Fluorine electrostatic potential map

Figure 3.1 Electrostatic potential maps for BF3, NH3, and the product that results from reaction between them. Attraction between the strongly positive region of BF3 and the negative region of NH3 causes them to react. The electrostatic potential map for the product for the product shows that the fluorine atoms draw in the electron density of the formal negative charge, and the nitrogen atom, with its hydrogens, carries the formal positive charge.

The electronegativity of the three fluorine atoms causes the trifluoromethyl group to be electron-withdrawing and deactivating toward electrophilic substitution. The electrostatic potential map shows that the aromatic ring of (trifluoromethyl)benzene is more electron-poor, and thus less reactive, than the ring of toluene shown in Figure 16.12. 

Scheme 2.161 Despite the resonance stabilization of singlet difluorocarbene by T-donation (+R) (box) from the a-fluorine atom, the carbon atom still has quite a large positive natural charge qc of -1-0.84 e = -0.42 e), rendering the species moderately electrophilic (geometry optimization at the MP2/6-311 - -C level of theory, electrostatic potential mapped on electron isodensity surface blue and red denote positive and negative partial charges, respectively) [33, 34). Difluorocarbene reacts readily with electron-rich olefins to yield gem-difluorocyclopropanes.

An electrostatic potential map of HR Red indicates the most electron-rich area (the fluorine atom) and blue indicates the most electron-poor region (the hydrogen atom). 

The covalent bond in H2 joins two hydrogen atoms. Because the bonded atoms are identical, so are their electronegativities. There is no polarization of the electron distribution, the H—H bond is nonpolar, and a neutral yellow-green color dominates the electrostatic potential map. Likewise, the F—F bond in F2 is nonpolar and its electrostatic potential map resembles that of H2. The covalent bond in HF, on the other hand, unites two atoms of different electronegativity, and the electron distribution is very polarized. Blue is the dominant color near the positively polarized hydrogen, and red the dominant color near the negatively polarized fluorine. 

Electrostatic potential maps of benzene and fluorobenzene. The high electronegativity of fluorine causes the TT electrons of fluorobenzene to be more strongly held than those of benzene. This difference is reflected in the more pronounced red color associated with the TT electrons of benzene. The color scale is the same for both models. 

The following illustration shows electrostatic potential maps for the preceding second example, the reaction of NH3 with BF3. The electron-rich (red) region of NH3 attacks the electron-poor (blue) region of BF3. The product shows high electron density on the boron atom and its three fluorine atoms and low electron density on nitrogen and its three hydrogen atoms. 

Figure 3.5 shows the dipole moment for ethyl fluoride (fluoroethane). The distribution of negative charge around the electronegative fluorine is plainly evident in the calculated electrostatic potential map. 

Fluorine, chlorine, and bromine are all more electronegative than carbon (Table 1.5) as a result, C—bonds with these atoms are polarized with a partial negative charge on halogen and a partial positive charge on carbon. Table 8.1 shows that each of the halomethanes has a substantial dipole moment. The electrostatic potential map of fluo-romethane shows the large charge separation in this compound caused by the dipole. 

Structures and electrostatic potential maps of the interhalogen compounds of iodine and fluorine... 

Figure 1.9 Resonance stabilization of the carbon—fluorine bond in tetrafluoromethane, and electrostatic and steric shielding against nucleophilic attack on the central carbon atom. The electrostatic potentials are mapped on the electron isodensity surface (calculation at the MP2/6-31+G level of theory [7, 14] red denotes negative, blue positive partial charges).

Table 4.14 Structure (top) and physical properties [41] table below) of semi-fluorinated n-alkanes (60-63) and the homologous dialkyl bicyclohexyl liquid c stals 64 and 65 from which they are structurally derived. The spacefill model of 63 shows the helical conformation of the central perfluoroalkylene segment in contrast with the pentyl side-chains with their typical hydrocarbon zigzag conformation. The differences in charge distribution (red and blue denote negative and positive partial charges, respectively) are visualized by mapping of the electrostatic potential on to the electron density of 63 (B3LYP/6-31C //PM3 level of theory) [44, 50].

Halogen atom size increases as we go down the periodic table fluorine atoms are the smallest and iodine atoms the largest. Consequently, the carbon-halogen bond length increases and carbon-halogen bond strength decreases as we go down the periodic table (Table 6.1). Maps of electrostatic potential (see Table 6.1) at the van der Waals surface for the four methyl... 

MO Calculations and Photoelectron Spectroscopy. Some all-valence-electron CNDO/2 SCF-MO calculations on fluorobenzene, hexafluorobenzene, pentafluoro-anisole, and some derived Wheland intermediates have been reported in a paper which is mainly concerned with derivatives of pyridine and the diazines (see p. 467). An MO-LCAO-SCF study of the electronic structure of fluorobenzene has yielded the electrostatic molecular potential and isopotential maps which are consistent with a poru-directing influence of fluorine in electrophilic substitution, ... 

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