Spectroscopic separation of perfluorinated liquid mixtures using two-dimensional NMR

Abstract

One-dimensional nuclear magnetic resonance (NMR) Is a valuable tool for structural elucidation of perfluorinated materials. The value of ieF 1-D NMR diminishes, however, In multicomponent systems due to rapidly Increasing spectral complexity. The alternative of separatlon/purtflcatlon of these complex liquid mixtures can be prohibitively costly and time consuming. Physical-chemical property similarities potentially prohibit component purification using standard chromatographic techniques. The application of two-dimensional 19F NMR techniques to complex perfluorinated liquid mixtures has provided previously unobtainable Information about the number and structure of components. The Impact of homonuclear Hartmann Hahn (HOHAHA) and homonuclear relayed coherence transfer (COSYRCT) on component Identification In Isomer mixtures was explored. With component Identification and subsequent 1-D 19F spectral assignments completed, quantitation via routine methods could be performed. The ability to extract one-dlmenslonal 19F spectra of "pure" Individual components from both the 2-D HOHAHA and COSYRCT experimental spectra and their unique advantages, disadvantages, and limitations as tools for spectroscopic separation of complex liquid mixtures were demonstrated.

2D 19F/19F NOESY for the assignment of NMR spectra of fluorochemicals

“…The 2-CF 2 is distinguished from the 5-CF 2 since no NOESY correlations in the CF 2 region are observed for the equatorial 2-CF 2 whereas the equatorial fluorine in the 5-CF 2 shows correlations to both of the 4-CF 2 fluorines. The chemical shifts and assignments are given for the cis and trans isomers in Table 1 and agree with those given by McDaniel and coworkers [3], but also provide the axial/equatorial assignments not previously available. Further, the large 5 J FF and NOESY correlation between the CF and one of the fluorines in the 5-CF 2 indicates both of these coupled fluorines must be axial, proving that the CF 3 's must be equatorial in the more stable cis conformer.…”

“…Table 1 Chemical shifts (in ppm) and some coupling constants (in Hz) in 1,3-and 1,4-dimethylperfluoro cyclohexane Axial (ax) and equatorial (eq) refer to the major conformer in cis-1,3 or trans-1,4; there is no preferred conformation in the trans-1,3 or cis-1,4 isomers. a Chemical shifts reported in [3] on a neat sample. b The À120.44 resonance is cis to the CF 3 .…”

“…Experimental problems include insufficient power to generate uniform 908 and 1808 pulses over the entire chemical shift range and systematic errors that arise when coupling constants are not negligible compared to chemical shift differences. The 19 F/ 19 F TOCSY experiment has been applied to fluorochemicals [3], but it is difficult to create a spin-lock over the entire fluorine chemical shift range of interest. Carbon-fluorine heteronuclear correlation experiments (both 19 F and 13 C detected) have been published, but are of limited use in most fluorochemicals because of the limited chemical shift range of 13 C nuclei in fluorochemicals and the difficulty of decoupling the entire fluorine spectrum in order to observe singlets in the 13 C spectrum [4][5][6][7].…”

“…Carbon-fluorine heteronuclear correlation experiments (both 19 F and 13 C detected) have been published, but are of limited use in most fluorochemicals because of the limited chemical shift range of 13 C nuclei in fluorochemicals and the difficulty of decoupling the entire fluorine spectrum in order to observe singlets in the 13 C spectrum [4][5][6][7]. Vicinal 3 J FF is very dependent on electronegativity and ranges from þ3.5 Hz in CF 3 -CF 3 [8] to À24 Hz in CF 2 X-CFH 2 , for X ¼ Br and I [9]. Although 19 F/ 19 F COSY experiments are routinely used for assignments in perfluorochemicals, assignment are often not obvious because four-bond ( 4 J FF ) coupling constants, typically 10 Hz, are much larger than the near zero vicinal ( 3 J FF ) coupling constants in linear CF 2 CF 2 groups.…”

Carbon–fluorine two-dimensional NMR correlation spectra

“…The 19 F-NMR spectra have been published [10,11]. The trans isomers 1a consists of two equivalent conformers with one axial and one equatorial CF 3 group (ae and ea), whereas the cis isomer 1b consists of two in-equivalent conformers, diequatorial (ee) and diaxial (aa).…”