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¹⁹F NMR fingerprints: identification of neutral organic compounds in a molecular container.

Zhao Y, Markopoulos G, Swager TM - J. Am. Chem. Soc. (2014)

Bottom Line: We report a new approach to effectively "fingerprint" neutral organic molecules by using (19)F NMR and molecular containers.Spatial proximity of the analyte to the (19)F is important to induce the most pronounced NMR shifts and is crucial in the differentiation of analytes with similar structures.This new scheme allows for the precise and simultaneous identification of multiple analytes in a complex mixture.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemistry, Massachusetts Institute of Technology , Cambridge, Massachusetts 02139, United States.

ABSTRACT
Improved methods for quickly identifying neutral organic compounds and differentiation of analytes with similar chemical structures are widely needed. We report a new approach to effectively "fingerprint" neutral organic molecules by using (19)F NMR and molecular containers. The encapsulation of analytes induces characteristic up- or downfield shifts of (19)F resonances that can be used as multidimensional parameters to fingerprint each analyte. The strategy can be achieved either with an array of fluorinated receptors or by incorporating multiple nonequivalent fluorine atoms in a single receptor. Spatial proximity of the analyte to the (19)F is important to induce the most pronounced NMR shifts and is crucial in the differentiation of analytes with similar structures. This new scheme allows for the precise and simultaneous identification of multiple analytes in a complex mixture.

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19F NMR spectra (64 scans) of complex 1 alone and mixturesof complex 1 (1.0 mM in CDCl3) and differentanalytes (2.0 mM): (a) complex 1 alone, (b) nine nitrilesadded to a solution of 1 inCDCl3, (c) superimposition of the spectra of complex 1 with each of the nine nitriles from (b) collected independently,(d)–(o) complex 1 bound to various nitriles.
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fig1: 19F NMR spectra (64 scans) of complex 1 alone and mixturesof complex 1 (1.0 mM in CDCl3) and differentanalytes (2.0 mM): (a) complex 1 alone, (b) nine nitrilesadded to a solution of 1 inCDCl3, (c) superimposition of the spectra of complex 1 with each of the nine nitriles from (b) collected independently,(d)–(o) complex 1 bound to various nitriles.

Mentions: To evaluatethe fidelity of this strategy in the precise identification of structurallysimilar molecules, we selected a series of nitriles with an interestin differentiating pesticides and pharmaceuticals.16 The Lewis basic nitrile can be encapsulated in the molecularcontainers 1–5 and 5a through the formation of a coordination bond with the tungsten atom.Sensing experiments are performed by adding analytes to chloroformsolutions of 1 at ambient temperature. The formationof a static complex with 1 is critical to create a clearshift rather than a dynamic structure that will produce shifts thatare more akin to a solvent effect.15 Inthis way, the fluorine atoms provide discrete signals at precise shiftsthat are uniquely assignable to the encapsulated analytes. Notably,the −OCF3 group in tungsten complex 1 appears as a singlet at −56.63 ppm (Figure 1a), which is very close to the shift found with parent calix[4]arene 7 (−56.51 ppm), indicating the remote through-bondeffects are not efficient to induce a 19F NMR shift. Incontrast, the binding of nitriles to 1 produces 0.2–0.9ppm downfield shifts in 19F NMR as a result of the disturbanceof the environment through replacement of solvent molecules by theanalyte. Consistent with this model, acetonitrile induces a much smallershift than less electron-donating 3-bromopropionitrile. All of ourresults are consistent with the differences in 19F NMRof free and bound complex 1 being caused by spatial proximityrather than through-bond electron transmission (Figure 1d,g). The precision in the identification of molecules isillustrated by comparison of the differences induced by the bindingof acetonitrile, propionitrile, and nonanenitrile with 1. In this experiment, nonanenitrile induces a more pronounced downfieldshift than propionitrile and acetonitrile (Figure 1d–f). The power of this method was further evaluatedby the analysis of a mixture with a number of potential guest molecules.In this experiment, a mixture of nine different nitriles and 1 gave the same spectrum as obtained by superimposing thespectra recorded with each analyte independently (Figure 1b,c). It is notable that the precise identificationof the multiple neutral organic analytes in a mixture represents apowerful advance in chemical sensing.


¹⁹F NMR fingerprints: identification of neutral organic compounds in a molecular container.

Zhao Y, Markopoulos G, Swager TM - J. Am. Chem. Soc. (2014)

19F NMR spectra (64 scans) of complex 1 alone and mixturesof complex 1 (1.0 mM in CDCl3) and differentanalytes (2.0 mM): (a) complex 1 alone, (b) nine nitrilesadded to a solution of 1 inCDCl3, (c) superimposition of the spectra of complex 1 with each of the nine nitriles from (b) collected independently,(d)–(o) complex 1 bound to various nitriles.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4120996&req=5

fig1: 19F NMR spectra (64 scans) of complex 1 alone and mixturesof complex 1 (1.0 mM in CDCl3) and differentanalytes (2.0 mM): (a) complex 1 alone, (b) nine nitrilesadded to a solution of 1 inCDCl3, (c) superimposition of the spectra of complex 1 with each of the nine nitriles from (b) collected independently,(d)–(o) complex 1 bound to various nitriles.
Mentions: To evaluatethe fidelity of this strategy in the precise identification of structurallysimilar molecules, we selected a series of nitriles with an interestin differentiating pesticides and pharmaceuticals.16 The Lewis basic nitrile can be encapsulated in the molecularcontainers 1–5 and 5a through the formation of a coordination bond with the tungsten atom.Sensing experiments are performed by adding analytes to chloroformsolutions of 1 at ambient temperature. The formationof a static complex with 1 is critical to create a clearshift rather than a dynamic structure that will produce shifts thatare more akin to a solvent effect.15 Inthis way, the fluorine atoms provide discrete signals at precise shiftsthat are uniquely assignable to the encapsulated analytes. Notably,the −OCF3 group in tungsten complex 1 appears as a singlet at −56.63 ppm (Figure 1a), which is very close to the shift found with parent calix[4]arene 7 (−56.51 ppm), indicating the remote through-bondeffects are not efficient to induce a 19F NMR shift. Incontrast, the binding of nitriles to 1 produces 0.2–0.9ppm downfield shifts in 19F NMR as a result of the disturbanceof the environment through replacement of solvent molecules by theanalyte. Consistent with this model, acetonitrile induces a much smallershift than less electron-donating 3-bromopropionitrile. All of ourresults are consistent with the differences in 19F NMRof free and bound complex 1 being caused by spatial proximityrather than through-bond electron transmission (Figure 1d,g). The precision in the identification of molecules isillustrated by comparison of the differences induced by the bindingof acetonitrile, propionitrile, and nonanenitrile with 1. In this experiment, nonanenitrile induces a more pronounced downfieldshift than propionitrile and acetonitrile (Figure 1d–f). The power of this method was further evaluatedby the analysis of a mixture with a number of potential guest molecules.In this experiment, a mixture of nine different nitriles and 1 gave the same spectrum as obtained by superimposing thespectra recorded with each analyte independently (Figure 1b,c). It is notable that the precise identificationof the multiple neutral organic analytes in a mixture represents apowerful advance in chemical sensing.

Bottom Line: We report a new approach to effectively "fingerprint" neutral organic molecules by using (19)F NMR and molecular containers.Spatial proximity of the analyte to the (19)F is important to induce the most pronounced NMR shifts and is crucial in the differentiation of analytes with similar structures.This new scheme allows for the precise and simultaneous identification of multiple analytes in a complex mixture.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemistry, Massachusetts Institute of Technology , Cambridge, Massachusetts 02139, United States.

ABSTRACT
Improved methods for quickly identifying neutral organic compounds and differentiation of analytes with similar chemical structures are widely needed. We report a new approach to effectively "fingerprint" neutral organic molecules by using (19)F NMR and molecular containers. The encapsulation of analytes induces characteristic up- or downfield shifts of (19)F resonances that can be used as multidimensional parameters to fingerprint each analyte. The strategy can be achieved either with an array of fluorinated receptors or by incorporating multiple nonequivalent fluorine atoms in a single receptor. Spatial proximity of the analyte to the (19)F is important to induce the most pronounced NMR shifts and is crucial in the differentiation of analytes with similar structures. This new scheme allows for the precise and simultaneous identification of multiple analytes in a complex mixture.

Show MeSH