Excitation and dissociation of polyatomic molecules under the action of femtosecond infrared laser pulses

The effect of high-intensity femtosecond laser pulses (100–200 fs) in the near (0.8–1.8 μm) and medium (4.6–5.8 μm) IR ranges on the CF₂HCl, CF₃H, (CF₃)₂C=C=O, and C₄F₉COI molecules is examined. Irradiation of CF₂HCl and CF₃H molecules by 0.8-to 1.8-μm laser pulses with intensities of >40 TW/cm² (>4 × 10¹³ W/cm²) makes them dissociate to yield CF₃H and CF₄, respectively. The key mechanism of the dissociation of these molecules is field ionization and fragmentation. The excitation of the stretching vibrations of the C=O bond in the (CF₃)₂C=C=O and C₄F₉COI molecules by 4.5-to 5.8-μm femtosecond pulses produced no detectable dissociation up to a fluence of ∼0.5 J/cm² (or a intensity of ∼2.5 TW/cm²). Probable explanations of this observation are discussed. Original Russian Text ? V.M. Apatin, V.O. Kompanets, V.B. Laptev, Yu.A. Matveets, E.A. Ryabov, S.V. Chekalin, V.S. Letokhov, 2007, published in Khimicheskaya Fizika, 2007, Vol. 26, No. 4, pp. 18–25.     

The effects of backbone conformation on hydration and proton transfer in the ‘short-side-chain’ perfluorosulfonic acid membrane

Presented here is a first principles based molecular modeling investigation of the effects of conformational changes in the backbone on hydration and proton transfer in the short-side-chain perfluorosulfonic acid fuel cell membrane under minimal hydration conditions. Extensive searches for minimum energy structures (at the B3LYP/6-311G^⁎⁎ level) of the two pendant side chain polymeric fragment: CF₃CF(–O(CF₂)₂SO₃H)–(CF₂)₇–CF(–O(CF₂)₂SO₃H)CF₃ with from 4 to 7 explicit water molecules revealed that the perfluorocarbon backbone may adopt both an elongated geometry with all carbons in a trans configuration and also a folded conformation as a result of the hydrogen bonding of the terminal sulfonic acids with the water. Our calculations show that the fragments displaying the latter ‘kinked’ backbone possessed stronger binding of the water to the sulfonic acid groups and also effect proton dissociation with fewer water molecules. These calculations point to the importance of the flexibility of the backbone in the transport of protons for membranes with low water content.     

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).     

The basicity of sulfonamides and carboxamides. Theoretical and experimental analysis and effect of fluorinated substituent

The basicity of a series of sulfonamides and carboxamides with respect to protonation and hydrogen‐bonded complex formation with phenol was investigated by calculations using the Becke three‐parameter hybrid functional combined with Lee–Yang–Parr correlation functional with the 6‐311G** and 6‐311++G** basis sets and by infrared spectroscopy. The effect of fluorinated substituent was studied for the two series. The proton affinity of nitrogen in sulfonamides is higher than oxygen, in contrast to carboxamides, which are protonated at oxygen. The phenyl group in benzenesulfonamide increases the basicity of both heteroatoms, but more strongly of the nitrogen, whereas in benzamide the effect on the two heteroatoms is about the same. The CF₃ group equally decreases the basicity of nitrogen and oxygen atoms in sulfonamides and carboxamides. The second fluorinated substituent decreases the basicity of oxygen in (CF₃CO)₂NH more strongly than of nitrogen. For sulfonamides, the same effect results in the reverse of the center of basicity from nitrogen in (MeSO₂)₂NH to oxygen in (CF₃SO₂)₂NH. All studied carboxamides give H‐complexes via the carbonyl oxygen, whereas for sulfonamides two types of H‐complexes, with the OH···N and OH···O=S, were found theoretically, the latter being more stable. The exception is bisimide (CF₃SO₂)₂NH, for which only the OH···O=S complex is stable. Experimentally, only the oxygen‐bound complexes are observed. Analysis of the natural charges revealed an ‘abnormal’ increase of the electron density on the NH group by electron‐acceptor substituents in CF₃SO₂NHR, which was explained using the natural bond orbital analysis by loosening of the S–N bond because of orbital interactions with the σ*_S?N orbital. Copyright © 2012 John Wiley & Sons, Ltd.     

Composition and Structure of Complexes Formed in Aqueous Solutions of Trifluoroacetic Acid According to IR Spectroscopy Data

The composition and structure of complexes that formed in aqueous solutions of trifluoroacetic acid were studied by frustrated multiple total internal reflection IR spectroscopy (FMTIR). Two types of complexes with a molecular structure formed: trimers CF₃COOH · (H₂O)₂ and cyclic tetramers (CF₃COOH)₂ · (H₂O)₂, in which the molecules of the components are arranged in pairs. In the range of acid concentrations from 100% to [H₂O]/[CF₃COOH] = 1: 1, only these tetramers formed, and all added water was bound into these hydrates. In more dilute solutions (up to [H₂O]/[CF₃COOH] = 2: 1), CF₃COOH · (H₂O)₂ complexes formed along with tetramers; at a double excess of H₂O, the components of the solution were completely bound into these trimers. In dilute solutions (from 0 to 3.6 M CF₃COOH), the acid is completely dissociated into H₅O ₂ ⁺ and CF₃COO^– ions hydrated with water molecules. In the range of medium concentrations (from 3.6 M to [H₂O]/[CF₃COOH] = 2: 1), the solutions contain both these ions and CF₃COOH · (H₂O)₂ dihydrates. For this range of compositions of the CF₃COOH?H₂O system, the concentrations of H₅O ₂ ⁺ ions and CF₃COOH · (H₂O)₂ dihydrates were calculated.     

18O-selective IR MPD of perfluorodimethyl ether

The ¹⁸O-selective IR MPD of perfluorodimethyl ether (CF₃)₂O has been studied. The dissociation yields of (CF₃)₂ ¹⁸O (₁₈) and (CF₃)₂ ¹⁶O (₁₆) and the isotope selectivity (18/16) have been measured as functions of TEA CO₂-laser frequency, laser fluence and ether parent pressure. The (CF₃)₂O molecule has been found to provide highly efficient ¹⁸O separation. The MPD yield of the desired isotope ¹⁸O varies in the range 3–13%; the selectivity (18/16) achieves a value of 95 at the laser line 10P22, at moderate fluence Ø=4 J/cm² and at P _{(CF 3)2 O}=0.5 Torr.     

Characterization of low pressure chemical vapor deposited polymeric fluorinated carbon m (C:FX)n thin films with low dielectric constant

Thermal chemical vapor deposition of fluorinated carbon thin films in the polymeric form is described by hot filament decomposition of the gaseous C₃F₆O precursor. Decomposition at filament temperatures, ≤450 °C produces films in the ordered (CF₂)_2n polymeric chain structure as in a tetrafluoroethylene polymer. A composite of (CF₂)_2n chains structure and crosslinked m(C:F_x)_n phases are formed in films deposited at filament temperature ≥600 °C. Polymerization of :CF₂ radicals results in (CF₂)_2n chain structure and the crosslinked phase emerges from a separate process involving reaction among the CF₃, CFO and CF₃CO radicals and including CF₂. Substrate temperature affects both the C-to-F bonding configuration and the relative ratio of the composite phases. Dominant C–CF bonding structure in the low (<-5 °C) substrate temperature films is thermally less stable compared to the C–F structure, which dominates the crosslinked structure in films deposited at high (∼70 °C) substrate temperatures. Dielectric properties of the composite films are studied using the electrical equivalent model and a correlation with the C-to-F bond structure is established. High polymeric (CF₂)_2n phase determines the electrical impedance and the dielectric constant of the film, and the crosslinked phase imparts structural stability. PACS 81.15.Gh; 73.61.Ph; 77.84.Jd; 79.60.Fr     

The mechanism of formation of the hydroperoxyl radical in the CF<Subscript>3</Subscript>COOH + <Superscript>3</Superscript>O<Subscript>2</Subscript> system: a quantum-chemical study

Ab initio quantum-chemical calculations of the (CF₃CO₂H₂⁺…³O₂) and (CF₃CO₂⁻…³O₂) complexes were performed by the MP2 method. It was found that these complexes were characterized by low complex formation energies, of 2.97 and 1.72 kcal/mol, respectively. According to the MP2(full)/6-311++G(d, p) calculation data, the bridge stabilization of oxygen by linking with both the CF₃CO₂H₂⁺ cation and CF₃CO₂⁻ anion is much more favorable energetically. A study of the potential energy surface of the joint molecular system (CF₃CO₂H₂⁺…³O₂…CF₃CO₂⁻) shows that proton experiences activationless transfer from the cation to the ³O₂ molecule accompanied by electron transfer from the CF₃COO⁻ anion. An analysis of spin density distribution shows that two radicals are stabilized in the (CF₃CO₂….OOH….O=C(OH)CF₃) complex in the triplet state observed on the potential energy surface.     

Scheibe-aggregates of Pseudoisocyanine in solution and in molecular monolayers

A new working molecule 1,1,1,3,3,3-hexafluoro-2-(trifluoromethyl)-propane-2-t (CF₃)₃CT, is reported for the isotope separation of tritium by TEA CO₂-laser-induced multiphoton dissociation (MPD). Selective and efficient dissociation of (CF₃)₃CT was observed by irradiation at about 980 cm^–1 where (CF₃)₃CH was nearly transparent. The critical fluence for dissociation of (CF₃)₃CT at 10R(28) 980.9 cm^–1 was estimated to be as low as 4.6 J/cm², which is the lowest of the tritiated halocarbons that we have ever reported. A detailed study was made of the pressure dependence of the dissociation rate constants for (CF₃)₃CT and (CF₃)₃CH to clarify the collisional effects in their MPD. The hydrogen isotope exchange between (CF₃)₃CH and HTO was found to be extremely rapid, which is advantageous in the practical laser separation cycle for tritium removal from water.     

Photolysis of compounds with P−I and As−I bonds

Detailed experimental investigations are reported of the parameters of lasing on the transition²P_1/2→²P_3/2, λ = 1.3 μm of atomic iodine upon photodissociation of the molecules (CF₃)₂AsI and (CF₃)₂PI. The light source was an IFP-5000 flash lamp. The experimental data are used to determine the sequence of the basic reactions that accompany the photolysis of the molecules (CF₃)₂AsI and (CF₃)₂PI and to determine the energy parameters of the generated radiation.     

Absorption spectrum of the (CF4) dimer in liquid argon solution

The concentration dependence of the shape of absorption bands in the spectrum of CF₄ in liquid argon is studied in the concentration range (0.01–17)×10^?3 molar fractions at 93 K. In all spectral regions related to ν₃, the shape of the spectral function is determined, along with the Fermi resonance 〈ν_i,ν₃+1,ν₄|≈〈νi,ν₃,ν ₄+2|, by the resonance dipole-dipole interaction. In the spectral region of the Fermi doublet ν ₁+ν₃≈ ν₁+2ν₄, the spectrum of the contact (CF₄)₂ dimer is identified. Agreement between this spectrum and the calculated spectrum is achieved by simultaneously taking intramolecular and intermolecular resonances into account. The distance R _C-C in the dimer is 4.85(15) Å. The calculations of the spectra of (¹²CF₄)₂ and (¹³CF₄?¹²CF₄) dimers with this value of R _C-C in the region ν ₃≈2ν₄ agree with the experiment.     

Counter anion effect on structural,opto-electronic and charge transport properties of fused π-conjugated imidazolium compound

The structure–activity relationship of fused π-conjugated imidazolium cation with three counter anion molecules, BF₄^?, CF₃SO₃^? and (CF₃SO₂)₂N^?, was studied using electronic structure calculations. The structural, opto-electronic and charge transport properties of these complexes were studied. The charge transfer from π-conjugated imidazolium(I) to counter anion was confirmed in all the studied complexes. Interaction energy varies significantly depending on the counter anion and the stability was found higher for I-BF₄ complex than both I–CF₃SO₃ and I–(CF₃SO₂)₂N complexes. The strong (C–H)⁺···F^? hydrogen bond of length 1.95 Å between fused π-conjugated imidazolium and BF^?₄ anion is the driving force for the strongest interaction energy in I–BF₄ complex. The energy decomposition analysis confirms that the interaction between imidazolium and counter anion is mainly driven by electrostatic and orbital interaction. It has been observed that the absorption spectra of the complex are independent of anion nature but the influence of anion character is observed on frontier molecular orbital pattern. The charge transport property of I–BF₄ complex was studied by using tight-binding Hamiltonian approach and found that the hole mobility in I–BF₄ is 1.13 × 10^?4 cm² V^?1 s^?1.     

A theoretical study on the mechanism and dynamics of reactions (CF3)2CHOCH2F/(CF3)2CHOCHF2 with OH radical

The information related with the mechanism of reactions (CF₃)₂CHOCH₂F + OH (R1) and (CF₃)₂CHOCHF₂ + OH (R2) was explored theoretically at the BMC-CCSD//BMK/6-311 + G(d,p) level. Based on the optimised structures, energies, and other information, the rate constants were evaluated by the canonical variational transition-state theory with small curvature tunneling contributions in a temperature range of 220–2000 K. For each reaction, there are both hydrogen-abstraction and displacement channels. In addition, more than one hydrogen atom can be abstracted. The relationship between hydrogen abstraction and displacement, between different hydrogen-abstraction channels, and between reactions R1 and R2 are elucidated.     

On the energy dependence of the yield of doubly charged negative ions during the capture of free electrons by C60(CF3)12 trifluoromethylfullerene molecules

Peaks of C₆₀(CF₃) _n ^2? doubly charged negative ions (n = 6–12) have been observed in the mass spectra of the resonance electron capture by trifluoromethylfullerene C₆₀(CF₃)₁₂ molecules. It has been established that these ions are formed owing to the attachment of two free isoenergetic electrons. The autodetachment of an extra electron has been detected for the doubly charged molecular ions (n = 12). It has been established from the observation of the delayed fragmentation of the most abundant ions with n = 8 and 10 that the doubly charged negative ions, like their singly charged analogs, are metastable with respect to the separation of the CF₃ fragment(s). The yield of doubly charged negative ions has been obtained as a function of the electron energy. By comparing them with the analogous dependences for the singly charged ions, the specific features have been revealed which were associated with the presence of the repulsive Coulomb barrier and the regular effect of the doubled energy of two additional electrons on the energy dependence of the dissociative decay of the doubly charged negative ions. The absolute cross section for the formation of the C₆₀(CF₃) ₁₀ ^2? ions has been measured. At the energy of their yield maximum near the 5 eV, it is ～1 × 10^?19 cm².     

Study of femtosecond dynamics in vibrationally excited free bis(trifluoromethyl)ketene and metal carbonyl molecules induced by CO bond resonant excitation

The dynamics of relaxation processes in free bis(trifluoromethyl)ketene (CF₃)₂CCO and Fe(CO)₅ and Cr(CO)₆ metal carbonyl molecules after multiphonon excitation of the C=C=O and C=O vibrations by femtosecond laser infrared radiation was studied. The temporal and spectral dependences of the relaxation of the excited vibrational states were measured. Kinetics with a characteristic decay time of about 5 ps was obtained for (CF₃)₂CCO molecules. Its behavior is interpreted as a manifestation of the intramolecular relaxation of the excited vibration states of the resonance mode. Kinetic curves with characteristic times of about 250–500 fs were observed for Fe(CO)₅ and Cr(CO)₆. The behavior of these curves depends on the mutual orientation of the polarizations of the pump and probe pulses.     

Theoretical study on the electronic structures and phosphorescent properties of four Ir(III) complexes with different substituents on the ancillary ligand

The geometry structures, electronic structures, absorption and phosphorescent properties of four Ir(III) complexes {[(F₂-ppy)₂Ir(pta-X)], where F₂-ppy = (2,4-difluoro)phenylpyridine; pta = pyridine-1,2,4-triazole; X = –CF₃; –H; –CH₃; –N(CH₃)₂}, are investigated using the density functional method. The results reveal that the electron-accepting group –CF₃ has no obvious effect on absorption and emission properties, while the substitutive group –N(CH₃)₂ with strong electron-donating ability has obvious effect on the emission properties. The mobility of hole and electron were studied computationally based on the Marcus–Hush theory. Calculations of ionisation potential and electron affinity were used to evaluate the injection abilities of holes and electrons into these complexes. We hope that this theoretical work can provide a suitable guide to the future design and synthesis of novel phosphorescent materials for use in the organic light-emitting diodes.     

A high level computational study of the CH4/CF4 dimer: how does it compare with the CH4/CH4 and CF4/CF4 dimers?

The interaction within the methane–methane (CH₄/CH₄), perfluoromethane–perfluoromethane (CF₄/CF₄) methane–perfluoromethane dimers (CH₄/CF₄) was calculated using the Hartree–Fock (HF) method, multiple orders of Møller–Plesset perturbation theory [MP2, MP3, MP4(DQ), MP4(SDQ), MP4(SDTQ)], and coupled cluster theory [CCSD, CCSD(T)], as well as the PW91, B97D, and M06-2X density functional theory (DFT) functionals. The basis sets of Dunning and coworkers (aug-cc-pVxZ, x?=?D, T, Q), Krishnan and coworkers [6-311++G(d,p), 6-311++G(2d,2p)], and Tsuzuki and coworkers [aug(df, pd)-6-311G(d,p)] were used. Basis set superposition error (BSSE) was corrected via the counterpoise method in all cases. Interaction energies obtained with the MP2 method do not fit with the experimental finding that the methane–perfluoromethane system phase separates at 94.5?K. It was not until the CCSD(T) method was considered that the interaction energy of the methane–perfluoromethane dimer (?0.69?kcal?mol^?1) was found to be intermediate between the methane (?0.51?kcal?mol^?1) and perfluoromethane (?0.78?kcal?mol^?1) dimers. This suggests that a perfluoromethane molecule interacts preferentially with another perfluoromethane (by about 0.09?kcal?mol^?1) than with a methane molecule. At temperatures much lower than the CH₄/CF₄ critical solution temperature of 94.5?K, this energy difference becomes significant and leads perfluoromethane molecules to associate with themselves, forming a phase separation. The DFT functionals yielded erratic results for the three dimers. Further development of DFT is needed in order to model dispersion interactions in hydrocarbon/perfluorocarbon systems.     

Anomalous Broadening of CF4 Molecule Lines. Observation of Carbon Tetrafluoride Hydrates?

Spectroscopic studies of pure carbon tetrafluoride and carbon tetrafluoride in the presence of water vapor have been carried out. Studies have revealed changes in the absorption spectrum of the 1280 cm^‒1 band of CF₄, indicating the formation of new molecules, CF₄–H₂O hydrates. The bond between CF₄ and H₂O is not chemical in nature. The formation of these molecules can accelerate the removal of carbon tetrafluoride from the atmosphere with precipitation in the form of rain or snow.

     

Frequency dependent kinetics of the cw CO2 laser induced reaction of CF3Br

Kinetics of the decomposition of CF₃Br at 20 torr by a cw CO₂ laser have been studied over the range of laser frequencies 1043–1085 cm^-1. At constant translational temperature the change in the rate constant with laser frequency over the frequency range is a factor of 500, comparable to the effect previously observed in CF₂ClCF₂Cl and CF₃CF₂Cl. Arrhenius plots show an activation energy of 64.9 kcal/mole, independent of frequency. The laser induced optical absorption of CF₃Br exhibits a hysteresis effect at the lower laser frequencies.     

Di-urethanesil hybrid electrolytes doped with Mg(CF3SO3)2

Two siloxane-based di-urethanesil frameworks incorporating poly(oxyethylene) (POE) chains have been synthesized by the sol–gel process and doped with magnesium triflate (Mg(CF₃SO₃)₂) with the goal of developing electrolytes for the fabrication of solid-state rechargeable magnesium batteries. In these matrices, short POE chains are covalently bonded to the siloxane network via urethane linkages. The xerogels have been represented by the notation d-Ut(Y) _n Mg(CF₃SO₃)₂, where Y?=?300 and 600 represents the average molecular weight of the POE chains and n stands for salt composition (molar ratio of OCH₂CH₂ units per Mg²⁺). Xerogels with compositions ranging from 2?≤?n?<?∞ were prepared. A crystalline POE/Mg(CF₃SO₃)₂ complex of unknown stoichiometry is formed in the d-Ut(300) _n Mg(CF₃SO₃)₂ materials with n?≤?6 and in the d-Ut(600) _n Mg(CF₃SO₃)₂ materials with n?≤?5. The organically modified silicate electrolytes with the highest conductivity of the d-Ut(300) _n Mg(CF₃SO₃)₂ and d-Ut(600) _n Mg(CF₃SO₃)₂ series are the samples with n?=?6 (3.9?×?10^?8 S cm^?1 at 26 °C and 8.7?×?10^?5 S cm^?1 at 97 °C) and n?=?100 (2.63?×?10^?7 S cm^?1 at 20 °C and 1.4?×?10^?5 S cm^?1 at 85 °C), respectively. Since the electrolytes for Mg batteries that have been proposed up to now have many intrinsic problems and although the room temperature conductivity values exhibited by the systems developed in the present study are still low in view of practical application, this work opens new directions for the development of solid-state Mg ion electrolytes.