Fluorine nonequivalence

With a steric number of 5, chlorine has trigonal bipyramidal electron group geomehy. This means the inner atom requires five directional orbitals, which are provided hymsp d hybrid set. Fluorine uses its valence 2 p orbitals to form bonds by overlapping with the hybrid orbitals on the chlorine atom. Remember that the trigonal bipyramid has nonequivalent axial and equatorial sites. As we describe in Chapter 9, lone pairs always occupy equatorial positions. See the orbital overlap view on the next page. 

There is a second, more complicated and for fluorine NMR spectra more common situation that will lead to second-order spectra, that in which chemically equivalent fluorines (same chemical shift) are magnetically nonequivalent. This occurs when the chemically equivalent fluorines do not have the same coupling constants to specific other nuclei in the molecule. 

Magnetic nonequivalence is not uncommon, often deriving from the constraints of a ring, as in pentafluorophenyl derivatives or other symmetrically fluorine substituted ring systems such as those shown in Scheme 2.10. The fluorine and proton NMR spectra of 1,2-difluoroben-zene are both representative of the appearance of second order spectra of polyfluoroaromatics. They can be found in Chapter 3, Section 3.9.3. 

Another common situation that can lead to second order spectra is an open chain system such as meso-l,2-difluoro-l,2-phenylethane whose magnetically nonequivalent spin system and resultant second order fluorine NMR spectrum (Fig. 2.7) can only be understood by examination of the contributing conformations about its fluorine bearing carbons.10... 

In spite of bearing nonequivalent fluorine substituents, both PF5 and all compounds of the type R-PF4 exhibit only a single signal in their fluorine NMR spectra. The observed magnetic equivalence of the fluorines in such compounds is believed to derive from a rapid intramolecular, pseudorotational exchange process that is too rapid, even at -80 °C, to allow distinction of the axial and equatorial fluorine atoms (Scheme 7.7). 

There are some unique structural aspects of some of the sulfur fluorides that will need to be discussed in order to understand the 19F NMR spectra. The geometry of tetracoordinate group VI compounds is predicted on the basis of Gillespie s electron-pair repulsion theory to be trigonal bipyramid, with an electron pair occupying one of the equatorial sites.2 Thus, the SF3 substituent as well as the molecule SF4 have structures as depicted in Scheme 7.12, with nonequivalent (axial and equatorial) fluorines, and thus their 19F NMR spectra consist of two 19F signals, with the fluorines being coupled if the system is scrupulously dry. 

Hexacoordinate, hypervalent sulfur fluorides have an octahedral geometry that is symmetrical for SF6, which appears as a sharp singlet at +56 ppm, but which has magnetically nonequivalent (axial and equatorial, ab4 system) fluorines for compounds of the structure R-SF5. Compounds with the general structure R-SF4-X can exist as cis- and trans-isomers, the former having three types of fluorine, and the latter only one (Scheme 7.17). 

This spectral nonequivalence for the diastereomeric solvates was originally rationalized in terms of conformers 28 (shown for TFPE as solute) where (/2)-NEA as its preferred rotamer (dictated by the peri interaction) interacts primarily with the carbinol hydroxyl and populates the conformers shown through an aryl-aryl attraction. Thus, in (5, )-28, the carbinyl proton is held more closely to the naphthyl ring, whereas in R,R)-28 its position is reversed. This situation results in the observed highfield sense of nonequivalence for the carbinyl proton of carbinols 27 and accounts for the opposite sense shown by the fluorine resonances. In one instance, aryl nonequivalence was also identified. AU three ring protons of trifluoromethyl-a-thienylcarbinol (13) show the same nonequivalence sense as the carbinyl proton (opposite to that of trifluoromethyl). Such also is expected from the proposed interactions 28, since the aryl substituent... 

Only fluorine and carbon nuclei have been studied in combinations for which solvation modes are understood. Fluorine appears to be the more well behaved. NEA-induced nonequivalence senses in fluorine nuclei of trifluoromethylarylcarbinols are correctly predicted by the usual solvation model (Sect. IV-C) (28). 

In 1965, the determination of the enantiomeric purity (ee) by NMR spectroscopy using a chiral solvating agent (CSA)69- 73 was first postulated17 and demonstrated experimentally by Pirkle in 1967. An example is the nonequivalence of the proton and fluorine resonances of racemic 2,2,2-trifluoro-l-phenylethanol in the presence of optically active 1-phenylethanamine78 or l-(l-naphthyl)ethanamineS3 (Figure 5). 

Superimposed on each of the SF4 resonances is a triplet fine structure of 1 2 1 intensity ratios (Fig. 4) which reflects spin-spin coupling between nonequivalent sets of fluorine atoms. This fine structure establishes the number of nuclei per environment as two by the following line of reasoning. 

Only when the rate of alcohol exchange is lowered by cooling is the nonequivalence of fluorine atoms evident in the NMR spectrum. These exchange effects are observed because at room temperature the chemical shift between fluorine atoms in the two environments is comparable to the exchange rate. Chemical shiftsf and coupling constants of nuclei in molecules are comparable to the rates of many chemical processes. Consequently this possible complication must always be kept in mind. Exchange phe-... 

All nuclear multiplet structures due to coupling of nonequivalent nuclei are, as noted earlier, subject to effects on line shapes by chemical or positional exchange. For those multiplet structures arising from coupling of nuclei, one of which has a nonzero nuclear quadrupole moment, effects of quadrupole relaxation must be considered. For example, if a proton or fluorine atom is bonded to a nitrogen nucleus (I = 1), a triplet resonance will be expected in the proton or fluorine spectrum. For observation of this fine structure it is necessary that the lifetimes of the nuclear spin states of nitrogen (m = 1, 0, —1) be greater than the inverse frequency separation between multiplet components, i.e., t > l/ANx (106). The lifetimes of N14 spin states can become comparable to or less than 1 /A as a result of quadrupole relaxation. When the N14 spin-state lifetimes are comparable... 

Examples of kinetic analysis of NMR spectra in the transition between slow and fast exchange (on the NMR time scale) are somewhat limited. Treatment of fluorine exchange in sulfur tetrafluoride is selected here because this exchange process exemplifies the type of kinetic process ideally suited to NMR study. The fluorine atoms of the two nonequivalent environments in this molecule of C2v symmetry give rise to two triplets under conditions of very slow exchange at temperatures below —85° (at 40 Mc/sec). 

There are two nonequivalent pairs of S-F bonds in the sulfur tetrafluoride molecule, the shorter equatorial S-F bonds (1.545 A) and the longer axial S-F bonds (1.646 A). The bond angles arc equally 101 33 and 186" 56 for equatorial and axial bonds, respectively.5 The shorter and the longer S-F bond energies are, respectively, —20.31 and — 13.74 eV. The electric charge distribution in the sulfur tetrafluoride molecule is as follows + 1.70 on the sulfur atom, — 0.35 on the equatorial fluorine atoms and —0.50 on the axial fluorine atoms.5... 

The relative fluorination rates for nonequivalent C = C bonds in substituted perfluorocyclo-hexa-1,4-dienes depend on the electronic properties of the substituents and indicate the electrophilic nature of vanadium(V) fluoride."4... 

The 29 Si NMR (Table 20) supports a hexacoordinate structure in solution which is similar to that in the solid state. (529Si for the hexacoordinate fluorosilicates is ca 50 ppm shifted upheld relative to the pentacoordinate precursors. The multiplicity of the 29 Si resonances indicates the nonequivalence of fluorine ligands at low temperatures (between 25 and — 60 °C). These become equivalent as the temperature is raised, such that a symmetrical 29Si multiplet is observed. 

This outcome derives from the fact that the carbon that bears the fluorine is chiral, which makes the two vicinal hydrogens diastereotopic and thus magnetically nonequivalent. In such a case, the two diastereotopic protons will not only appear as separate signals (an AB system), but they usually will also couple to vicinal fluorines (and hydrogens) with different coupling constants. Examining Figure 2.8b, which represents... 

On the other hand, compounds of the structure R-SF5 have nonequivalent (axial and equatorial, AB4 system) fluorines. The fluorine NMR spectrum of SF5C1 exemplifies this AB4 system (Figure 7.3), with a pen-tet representing the axial fluorine at +62.3 ppm and a doublet representing the four equatorial fluorines at 125.8 ppm (2/ff = 151 Hz). 

Note the resemblance of the fluorine atom absorption to the symmetry of the absorptions of the magnetic nonequivalent aromatic protons described in Section 3.9. The complexity in all of these spectra is not a result of an inadequate magnetic field. In fact, a more powerful magnetic field would result in greater complexity. This is quite different from the response of a first-order system, which may not be resolved in a weak magnetic field, but would show a first-order spectrum at a sufficiently high field. 

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