Electronegative Fluorine Research Articles

A high stable long life-span reflective GaAs(Cs, F) photocathode

A stable surface layer is essential for a high stable long life-span photocathode. By using fluorine instead of oxygen in activating the surface of GaAs substrate, the strong electronegativity of fluorine may improve the stability of photocathode. Life-span of reflective GaAs(Cs, F) photocathode has been made up to 8, 500 hours. Furthermore, Its technological reproducibility and anti-contamination ability are better than those of GaAs(Cs, O2) photocathodes.

Pyridoxal phosphate site in glycogen phosphorylase b: structure in native enzyme and in three derivatives with modified cofactors.

The detailed environment of the essential cofactor pyridoxal 5'-phosphate in glycogen phosphorylase b, resulting from crystallographic refinement at 1.9-A resolution, is described. The pyridoxal ring is buried in a nonpolar site containing three aromatic rings while the 5'-phosphate group is highly solvated and makes only three direct contacts to the protein. The pyridine nitrogen interacts via a water with protein atoms [main chain carbonyl oxygen (Asn-133) and OH of tyrosine (Tyr-90)]. The crystal structures of three active derivatives of phosphorylase reconstituted with 5'-deoxypyridoxal 5'-methylenephosphonate (PDMP), 6-fluoropyridoxal 5'-phosphate (6-FPLP), and pyridoxal (PL) in place of the natural cofactor have been determined at 2.5-A resolution. The results for PDMP-phosphorylase show a closer proximity of the phosphonate group to the NZ atom of a lysine (Lys-574) than that observed in the native enzyme, consistent with 31P NMR studies that have shown a change in ionization state of the phosphonate group compared to the native cofactor phosphate. The replacement of the polar 5'-ester linkage by a CH2 group results in a small shift of a water and its hydrogen-bonded tyrosine (Tyr-648). In 6-FPLP-phosphorylase the fluorine is accommodated with no significant change in structure. It is suggested that substitution of the electronegative fluorine at the 6-position may result in lower activity of 6-FPLP-phosphorylase through a strengthening of hydrogen-bonded interactions to the pyridine nitrogen N1.(ABSTRACT TRUNCATED AT 250 WORDS)

Exciplexes in Perfluorohydrocarbon. Charge-Transfer Interaction between Exciplexes and Perfluorohexane

Abstract The rate constants for photochemical processes of pyrene in perfluorohexane (PFH) and hexane containing N,N-dimethylaniline (DMA) have been determined by analyzing the rise and decay curves of the pyrene–DMA exciplex to study the interaction between electronically polar exciplex and an extremely nonpolar solvent, PFH. In PFH, the dissociation of the exciplex to the original pyrene in the excited singlet state and DMA was decelerated and the radiationless deactivation of the exciplex to the ground states was accelerated. In the case of the naphthalene–triethylamine exciplex system, the radiationless deactivation of the exciplex was also much faster in PFH than in hexane. These results suggest charge-transfer interactions between the polar exciplexes and PFH. The negative charges of the exciplexes may transfer to the strongly electronegative fluorine atom(s) of PFH. The deceleration of the back reaction and the acceleration of the radiationless deactivation seem to lead the efficient fluorescence quenching of pyrene and naphthalene by amines in PFH.

Electrical Conduction and Chemical Structure of Insulating Polymers

Currents in various polymers were measured using both a corona electrode and an evaporated metal electrode. The former could usually inject more carriers into polymers than the latter. The current in PE was two or three orders of magnitude larger with the corona electrode than with the evaporated metal electrode and injected negative carriers made a greater contribution to the current than positive ones. On the other hand, injected positive carriers played an important role in the currents in PTFE, FEP, PFA, and ETFE. Similar polarity effects were observed in the surface potential decay characteristics for coronacharged polymers. The polarity of injected carriers which dominated the current changed from negative to positive as the degree of fluorination of a polymer increases. Electronegative fluorine atoms were considered to provide negative carrier traps and suppress the current due to negative carriers.

Infrared studies and thermodynamics of hydrogen bonding in 2-halogenoethanols and 3-halogenopropanols

From a non-linear least-squares curve analysis of the spectra of 2-halogenoethanols and 3-halogenopropanols in the hydroxy stretching region in dilute carbon tetrachloride solution at 298–338 K, the thermodynamics of the equilibrium between free and intramolecularly hydrogen-bonded species have been determined. The amount of intramolecularly hydrogen-bonded species decreased with increasing size of halogen and increasing steric repulsion. The similar frequency shifts of the intramolecularly hydrogen-bonded hydroxy group in both the 2-halogenoethanols (17–50 cm–1) and 3-halogenopropanols (15–52 cm–1) established the poor proton-acceptor properties of the halogens, including the highly electronegative fluorine. The absence of intramolecularly hydrogen-bonded species in 4-chlorobutanol is a case where the entropy change over-rides the enthalpy change due to hydrogen bonding. Hydroxy frequency shifts due to intramolecular hydrogen-bonding ran opposite to the enthalpy changes disobeying Badger–Bauer rule. These alcohols associate forming dimers (ca. 3500 cm–1) in which the hydroxy groups alone participate in intermolecular hydrogen bonding.

Influence of Chemical Structure on Electrical Conductionin Insulating Polymers

The electrical conductivities of polyethylene, polypropylene and related fluorinated polymers such as polytetrafluoroethylene and fluorinated ethylene propylene copolymer were measured using a corona electrode. The polarity of the dominant carriers making the greatest contribution to the current changed from negative to positive as the ratio of fluorine atoms to hydrogen atoms was increased. Electronegative fluorine atoms were concluded to provide deep trap sites for negative carriers and to suppress the current due to negative carriers. The results of surface potential decays and TSC's of corona-charged specimens also supported the above conclusion.

14N nuclear quadrupole double-resonance and the BN bond in amine-boranes and amine-trifluoroboranes

The nitrogen nuclear quadrupole double-resonance spectra of several amine-trifluoroboranes are reported and compared with the corresponding data of the free amines and amine-boranes. The observed increase in the nitrogen coupling constants with increasing substitution of the nitrogen-bonded hydrogens by organic groups in the amine-borane coordinative compounds is interpreted in analogy to the case of the free amines as being due to the higher electronegativity of carbon compared to hydrogen. Corresponding H 3B- and F 3B compounds have similar nitrogen coupling constants in spite of the high electronegativity of fluorine. This experimental finding is in accord with several quantum-chemical calculations of the literature which show that equal charge transfers from the amine to the borane take place on formation of the hydrogen and fluorine compounds from their two constituents. Yet, the higher electronegativity of fluorine results in a stronger polarization of the amine which leads to a uniform increase in the population of all p orbitals centered at the nitrogen at the expense of the N substituents, the nitrogen thus serving as an electron buffer. The experimental nitrogen coupling constants can only give an upper limit of 0.45 e for the charge transfer from N to B which is hardly affected by the N substituents. The figures obtained theoretically vary considerably depending on the method of calculation employed and range from 0.2 e for ab initio methods to 0.4 e for CNDO/2 calculations.

ChemInform Abstract: PHOTOEFFECTS ON THE FLUORINATION OF SILICON. I. INFLUENCE OF DOPING ON STEADY‐STATE PHENOMENA

AbstractBez. der Thematik Laser‐induzierter oder ‐verstärkter chemischer Wechselwirkungen zwischen Gasen und Oberflächen wurde der Einfluß von Licht (Ar ‐Laser) auf die Reaktion zwischen Si und XeF2 als Funktion von Druck, Lichtintensität und Dotierung untersucht.

Photoeffects on the fluorination of silicon. I. Influence of doping on steady-state phenomena

The effect of visible light on the reaction between silicon and XeF2 has been studied as a function of pressure, light intensity, and dopant. Steady-state etch rates are sensitive to all three variables. Two major light-induced processes have been identified. The reaction can be suppressed by diffusion of fluorine into unreactive sites and enhanced by an increased rate of addition of fluorine to silicon centers relative to the rate of desorption of the intermediate fluorosilyl species. Experiments indicate that these unreactive sites are located beneath the surface. The data have been interpreted in terms of a model in which adsorption of electronegative fluorine causes band bending in the semiconductor, the direction of which depends on whether the material is doped n or p type. Photogenerated charge carriers induce faster fluorine insertion rates into Si–Si bonds, and enhance diffusion below the surface. N- and p-type silicon differ mainly in their relative ability to stabilize subsurface fluorine. The data have important implications for the mechanism of the spontaneous reaction of fluorine with silicon. They may also be relevant to photo-oxidation of clean semiconductor surfaces and issues related to the stability of amorphous Si:H:F alloys.

93] Fluorine-labeled retinals and rhodopsins

Publisher Summary This chapter discusses the fluorine (F)-labeled retinals and rhodopsins. The use of F as a label to probe for structural information of complex biomolecules by nuclear magnetic resonance (nmr) spectroscopy is an established technique. Its advantages over other NMR active nuclei are several. Among them is the absence of background signals from proteins or detergents. The disadvantages associated with the method are the synthetic difficulties sometimes encountered in introducing the fluorine labels at strategic positions and the possibility that the highly electronegative fluorine atom might change the overall properties of the biomolecules of interest. The absorption maxima of isomers of several fluorinated rhodopsins are listed in the accompanying table along with those of the corresponding retinals. That the selectivity of fluororetinal isomers in their reaction with cattle opsin is virtually the same as the parent retinals suggests that the fluorine substituents do not exert a vastly different steric effect.

The Mass Spectral Studies of Halogenated Vinyl Methanesulfonates

AbstractThe mass spectra of saturated sulfonate esters1,2) have been reported recently. No study has been made on the vinylic methanesulfonates. In order to understand their breakdown patterns, several chloro‐ and fluoro‐containing vinyl methanesulfonates have been made3) and their mass spectra studied. The chlorine compounds are chosen because the natural abundance of isotope 35Cl and 37Cl can serve as a internal tag. The vinyl methanesulfonates containing electronegative fluorine are used as a comparison to the chloro‐compounds because of their similar electronegativities.

The CF 3O − anion: Vibrational spectrum of an unusual CF 3 compound

The i.r. and Raman spectra of solid CsOCF 3 and the Raman spectra of liquid and gaseous NF 3O have been recorded. The vibrational spectrum of CF 3O − closely resembles that of isoelectronic NF 3O. For CF 3O −, six fundamentals were observed and assigned consistent with a pseudo-tetrahedral structure of symmetry C 3 v . It is shown that the unusual bonding in NF 3O is not unique but also occurs in CF 3O −. In both isoelectronic XF 3O species, the XO bond possesses almost double bond character whereas the three XF bonds are highly polar implying very strong contributions from resonance structures such as ▪ The occurrence of this type of bonding can be rationalized by the high electronegativity of fluorine and by the importance of the octet rule for first row elements limiting the sum of the bond orders to a maximum of four.

The induction constant ? F * in fluoronitro-compounds

The empirical induction constant σ F * in fluoronitro-compounds is related to the induction constant of a constant portion of the molecule and the electronegativity of fluorine.

The Fluorination of Decachlorocyclopentaphosphonitrile with Potassium Fluorosulfite

Decachlorocyclopentaphosphonitrile N5P5Cl10 is fluorinated by potassium fluorosulfite to give a mixture of chloride–fluorides N5P5Cl10−xFx (x = 1–9 ), no compound containing more than one PClF group. As for the comparable reactions of N3P3Cl6 and N4P4Cl8, the dominance of the geminal substitution pattern is attributed to the electrostatic or σ-inductive effect of the electronegative fluorine atom. The relative rates of the first two fluorination steps, in comparison with similar information for N3P3Cl6 and N4P4Cl8, confirm the importance of π-inductive effects in determining the rates of the reactions.

The question of intramolecular hydrogen bonding in 2-flouroethanol

Several investigations have shown, by a variety of spectral techniques, that 2-fluoroethanol (1) exists both in the liquid and vapor state, predominantly (>95%) in the conformation. with hydroxyl and fluorine gauche to one another. Notable is the recent electron-diffraction study in which 1 was shown to be greater than 90% gauche in the vapor phase at 156°. In every instance, this marked conformational preference for the gauche form has been attributed to the formation of a strong intramolecular hydrogen bond between the electronegative fluorine atom and the hydroxyl proton which are in close proximity in this configuration. That this is the case is presumably also supported by the observation that the CCF bond angle of 107.7° is compressed relative to fluoroethane.

Lennard-Jones potential calculations of the barrier to rotation around the glycosidic CN linkage in selected purine nucleosides and nucleotides. A direct comparison of the results of 6–12 potential calculations with results of semiempirical molecular orbital studies

Lennard-Jones (6–12) potential energy calculations were performed on the barrier to rotation around the CN glycosidic linkage in purine nucleosides and nucleotides. While such calculations are strictly valid only for relatively non-polar substances the results of these simple and very inexpensive calculations appear to be in reasonable accord with the results of semi-empirical molecular orbital calculations. The effect of the assumed hydrogen van der Waals radius on rotational barrier was examined and use of the accepted value of 1·2 along with the newer 1·0 simultaneously is shown to be useful in determining the source of such barriers. The detailed curves indicated that those molecules found in the syn conformation in the solid state possessed relatively small barriers to rotation. Adenine nucleotides and nucleosides invariably were indicated to have very large barriers to rotation with notable exception of the 3′- O-acetyladenosine and 2′ and 3′-AMP. A systematic variation of the C(8) substituent from H to F to Cl to Br in guanosine and 8-bromoguanosine indicates that the preference of the syn conformation is primarily due to steric factors. However, a minor barrier which appears to be due to the fluorine electronegativity is also noticed when the C(8)F passes near the ribose O(1′)atom. Inclusion of electrostatic interaction (monopole-monopole) in the potential barrier calculation indicated a minor contribution from this term even in the molecules possessing considerable charge asymmetry.

Inductive effect of polar groups on CH and CF stretching vibrations of alkyl and perfluoroalkyl groups and its relation to adsorption

The trifluoromethyl-stretching vibrations for a series of perfluorocarboxylic acids and dihydroperfluoro alcohols were determined using an infrared spectrophotometer, and the results were compared with published information on the methyl-stretching vibrations of the corresponding hydrocarbon compounds. The frequency shifts of the CF 3-stretching vibrations are opposite in direction to those of the CH 3-stretching vibrations. This opposite shift may be attributed to the inductive effect of highly electronegative fluorine atoms. As a result, the alkyl groups are expected to become somewhat electropositive whereas the perfluoroalkyl groups electronegative. These induced polarities of the nonpolar groups were confirmed by the surface potential measurements at an air-aqueous solution interface and by the electrocapillary and differential capacity measurements at a mercury-aqueous solution interface.

United atom models for polyatomic ligands. Part II. Methyl and fluoro derivatives

The United Atom model is extended to the fluoro and methyl derivatives of Group V and VI central atoms. An additional second-order effect, the Anisotropic Potential Stabilization (APS) is identified associated with the orbitals of the highly electronegative fluorine substituent. The current spectroscopic assignments for these molecules are compared to those predicted by the correlation diagrams and in some cases alternative assignments are proposed. In all of the methyl derivatives the highest occupied orbital appears to be localized in the methyl groups. The Lewis σ basicity of the highest occupied and the π acidity of the first unoccupied orbitals are described using spectroscopic criteria of Part 1. Both π acidity and its extreme form, synergism are important if the frontier orbitals display molecular d or f quantum members and originate on the central atom.

Étude spectroscopique des complexes HCo(PF 3) 4 et KCo(PF 3) 4

Raman and IR spectra have been studied of the complexes HCo(PF 3) 4 and KCo(PF 3) 4, especially in the low frequencies region. The study of these spectra confirms the T d symmetry for KCo(PF 3) 4 and shows also that the symmetry of HCo(PF 3) 4 is C 3 v . The complex HCo(PF 3) 4 is probably the first transition metal hydride complex in which the E δ(MH) band has been unequivocally determined. The exceptional width of this deformation band in Raman and IR is explained by the presence of the neighbouring electronegative fluorine atoms. The difference of the CoP and PF vibration frequencies observed in the potassium salt when compared to the hydride, has been related to the difference in charge densities located on the cobalt atom in these two compounds.

The possibility of different electronegativities of fluorine

1. A scheme according to which the electronegativities of the halogens were calculated from the dipole moments of halomethanes was substantiated, and a low value of the electronegativity of fluorine, xF= 2.79, was obtained. In particular, the introduction of a high electronegativity of fluorine, xF=3.5, into this scheme does not lead to satisfactory results. 2. The eleetronegativity of fluorine, xF=2.92, was calculated according to data on the dipole moments of fluorosilanes. 3. In a calculation of the electronegativity of fluorine according to thermochemical data, an approximate consideration of the partial double-bonded character of the “single” bond lowers the value of xF to 2.65–2.9. 4. We must reckon not only with the possibility of different electronegativities of fluorine in principle, but also concretely with its low value xF ≈ 2.8.