Fluorine-induced electronic effects

Fluorine-Induced Electronic Effects and Adrenergic Selectivities. It... 

By far, 2-fluoro-2-deoxyfuranoses have been the most studied compounds. Indeed, at a structural level they are the closest analogues of 2-deoxynucleosides. Due to its electronic effect, the fluorine atom in the 2 position inhibits development of a positive charge on the anomeric carbon (which is responsible for the hydrolytic cleavage of nucleosides). In order to enhance the stability of 2-deoxynucleosides in acidic medium, and thus make oral administration of an antiviral compound easier, introduction of a fluorine atom in the 2 position is a commonly used strategy. The resulting protective effect toward proteolysis has been well demonstrated, as exemplified by the fluorinated analogues of ddl and ddA (cf. Chapter 3, Figure 3.13). However, the presence of this fluorine atoms often induces modifications in the antiviral properties of the molecule. ... 

R. M. Lambert, R. L. Cropley, A. Husain, M. S. Tikhov, Halogen-induced selectivity in heterogeneous epoxidation is an electronic effect—fluorine, chlorine, bromine and iodine in the Ag-catalysed selective oxidation of ethene, Chem. Comm. (2003) 1184. 

Most of the effects induced by the presence of fluorine atoms in a molecule come from both the structure and the fundamental atomic properties of the fluorine atom (Table 1.1). Because of its electronic structure ls 2s 2p, fluorine has very specific properties, as indicated by the extreme values of the atomic parameters given in Table 1.1. ... 

Light-induced transformations over fluorinated titania (TiOi/F) cannot be initiated either by =Ti— 0 (OHads), due to the lack of =Ti—OH groups, or by SET from a surface complexed substrate, due to the fluoride competition. In addition to these major effects, the adsorption of molecular oxygen can be affected also and the surface charge is dramatically decreased. The last effect may be important particularly for charged substrates and intermediates and for the possibility of interfacial electron transfer. 

From Eq, (1) it is clear that a model of crystal polarization that is adequate for the description of the piezoelectric and pyroelectric properties of the P-phase of PVDF must include an accurate description of both the dipole moment of the repeat unit and the unit cell volume as functions of temperature and applied mechanical stress or strain. The dipole moment of the repeat unit includes contributions from the intrinsic polarity of chemical bonds (primarily carbon-fluorine) owing to differences in electron affinity, induced dipole moments owing to atomic and electronic polarizability, and attenuation owing to the thermal oscillations of the dipole. Previous modeling efforts have emphasized the importance of one more of these effects electronic polarizability based on continuum dielectric theory" or Lorentz field sums of dipole lattices" static, atomic level modeling of the intrinsic bond polarity" atomic level modeling of bond polarity and electronic and atomic polarizability in the absence of thermal motion. " The unit cell volume is responsive to the effects of temperature and stress and therefore requires a model based on an expression of the free energy of the crystal. 

The high electrophilicity of perfluoroolefins is the result of the profound electron-accepting effect of fluorine atoms and CF3 groups combined with the capability of vinyl fluorine atoms to be effectively conjugated with the C=C bond. The reactivity of perfluoroolefins does not depend so strongly on the statistical distribution of electron density (or on the induced positive charge on the C=C carbon atoms in perfluoroolefins) as it does on the dynamic properties of this distribution. 

Fluorine has large electron negativity which draws electrons from sulfur. As we have shown for the other sulfur fluorides, the effective charge for fluorine is -0.11 -0.15. The fluorine in S2F2 has a similar effective charge. However, for S2F2, electrons are drawn from both the central and the terminal sulfur atoms. Both the terminal sulfur atom and the central sulfur atom bear charge induced by... 

Analyzing the frequency factor v, Bursey43 was able to show that the details of the reaction mechanisms of the electron impact induced ketene elimination from ortho substituted phenyl acetates and from acetanilides, 36, are dramatically influenced by the nature of the substituent R = F, Cl, Br, J. The results demonstrate that the decrease of v going from the voluminous iodine to the small fluorine is not caused by steric effects (which should operate in an opposite direction) but is the result of an electronic interaction of both substituents, leading to a tighter transition state in the case of the more electronegative fluorine. Additional factors which are not yet completely understood play a decisive part in the fragmentation of the anilides. 

---