Direct and initiated halogenation of carbon nanomaterials at low temperatures

Cryochemical reactions of the direct and initiated (by photolysis and radiolysis) halogenation of carbon nanomaterials (C₆₀ fullerene, nanotubes, and nanofibers) at 77–240 K were investigated by the ESR, IR spectroscopy, and elemental analysis techniques. A high reactivity of C₆₀ in reactions with fluorine and chlorine with the formation of corresponding derivatives was shown. High concentrations of radical intermediates indicating the radical chain halogenation of C₆₀ were detected (the kinetic chain length for the chlorination process reaches 10⁴–10⁶ units). The amount of chlorine attached to fullerene is ～35% and practically does not depend on the initiation mode (UV or γ-irradiation at doses up to 350 kGy). The mechanism of the cryochemical halogenation of C₆₀ is considered within the limits of the model of multicenter synchronous transitions in a molecular complex consisting of several reactant molecules including molecular fluorine or chlorine and ensuring a net exothermic effect. The amount of chlorine added to nanotubes and nanofibers did not exceed 2.5–8%, thereby indicating a low reactivity of these materials under cryogenic conditions.     

Mechanistic Aspects of the Gas‐Phase Reactions of Halobenzenes with Bare Lanthanide Cations: A Combined Experimental/Theoretical Investigation

The gas‐phase reactions of chlorobenzene with all atomic lanthanide cations Ln⁺ (except Pm⁺) have been investigated by using Fourier transform ion cyclotron resonance mass spectrometry in conjunction with density functional theory calculations. According to the latter, a direct chlorine transfer to the lanthanide cation, which has been observed previously for fluorine abstraction from fluorobenzene, is not operative for the C₆H₅Cl/Ln⁺ couples; rather, chlorine transfer proceeds through an initial coordination of the lanthanide cation to the aromatic ring of the substrate. Both, the product distribution and the chlorine abstraction efficiencies are affected by the bond dissociation energy (BDE(Ln⁺?Cl)) as well as the promotion energies of Ln⁺ to attain a 4fⁿ 5d¹ 6s¹ configuration. In addition, mechanistic aspects of some C?H and C?C bond activations are presented. Where appropriate, comparison with the previously studied C₆H₅F/Ln⁺ systems is made.     

Reactions de redistribution avec des molecules dinucleaires fluorees et chlorees. Mise en evidence d'un heterocomplexe contenant du molybdene et du tungstene en equilibre avec les molecules dinucleaires symetriques

Redistribution reactions have been undertaken in dinuclear ions with μ-oxalato group. The fluoro-chloro species appear in solution.The accumulation of fluoride on tungsten atoms is always favored, whereas chlorine redistributes essentially on molybdenum, there is a preferential affinity of fluorine for tungsten.In solution a slow dynamic exchange is observed to occur between the two symetrical fluorospecies, the formation of the F₂O₂W(C₂O₄)Mo0₂F₂^2- heterofluoroanion is reported.     

Gas-phase C-F bond cleavage in perfluorohexane using W-, Si-, P-, Br-, and I-containing ions: Comparisons with reactions at fluorocarbon surfaces

Gas-phase reactions of W-, Si-, P-, Br-, and I-containing ions with the target molecule perfluorohexane at low collision energies (<15 eV) parallel known ion/surface reactions of the same projectile ions at fluorinated self-assembled monolayer surfaces. Charge exchange, dissociative charge exchange, and fluorine atom abstraction are observed and the majority of the projectile ions also undergo reactive charge exchange to produce specific fluorocarbon fragment ions of the target molecule in distinctive relative abundances. Abstraction of up to five fluorine atoms is observed upon collision of W⁺ with gaseous perfluorohexane, while similar experiments with CI⁺, SiCl⁺, and PCl^+· show abstraction of one or two fluorine atoms. Other projectiles, including Si^+·, PCl ₂ ⁺ , Br⁺, CBr⁺, and I⁺, abstract only a single fluorine atom. These patterns of fluorine atom abstraction are similar to those observed in ion/surface collisions. Also paralleling the ion/surface reactions, halogen exchange (Cl-for-F) reactions occur between the Cl-containing projectile ions and perfluorohexane to produce C₆F₁₂Cl⁺, a product of chemical modification of the target. Collisions of PCl^+· and PCl ₂ ⁺ also result in production of C₆F ₁₂ ^+· , indicating that the corresponding surface modification reaction involving molecular defluorination should be sought. Implications for previously proposed mechanisms, new ion/surface reactions, and for the use of gas-phase studies to guide investigations of the ion/surface reactions are discussed.     

Expanding molecular dynamics simulations to the NMR time scale. I. Studies of conformational interconversions of 1, 1-difluoro-4, 4-dimethylcycloheptane using MM3-MD

A molecular dynamics (MD) simulation of 35,000 picoseconds (ps) has been carried out to study the conformational interconversions of 1,1-difluoro-4,4-dimethylcycloheptane at room temperature using the MM3 force field. The exchange between axial and equatorial fluorine atoms was the only conformational interconversion that occurred, and it took place via the process of pseudorotation. Ring inversions (twist–chair < twist–boat < twist–chair) were not observed. The axial-equatorial exchange of the two fluorine atoms took place five times during the MD trajectory of 35,000 ps. The two CH₃ groups occupied symmetrical positions (exchangeable by a C₂-like rotations, where C₂-like means it would be C₂ if the fluorines were not present) in the MM3 structures, and during most of the time of the MD trajectory. The methyls occasionally moved off the C₂-like axis in the simulated process, mostly because the C₂-like axis was momentarily moved so that it did not pass through the ring atom to which the two CH₃ groups are bonded. A C₂-like symmetry of the twist-chair conformation was maintained approximately during most of the MD simulation. The conformational geometry with the highest energy obtained during the axial-equatorial exchange process was found and used to locate the transition state. The energy barrier for this axial-equatorial exchange was calculated to be 4.7 kcal/mol, and it compares with the value (5.0 kcal/mol) determined by dynamic nuclear magnetic resonance (NMR). © 1994 by John Wiley & Sons, Inc.     

The thiolate anion as a nucleophile Part XII. Reactions of Lead(II) Benzenethiolate

The reactions of various fluoroaromatics, C₆F_6?xH_x, and C₆F₅X (X = C₆F₆, Cl, Me, NO₂, CF₃, COCl, CH₂Br, OMe, and NH₂) with lead(II) benzenethiolate in DMF have been examined. Lead thiolate acted as an excellent source of benzenethiolate anions and displacement of fluorine, chlorine or the nitro group was observed. The new products have been characterized by elemental analysis, and NMR (H?1 and F?19), infrared and mass spectroscopy.     

Fluorinated heterocyclic compounds: Selective chlorine/fluorine exchange reactions on pyrimidines

Selective chlorine/fluorine exchange reactions on tetrachloropyrimidine, 6-methyl-, 6-chloromethyl-, 6-dichloromethyl-, and 6-trichloromethyl-2,4,5-trichloropyrimidine are described. Sodium fluoride, potassium fluoride, hydrogen fluoride, and antimony trifluoride were used as the fluorinating agents. It was found that NaF and KF fluorinate only in the heterocyclic nucleus, HF in the nucleus and in the chlorinated methyl group, and SbF₃ only in the chlorinated methyl group. In the first stage of fluorination with NaF only chlorine bound in position 4 of the pyrimidine ring is exchanged. The HF reaction is an equilibrium reaction in which the substitution of the fluorine for the first chlorine atom occurs preferentially in position 2. The behaviour of partly fluorinated pyrimidines in nucleophilic exchange reactions is also discussed.     

The Crystal Structure of α-F2: Solving a 50 Year Old Puzzle Computationally

The cohesive energy of α-fluorine, with C2/c space group symmetry, was calculated at benchmark quality by applying the method of increments. The known experimental X-ray structure data needed to be refined, since the reported intramolecular bond length was unrealistically large. At the CCSD(T) level, including corrections for zero-point energy, the basis set superposition error, and extrapolated to the complete basis set limit, a cohesive energy of −8.72 kJ mol⁻¹ was calculated, which agrees well with the 0 K-extrapolated experimental value of −8.35 kJ mol⁻¹. 1 Comparison of the C2/c structure with a Cmca structure, isotypic to that of chlorine, bromine, and iodine reveals that the origin of the different structure of solid fluorine, compared to the heavier halogens, is the lack of significantly stabilizing σ-hole interactions. In addition, the wave numbers of the stretching mode in solid fluorine were calculated at coupled cluster level and compared to newly recorded Raman spectra of condensed fluorine. Both experiment and calculation indicate a slight up-shift for the stretching mode by 2 or 5 cm⁻¹, respectively, with respect to a free F₂ molecule in the gas phase.     

Extent of exchange of36Cl with CAB in strong acid medium

Aqueous chloramine-B /C₆H₅SO₂NCl Na/ solution is known to contain species like RNCl^–, RNHCl, RNCl₂, RN⁺H₂Cl, HOCl and H²⁺OCl where R=C₆H₅SO₂. The exchange studies between³⁶Cl and CAB carried out in various media by ion-exchange method indicated that there is no exchange in solution at pH7. As the pH is decreased below 7, the extent of exchange increases reaching a maximum at pH 3. 3. The exchange decreases as the acidity is increased between pH 3.3 and 1N and again the exchange increases beyong 1N. The observed increase in exchange in strong acid medium is due to the evolution of chlorine.     

Fluorination of Hydrogen-containing with elemental fluorine

The hydrohalo-olefins cis and trans CHC1=CHC1, CHC1=CC1₂, CHC1=CH-CH₂C1 and CH₂=CC1-CH₂C1 have been fluorinated with elemental fluorine to give good yields of hydrohalofluoroalkanes. The best operational conditions for F₂ addition to the double bond rather than hydrogen and/or chlorine atom substitution, dimerization and oligomerization of radical intermediates have been studied. Preliminary studies on the reaction of Freon 12 and Freon 22 towards elemental fluorine have also been carried out.     

Does the thermal motion of macromolecules really influence the fluorine-atomic concentration of plasma-fluorinated polyolefin surfaces?

The relative fluorine-atomic concentrations and the abundance of CF_x functionalities from CF₄- and C₆F₆-RF-plasma treated polypropylene (PP) film surfaces were evaluated. Survey and high resolution (HR) ESCA data indicate that intense surface fluorination can be carried out, from both fluorine precursors, under relatively low power and treatment time conditions. However, it was found that the stability (under open laboratory conditions and under various solvent and temperature environments) of plasma implanted fluorine based groups significantly depend on the nature of plasma gases involved. Simulation of plasma induced molecular fragmentation, at different electron energy MS conditions, indicates the presence of a much higher fluorine atom concentration from a CF₄-plasma in comparison to a C₆F₆-plasma. It is suggested that fluorine atom mediated fragmentation of macromolecular backbones is probably responsible for the erosion of plasma fluorinated surfaces, rather than thermal motion induced burying processes.     

Quantum chemical study of the reaction mechanism of ozone and methane with fluorine and chlorine atoms

Ab initio calculations of the potential energy surface for the F + O₃ and Cl + O₃ reactions have been performed using the G3 and G3MP2 methods, which optimize the geometry configuration of reactants, products, intermediates, and transition states. The results show that fluorine atoms react with ozone as violently as chlorine atoms. At the same time, we have studied the reaction mechanisms of F atoms and Cl atoms with methane. It is found that fluorine atoms prefer to react with methane and chlorine atoms with ozone when there is competition between ozone and methane. Therefore, we can reasonably explain why chlorine atoms play the main role of reactants depleting ozone, while the more active fluorine atoms deplete less ozone. © 2002 Wiley Periodicals, Inc.; DOI 10.1002/qua.10119     

Reaktionen an Isocyanidverbindungen

By the reaction of FSO₂N?PCl₃ with perfluorpropionic acid FSO₂NHC(O)C₂F₅ is formed, which yields FSO₂N?C(Cl)? C₂F₅ (I) with PCl₅. The chlorine atom in (I) could be replaced by the substituents NH₂ (II) and N(C₂H₅)₂ (III). FSO₂N?C(Cl)? CF₃ reacts with AgOCN, AgSCN, unhydrous HF and 2,3-dimethylbutadiene. FSO₂N(CH₃)? C(O)F reacts with elemental fluorine under exchange of a proton against a fluorine atom to give FSO₂N(CH₂F)? C(O)F, which liberates at room temperature COF₂ and trimerises to form 1,3,5-Tris-fluorosulfonyl-s-triazine (VIII). The amides FSO₂N?C(CH₃)NH₂ and FSO₂N?C(CF₃)NH₂ react with SF₄ in the presence of NaF to yield the iminosulfur difluorides FSO₂N?C(CH₃)? NSF₂ (IX) and FSO₂N?C(CF₃)? NSF₂ (X)     

Nature of halogen-centered intermolecular interactions in crystal growth and design: Fluorine-centered interactions in dimers in crystalline hexafluoropropylene as a prototype

The wide occurrence of halogen-centered noncovalent interactions in crystal growth and design prompted this study, which includes a mini review of recent advances in the field. Particular emphasis is placed on providing compelling theoretical evidence of the formation of these interactions between sites of positive electrostatic potential, as well as between sites of negative electrostatic potential, localized on the electrostatic surfaces of the bound fluorine atoms in a prototypical system, hexafluoropropylene (C₃F₆), upon its interaction with another same molecule to form (C₃F₆)₂ dimers. The existence of σ- and π-hole interactions is shown for the stable dimers. Even so, weakly bound interactions locally responsible in holding the molecular fragments together cannot and should not be overlooked since they are partly responsible for determining the overall geometry of the crystal. The results of combined quantum theory of atoms in molecules, molecular electrostatic surface potential, and reduced density gradient noncovalent interaction analyses showed that these latter interactions do indeed play a role in the stability and growth of crystalline C₃F₆ itself and the (C₃F₆)₂ dimers. A symmetry adapted perturbation theory energy decomposition analysis leads to the conclusion that a great majority of the (C₃F₆)₂ dimers examined are the consequence of dispersion (and electrostatics), with nonnegligible contribution from polarization, which together competes with an exchange repulsion component to determine the equilibrium geometries. In a few structures of the (C₃F₆)₂ dimer, the fluorine is found to serve as a six-center five-bond donor/acceptor, as found for carbon in other systems (Malischewski and Seppelt, Angew. Chem. Int. Ed. 2017, 56, 368). © 2019 Wiley Periodicals, Inc.     

Novel method of synthesis of C60F48 with improved yield and selectivity

A novel two-stage method of preparation of C₆₀F₄₈ with 96% purity and 80% yield is reported. A C₆₀ embedded into a MnF₂ matrix is reacted with molecular fluorine under dynamic conditions, i.e. in flow of fluorine gas and with sublimation of volatile products, which results in formation of C₆₀F₃₄-C₆₀F₃₈ mixtures with >90% yield. Subsequent fluorination of the mixture thus obtained in the closed reactor at elevated pressure directly leads to the final product. C₆₀F₄₈ thus synthesized has been characterized by means of EI-MS, MALDI-MS, IR-spectroscopy and X-ray photoelectron spectroscopy (XPS). The problems of fullerene burning and degradation in the fluorine atmosphere are discussed.     

Zur Darstellung von N,N′-Dimethyldiaza-bis-fluorodichlorophosphetidin Stereoselektive Fluorierung von Phosphor(V)-Verbindungen mittels Bortrifluorid 19F- und 31-NMR-spektroskopischer Nachweis von Brenzcatechylphosphordifluoridchlorid und Brenzcatechylphosphortrifluorid

The reaction between BF₃ and (CH₃NPCl₃)₂ proceeds in benzene solution under substitution of two chlorine atoms by fluorine to give the compound (CH₃NPFCl₂)₂. The two fluorine atoms are located at the axial positions of the two trigonal bipyramidal phosphorus atom environments. The reaction between C₆H₄O₂PCl₃ and BF₃ in benzene solution results in the formation of the known products C₆H₄O₂PFCl₂ and (C₆H₄O₂)₂PCl, and the new compounds C₆H₄O₂PF₂Cl and C₆H₄O₂PF₃. ³¹P and ¹⁹F n.m.r. spectra are discussed.     

Dismutierungs- und Isomerisierungsreaktionen an C?Cl?F-Verbindungen

Dismutation and Isomerization Reactions of C? Cl? F Compounds Results of dismutation and isomerization reactions of the compounds C₂Cl_6?nF_n (n = 1 to 5) are shown. Chromium oxides and aluminium fluoride are used as catalysts in heterogeneous reactions. Reaction mechanisms are formulated. Under the applied conditions no thermal decompositions occur. Thermal stability of the compounds is described.     

Chemical bonding and molecular dynamics in inclusion compounds of fluorinated graphite according to 19F NMR spectroscopy data

In Inclusion compounds of fluorinated graphite with chlorine trifluoride C₂. xClF₃ and hexafluoro-benzene C₂F. xC₆F₆ , the guest molecules are characterized by rotational mobility and weak bonds with the host matrix. ¹⁹F NMR chemical shift tensors are determined for the fluorine nuclei of the matrix and the guest molecules, including the structurally nonequivalent fluorine atoms ofClFj molecules [δ (Fl) = −700, δ(F1) = −280; δ|| (F2) = −440, δ±(F₂) = −220ppm relative to F2]. It is shown that C-F bonds in the host matrix are close to those in aromatic fluorocarbons. Translated from Zhumal Stmktumoi Khimii, Vol. 41, No. 1, pp. 80-85, January–February, 2000.     

Effect of the Structure of 1- and 3-Methyl-5-fluoropyrimidin-4-ones on H-D Exchange in Position 6under Conditions of Base Catalysis

Hydrogen atom in position 6of 5-fluoro-1-methylpyrimidin-4(1H)-one and its 2-methylsulfanyl, 2methoxy and 2-butylamino derivatives is readily replaced by deuterium in 90% methanol-d ₄ at ~20°C under conditions of base catalysis. The rate of H-D exchange decreases in the series H, SMe > OMe >> NHBu. Isomeric 5-fluoro-3-methylpyrimidin-4-ones, as well as their 5-unsubstituted analogs, do not undergo H–D exchange. Ready deuterium exchange in 5-fluoro-1-methylpyrimidin-4-ones is explained by synergistic effect of the zwitterionic structure and 5fluorine atom on the C⁶-H acidity. The gain in energy due to this effect, expressed through the enthalpy of dissociation of the C⁶-H bond, is ~15 kcal/mol provided that effect of the medium is absent; the contribution of the 5-fluorine atom is 5.3-6.0 kcal/mol.