Fluorine-19 ($\mathbf{^{19}\text{F}}$) NMR Spectroscopy
Fluorine-19 ($\mathbf{^{19}\text{F}}$) NMR is a powerful analytical technique, second only to $^1\text{H}$ and $^{13}\text{C}$ NMR in its importance for studying fluorinated compounds. Its properties make it highly sensitive and informative.
1. Fundamental Properties
- Natural Abundance: $100\%$. (Unlike $^{13}\text{C}$, every fluorine atom contributes to the signal.)
- Nuclear Spin: $I = \frac{1}{2}$. (This is the same as $^1\text{H}$ and $^{13}\text{C}$, leading to sharp, easy-to-interpret signals.)
- Sensitivity: Very High. Its sensitivity is about $83\%$ that of the proton, making it one of the most sensitive NMR-active nuclei.
- Reference Standard: The common external standard is pure $\mathbf{\text{CFCl}_3}$ (Trichlorofluoromethane), which is assigned a chemical shift ($\delta$) of $\mathbf{0 \text{ ppm}}$.
2. Chemical Shift Range ($\delta$)
The total chemical shift range for $^{19}\text{F}$ is extremely large—typically $\mathbf{+200 \text{ ppm} \text{ to } -300 \text{ ppm}}$ (a span of $\sim 500 \text{ ppm}$). This wide range is beneficial as it minimizes peak overlap, leading to excellent signal resolution.
Key Observation: Unlike $^1\text{H}$ and $^{13}\text{C}$ NMR, the $\delta=0 \text{ ppm}$ standard ($\text{CFCl}_3$) is at the deshielded (downfield) end of the spectrum. Most fluorine signals are upfield (negative $\delta$ values).
General Trends (Relative to $\text{CFCl}_3$ at 0 ppm):
- Highly Shielded (More Negative $\delta$): $\text{F}$ atoms in highly ionic environments or attached to electropositive elements. (e.g., $\text{F}^{-}$ or aliphatic $\text{R-CF}_3$)
- Highly Deshielded (Less Negative/Positive $\delta$): $\text{F}$ atoms involved in double bonds, aromatic rings, or attached to elements/groups that allow for strong $p\pi-p\pi$ bonding. (e.g., $\text{COF}_2$).
3. Spectral Interpretation (The Same Rules Apply)
The interpretation of a $^{19}\text{F}$ NMR spectrum follows the same basic principles as $^1\text{H}$ NMR:
- Number of Signals: Equals the number of chemically non-equivalent fluorine atoms in the molecule (determined by symmetry).
- Chemical Shift ($\delta$): Indicates the chemical environment of the fluorine atom (aliphatic, aromatic, $\text{S-F}$, $\text{P-F}$, etc.).
- Signal Multiplicity (Splitting): Determined by the spin-spin coupling constants ($J$) to neighboring $\mathbf{^{19}\text{F}}$, $\mathbf{^1\text{H}}$, or other $\text{NMR}$-active nuclei ($\text{P}$, $\text{Si}$, etc.). The $n+1$ rule applies.
- Coupling Constants ($J$): $\mathbf{^{19}\text{F}\text{-}^{19}\text{F}}$ coupling is very strong, often occurring over many bonds (long-range coupling). $\mathbf{^{19}\text{F}\text{-}^1\text{H}}$ coupling is also common.
Example: Perfluoro-1-Butene ($\mathbf{\text{CF}_3-\text{CF}_2-\text{CF}=\text{CF}_2}$)
This molecule has 8 fluorine atoms, but its spectrum will only show $\mathbf{5}$ signals because of chemical equivalence. The $^1\text{H}$-decoupled $^{19}\text{F}$ spectrum reveals the following:
Due to the double bond, the two $\text{CF}_2$ fluorines at the end of the chain ($\text{F}_{\text{d}}$ and $\text{F}_{\text{e}}$) are non-equivalent (cis vs. trans relative to the central $\text{CF}$ group).
| Set | Location | $\delta$ Range (ppm) | Splitting (Multiplicity) |
|---|---|---|---|
| $\mathbf{\text{F}_{\text{a}}}$ | $\text{CF}_3$ (Shielded end) | $\sim -60 \text{ to } -80$ | Complex Multiplet ($J$ to $\text{F}_{\text{b}}$, $\text{F}_{\text{c}}$) |
| $\mathbf{\text{F}_{\text{b}}}$ | $\text{CF}_2$ (Aliphatic) | $\sim -100 \text{ to } -120$ | Complex Multiplet ($J$ to $\text{F}_{\text{a}}$, $\text{F}_{\text{c}}$, $\text{F}_{\text{d}}$, $\text{F}_{\text{e}}$) |
| $\mathbf{\text{F}_{\text{c}}}$ | $\text{CF}$ (Vinylic) | $\sim -130 \text{ to } -150$ | Complex Multiplet ($J$ to all other $\text{F}$'s) |
| $\mathbf{\text{F}_{\text{d}}}$ | $\text{CF}_2$ (Vinylic, cis to $\text{CF}$) | $\sim -10 \text{ to } -30$ | Multiplet (Distinct from $\text{F}_{\text{e}}$) |
| $\mathbf{\text{F}_{\text{e}}}$ | $\text{CF}_2$ (Vinylic, trans to $\text{CF}$) | $\sim -10 \text{ to } -30$ | Multiplet (Distinct from $\text{F}_{\text{d}}$) |
The spectrum shows 5 distinct signals because the two vinylic $\text{F}$ atoms ($\text{F}_{\text{d}}$ and $\text{F}_{\text{e}}$) and the two aliphatic $\text{F}$ atoms ($\text{F}_{\text{b}}$) are non-equivalent due to hindered rotation and the asymmetric environment created by the $\text{CF}$ group.
Total Signals: 5
Q. Of the following, the compounds that show two signals in $\mathbf{^{19}\text{F}}$ NMR spectra are: (i) $\text{SF}_6$ (ii) $\text{SF}_4$ (iii) $\text{ClF}_5$ (iv) $\text{XeOF}_4$
(A) (i) and (ii)
(B) (i) and (iii)
(C) (ii) and (iii)
(D) (iii) and (iv)
(i) $\text{SF}_6$ (Sulfur Hexafluoride)
Geometry: Perfect Octahedral ($O_h$ symmetry).
Fluorine Equivalence: All six $\text{F}$ atoms are in identical chemical environments (axial and equatorial positions are equivalent).
Number of $\text{NMR}$ Signals: 1 signal
(ii) $\text{SF}_4$ (Sulfur Tetrafluoride)
Geometry: Seesaw (derived from trigonal bipyramidal, $C_{2v}$ symmetry).
Fluorine Equivalence: The four $\text{F}$ atoms are divided into two non-equivalent sets:
a. 2 Axial $\text{F}$ atoms ($\text{F}_{\text{ax}}$).
b. 2 Equatorial $\text{F}$ atoms ($\text{F}_{\text{eq}}$).
Number of $\text{NMR}$ Signals: 2 signals
(iii) $\text{ClF}_5$ (Chlorine Pentafluoride)
Geometry: Square Pyramidal ($C_{4v}$ symmetry).
Fluorine Equivalence: The five $\text{F}$ atoms are divided into two non-equivalent sets:
a. 4 Basal $\text{F}$ atoms ($\text{F}_{\text{basal}}$).
b. 1 Axial $\text{F}$ atom ($\text{F}_{\text{ax}}$).
Number of $\text{NMR}$ Signals: 2 signals
(iv) $\text{XeOF}_4$ (Xenon Oxytetrafluoride)
Geometry: Square Pyramidal ($C_{4v}$ symmetry).
Fluorine Equivalence: The $\text{O}$ atom is axial, and the four $\text{F}$ atoms occupy the square base. In this stable structure, all four $\text{F}$ atoms are equivalent.
Number of $\text{NMR}$ Signals: 1 signal
Conclusion: The compounds showing two signals are (ii) $\text{SF}_4$ and (iii) $\text{ClF}_5$. This matches Option (4).
Q. Consider a 0.3 M solution of cis-OsO2F4 in neat SbF5. The 19F NMR spectrum of the Os containing species in this solution shows a doublet and a triplet at 122.4 ppm and 129.5 ppm respectively. The Os species generated is
Answer: A
$$\text{cis-}\text{OsO}_{2}\text{F}_{4} + \text{SbF}_{5} \rightarrow [\text{OsO}_{2}\text{F}_{3}]^{+} + [\text{SbF}_{6}]^{-}$$- Doublet: A doublet in the 19F NMR spectrum indicates that the fluorine atom is coupled with one neighboring fluorine atom indicates the presence of one set of equivalent fluorines.
- Triplet: A triplet indicates that the fluorine atom is coupled with two equivalent fluorine atoms, suggesting another set of equivalent fluorines.
In option A, Two equivalent fluorine gives a doublet and one axial fluorine gives a triplet.