Electronic excitation of tetrafluoroethylene, C2F4

Perfluoro effect on the electronic excited states of para-benzoquinone revealed by experiment and theory

We report a comprehensive study on the electronic excited states of tetrafluoro-1,4-benzoquinone, through high-resolution vacuum ultraviolet photoabsorption spectroscopy and time-dependent density functional theory calculations performed within the nuclear ensemble approach. Absolute cross section values were experimentally determined in the 3.8-10.8 eV energy range. The present experimental results represent the highest resolution data yet reported for this molecule and reveal previously unresolved spectral structures. The interpretation of the results was made in close comparison with the available data for para-benzoquinone [Jones et al., J. Chem. Phys., 2017, 146, 184303]. While the dominant absorption features for both molecules arise from analogous π* ← π transitions, some remarkable differences have been identified. The perfluoro effect manifests in different ways: shifts in band positions and cross sections, appearance of features associated with excitations to σCF* orbitals, and spectrum broadening by quenching of either vibrational or Rydberg progressions. The level of agreement between experiment and theory is very satisfactory, yet that required the inclusion of nuclear quantum effects in the calculations. We have also discussed the role of temperature on the absorption spectrum, as well as the involvement of core-excited resonances in promoting dissociative electron attachment reactions in the 3-5 eV range.

The Influence of O2 on Decomposition Characteristics of c-C4F8/N2 Environmental Friendly Insulating Gas

The c-C4F8 gas is considered to have great potential as a gaseous medium for gas-insulated equipment, due to its good insulation properties and its relatively low greenhouse gas potential (GWP) relative to SF6. However, the decomposition is an important indicator of its use in equipment. In this paper, the decomposition characteristics of c-C4F8 and the influence by oxygen have been explored through experiments and theoretical calculations. Firstly, the breakdown test of mixed gas was carried out and the precipitated elements of the electrodes and breakdown products of gas mixture were analyzed by X-ray photoelectron spectroscopy (XPS) and gas chromatography mass spectrometry (GC-MS). At the same time, the differences in decomposition products have also been studied when a small amount of O2 was present. The path and mechanism of c-C4F8 decomposition is then discussed, based on density functional theory (DFT). The results show that the black powdery substance descends on the electrode surface after the breakdown of the mixture of c-C4F8/N2 gas containing O2, and its main constituent elements are C, O and F. O2 can promote the decomposition of c-C4F8. The mixture with O2 produced a large number of additional toxic and corrosive COF2 in addition to generating more CF4, C2F4, C2F6, C3F6 and C3F8. The GWP values of the products are lower than SF6. Comprehensive insulation properties and decomposition characteristics, c-C4F8 should not be mixed with dry air for use, and the oxygen content should be strictly controlled in c-C4F8 mixed gas.

Conical intersections and low-lying electronic states of tetrafluoroethylene

The low-lying electronic states of tetrafluoroethylene (C2 F4 ) are characterized theoretically for the first time using equation-of-motion coupled cluster theory (EOM-CCSD), and complete active space self-consistent field (CASSCF) and second-order perturbation theory (CASPT2). Computations are performed for vertical excitation energies, equilibrium geometries, minimum-energy conical intersections, and potential energy curves along three geometric coordinates: 1) twisting of the FCCF dihedral angle, 2) pyramidalization of the CF2 group, and 3) migration of a fluorine atom resulting in an ethylidene-like (CF3 CF) structure. The results suggest two relaxation pathways from the Rydberg-3s excited electronic state to the ground state. These relaxation pathways are discussed in conjunction with the femtosecond photoionization spectroscopy results of Trushin et al. [ChemPhysChem- 2004, 5, 1389].

The kinetics and product state distributions from gas-phase reactions of small atomic and molecular cations with C2H4, C2H3F, 1,1-C2H2F2, C2HF3 and C2F4

The reactions of twenty one gas-phase cations with C2H3F, 1,1-C2H2F2, C2HF3 and C2F4 have been studied in a selected ion flow tube at 298 K. The cations are both atomic and molecular with recombination energies in the range 6-22 eV, and the kinetics and branching ratios into product ions are revealed for all the reactions. These data, together with that from an earlier study of reactions of C(x)F(y)(+) with these four fluorinated ethenes (J. Phys. Chem. A., 2012, 116, 8119), are compared with the reactions of these ions with C2H4, where available. Nearly all the reactions have a rate coefficient close to the collisional value calculated by either Langevin or modified average dipole orientation theories. The products of the reactions of N(+) and N2(+) with C2H4 are found to be anomalous, compared to their reactions with the four fluorinated ethenes. The branching ratios into product cations are compared with those from a high resolution (ca. 0.002 eV) photoionisation (hν = 10-22 eV) study of C2H3F, 1,1-C2H2F2, C2HF3 and C2F4 (Phys. Chem. Chem. Phys., 2012, 14, 3935) in order to gauge the importance of electron transfer in ion-molecule reactions. The higher the recombination energy of the cation, the better the agreement between the two sets of product branching ratios. Where there is disagreement at lower recombination energies, it appears that there is more fragmentation of the products in the photoionisation experiment compared to the ion-molecule reactions.

Charged particle velocity map image reconstruction with one-dimensional projections of spherical functions

Velocity map imaging (VMI) is used in mass spectrometry and in angle resolved photo-electron spectroscopy to determine the lateral momentum distributions of charged particles accelerated towards a detector. VM-images are composed of projected Newton spheres with a common centre. The 2D images are usually evaluated by a decomposition into base vectors each representing the 2D projection of a set of particles starting from a centre with a specific velocity distribution. We propose to evaluate 1D projections of VM-images in terms of 1D projections of spherical functions, instead. The proposed evaluation algorithm shows that all distribution information can be retrieved from an adequately chosen set of 1D projections, alleviating the numerical effort for the interpretation of VM-images considerably. The obtained results produce directly the coefficients of the involved spherical functions, making the reconstruction of sliced Newton spheres obsolete.

Selected ion flow tube study of the gas-phase reactions of CF+, CF2+, CF3+, and C2F4+ with C2H4, C2H3F, CH2CF2, and C2HF3

We study how the degree of fluorine substitution for hydrogen atoms in ethene affects its reactivity in the gas phase. The reactions of a series of small fluorocarbon cations (CF(+), CF(2)(+), CF(3)(+), and C(2)F(4)(+)) with ethene (C(2)H(4)), monofluoroethene (C(2)H(3)F), 1,1-difluoroethene (CH(2)CF(2)), and trifluoroethene (C(2)HF(3)) have been studied in a selected ion flow tube. Rate coefficients and product cations with their branching ratios were determined at 298 K. Because the recombination energy of CF(2)(+) exceeds the ionization energy of all four substituted ethenes, the reactions of this ion produce predominantly the products of nondissociative charge transfer. With their lower recombination energies, charge transfer in the reactions of CF(+), CF(3)(+), and C(2)F(4)(+) is always endothermic, so products can only be produced by reactions in which bonds form and break within a complex. The trends observed in the results of the reactions of CF(+) and CF(3)(+) may partially be explained by the changing value of the dipole moment of the three fluoroethenes, where the cation preferentially attacks the more nucleophilic part of the molecule. Reactions of CF(3)(+) and C(2)F(4)(+) are significantly slower than those of CF(+) and CF(2)(+), with adducts being formed with the former cations. The reactions of C(2)F(4)(+) with the four neutral titled molecules are complex, giving a range of products. All can be characterized by a common first step in the mechanism in which a four-carbon chain intermediate is formed. Thereafter, arrow-pushing mechanisms as used by organic chemists can explain a number of the different products. Using the stationary electron convention, an upper limit for Δ(f)H°(298)(C(3)F(2)H(3)(+), with structure CF(2)═CH-CH(2)(+)) of 628 kJ mol(-1) and a lower limit for Δ(f)H°(298)(C(2)F(2)H(+), with structure CF(2)═CH(+)) of 845 kJ mol(-1) are determined.

Electronic excitations of fluoroethylenes

Several lowest-lying singlet electronic states of vinyl fluoride, trans-, cis-, and 1,1-difluoroethylene, trifluoroethylene, and tetrafluoroethylene were investigated by using symmetry-adapted cluster configuration interaction theory. Basis sets up to Dunning's aug-cc-pVTZ augmented with appropriate Rydberg functions were utilized for the calculations. Calculated excitation energies show a good agreement with the available experimental values. Even in the troublesome pi-->pi(*) transitions, the excitation energies obtained in the present study agree well with the experimental values except in one or two fluoroethylenes. Strong mixing between different states was noticed in a few fluoroethylenes; especially the mixing is very strong between pi-pi(*) and pi-3ppi states in trifluoroethylene. No pure pi-sigma(*) excited state was found in almost all the fluoroethylenes. Several assignments and reassignments of features in the experimental spectra were suggested. The present study does not support the existing argument that the interaction between the pi-pi(*) and sigma-sigma(*) states is the reason behind the blueshift of around 1.25 eV in the pi-pi(*) excitation energy of tetrafluoroethylene. Possible reasons, including structural changes, for this shift are discussed in detail. Several low-lying triplet excited states were also studied.