Hydrogen abstraction from CF3CF2CFH2 and CF3CFHCF2H by OH radicals and Cl atoms: theoretical enthalpies and rate constants

The hydrogen abstraction reactions of CF₃CF₂CFH₂ and CF₃CFHCF₂H with OH radicals and Cl atoms have been studied theoretically by a dual-level direct dynamics method. Two stable conformers of CF₃CF₂CFH₂ with C _s and C ₁ symmetries and all possible abstraction channels for each reaction are all taken into consideration. Optimized geometries and frequencies of all the stationary points and extra points along minimum-energy path (MEP) have been computed at the BB1K/6-31+G(d, p) level of theory. To refine the energy profile of each reaction channel, single point energy calculations have been performed by the BMC-CCSD method. The rate constants are evaluated by canonical variational transition state theory (CVT) with the small-curvature tunneling correction method (SCT) over a wide temperature range of 200–1,000 K. The detailed branching ratios of four reactions are discussed. The good agreement found between our theoretical rate constants and the available experimental data suggests that the present approach could provide a reliable prediction for the CF₃CFHCF₂H + Cl reaction about which there is little experimental information. The kinetic calculations show that the SCT effect plays an important role in all channels. In addition, in order to further reveal the thermodynamic properties, the enthalpies of formation of the reactants (CF₃CF₂CFH₂ and CF₃CFHCF₂H) and the product radicals (CF₃CF₂CFH, CF₃CFCF₂H, and CF₃CFHCF₂) are evaluated by applying isodesmic reactions at both BMC-CCSD//BB1K/6-31+G(d, p) and MC-QCISD//BB1K/6-31+G(d, p) levels of theory.     

Design and synthesis of the novel branched fluorinated surfactant intermediates with CF3CF2CF2C(CF3)2 group

Here we reported a novel and efficient method for the synthesis of the critical intermediates of branched fluorinated surfactants with CF₃CF₂CF₂C (CF₃)₂- group using HFPD as starting material. The reaction conditions were mild and easy to handle, which was promisingly applied to the industrial production.     

Experimental and theoretical vibrational study of 2,2,2-trifluoroethyl trifluoromethanesulfonate, CF3SO2OCH2CF3

The infrared spectra of 2,2,2-trifluoroethyl trifluoromethanesulfonate (CF3SO2OCH2CF3) were obtained in the gaseous, liquid and solid states as well as the Raman spectrum of the liquid. Quantum chemistry calculations using the density functional theory were used to predict the most stable geometry and conformation of the studied molecule. Subsequently, the harmonic vibrational frequencies and force field were calculated. An assignment of the observed spectral features made after comparison with the related molecules and with the predicted frequencies was used as the basis of a scaling of the original force field in order to reproduce as well as possible the experimental frequencies. With this purpose a set of scale factors was calculated by a least square procedure, leading to a final root mean square deviation (RMSD) of 9.7 cm(-1).     

Vibrational spectra and normal coordinate analysis of CF3 compounds: Part XXXV. Vibrational spectra,normal coordinate analysis and electron diffraction studies of (CF3)4Sn

The IR and Raman spectra ofSn(CF₃)₄ have been recorded and analyzed by means of a normal coordinate analysis. The molecular structure has been determined by electron diffraction yielding the geometric parameters (T_d symmetry) τ_a(CF) 133.8(2) pm, τ _a(SnC) 220.1(5) pm and ∠FCF 108.2(2)°. Compared with Sn(CH₃)₄, the SnC bonds are weakened and lengthened by 5.8(6) pm.     

Comprehensive theoretical studies on the CF3H dissociation mechanism and the reactions of CF3H with OH and H free radicals

The potential energy surfaces for the CF3H unimolecular dissociation reaction and reactions of CF3H with free radical OH and H were investigated at the B3LYP6-311++G(**) and QCISD(T)6-311++G(**) levels and by the G3B3 theory. All the possible stationary and first-order saddle points along the reaction paths were verified by the vibrational analysis. The calculations account for all the product channels. The reaction enthalpies obtained at the G3B3 level are in good agreement with the available experiments. Canonical transition-state theory with Wigner tunneling correction was used to predict the rate constants for the temperature range of 298-2500 K without any artificial adjustment, and tshe computed rate constants for elementary channels can be accurately fitted with three-parameter Arrhenius expressions. The theoretical rate constants of the CF3H+H reaction agree with the available experimental data very well. The theoretical and experimental rate constants for the CF3H+OH reaction are in reasonable agreement. The H abstraction of CF3H by OH is found to be the main reaction channel for the CF3H fire extinguishing reactions while the CF3H unimolecular dissociation reaction plays a negligible role.     

Bromodifluoroacetyl fluoride,CF2BrC(O)F: Experimental and theoretical studies

Bromodifluoroacetyl fluoride, CF₂BrC(O)F, was prepared through the gas-phase reaction of bromotrifluoroethene, CF₂CFBr, with molecular oxygen initiated either by NO₂ or CF₃OF. The compound was experimentally studied by FTIR spectroscopy of the gas phase and also isolated in Ar and N₂ matrices at low temperature. The energy differences between the possible conformers were theoretically studied, as well as the vibrational spectra of the conformers.     

Reflected shock tube and theoretical studies of high-temperature rate constants for OH+CF3H<-->CF3+H2O and CF3+OH-->products

The reflected shock tube technique with multipass absorption spectrometric detection of OH radicals at 308 nm, using either 36 or 60 optical passes corresponding to total path lengths of 3.25 or 5.25 m, respectively, has been used to study the bimolecular reactions, OH+CF3H-->CF3+H2O (1) and CF3+H2O-->OH+CF3H (-1), between 995 and 1663 K. During the course of the study, estimates of rate constants for CF3+OH-->products (2) could also be determined. Experiments on reaction -1 were transformed through equilibrium constants to k1, giving the Arrhenius expression k1=(9.7+/-2.1)x10(-12) exp(-4398+/-275K/T) cm3 molecule(-1) s(-1). Over the temperature range, 1318-1663 K, the results for reaction 2 were constant at k2=(1.5+/-0.4)x10(-11) cm3 molecule(-1) s(-1). Reactions 1 and -1 were also studied with variational transition state theory (VTST) employing QCISD(T) properties for the transition state. These a priori VTST predictions were in good agreement with the present experimental results but were too low at the lower temperatures of earlier experiments, suggesting that either the barrier height was overestimated by about 1.3 kcal/mol or that the effect of tunneling was greatly underestimated. The present experimental results have been combined with the most accurate earlier studies to derive an evaluation over the extended temperature range of 252-1663 K. The three parameter expression k1=2.08x10(-17) T1.5513 exp(-1848 K/T) cm3 molecule(-1) s(-1) describes the rate behavior over this temperature range. Alternatively, the expression k1,th=1.78x10(-23) T3.406 exp(-837 K/T) cm3 molecule(-1) s(-1) obtained from empirically adjusted VTST calculations over the 250-2250 K range agrees with the experimental evaluation to within a factor of 1.6. Reaction 2 was also studied with direct CASPT2 variable reaction coordinate transition state theory. The resulting predictions for the capture rate are found to be in good agreement with the mean of the experimental results and can be represented by the expression k2,th=2.42x10(-11) T-0.0650 exp(134 K/T) cm3 molecule(-1) s(-1) over the 200-2500 K temperature range. The products of this reaction are predicted to be CF2O+HF.     

Vibrational and theoretical studies of urea and magnesium-urea complexes

The molecular structure and IR spectra of urea, H₂NCONH₂, in gas phase and in acetonitrile solution, as well as of the two complexes [MgU₄Cl₂] and [MgU₆]Cl₂ have been observed. The influence of environmental changes to geometry and spectra are shown. Various basis sets have been employed to safeguard the validity of the reported findings, using polarization functions for all calculations to get the correct pyramidal amide configuration. The erroneous low energy of the C_2v symmetry group, after the addition of the ZPVE correction, is discussed. For the solvated urea molecule a reduction of the energy barrier, compared to the gas phase urea, between the two minimum configurations, C₂ and C_s, and the planar geometry, is observed. The lowest energy minimum in acetonitrile is found to be the C₂ symmetry group, while for the two complexes, the local symmetry of urea is C_s or C₂ depending on the complex, or even on the coordination position of urea in the complex. The wagging motion of the amide group is also discussed in all the studied urea species. The computed geometries and most of the spectroscopic results are in good agreement with the available experimental data. Received: 18 July 2000 / Accepted: 31 July 2000 / Published online: 2 November 2000     

Vibrational and theoretical study of the 2',6'-dimethoxyflavone cis-formic acid inclusion compound

The FT-infrared and Raman microscopy spectra of the 2',6'-dimethoxyflavone and its 1:1 complex with formic acid in solid state have been recorded and analysed. Some vibrational components appear as specific to the cis-rotamer of formic acid in the crystalline sample, especially the CH group stretching vibration feature. The broad and intense infrared absorption observed in the range 3400-1900 cm(-1) and assigned to the hydrogen bonded OH group stretching vibration exhibits the characteristic ABC structure of strong hydrogen bonded complexes. This ABC pattern corroborates previous X-ray crystallographic data showing that cis-formic acid is strongly hydrogen bonded to the flavonic compound. The inclusion complex is quite unstable and the infrared spectrum clearly shows that formic acid disappears after a period of a few months. In order to get some information on the stability criterions of the intermolecular hydrogen bonded complex, semiempirical AM1 calculations have been investigated. The comparison of the calculated heats of complexation (deltacH) for chelates involving the cis- and trans-conformers of formic acid suggests that the reaction of hydrogen bonding complexation with the cis-rotamer is surely favoured.     

Vibrational spectra of the trifluoromethylsulfinate anion and scaled quantum mechanical force fields for CF3SO2- and CF3SeO2-

The infrared and Raman spectra of KCF(3)SO(2) were obtained and the observed spectral features assigned to the expected normal modes of vibration. Besides, the vibrational properties of the CF(3)SO(2)(-) and CF(3)SeO(2)(-) related anions were studied by means of density functional theory (DFT) techniques. After obtaining the optimized geometrical parameters and conformations, the vibrational wavenumbers and the associated force constants were calculated. The original force fields in cartesian coordinates were transformed to local symmetry coordinates and subsequently scaled to reproduce the experimental wavenumbers. Some trends observed in the force constants of the studied species and of the related CF(3)SO(3)(-) anion could be explained by the differences in geometrical parameters.     

A quantum mechanical study of the decomposition of CF3OCF3 and CF3CF2OCF2CF3 in the presence of AlF3

The effect of AlF3 on the decomposition of CF3OCF3 and CF3CF2OCF2CF3 is investigated using ab initio theory. Previous work by Pancansky et al. [Pacansky, J.; Waltman, R. J. J Fluorine Chem. 1997, 83, 41] showed that AlF3 significantly reduces the activation energy of the decomposition of CF3OCF3 due to the strong electrostatic interaction between the aluminum trifluoride and the reactant. In this work, a new transition-state structure and reaction mechanism have been identified for the decomposition of CF3OCF3 in the presence of AlF3. This new mechanism shows that AlF3 functions by accepting a fluorine atom from one carbon and simultaneously donating a fluorine atom to the other carbon. We show that the same pathway is obtained independently of the level of theory. The reaction rate, generated via statistical mechanics and transition-state theory, is 2-3 orders of magnitude higher for the new transition state when compared to that of the old one. The study was also performed for CF3CF2OCF2CF3 in order to ascertain the effect of chain length on the reaction mechanism and rate. We find that an analogous transition state, with lower activation energy, provides the lowest-energy path for decomposition of the longer chain.     

Rearrangement reactions of the transient lewis acids (CF(3))(3)B and (CF(3))(3)BCF(2): an experimental and theoretical study

Short-lived (CF(3))(3)B and (CF(3))(3)BCF(2) are generated as intermediates by thermal dissociation of (CF(3))(3)BCO and F(-) abstraction from the weak coordinating anion [B(CF(3))(4)](-), respectively. Both Lewis acids cannot be detected because of their instability with respect to rearrangement reactions at the B-C-F moiety. A cascade of 1,2-fluorine shifts to boron followed by perfluoroalkyl group migrations and also difluorocarbene transfer reactions occur. In the gas phase, (CF(3))(3)B rearranges to a mixture of linear perfluoroalkyldifluoroboranes C(n)()F(2)(n)()(+1)BF(2) (n = 2-7), while the respective reactions of (CF(3))(3)BCF(2) result in a mixture of linear (n = 2-4) and branched monoperfluoroalkyldifluoroboranes, e.g., (C(2)F(5))(CF(3))FCBF(2). For comparison, the reactions of [CF(3)BF(3)](-) and [C(2)F(5)BF(3)](-) with AsF(5) are studied, and the products in the case of [CF(3)BF(3)](-) are BF(3) and C(2)F(5)BF(2) whereas in the case of [C(2)F(5)BF(3)](-), C(2)F(5)BF(2) is the sole product. In contrast to reports in the literature, it is found that CF(3)BF(2) is too unstable at room temperature to be detected. The decomposition of (CF(3))(3)BCO in anhydrous HF leads to a mixture of the new conjugate Br?nsted-Lewis acids [H(2)F][(CF(3))(3)BF] and [H(2)F][C(2)F(5)BF(3)]. All reactions are modeled by density functional calculations. The energy barriers of the transition states are low in agreement with the experimental results that (CF(3))(3)B and (CF(3))(3)BCF(2) are short-lived intermediates. Since CF(2) complexes are key intermediates in the rearrangement reactions of (CF(3))(3)B and (CF(3))(3)BCF(2), CF(2) affinities of some perfluoroalkylfluoroboranes are presented. CF(2) affinities are compared to CO and F(-) affinities of selected boranes showing a trend in Lewis acidity, and its influence on the stability of the complexes is discussed. Fluoride ion affinities are calculated for a variety of different fluoroboranes, including perfluorocarboranes, and compared to those of the title compounds.     

Experimental and theoretical vibrational study of silyl trifluoromethanesulfonate, CF3SO2OSiH3

Infrared and Raman spectra were obtained for liquid silyl trifluoromethanesulfonate, a silylating agent of limited stability. The molecular geometry was optimized by means of density functional theory and M?ller-Plesset second order perturbation theory methods, using different basis sets. The optimized structure presents a gauche conformation, similar to that adopted by methyl trifluoromethanesulfonate, which was determined experimentally a short time ago. The wavenumbers for the normal modes of vibration and the corresponding force constants were also calculated, facilitating the interpretation of the vibrational data. The harmonic force constants given by theory were scaled to reproduce adequately the experimental wavenumbers.     

The complexes between CH3OH and CF4. Infrared matrix isolation and theoretical studies

The complex formed between methanol and tetrafluoromethane has been identified in argon and neon matrixes by help of FTIR spectroscopy. Three fundamentals (nu(OH), nu(FCF), and nu(CO)) were observed for the complex isolated in the two matrixes, and the OH stretch was red shifted in a neon matrix and blue shifted in an argon matrix with respect to the corresponding vibration of the methanol monomer. The theoretical studies of the structure and spectral characteristics of the complexes formed between CH(3)OH and CF(4) were carried out at the MP2 level of theory with the 6-311+G(2df,2pd) basis set. The calculations resulted in three stationary points from which two (I-1, I-2) corresponded to structures involving the O-H...F hydrogen bond and the third one (I-3) to the non-hydrogen-bonded structure. The topological analysis of the distribution of the charge density (AIM theory) confirmed the existence of the hydrogen bond in I-1, I-2 complexes and indicated weak interaction between the oxygen atom of CH(3)OH and three fluorine atoms of CF(4) in the I-3 complex. The comparison of the experimental and theoretical data suggests that in the matrixes only the non-hydrogen-bonded complex I-3 is trapped. The blue/red shift of the complex OH stretching vibration with respect to the corresponding vibration of CH(3)OH in argon/neon matrixes is explained by the different sensitivity of the complex and monomer vibrations to matrix material. The ab initio calculations performed for the ternary CH(3)OH-CF(4)-Ar systems indicated a negligible effect of an argon atom on the binary complex frequencies.     

Vibrational spectra of cyclopentadienylphosphine: infrared and theoretical studies from DFT anharmonic potentials

Both experimental and theoretical infrared investigations of cyclopentadienylphosphine (CpP) are reported. The infrared spectra (3500-500 cm(-1)) in the gas phase have been recorded at 0.5 cm(-1) resolution. Infrared absorptions bands of the two lowest stable conformers were observed and assigned. Average integrated intensities of isolated and overlapping vibrational bands were also determined experimentally. The vibrational frequencies of the CpP system and its P-dideuterated isotopologue have been calculated by means of density functional theory. The Becke exchange functional and Lee-Yang-Parr correlation functional method with a combination of the two basis sets, namely 6-31+G(d,p) and the correlation-consistent triple-zeta cc-pVTZ set of Dunning, were used. Hybrid B3LYP/B3LYP//cc-pVTZ/6-31+G(d,p) anharmonic frequencies of the fundamental, overtone, and combination transitions were calculated in the 3500-200 cm(-1) area with the use of a variational approach, implemented in the P_Anhar_v1.1 code, to assign the experimental data for each conformer.     

Vibrational relaxation and dissociation in the perfluoromethyl halides,CF3Cl,CF3Br,andCF3I

The processes of vibrational relaxation and unimolecular dissociation of the perfluoromethyl halides CF₃Cl, CF₃Br, and CF₃I have been studied in the shock tube with the laser-schlieren technique. Vibrational relaxation was resolved in pure CF₃Cl and CF₃Br (400–484 K and 400–500 K, respectively), and in the mixtures; 2% CF₃Cl/Kr (500–1000 K), 10% CF₃Cl/Kr (440–670 K), 4% CF₃Br/Kr (450–850 K), and 2% CF₃I/Kr (620–860 K). Relaxation in the pure gases is extremely rapid, but shows a well-resolved, accurately exponential decay which provides very precise relaxation times in close agreement with ultrasonic results. Relaxation times as short as 0.1 μs-atm can be resolved, showing the method has a resolution within a factor 2–3 of the best ultrasonic methods. Relaxation dilute in rare gas shows a complex double exponential behavior consistent with a two-stage series process. Rates of CF₃(SINGLEBOND)X fission in these mixtures were measured over 1800–3000 K, P<0.55 atm, for CF₃Cl; 1600–2500 K, P<0.55 atm, in CF₃Br; and 1260–2100 K, P<0.34 atm, in CF₃I. Rates for dissociation were derived from a full profile modeling using a secondary mechanism of six CF₃ reactions. RRKM analysis showed all dissociations to lie near the low pressure limit. Using literature barriers, these rates are best fit with (ΔE)_all=−270 cm⁻¹ for CF₃Cl, 〈ΔE〉_down=0.3 T for CF₃Br, and 〈ΔE〉_down=800 cm⁻¹ for CF₃F. All these transfers are on the large side, similar to those found in other halogenated methanes. © 1997 John Wiley & Sons, Inc.     

Experimental and theoretical studies on the organic-inorganic hybrid compound: aluminum-NTCDA Co-deposited film

Electronic absorption, Fourier transform-infrared (FT-IR), and electron spin resonance spectra of aluminum-naphthalene tetracarboxylic dianhydlide (Al-NTCDA) co-deposited film have been measured at room temperature, and hybrid density functional theory (DFT) calculations have been carried out in order to elucidate the electronic states for the ground and low-lying excited states of the complexes. After the interaction of NTCDA with Al atom, the new electronic transition bands were appeared at near-IR region. The C=O stretching modes of NTCDA are red-shifted by the interaction with Al. From the DFT calculations, it was found that the electronic state of the complex at the ground state is characterized by a slight charge-transfer state expressed by (Al(4))(delta+)(NTCDA)(delta-). The binding of Al to NTCDA is strong. The C=O double-bond character of NTCDA is changed to C-O single-bond-like character by the strong interaction of Al to the C=O bond. This is the origin of the red-shift of the FT-IR spectrum. The electronic states of organic-inorganic hybrid material were discussed on the basis of theoretical results.     

Vibrational spectra, structure, and theoretical calculations of 2-fluoro- and 3-fluoropyridine

The infrared and Raman spectra of liquid and vapor-phase 2-fluoropyridine and 3-fluoropyridine have been recorded and assigned. Ab initio and DFT calculations were carried out to compute the molecular structures and to verify the vibrational assignments. The observed and calculated spectra agree extremely well. The ring bond distances of the fluoropyridines are very similar to those of pyridine except for a shortening of the C-N(F) bond in 2-fluoropyridine. The C-F bond stretching frequencies are similar to that in fluorobenzene reflecting the influence of the ring π bonding.