Substitution Of Fluorine For Hydrogen Research Articles

Thermodynamics and performance of the Mg-H-F system for thermochemical energy storage applications.

Magnesium hydride (MgH2) is a hydrogen storage material that operates at temperatures above 300 °C. Unfortunately, magnesium sintering occurs above 420 °C, inhibiting its application as a thermal energy storage material. In this study, the substitution of fluorine for hydrogen in MgH2 to form a range of Mg(HxF1-x)2 (x = 1, 0.95, 0.85, 0.70, 0.50, 0) composites has been utilised to thermodynamically stabilise the material, so it can be used as a thermochemical energy storage material that can replace molten salts in concentrating solar thermal plants. These materials have been studied by in situ synchrotron X-ray diffraction, differential scanning calorimetry, thermogravimetric analysis, temperature-programmed-desorption mass spectrometry and Pressure-Composition-Isothermal (PCI) analysis. Thermal analysis has determined that the thermal stability of Mg-H-F solid solutions increases proportionally with fluorine content, with Mg(H0.85F0.15)2 having a maximum rate of H2 desorption at 434 °C, with a practical hydrogen capacity of 4.6 ± 0.2 wt% H2 (theoretical 5.4 wt% H2). An extremely stable Mg(H0.43F0.57)2 phase is formed upon the decomposition of each Mg-H-F composition of which the remaining H2 is not released until above 505 °C. PCI measurements of Mg(H0.85F0.15)2 have determined the enthalpy (ΔHdes) to be 73.6 ± 0.2 kJ mol-1 H2 and entropy (ΔSdes) to be 131.2 ± 0.2 J K-1 mol-1 H2, which is slightly lower than MgH2 with ΔHdes of 74.06 kJ mol-1 H2 and ΔSdes = 133.4 J K-1 mol-1 H2. Cycling studies of Mg(H0.85F0.15)2 over six absorption/desorption cycles between 425 and 480 °C show an increased usable cycling temperature of ∼80 °C compared to bulk MgH2, increasing the thermal operating temperatures for technological applications.

A quantum chemical NMR study of branched and unbranched fluoropolymers

The work demonstrates the results of quantum chemical calculations of 19F and 13C NMR spectra of model fluorocarbon CnF2n+2 molecules with various configurations and hydrocarbon chain molecules. The possibilities to determine the chain length, formation of branches, identification of fluorine substitution for hydrogen during the fluorination of hydrocarbon paraffins and polymers are discussed.

The structural features of fluorinated paraffins

A systematic study of the molecular and supramolecular structural features of fluorinated paraffins has been conducted with the use of a set of physical methods and quantum-chemical calculations of the parameters of the IR and NMR spectra. The presence of nonuniformity in the morphological structures of samples obtained from different initial products has been shown. Incomplete substitution of fluorine for hydrogen during synthesis has been revealed, and the types of structural groups in which the substitution occurs have been determined. Fluoroparaffins have been found to effectively dissolve in supercritical carbon dioxide, a phenomenon that opens new fields of application for these materials.

Fluorination, and tunneling across molecular junctions.

This paper describes the influence of the substitution of fluorine for hydrogen on the rate of charge transport by hole tunneling through junctions of the form Ag(TS)O2C(CH2)n(CF2)(m)T//Ga2O3/EGaIn, where T is methyl (CH3) or trifluoromethyl (CF3). Alkanoate-based self-assembled monolayers (SAMs) having perfluorinated groups (R(F)) show current densities that are lower (by factors of 20-30) than those of the homologous hydrocarbons (R(H)), while the attenuation factors of the simplified Simmons equation for methylene (β = (1.05 ± 0.02)n(CH2)(-1)) and difluoromethylene (β = (1.15 ± 0.02)n(CF2)(-1)) are similar (although the value for (CF2)n is statistically significantly larger). A comparative study focusing on the terminal fluorine substituents in SAMs of ω-tolyl- and -phenyl-alkanoates suggests that the C-F//Ga2O3 interface is responsible for the lower tunneling currents for CF3. The decrease in the rate of charge transport in SAMs with R(F) groups (relative to homologous R(H) groups) is plausibly due to an increase in the height of the tunneling barrier at the T//Ga2O3 interface, and/or to weak van der Waals interactions at that interface.

The hapticity of octafluorocyclooctatetraene in its first-row mononuclear transition metal carbonyl complexes: effect of perfluorination

The iron tricarbonyl complex of octafluorocyclooctatetraene was synthesized by Hughes and co-workers and shown by X-ray crystallography to have a trihapto–monohapto structure (η3,1-C8F8)Fe(CO)3 in contrast to the tetrahapto structure (η4-C8H8)Fe(CO)3 formed by the non-fluorinated cyclooctatetraene. This difference has stimulated a comprehensive density functional theoretical study of the octafluorocyclooctatetraene metal carbonyl complexes (C8F8)M(CO)n (n = 4, 3, 2, 1 for M = Ti, V, Cr, Mn, and Fe; n = 3, 2, 1 for M = Co, Ni) for comparison with their hydrogen analogues (C8H8)M(CO)n. In most such systems, the substitution of fluorine for hydrogen leads to relatively small changes in the preferred structures. However, for the iron carbonyl derivatives (C8X8)Fe(CO)3 (X = H, F), the difference observed experimentally has been confirmed by theory with (η3,1-C8F8)Fe(CO)3 and (η4-C8H8)Fe(CO)3 being the lowest energy structures by 4 and 14 kcal/mol, respectively. The ligand exchange reactions C8H8 + (C8F8)M(CO)n → C8F8 + (C8H8)M(CO)n are predicted to be exothermic for almost all of the systems considered, with the (η3,1-C8X8)Fe(CO)3 system being the main exception. This suggests that the C8F8 ligand generally bonds more weakly to transition metals than the C8H8 ligand in accord with the electron-withdrawing effect of the ligand fluorine atoms.

First-principles investigations of F and Cl impurities in NaAlH 4

We study halide interactions with NaAlH 4 using first-principles calculations based on density functional theory. First, we determine which products can be formed when TiX 3 ( X = F , Cl) is ball milled with NaAlH 4. Then we investigate impurity incorporation, finding that fluorine may substitute for hydrogen, or form a charged interstitial defect. Fluorine substitution for hydrogen has been proposed as a means of destabilizing hydrides, but we find this to be unlikely for either NaAlH 4 or Na 3AlH 6. Fluorine interstitials are most stable as charged defects; however, their concentration will likely be too low to shift the Fermi level. The migration barrier for interstitial fluorine is low enough to easily allow diffusion during cycling through the hydride phases. Chlorine has a large size mismatch with hydrogen and every chlorine-related defect studied is found to have a high formation energy. Chlorine is therefore unlikely to incorporate in NaAlH 4.

Theoretical studies of the Diels–Alder reaction of 1-fluoroethylene with some model dienes

3-Vinylcoumarin and 3-trifluorovinylcoumarin have been chosen as model compounds for [4+2]-cycloaddition reaction. Density functional theory (DFT) has been applied to the study of concerted mechanism of the Diels–Alder reaction with fluorinated ethylene. The main goal of this work was to determine the transition state (TS) on several concerted paths of reaction to examine the regioselectivity of the Diels–Alder reaction in our model systems. The effect of fluorine substitution on TS formation has been estimated. Theoretical investigation has led to the conclusion, that the introduction of one electron-withdrawing substituent (fluorine atom) into dienophile molecule makes the process of reaching the TS more difficult due to an increase in the ‘energy of concert’ ( Δ G act 0 ) compared to non-fluorinated system. The substitution of fluorine for hydrogen in exocyclic vinyl group results (with one exception) in a lower activation energy and higher energy gain of reaction for systems with 1-fluoroethylene.

Characteristic Substituent-Shift Models for Carbon 1s Ionization Energies and Mean Dipole-Moment Derivatives

Characteristic substituent-shift models for carbon mean dipole-moment derivatives are determined for the halomethanes, fluorochloroethanes, and some other small molecules. These models are analogous to those reported earlier for core ionization energies measured by X-ray photoelectron spectroscopy and are to be expected since Siegbahn's simple potential model relates these to mean dipole-moment derivatives obtained from infrared spectral data. Linear models relating carbon 1s ionization energies and mean dipole-moment derivatives to the number of fluorine, chlorine, bromine, and iodine atoms substituting hydrogen atoms in the halomethanes are reported. The regression coefficients in these models are similar to the coefficients for the fluorine and chlorine atoms found in linear models derived for the mean dipole-moment derivatives and carbon 1s ionization energies of the fluorochloroethanes. The signs of the coefficients in the fluorochloroethane model indicate that the a carbon becames more positive and the β carbon more negative upon fluorine substitution for hydrogen. Standard derivative values of -0.52 ′ 0.05, -0.25 ′ 0.05, -0.18 ′ 0.03, -0.17 ′ 0.03, and -0.01 ′ 0.01 e are proposed for the fluorine, chlorine, bromine, iodine, and hydrogen atoms of saturated fluorochlorohydrocarbons. Characteristic substituent shifts for Mulliken, CHELPG, and Bader charges of the carbon atoms in these molecules are also investigated.

Influence of Fluorine Substitution for Hydrogen on the Appearance of SmC* Phase Between SmA* and SmCa* Phases

Recently several fluorinated molecules have been investigated by many means [1]. We investigated in details physical properties of two antiferroelectric liquid crystals [2]: MHPB(H)PBC and MHPB(F)PBC. In MHPB(H)PBC paraelectric SmA∗ phase neighbours antiferroelectric SmCA∗ phase, while in MHPB(F)PBC the substitution of fluorine for hydrogen leads to formation of additional narrow ferroelectric phase SmC∗ phase, placed between the mentioned above phases. We also found several other phenomena, namely: increasing of the phase transition temperatures for fluorinated compound in comparison with hydrogenated one, decreasing of the smectic layer thickness, and increasing of spontaneous polarisation. In our work we focused on the explanation of these phenomena from the molecular point of view using computer molecular modelling.

Structure and mechanical properties of PACVD fluorinated amorphous carbon films

Fluorinated amorphous carbon films were deposited by plasma assisted chemical vapour deposition from C 2H 2 and CF 4 gases. Gas mixture and bias voltage were changed. We analysed film structure by Raman and infrared spectroscopies and film hardness by micro-indentation. Raman spectra show a transition from diamond-like carbon films to polymer-like films as the CF 4 content in the plasma increases. The presence of a strong fluorescence background in the spectra of fluorine rich films indicates a polymer-like structure. Infrared spectra confirm the presence of CF x , F 2CCF 2 and F 2CC(HF) groups in the films. Hardness values are correlated with the trends of the D to G peak intensity ratio, I D/ I G. Mechanisms involved in film growth and formation are discussed especially regarding fluorine substitution for hydrogen.

Systematic study of amorphous hydrogenated and fluorinated carbon films

Amorphous fluorinated carbon films were grown from CF4 and C2H2 mixtures, using a Plasma Assisted Chemical Vapour Deposition (PACVD) apparatus. Two sets of films were deposited, changing in a systematic way the CF4 flux and the bias voltage (Vb). Film composition and structure were analysed by secondary ion mass spectroscopy (SIMS), infrared (IR) and Raman spectroscopies. Film hardness was obtained by micro-indentation measurements. On increasing fluorine content in films, hardness decreases and a fluorescence background in Raman spectra appears at high fluorine content, showing a diamond- to polymer-like structural transition. Infrared spectra indicate the presence of CFx, CCHF and CCF2 groups in the films. Our data are compared with previous results in the literature and the mechanisms involved in film formation are discussed, especially regarding fluorine substitution for hydrogen.

Temperature Effect on the Adsorption of Fluorooctanols at the Hexane/Water Interface

The interfacial tensions of the hexane solution of fluorooctanols (1,1,2,2-tetrahydrotridecafluorooctanol, TFC8OH, and 1,1-dihydropentadecafluorooctanol, DFC8OH) against water were measured as a function of temperature and molality under atmospheric pressure. By drawing the interfacial pressure π vs mean area per adsorbed molecule A curves, it was concluded that the adsorbed film of TFC8OH exhibits a first-order phase transition between the gaseous and expanded states and that of DFC8OH shows the two types of phase transitions from the gaseous to the expanded state and from the expanded to the condensed one at the hexane/water interface. The comparison of the π vs A curve between TFC8OH and DFC8OH shows that the intermolecular interaction is enhanced by the substitution of fluorine for hydrogen on the β-carbon of TFC8OH. Furthermore, the difference in the transition pressure between DFC8OH and TFC10OH (1,1,2,2-tetrahydroheptadecafluorodecanol) is explained by the differences in London dispersion force between hydrophobic chains and the dipole moment of their hydroxyl group. The partial molar entropy s̄sH − ssO and energy ūsH − usO changes of adsorption were evaluated and compared to those of TFC10OH. The s̄sH − ssO value is negative and therefore alcohol molecules have smaller entropy at the interface than in the solution, which is attributable to the orientation of the molecules at the interface. The phase transition from the expanded to the condensed state in the adsorbed TFC10OH film causes larger decrease in partial molar entropy than that in the DFC8OH one. This may arise from the larger partial molar entropy of TFC10OH molecules due to the larger entropy of mixing of longer fluorocarbon chain with hexane in the expanded state and the smaller entropy of TFC10OH due to the stronger attractive interaction in the condensed state than that of DFC8OH molecules. The ūsH − usO value is less negative for DFC8OH than for TFC10OH and therefore the energetical stabilization of DFC8OH accompanied by the adsorption from the solution is less than that of TFC10OH. Furthermore, it was concluded that the DFC8OH molecules are stabilized by forming the condensed film at the interface because of the strong molecular interaction between them, and the TFC8OH molecules form mainly tetramers in the hexane solution to lower the energetical state of the system.

Temperature-dependent surface potentials of fluorinated alkanethiolate self-assembled monolayers

Self-assembled monolayers (SAMs) consisting of twenty-two carbon, methyl-terminated alkanethiolates adsorbed on vapor-deposited gold have been fluorinated in vacuum using an effusive F atom source. The reactive uptake of fluorine as a function of F atom exposure was calibrated using X-ray photoelectron spectroscopy. The surface potentials ( V s) of SAMs that were fluorinated to different degrees were measured as a function of temperature using a high-sensitivity vibrating probe electrostatic voltmeter. The surface potential grew increasingly negative with increasing fluorine uptake, reflecting the charge asymmetry that is induced in the alkanethiolate chains as a result of the substitution of fluorine for hydrogen. The V s of the most highly fluorinated SAMs displayed a negative temperature dependence. This observation may be indicative of a pyroelectric effect in these monolayers although a definitive conclusion awaits further measurements.

Mechanisms of clinical resistance to 5-fluorouracil chemotherapy.

In the late 1950s, the era of fluoropyrimidine antimetabolite chemotherapy was ushered in with the synthesis of 5-fluorouracil (5-FU) by Heidelberger and colleagues [1]. The rationale for the development of this class of compounds arose from studies that showed preferential utilization of the nucleobase uracil for nucleic acid biosynthesis by rat hepatoma cells when compared with normal rat intestinal mucosa [2]. In view of this finding and the fact that profound biological effects had been observed upon substitution of fluorine for hydrogen in several classes of compounds, it was postulated that fluorine-substituted pyrimidine analogs might display selective antitumor activity.KeywordsClin OncolReduce Folate CarrierThymidylate SynthetaseHuman Colon Tumor Cell LineCysteine Sulfhydryl GroupThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Improved Synthesis of Zebularine [1-(β-D-Ribofuranosyl)-Dihydropyrimidin-2-One] Nucleotides as Inhibitors of Human Deoxycytidylate Deaminase

The 2'-deoxy (2a) and 2'-ara-fluoro (3a) derivatives of zebularine [1-(beta-D-ribofuranosyl)-dihydropyrimidin-2-one, 1a] were phosphorylated in high yield to the 5'-nucleotides 2b and 3b, respectively, and characterized by HPLC, enzyme degradation, 1H, 13C and 31P NMR, and high resolution mass spectral analysis. Their inhibitory activity against partially purified MOLT-4 deoxycytidylate deaminase (dCMPD) in the presence of the allosteric effector deoxycytidine triphosphate (dCTP) and Mg+2 ion was examined. Compounds 2b and 3b inhibited dCMPD with Ki values of 2.1 x 10(-8) M and 1.2 x 10(-8) M, respectively. The parent nucleotide, zebularine monophosphate 1b was ineffective at concentrations > 100 mumol. The effect of the nucleosides, 1a-3a, as well as tetrahydrouridine (THU) and 2'-deoxy THU (dTHU), on the cellular production of DNA precursors was examined in human MOLT-4 peripheral lymphoblasts. It was shown that 1a, 2a and 3a all elevated intracellular dCTP and TTP levels in whole cells with the most powerful effect elicited by 1a. The 2'-fluoro derivative 3a was chemically phosphorylated much more cleanly and higher yield than 2a, without the formation of diphosphorylated by-products. This compound was found to be infinitely less sensitive to acid-catalyzed degradation than 2a. Since the substitution of fluorine for hydrogen had a slight potentiating effect on the dCMPD inhibitory activity while stabilizing the compound toward acid-catalyzed and enzymatic depyrimidination, compound 3b emerges as a very attractive tool for the pharmacological modulation of pyrimidine deaminase activity.

Polyfluorinated arenium and arylmethyl cations: structure and synthetic application

A variety of methods is applied to generate long-lived polyfluorinated arenium and arylmethyl cations on the basis of the reactions of CF bond heterolysis by Lewis acid and the electrophilic attack of polyfluoroarene substrates in superacidic media. Using the 19F NMR spectroscopy as a method of electronic structure investigation and the competitive generation method to determine carbocation relative stability, the regularities have been revealed that characterize the influence of the substitution of fluorine for hydrogen on the positive charge distribution and the correlation between the electronic and stabilizing substituent effects. Synthetic approaches have been developed based on the application of polyfluorinated arenium and arylmethyl cations as electrophiles towards arenes and isomerizational transformations. The dependence of the character of these reactions in the case of polyfluorinated arenium ions on the nature of substituents located at sp 3-hybridized carbon is considered.

Changes in retention and polarity accompanying the replacement of hydrogen by fluorine in tetraalkylammonium alkyl- and arylsulfonate salts used as stationary phases in gas chromatography

Fourteen tetraalkylammonium alkanesulfonate, perfluoroalkanesulfonate, and benzene- and pentafluorobenzenesulfonate salts possessing useful chromatographic properties were synthesized. All salts had wide liquid temperature ranges with an upper column operating temperature limit in the range 160–220°C and yielded efficient column packings. The substitution of fluorine for hydrogen in the alkanesulfonate chain resulted in a general reduction in solute non-specific and proton donor interactions accompanied by an increase in solute electron donor interactions. These findings are rationalized in terms of the weak dispersive interactions for solutes with the perfluoroalkane side chains and the ability of fluorine to effect changes in the electron density of the sulfonate group through its inductive effect. On over half the salts studied retention is shown to depend on both gas—liquid partitioning and interfacial adsorption for a wide range of solutes but the relative importance and magnitude of the adsorption contribution to retention is not simply related to the structure of the salts or the incorporation of fluorine into the anion.

Comparison of the structural, electrical, and optical properties of amorphous silicon-germanium alloys produced from hydrides and fluorides.

Amorphous silicon-germanium alloys have been prepared in the same rf glow-discharge reactor from (${\mathrm{SiH}}_{4}$+${\mathrm{GeH}}_{4}$) and (${\mathrm{SiF}}_{4}$+${\mathrm{GeF}}_{4}$+${\mathrm{H}}_{2}$) mixtures at substrate temperatures between 200 and 400 \ifmmode^\circ\else\textdegree\fi{}C. The principal aim of the investigation has been to discover whether preparation of these alloys from fluorides rather than from hydrides will result in better photoelectronic properties, and if so, whether the underlying cause is the substitution of fluorine for hydrogen in the alloy, or some other structure-related alteration of the material. Thus the apparatus, preparation procedures, characterizational techniques, and property measurements follow those of an earlier publication on alloys produced from hydrides alone. The only significant difference in photoelectronic properties found has been an order-of-magnitude improvement of the photoconductivity of alloys with band gaps near 1.5 eV. Evidence is assembled to assert that fluorine substitution for hydrogen in the alloy is not the cause of the changes in photoelectronic properties, but that these are more probably related to changes in a two-phase heterostructure, which are revealed most directly by transmission electron microscopy. A two-phase, two-transport-path model is proposed to explain the improved photoconductivity obtained with the fluoride-derived alloys.

Comparative metabolism of 7,12-dimethylbenz[ a]anthracene and its non-carcinogenic 2-fluoro analogue by Syrian hamster embryo cells

The metabolism of 7,12-dimethylbenz[ a]anthracene (DMBA) and its non-carcinogenic 2-fluoro analogue (2F-DMBA) by Syrian hamster embyro (SHE) cells has been studied using high pressure liquid chromatography (HPLC) and fluorescence spectroscopy. Metabolites produced by SHE cells were compared chromatographically to those produced on a larger scale by liver microsomal preparations and previously identified by gas chromatography and mass spectrometry. At least 2 (possibly 3) phenol metabolites, none of which appear to be in the A-ring, were formed from [ 3H] 2F-DMBA and totalled only 3% of the organic extractable activity present in the media at 24 h. On the other hand, 3 A-ring phenols (DMBA-2-ol, DMBA-3-ol and DMBA-4-ol) comprising almost 12% of the total organic extractable radioactivity at 24 h were identified as metabolites in SHE cell culture media. For both hydrocarbons the major organic extractable metabolite present at 24 h was the respective 8,9-dihydro-dihydroxydiol (DMBA 45%, 2F-DMBA 39%). Thus, substitution of fluorine for hydrogen at the 2-carbon of DMBA appears to block or greatly reduce the A-ring metabolism of this compound but has relatively little effect on D-ring oxidation. Therefore loss of the carcinogenic/mutagenic activity of DMBA correlates with the extent of A-ring metabolism including, possibly, the bay region diol epoxide.

Structures of polyfluoroaromatic compounds. Part IX [1]. Crystal structure of decafluorofluoranthene

Crystals of decafluorofluoranthene, C 16F 1 0, are monoclinic, space group C2/c with unit cell dimensions a = 18.92(1), b = 4.84(1), c = 29.01(2) Å, β = 105.34(5)°. The structure was refined to a conventional discrepancy factor R of 4.7% and weighted discrepancy factor R w of 4.9% for 1841 observed counter amplitudes. Estimated standard deviations average 0.004 Å; for bond lengths and 0.3° for bond angles. The molecule is essentially planar. Detailed comparison with the parent hydrocarbon indicates that substitution of fluorine for hydrogen has brought about only quite small changes in molecular geometry.