chemistry of perfluoroethyl iodide (C₂F₅I) adsorbed on Cu(1 1 1) has been investigated by temperature-programmed reaction/desorption (TPR/D), reflection-absorption infrared spectroscopy (RAIRS), and X-ray photoelectron spectroscopy (XPS). I 4d and F 1s XPS spectra show that dissociative adsorption of C₂F₅I to form the surface-bound perfluroethyl (Cu-C₂F₅) moieties occurs at very low temperature (T < 90 K), while the C-F bond cleavage in adsorbed perfluroethyl (Cu-C₂F₅) begins at ca. 300 K. XPS and TPR/D studies further reveal that the reactions of ^βCF₃^αCF_2(ad) on Cu(1 1 1) are strongly dependent on the surface coverage. At high coverages (?0.16 L exposure), the adsorbed perfluroethyl (Cu-C₂F₅) evolves, via α-F elimination, into the surface-bound tetrafluoroethylidene moieties (CuCF-CF₃) followed by a dimerization step to form octafluoro-2-butene (CF₃CFCFCF₃) at 315 K as gas product. The surface-bound (Cu-C₂F₅) decomposes preferentially, at low coverages (?0.04 L), via consecutive α-F abstraction to afford intermediate, trifluoroethylidyne (CuCCF₃), resulting in the final coupling reaction to yield hexafluoro-2-butyne (CF₃CCCF₃) at 425 K. However, at middle coverages (ca. 0.08-0.16 L exposure), the adsorbed perfluroethyl (Cu-C₂F₅) first experiences an α-F elimination and then prefers to loss the second F from β position to yield the intermediate of Cu-CF₂-CFCu (μ-η,η-perfluorovinyl), which may further evolve into hexafluorocyclobutene (CF₂CFCFCF₂) at 350 K through cyclodimerization reaction. Our results have also shown that the surface reactions to yield the products, CF₃CFCFCF₃ and CF₃CCCF₃, obey first-order kinetics, whereas the formation of CF₂CFCFCF₂ follows second-order kinetics.     

Infrared intensity analysis of molecules C2H6, C2D6, C2F6 and CF4 zero and first order approximations

Infrared intensity formulae for C₂H₆ and C₂D₆ are derived following the first order approximations. Using the experimental intensities in the intensity equations, the first order coefficients are calculated. They are observed to be negligible compared to the accuracy limits within which the intensities can be measured. Correlating the experimental intensities to the intensity expressions of C₂F₆ and following the zero-order approximations, the bond dipole moment μ and its derivative e are calculated for the C–F bond. Substituting these in the intensity equations of CF₄, transferability of the bond moment parameters is discussed.     

Reactive scattering of a supersonic alkali atom beam: K + Br2, BrCN,SnCl4, PCl3, CCl4, CH3I

Angular distribution measurements of reactive scattering of a supersonic potassium atom beam by a series of molecules are reported with initial kinetic energy (～ 5 kcal mole^-1) above the thermal energy range. The narrow Laval nozzle velocity distribution gives improved resolution over thermal energy measurements. Total reaction cross sections are found to decrease with energy. Differential reaction cross sections for Br₂, BrCN and CCl₄ show increased forward scattering compared with thermal energies but for CH₃I there is no change to within experimental error. The BrCN scattering is compatible with spectator-stripping dynamics, though this limit has not been reached in the Br₂ scattering. The SnCl₄ differential reaction cross section appears not to be compatible with a single peak in the centre of mass recoil velocity distribution. It is suggested that high and low velocity contributions to the intensity may arise from a long-lived collision complex dissociating by two reaction paths.     

Double photoionisation spectra of HI,CH3I and CF3I from TOF-PEPECO measurements

Double photoionisation spectra of HI, CH₃I and CF₃I have been measured by the TOF-PEPECO technique, providing complete information on electron energy distributions. The lowest energy states of HI²⁺ and CH₃I²⁺ are identified and their vibrations are resolved. Possible reasons for the markedly different CF₃I spectrum are discussed. There is evidence for atomic autoionisation of iodine as a pathway contributing to the double photoionisation.     

Theoretical thermochemistry of the C60F18, C60F36, and C60F48 fluorofullerenes

Relative energies of C₆₀F_N fluorofullerenes are reproduced reasonably well at the B3LYP/6- 311G** level of theory employed in conjunction with isodesmic transfluorination reactions, although overestimation of steric repulsions among non-bonded atoms is evident for species with larger values of N. On the other hand, the MNDO method is found to be less suitable for studies of fluorofullerene thermochemistry. The gas-phase standard enthalpy of formation of the C₆₀F₁₈ species is predicted to lie between ?1500 kJ mol^?1 and ?1400 kJ mol^?1.     

Mechanisms of inelastic scattering of low-energy protons by C6H6, C60, C6F12, and C60F48 molecules

The mechanisms of inelastic scattering of low-energy protons with a kinetic energy of 2–7 eV by C₆H₆, C₆F₁₂, C₆₀, and C₆₀F₄₈ molecules are studied using the methods of quantum chemistry and nonempirical molecular dynamics. It is shown that, for the C₆H₆ + proton and C₆₀ + proton systems, starting from a distance of 6 Å from the carbon skeleton, the electronic charge transfer from the aromatic molecule to H⁺ occurs with a probability close to unity and transforms the H⁺ ion into a hydrogen atom and the neutral C₆H₆ and C₆₀ molecules into cation radicals. The mechanism of interaction of low-energy protons with C₆F₁₂ and C₆₀F₄₈ molecules has a substantially different character and can be considered qualitatively as the interaction between a neutral molecule and a point charge. The Coulomb perturbation of the system arising from the interaction of the noncompensated proton charge with the Mulliken charges of fluorine atoms results in an inversion of the energies of the electronic states localized, on the one hand, on the positively charged hydrogen ion and, on the other hand, on the C₆F₁₂ and C₆₀F₄₈ molecules. As a result, the neutral molecule + proton state becomes the ground state. In turn, this inversion makes the electronic charge transfer energetically unfavorable. Quantum-chemical and molecular-dynamics calculations on different levels of theory showed that, for fluorine derivatives of some aromatic structures (C₆F₁₂, C₆₀F₄₈), the barriers to proton penetration through carbon hexagons are two to four times lower than for the corresponding parent systems (C₆H₆, C₆₀). This effect is explained by the absence of active π-electrons in the case of fluorinated molecules.