The radiation chemistry of acyclic hydrofluoro and perhalogenated ether and hydrocarbon compounds

The radiolytic stability of some hydrofluoroethers and hydrofluorocarbons was investigated and compared with those of perfluoropolyethers (PFPEs) and the CCl₂FCClF₂ (CFC 113). The experimental results indicate that stability depends mainly on the relative abundance of hydrogen atoms in the molecule; however, a significant role is played also by the chemical structure (i.e. the relative positions of the hydrogen atoms in the molecule). As a result, molecules containing hydrogen atoms as OCF₂H chain ends show a higher stability compared with the other hydrofluoro compounds. Based on the analysis of the end products and on the nature of radicals detected by EPR, radiolysis mechanisms are proposed and discussed. Due to their high dipole moments the hydrofluoro compounds and CCl₂FCClF₂ degrade mainly through an ionic mechanism.     

Physical properties of four α, ω-dimethoxyfluoropolyethers

Several physical properties of four samples of linear α, ω-dimethoxyfluoropolyethers, varying in length from 6 to 14 chain atoms, i.e. carbon plus oxygen atoms, are reported and discussed. Among them are boiling points, glass and melting transition temperatures, specific volumes and their temperature coefficients, refractive indices and thermodynamic data such as cohesive energy density and melting enthalpies. The experimental data have been analysed, when possible, on the basis of the group contribution method, the results of which are in agreement with literature data. The above results are compared with those of two analogous series of fluoropolyethers whose chain ends are, respectively, OCF₂H and OCF₃. As expected, with increasing chain length, the effect of the OCF₂H and OCH₃ ends decreases progressively so that for chains longer than approximately 15-20 atoms, the differences among the three series should disappear.     

The comparison of thermal stability of some hydrofluoroethers and hydrofluoropolyethers

The thermal stability of representative hydrofluoropolyether (HFPE) and hydrofluoroether (HFE) compounds has been evaluated. The observed stability order appears to be correlated with the nature of the hydrogenated chain ends; in particular, molecules having fully hydrogenated chain ends (OCH₃ and OC₂H₅) show a significantly lower stability compared with the OCF₂H terminated compounds. The main degradation products suggest, however, that the same primary reaction is responsible for the decomposition of all the compounds examined; this reaction involves the fragmentation of the R_fOC_xH_yF_z bond with fluorine transfer between the two carbon atoms close to the oxygen, leading to the formation of a hydrofluorocarbon C_xH_yF_(z+1) and an acyl fluoride or a ketone.     

Catalytic properties of Phe41→His mutant of horseradish peroxidase expressed inE. coli

The recombinant horseradish peroxidase and its single-point F41H mutant have been reactivated fromE. coli inclusion bodies. The influence of the mutation on the heme entrapment, stability and activity of the enzyme was demonstrated. The catalytic rate constants for H₂O₂ cleavage and ammonium 2,2-azino-bis(3-ethylbenzothiazoline-6-sulfonate (ABTS) oxidation decrease by two and one orders of magnitude, respectively. Unlike the wild-type recombinant horseradish peroxidase, the elimination of the ABTS oxidation product is not a rate-determining step for the mutant. The F41H replacement results in significant changes of kinetics of iodide ion oxidation. The reaction rate is linear to the concentrations of iodide, H₂O₂, and the enzyme. The results suggest the direct interaction of iodide with the porphyrin ring of the heme. The decrease in ABTS oxidation activity accompanied by retention of activity in iodide oxidation in the course of low-dosage radiolysis of the F41H mutant is additional evidence of the direct electron transfer from iodide to the heme, in contrast to ABTS oxidation, in which the electron transfer chain in the protein molecule is involved.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2034–2038, November, 1994.     

Hydroxid-halogenid-verbindungen des di-t-butyl-substituierten zinns

The di-t-butyltin hydroxide halides t-Bu₂Sn(OH)X (X = F, Cl or Br) have been prepared starting from the dihalides t-Bu₂SnX₂ or the oxide (t-Bu₂SnO)₃. X-Ray analysis of the three compounds shows dimeric molecules: two 5-coordinated tin atoms and the oxygen atoms of the hydroxyl groups are linked to a four-membered ring. As confirmed by the IR spectra, the molecules in the crystal are held together by OH?X hydrogen bonds. These are strong in the hydroxide fluoride but are weak in the analogous chloride and bromide.     

Electron and hydrogen transfer in small hydrogen fluoride anion clusters

A new stable structure has been found for the anion clusters of hydrogen fluoride. The ab initio method was used to optimize the structures of the (HF)(3)(-), (HF)(4)(-), (HF)(5)(-), and (HF)(6)(-) anion clusters with an excess "solvated" electron. Instead of the well-known "zig-zag" (HF)(n)(-) structure, a new form, (HF)(n-1)F(-)···H, was found with lower energy. In this new form, the terminal hydrogen atom in the (HF)(n)(-) chain is separated from the other part of the cluster and the inner hydrogens transfer along the hydrogen bonds toward the outside fluoride. The negative charge also transfers from the terminal HF molecule of the chain to the center fluoride atoms. The (HF)(n)(-) clusters for n = 4, 5, and 6 have not yet been observed experimentally. These results should assist in the search for these systems and also provide a possible way to study the proton and electron transfer in some large hydrogen bonding systems.     

Bi-functional fluoroalkylation reagents: an introduction to halo-substituted 3-oxa-perfluoroalkanesulfonyl fluorides

X(CF₂CF₂)_nOCF₂CF₂SO₂F (X=I, Br, Cl; n=1, 2, 3, 4) are widely used fluoroalkylation reagents, which can incorporate ‘heavy’ fluorous tags into organic compounds. X(CF₂CF₂)_nOCF₂CF₂SO₂F have both sulfonyl and halo groups. They behave as bi-functional fluoroalkylation reagents. The cleavage of the C–I bonds of I(CF₂CF₂)_nOCF₂CF₂SO₂F by reductants (such as Na₂S₂O₄, Zn), single electron transfer reagents and radical initiator systems (like Bz₂O₂, AIBN, and (t-BuO)₂, or under UV and heat) gives, respectively, the sulfinatodehalogenated products, the hydrodehalogenated products, the homo-coupling products and the perfluoroalkylated products (if alkenes, alkynes or arenes were added). The functionalization of the sulfonyl groups (SO₂F) of X(CF₂CF₂)_nOCF₂CF₂SO₂F by esterification, amidation, and fluorination affords the corresponding perfluoroalkanesulfonates, fluoroalkanesulfonamide, and perfluoroalkanes. In many cases, both the halo and sulfonyl groups of X(CF₂CF₂)_nOCF₂CF₂SO₂F are transformed. These transformations finally lead to hundreds of useful highly fluorinated materials, such as supper acids, catalysts, surfactants, ion-exchange resins, electrolytes, polymers, and dense ionic liquids. Furthermore, X(CF₂CF₂)_nOCF₂CF₂SO₂F have commendable advantages, such as the easy preparation, the wide range of substrate tolerance, the mild reaction condition, and the high yields of desired products, which make them very promising. This review briefly summarizes the synthesis, reactivity, and applications of these intriguing reagents.     

Experimental and computational investigation of the uncatalyzed rearrangement and elimination reactions of isochorismate

The versatile biosynthetic intermediate isochorismate decomposes in aqueous buffer by two competitive pathways, one leading to isoprephenate by a facile Claisen rearrangement and the other to salicylate via elimination of the enolpyruvyl side chain. Computation suggests that both processes are concerted but asynchronous pericyclic reactions, with considerable C-O cleavage in the transition state but relatively little C-C bond formation (rearrangement) or hydrogen atom transfer to the enolpyruvyl side chain (elimination). Kinetic experiments show that rearrangement is roughly 8-times more favorable than elimination. Moreover, transfer of the C2 hydrogen atom to C9 was verified by monitoring the decomposition of [2-(2)H]isochorismate, which was prepared chemoenzymatically from labeled shikimate, by (2)H NMR spectroscopy and observing the appearance of [3-(2)H]pyruvate. Finally, the isotope effects obtained with the C2 deuterated substrate are in good agreement with calculations assuming pericyclic reaction mechanisms. These results provide a benchmark for mechanistic investigations of isochorismate mutase and isochorismate pyruvate lyase, the enzymes that respectively catalyze the rearrangement and elimination reactions in plants and bacteria.     

Synthesis of Tri‐ and Difluoromethoxylated Compounds by Visible‐Light Photoredox Catalysis

Trifluoromethoxy (OCF₃) and difluoromethoxy (OCF₂H) groups are fluorinated structural motifs that exhibit unique physicochemical characteristics. Incorporation of these substituents into organic molecules is a highly desirable approach used in medicinal chemistry and drug discovery processes to alter the properties of a parent compound. Recently, tri‐ and difluoromethyl ethers have received increasing attention and several innovative strategies to access these valuable functional groups have been developed. The focus of this Minireview is the use of visible‐light photoredox catalysis in the synthesis of tri‐ and difluoromethyl ethers. Recent photocatalytic strategies for the formation of O?CF₃, C?OCF_3, O?CF₂H, and C?OCF₂H bonds as well as other transformations leading to the construction of OR_F groups are discussed herein.     

Exploiting the Strong Hydrogen Bond Donor Properties of a Borinic Acid Functionality for Fluoride Anion Recognition

Borinic acids have typically not been considered as hydrogen bond donor groups in molecular recognition. Described herein is a bifunctional borane/borinic acid derivative ( 2 ) in which the two functionalities are connected by a 1,8‐biphenylenediyl backbone. Anion binding studies reveal that 2 readily binds a fluoride anion by formation of a unique B?F???H?O?B hydrogen bond. This hydrogen bond is characterized by a short H‐F distance of 1.79(3) Å and a large coupling constant (¹J_HF) of 57.2 Hz. The magnitude of this interaction, which has also been investigated computationally, augments the fluoride anion binding properties of 2 , thus making it compatible with aqueous environments.     

Mass spectrometry in structural and stereochemical problems-CCXXIX: Electron-impact-induced fragmentation of boric acid esters

The mass spectral decompositions of cyclic and acyclic borates have been examined. Straight chain trialkyl borates fragment by C? O cleavage reactions accompanied by transfer of one or two hydrogen atoms, whereas tri-(sec-alkyl) borates primarily undergo α-cleavage reactions. Both types of cleavage are observed in the fragmentation of cyclic borates. B? O bonds are not broken in the formation of most major fragment ions. The fragmentations of borates are compared with those of phenylboronates with special emphasis on the rearrangements leading to hydrocarbon ions in the spectra of the latter.     

5H-3-oxa-Octafluoropentanesulfonyl fluoride: a novel and efficient condensing agent for esterification, amidation and anhydridization

The use of 5H-3-oxa-octafluoropentanesulfonyl fluoride (HCF₂CF₂OCF₂CF₂SO₂F) as a novel and efficient condensing reagent for esterification of carboxylic acids with alcohols and amidation of carboxylic acids with amines in the presence of 1,3-diazabicyclo[5.4.0]-undec-7-ene (DBU) is reported. HCF₂CF₂OCF₂CF₂SO₂F cannot serve as a condensing agent for anhydridization of carboxylic acids, however, HCF₂CF₂OCF₂CF₂SO₂F/(CH₃)₃SiCN system can mediate anhydridization of some aromatic carboxylic acids.     

Dipole-Guided Electron Capture Causes Abnormal Dissociations of Phosphorylated Pentapeptides

Electron transfer and capture mass spectra of a series of doubly charged ions that were phosphorylated pentapeptides of a tryptic type (pS,A,A,A,R) showed conspicuous differences in dissociations of charge-reduced ions. Electron transfer from both gaseous cesium atoms at 100 keV kinetic energies and fluoranthene anion radicals in an ion trap resulted in the loss of a hydrogen atom, ammonia, and backbone cleavages forming complete series of sequence z ions. Elimination of phosphoric acid was negligible. In contrast, capture of low-energy electrons by doubly charged ions in a Penning ion trap induced loss of a hydrogen atom followed by elimination of phosphoric acid as the dominant dissociation channel. Backbone dissociations of charge-reduced ions also occurred but were accompanied by extensive fragmentation of the primary products. z-Ions that were terminated with a deaminated phosphoserine radical competitively eliminated phosphoric acid and H₂PO₄ radicals. A mechanism is proposed for this novel dissociation on the basis of a computational analysis of reaction pathways and transition states. Electronic structure theory calculations in combination with extensive molecular dynamics mapping of the potential energy surface provided structures for the precursor phosphopeptide dications. Electron attachment produces a multitude of low lying electronic states in charge-reduced ions that determine their reactivity in backbone dissociations and H- atom loss. The predominant loss of H atoms in ECD is explained by a distortion of the Rydberg orbital space by the strong dipolar field of the peptide dication framework. The dipolar field steers the incoming electron to preferentially attach to the positively charged arginine side chain to form guanidinium radicals and trigger their dissociations.     

Mechanistic investigation of vinylic carbon-fluorine bond activation of perfluorinated cycloalkenes using Cp*2ZrH2 and Cp*2ZrHF

Cp*₂ZrH₂ (1) (Cp*: pentamethylcyclopentadienyl) reacts with cyclic perfluorinated olefins to give Cp*₂ZrHF (2) and hydrodefluorinated products under very mild conditions. Initial C-F bond activation occurs selectively at the vinylic positions of the cycloolefin to exchange fluorine for hydrogen. Several mechanisms are discussed for this H/F exchange: (a) olefin insertion/β-fluoride elimination, (b) olefin insertion/α-fluoride elimination, and (c) hydride/fluoride σ-bond metathesis. Following H/F σ-bond metathesis exchange of both vinylic C-F bonds of perfluorocyclobutene, 1 then reacts with allylic C-F bonds by insertion/β-fluoride elimination. A similar sequence is observed with perfluorocyclopentene. Cp*₂ZrHF reacts selectively with vinylic C-F bonds of perfluorocyclobutene to give 3,3,4,4-tetrafluorocyclobutene and Cp*₂ZrF₂ without further hydrodefluorination occurring. In the presence of excess 1 and H₂, perfluorocyclobutene and perfluorocyclopentene are reduced to cyclobutane and cyclopentane in 46% and 16% yield, respectively. DFT calculations exclude the pathway by way of the olefin insertion/α-fluoride elimination and suggest that the pathway by way of hydride/fluoride σ-bond metathesis is preferred.     

Mass spectra of 1,2,4-triazoles—II: Alkyl 1,2,4-triazoles

The mass spectra of monomethyl 1,2,4-triazoles contain fragment ions produced by specific cleavage of the heterocyclic ring. A major fragmentation from many molecular ions involves the elimination of HCN, but loss of N₂ is either very small or completely absent. No N or H scrambling occurs within the triazole ring system, as evidenced by labelling studies. The loss of a hydrogen atom from the molecular ions of 3-alkyl-1,2,4-triazoles (alkyl ? C₂H₅) originates from hydrogens attached to the β carbon and nitrogen atoms.     

Insights on structural characteristic of Xilinguole lignite chars from low-temperature pyrolysis

The cleavage behavior of covalent bonds in Xilinguole (XLGL) lignite and changes in chemical structure of lignite and its chars during low-temperature pyrolysis were investigated by thermogravimetric (TG) analysis and Fourier-transform infrared (FTIR) spectroscopy. Based on the TG and differential thermogravimetric (DTG) analysis results, the cleavage of different types of chemical bonds in lignite occurred mainly at four certain temperatures, 170 °C, 376 °C, 432 °C, and 521 °C. The latter three were selected as the final pyrolysis temperatures of chars evaluated in this study. The FTIR analysis results indicate that thermal treatment increased the relative content of two and three adjacent H deformation structures but decreased that of four adjacent H deformation structure. This was caused by the cleavage of C_al–C_al and C_ar–C_al bonds. The oxygen-containing functional groups in lignite are dominated by C–O and C–OH groups with a lower chemical reactivity than C=O–C and conjugated C=O groups. Moreover, XLGL lignite has the highest ratio of CH₂/CH₃ which declines with increasing temperature, indicating the decrease in the length of aliphatic chains and increase in the degree of branching of aliphatic side chains. This change mainly resulted from the cleavage of C_al–O, C_al–C_al, and C_ar–C_al bonds. Furthermore, XLGL lignite and its chars contain five specific hydrogen bonds: OH–N, cyclic OH, OH–ether O, OH–OH, and OH–π hydrogen bonds. The relative content of OH–OH hydrogen bond was the highest, indicating that OH–OH hydrogen bond has the highest thermal stability.

     

Mechanism,kinetics, and environmental assessment of OH-initiated transformation of CTDE in the atmosphere

The transformation mechanism and kinetics of 2-chloro-1,1,2-trifluoroethyl-difluoromethyl-ether (CTDE, CHF₂OCF₂CHFCl) triggered by OH radicals are studied by density-functional theory methods and canonical variational transition state theory. The computational rate constant including small-curvature tunneling correction is found to be in commendable agreement with the experimental data. Two hydrogen abstraction channels to form the alkyl radicals of C·F₂OCF₂CHFCl and CHF₂OCF₂C·FCl are observed, and the formation of CHF₂OCF₂C·FCl is found to be more favorable than C·F₂OCF₂CHFCl kinetically and thermodynamically. Subsequent evolution of CHF₂OCF₂C·FCl in the presence of NO and O₂ indicates that the organic nitrate (CHF₂OCF₂CONO₂FCl) is the stable product. The dechlorinate of alkoxy radical (CHF₂OCF₂C(O·)FCl) is the most favorable degradation channel, and the estimated ozone depletion potential for CTDE relative to chlorofluorocarbon-11 is 0.0204, which could lead to ozone depletion as a consequence. The computed atmospheric lifetime for CTDE is found to be 3.69 years by considering the combined contributions from OH radicals and Cl atoms. The total radiative forcing and global warming potential of CTDE are, respectively, 0.547 W m⁻² ppbv and 628.58 (100 years) at 298 K, suggesting that the contribution of CTDE to the greenhouse effect is moderate.     

The hetero‐homogeneous pyrolysis of propane,in the presence or in the absence of dihydrogen,and the measurement of uptake coefficients of hydrogen atoms

Propane pyrolysis is studied in the presence and the absence of dihydrogen between 743 and 803 K, in the propane pressure range 10–100 Torr, and at 20–254 Torr dihydrogen pressure. In unpacked Pyrex reactors, dihydrogen accelerates propane dehydrogenation and demethanation. The reaction is modeled by a conventional homogeneous free‐radical chain mechanism. Propane pyrolysis is strongly inhibited by the walls of reactors packed with stainless steel, zirconium, or palladium foils. Adding dihydrogen to propane still increases the rates of product formation. The reaction in these packed reactors is modeled by the kinetic scheme proposed for the homogeneous reaction and by the heterogeneous process H. ⇄ ½H₂ (w₂)(−w₂) of chain termination and initiation. In the absence of dihydrogen, step (−w₂) is negligible and precise values of uptake coefficients of hydrogen atoms are obtained at 773 K: 0.31 for stainless steel 0.10 for zirconium 0.05 for palladium In the presence of dihydrogen, steps (w₂) and (−:w₂) are instantaneously at equilibrium. The latter system should be useful to study any reaction of hydrogen atoms in the temperature range. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 340–364, 2000     

Correlation of thermal degradation mechanisms: Polyacetylene and vinyl and vinylidene polymers

The thermal degradation of poly(vinyl bromide) (PVB), poly(vinyl chloride) (PVC), poly(vinyl alcohol) (PVA), poly(vinyl acetate) (PVAc), poly(vinyl fluoride) (PVF), poly(vinylidene chloride) (PVC₂), and poly(vinylidene fluoride) (PVF₂) has been studied by direct pyrolysis–mass spectrometry (DP-MS) and flash pyrolysis–gas chromatography–mass spectrometry techniques. Vinyl and vinylidene polymers exhibit two competitive thermal degradation processes: (1) HX elimination with formation of polyene sequences which undergo further moleculaar rearrangements, and (2) main-chain cleavage with formation of halogenated or oxigenated compounds. The overall thermal degradation process depends on the prevailing decomposition reaction in each polymer; therefore, different behaviors are observed. The thermal degradation of polyacetylene (PA) has also been studied and found important for the elucidation of the thermal decomposition mechanism of the title polymers.     

Kinetics of radiation-induced carbonization of poly(vinylidene fluoride) film surface

The kinetics of radiation-induced carbonization of PVDF surfaces aiming at carbyne (one-dimensional carbon allotrope) synthesis have been studied. A sample of poly(vinylidene fluoride) film was exposed to Mg Kα radiation (?ω = 1253.6 eV) in an ESCALAB Mk II spectrometer for 14 h with the aim of surface carbonization. Some 221 spectra of C 1s electrons were measured and expanded using 7 Gaussian curves to reveal and identify species being created on the film surface during its carbonization. A decrease in the content of CF₂ groups, the emergence of CF species in two different states, and growth of a number of fluorine-free carbon atoms have been detected. Simultaneous variations of CH/CH₂, CF and CF₂ peaks suggest elimination of H and F atoms as HF. A proposed model shows three probabilistic factors affecting the rate of degradation, one of which remains uncertain.