Anal. Chem. 1093, 65, 752-758
752
Spectroscopic Separation of Perfluorinated Liquid Mixtures Using Two-Dimensional NMR W. I. Bailey, A. L. Kotz, P. L. McDaniel,’ D. M. Parees, F. K. Schweighardt, and H. J. Yue* Air Products and Chemicals, Znc., 7201 Hamilton Boulevard, Allentown, Pennsylvania 18195-1501
C. Anklin Bruker Instruments, Znc., Manning Park, Billerica, Massachusetts 01821
One-dlmenrlonal nuclear magnetlc resonance (NMR) k a valuable tool for structural elucklatlon of perfluorhated materlak. The value of lgF1-D NMR dknlnlshes, however, In multlcomponent sydms due to rapldly Increarlng spectral complexlty. The alternative of separatlon/purlflcatlonof there complex llqukl mlxtures can be prohlbltlvely costly and thne consuming. Physkal-chemkalpmpetty dmllarltke potentially prohlblt component purllcatlon urlng standard chromatographic technlques. The appllcatlon of two-dhnemlonal lgF NMR technlques to complex perfluorlnatedllquld mlxtureshas provkledprevloustyunobtalnabk lnformatbnabout the number and structure of components. The Impact of homonuclear Hartmann Hahn (HOHAHA) and homonuclear relayed coherence tranafer (COSYRCT) on component ldentllcatlon In Isomer mlxtures was explored. WRh component Identllcatlon and subsequent 1-D lgF spectral adgnments completed, quantttatlon vla routlne methods could be performed. The ablllty to extract onedlmenrlonal lgF spectra of “pure” lndlvldual components from both the 2-D HOHAHA and COSYRCT experlmentalspectra and thelr unlque advantage8, dlsadvantages, and llmltatlons as tools for spectroscopic separetlon of complex llquld mlxtures were demonstrated.
INTRODUCTION The production of perfluorinated cyclic ring compounds using the CoF3 fluorination process generates complex reaction mixtures.’ Two byproductsare typically encountered when this reaction pathway to perfluorination is used. First, ring-open byproducts can be formed, and second, isomers can be created during the fluorination process due to fluorine addition either above or below the plane of the ring. As an example of the latter, fluorination of naphthalene in the production of perfluorinated decalin, results in mixtures of cis and trans isomers. Fortunately, capillary column gas chromatography can successfully isolate the two isomers.2 Even in the absence of physical separation, high-resolution fluorine-19 1-D nuclear magnetic resonance (19F NMR) can be used to characterize and quantify the two isomers by line width differences of the correspondingNMR signals resulting from the differing rigidity of the molecular structures.2 19F NMR has several advantagesover other analytical techniques for fluorocarbon structural characterization: (1)large fluorocarbon chemical shift range (-200 ppm) facilitating discriminationof a wide variety of fluorinated functionalities; (2) high NMR receptivity making the 19Fnucleus the second most sensitive naturally occurring nucleus in the periodic table; (3)long-rangeconfiguration-specific coupling constants
* To whom correspondence should be addressed.
(1) Haszeldine, R. N.; Smith, F. J. Chem. SOC.1950, 3617. (2) Fung, B.M. Org. Magn. Reson. 1983, 21 (6),397. 0003-2700/93/0365-0752$04.00/0
providing additional structural informationwhich is essential for perfluorinated cyclic derivative compounds.3 However, in the analysis of such reaction mixtures, overlap of the resonance signals causes ambiguity in chemical shift assignment. Structural identification of individual componentsby 19F NMR is preferentially done after the reaction mixture has undergone physical separationand purification. Although coupling of liquid chromatography (LC) and ‘9F NMRQhas provided a direct interface between physical separation and spectroscopiccharacterization, specialized equipment availability and inability to separate by LC are still problematical. In the analysis of perfluoro-l,3-dimethylcyclohexane (1,3DMC), more signals were observed in the 19F 1-D NMR spectrum than expected, indicating that the sample was a mixture. On the basis of the expected 1,345isomer structure (equatorial substitution of both-CF3 groups) and the resultant molecular symmetry, the 19F 1-D NMR spectrum should contain single methyllmethine peaks and six equatorWaxial doublet methylene pairs due to 2 J ~ ~ splitting. s,, Two methyl and methine peaks, however, were detected as well as several additional singlets and doublets in the methylene chemical shift region. In contrast to the example described previously, physical separation was both time-consuming and incomplete using available methods. Capillary gas chromatography suggested the sample was composed primarily of two major components. The assignment of two or more constituents motivated our interest in pursuing NMR spectroscopic avenuesfor identification of individual species in the mixture. Our approachwas to take advantageof the additional resolving power afforded by 2-D NMR techniques. In theory, multiquantum coherence5 created in an individual spin system or molecular component of a mixture should allow us to obtain a single slice from a 2-D NMR spectrum containing a group of resonances associated with only one of the individual components5 i.e., a 1-D spectrum of a pure component. In this way, resonance signal overlap which occurs in 1-D NMR spectra of complex liquid mixtures is removed. The goal is to obtain a spectroscopic extraction of a 1-D NMR spectrum of the pure component from the 2-D NMR data of a reaction mixture without physical separation. The advantage of obtaining a spectracopically separated 1-D NMR spectrum is the ability to obtain chemical shift values of individual components which cannot be obtained without ambiguityby other means. These values then aid in molecular structure determination through the subsequent use of established 19F chemical shift additivity (3)Jameson, C. J. In Multinuclear NMR;Mason, J., Ed.; Plenum Press: New York, 1987. (4)Allen, L. A,; Spratt, M. P.; Glass, T. E.; Dorn, H.C. And. Chem. 1988, 60,675. (5)Kessler, H.;Gehrke, M.; Griesinger, C. Angew. Chem., Int. Ed. Engl. 1988, 27,490. 0 I993 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 65, NO. 6, MARCH 15, lQQ3 715
By analogy, literature surveys indicated that 1H 2-D homonuclear Hartmann Hahn transfer (2-D HOHAHA) is useful in generating multiquantum coherences.10 This technique has been successfully applied to the separation of cyclic subunits in polysaccharides, peptides, and proteins by examining individual slices of 'H 2-D NMR spectra taken at various anomeric protons.lW1* However, 2-D HOHAHA is not directly translatable to l9F NMR spectroscopy using standard NMR hardware, due to the necessity for high 19F radio-frequency spin lock power covering a spectroscopic window of 120 ppm. The alternative is to take advantage of the long-range coupling constants which exist in perfluorinated cyclic systems. We are presently focused on the application of lQF,l9F-COSYRCT (Homonuclear Relayed Coherence Transfer Correlation Spectroscopy) to achieve spectroscopic separation of complex perfluorinated liquid mixtures; results on perfluorinated 1,3-DMC will be demonstrated. The advantages and limitations of a number of the more common 2-D toolsand a direct comparison of 19F,l9FCOSYRCT versus l9F,lgF-HOHAHAwill be discussed. Future development of selective excitation to 1-D19FCOSYRCT and 1-D 19F HOHAHA will be explored. EXPERIMENTAL SECTION Materials. Perfluoro-1,3-dimethylcyclohexane(1,3-DMC) was obtained from PCR, Inc. (Gainesville, FL). 2-Propanol, n-butanol, and perfluorinated n-hexanes were purchased from Aldrich Chemical Co. (Milwaukee,WI). These compounds and CFCl3 were used as purchased without further purification. Dimethyl sulfoxide-d6 (DMSO-de) was purchased from Cambridge Isotope Labs, Cambridge, MA. Nuclear Magnetic Resonance Spectroscopy. High-resolution room-temperature I9FNMR spectra were obtained using a Bruker AM-500 or a Bruker ACP-300 Fourier transform (FT) NMR spectrometer equipped with a 1H/19F 5-mm probe or a quad probe tuned to a fluorine-19resonancefrequency of 470.385 or 282.231 MHz, respectively. The lgF,l9F-HOHAHA was performed on a Bruker AMX-300 spectrometer using a quad (1H/W/31P/19F)5-mm probe at 282.231 MHz. A DIPSI-2 spinlocki3was used. High-resolution lH 1-D and 2-D spectra were collected using the AM-500 instrumentation described above. (I) 1-D NMR. For acquisition of 19F and 1H 1-D spectra, 1-2-8 relaxation delays and 30" observe pulses (3 and 2 ps) were used to ensure quantitative representation of all spectral features. Special parameters used in the acquisition and processing of I9F and lH NMR spectra are discussed in the appropriate text or figure captions. 1,bDMCwas diluted in CFCl3which also served as an internal reference (6(F)= 0.0 ppm). In other cases, the perfluorinated compound was examined in a pure state and l9F chemical shift values were externally referenced to CFC13. A capillary tube containing DzO was added as a deuterium lock solvent in each I9FNMR experiment. DMSO-& was chosen as a solvent for lH NMR spectroscopicexperiments to serve as both a lock solvent and internal chemical shift reference (6(H) = 2.49 ppm). DMSO-de alsosuppressed labile hydroxylproton exchange in the alcohol mixture. (11) Homonulcear Relay CoherenceTransfer. 'H,lH- and 19F,19F-COSYRCTspectra were collected using one of three pulse sequences shown in eq 1. One-step COSYRCT ( x = l),for an AMX spin system involves coherence transfer from spins A to M and M to X and is performed using the following pulse sequencei4 D1-90°-DO-900-(D2-1800-D2-900)x-acquire (1) D1, a relaxation delay, is followed by a normal COSY-liketransfer (6) Santini, G.; LeBlanc, M.; Riess, J. G. J. Fluorine Chem. 1977,10, 363. (7) Malinowski, E.R. J. Phys. Chem. 1972, 76 (lo), 1593. (8) Weigert, F.J.; Karel, K. J. J. Fluorine Chem. 1987, 37, 125. (9) Herrmann, E.C.;Hoyer, G. A.; Kelm, J. Spectrochim. Acta 1982, 38A (lo), 1057. (10) Davis, D.G.;Bax, A. J. Am. Chem. SOC.1985,107, 2820. 1986,108,918. (11) Edwards, M. W.;Bax, A. J. Am. Chem. SOC. (12) Spraul, M. Bruker Rep. 1991,1, 28.
(9O0-DO-9O0)from spins A to M. D2 is a coherence period = 0.5/ [ l.sJ,,,,] which is optimizedon the basis of the largest coupling An inversion pulse (180') is constant in the spin system, JmIU.l4 imbedded in the 2D2 delay in order to refocus chemical shifts. The second COSY-like M to X coherence transfer occurs with the final 90' pulse. Two- and three-step versions of the pulse sequence can be used to detect longer range correlations. In the above pulse sequence, x = 2 for two-step (AMQX) or x = 3 for three-step (AMQRX) coherence transfer processes. (111) Homonuclear Hartmann Hahn. 19F,lgF-HOHAHA spectra were collected using the pulse scheme15 D1-9O0-DO-(DIPSI-2spin lock)-acquire (2) Coherence transfer occurs via two pathways, both COSY-like through-bond interactions and through-space dipolar interactions. Mixing (i.e., coherence transfer) occurs in the HOHAHA sequence during the spin-lock pulse, DIPSI-2.13 The spin-lock pulse length is optimizedto maximize transfer throughout a given spin system. Capillary Gas Chromatography/Mass Spectrometry (GC/ MS). A VG Quattro quadrupole mass spectrometer interfaced to a Hewlett Packard 5890A gas chromatograph was used. Electron ionization was conducted at 70 eV. The mass range 50-500 Da was scanned in 1.0 s (total) with 0.1-8 settling time. The ionization source temperature was about 180 "C. The GC column was a Supelco SPB-5 fused-silica capillary, 30-m long, 0.25-mm i.d. and 1.0-pm stationary phase film thickness. The column was directly interfaced to the MS manifold, with the column exit placed at the ionization region GC entrance orifice. The helium carrier gas flow rate was 1.0 mL/min. Undiluted 1,3-DMCwas analyzedusing split injections, with a statically measured split ratio of 26:l and an injection volume of approximately 0.01 p L (a "wet needle" injection technique using a Hamilton 7001 1-pLvolume syringe was used). The injector was 250 "C, and the GC/MS interface was 310 "C. The column temperature was held at -10 "C for 1min and then programmed at 5 'C/min to 30 "C.
RESULTS AND DISCUSSION 1-D Spectroscopic Separation of Isomer Mixtures. The most rudimentary NMR spectroscopic separation of mixtures can be performed using line width variations among molecules with differing conformational stabilities. For example, perfluorinated decalin exists in two isomer forms, cis and trans. The rigidity of the trans isomer results in sharp readily distinguishable peaks in the 19F 1-D NMR spectrum (Figure l), while rapid ring flipping (fluorine exchange between equatorial and axial positions)16 broadens peaks associated with the cis isomer. The successful application of 1-D NMR spectroscopic separation to the decalin problem did not extend to the 1,3DMC perfluorocarbon mixture case. The major components in 1,3-DMC gave sharp peaks, and their relative molar concentrations were approximately equal; therefore, neither line width nor intensity arguments could be used to aid in mixture component identification (Figure 2). Unlike perfluorinated decalin, attempts at individual isomer structural elucidation failed due to incomplete separation using GC techniques (Figure 3). Alternatively, we focused on the development of 2-D NMR techniques which would allow us to observe the 1-D NMR spectrum of a pure isomer or component by extracting individual slices from the 2-D NMR spectrum. 2-DNMR Methods for Spectroscopic Separation. Correlation spectroscopy (COSY) and its analogues which detect long-range interactionsl7-l9have been extensively used (13) Shaka, A. J.; Lee, C. J.; Pines, A. J.Magn. Reson. 1988, 77,274. (14) Bax, A.;Drobny, G. J. Magn. Reson. 1985, 61, 306. (15) Bax, A.;Davis, D. G. J.Magn. Reson. 1985, 65, 355. (16) Harris, R. K. Nuclear Magnetic Resonance Spectroscopy; Longman Scientific & Technical: Essex, England, 1986.
754
ANALYTICAL CHEMISTRY, VOL. 65, NO. 6, MARCH 15, 1993 A
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to determine direct connectivities between J-coupled nuclei in hydrocarbon and fluorocarbon materials.20,21Examination of the family of 19F coupling constants (see Table I), however, reveals that some critical J values (such as 4J,,) are approximately zero.22 Consequently, any nuclear pair subject to this configurational relationship would not be coupled and, as a result, no crosspeak would be seen. Due to the absence of J-coupling among some fluorines in cyclic systems, an additional spin interaction must be tapped to observe crosspeaks corresponding to all fluorines in an isolated spin system or molecule. HOHAHA (homonuclear Hartmann Hahn) has been used extensively to characterize and spectroscopically separate distinct ring systems in polysaccharides and proteins.10-12 In these hydrocarbon cases, HOHAHA creates crosspeaks as a result of both Jand dipolar interactions. Therefore,coherence is transferred both through bond and space interactions. Application of this technique to perfluorocarbons, however, has not been previously documented. Thisapplicationproved more difficult than for hydrocarbon systems for several reasons. Although the results support the presence of two major components (Figure 41, phase twists in the 19F,19FHOHAHA experiment caused crosspeak distortions in the outer spectral regions. This experiment requires the application of a spin-lock pulse designed to hold all the spins along a common axis for coherence transfer. In perfluorinated materiale containing CF3, CF2, and CF functionalitiescovering a 120 ppm chemical shift range (56400 Hz at 11.7 T), conventional spin-lock fields cannot effectively 'lock" all the spins. Dephasingoccurswhich is manifested as phase twisting of the outer spectral features (Figure 4). Broadening the frequency range over which the spin-lock is effective by increasing the spin-lock field strength is limited by sample heating and probe arcing. Use of a DIPSI-2 composite spin(17) Aue, W. P.; Bartholdi, E.; Ernst, R. R. J. Chem. Phys. 1976,64, 2229. (18) Nagayama, K.; et al. J . Magn. Reson. 1980, 40, 321. (19) Bas, A.; Freeman, R. J.Magn. Reson. 1981,44,542. (20) Croasmun, W. R.; Carlson, R. M. K. Two-Dirnensioml NMR Spectroscopyfor ChernistsandBi0chemists;VCHPublishers;New York, 1987. (21) Ovenall,D. W.; Ferguson, R. C. In Puke Methods in I-D and 2-0 Liquid-Phase NMR; Brey, W. S.,Ed.; Academic Press; San Diego, CA, 1988. (22) Emsley, J. W.;Phillips,L.;Wray,V.Fluorine Coupling Constants; Pergamon Press; Oxford, England, 1977.
lock pulae,l3 instead of the more common MLEV-17,15 decreased, but did not totally remove, distortion of the outer spectral features. Figure 4 shows the 2-D HOHAHA spectrum when applied to 1,3-DMC. In contrast, the application of this technique to hydrocarbons is widely used. lH spectra of hydrocarbon systems have peaks covering a range less than 10 ppm in width, corresponding to 5000 Hz sweep width on an 11.7 T magnet. The hardware requirements in the hydrocarbon case are routinely available on most modern high resolution instrumente (15). No special high power pulae amplification is required. Extrapolation to the analogous 19F technique requires specialized '9F decoupling capabilities,a high-power probe capable of withstanding strong spin-locking fields, specialized composite spin-lock pulse sequences, and an efficientvariable temperature unit to prevent sample heating during the spin-lock pulse. Interest in exploring 2-D NMR techniques which could be readily implemented in laboratories with fluorine observe capabilities, but which lack more specialized hardware described above, direded our search to homonuclear relayed coherence transfer correlation spectroscopy (COSYRCT). Prior to the advent of the HOHAHA experiment, COSYRCT had been applied to individual component identification in polysaccharide and protein macromolecular systems.12~23-25 COSYRCT is loosely based on the traditional COSY experiment with the addition of one or more coherence relay steps (referto Experimental Section for pulae sequence discussion). Unlike COSY, the appearance of a crosspeak between spins A and X is not dependent on a nonzero J u ; instead, COSYRCT makes w e of mediating spins, M.14 JAM
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-
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In an AMX spin system, where JAM and Jm are nonzero, a crosspeak will appear at the intersection between A and X even if JAX is zero. Crosspeak intensity for a given coherence (23) King, G.; Wright, P. E. J. Magn. Reson. 1983,54,328. (24) Anderson, N. H.; Eaton, H. L.; Nguyen, K. T.;Hartzell, C.;Nelson, R. J.; Priest, J. H. Biochemistry 1988,27, 2782. (25) Homans, S. W.; Dwek, R. A.; Fernandes, D. L.; Rademacher, T. W. h o c . Natl. Acad. Sci. U.S.A.1984, 81, 6286.
ANALYTICAL CHEMISTRY, VOL. 65, NO. 6, MARCH 15, 1993
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Table I. Typical Long-Range lSF-l9F Coupling Constants in Cyclic FluorocarbonsZ1 3Jaa 3Jae 3Jee 2Jgem
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PPM IgF 1-0 NMR spectrum of perfluorinated 1,WMC (PCR, Inc.)acquired at 470.385 MHr (top). MWle and bottom spectra are slices extracted from the leF,lBFCOSYRCT spectrum indicated by dashed lines in Figure 68. (A) and (B) are the slices associated wkh the cis and trans isomers, respectively.
Figuro 2.
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relay pathway is optimized based on J-. For example, in an AMX spin system, J,, is the larger of the two coupling constants JAMor J m . One-, two-, and three-step relay experiments can be performed to maximize desired crosspeak intensities by increasing the availablepathways for coherence transfer.
2.2 Hz 14.7 Hz
10 Hz 300 Hz
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0 Hz -6t0-9Hz
To illustrate the use of this technique before applying it to a mixture with an unknown number of components,Figure 5 shows examples of the one-, two-, and three-step lH,lHCOSYRCT experiments on a physical mixture of known compounds: 2-propanol and n-butanol in DMSO-& The experiment was optimized using J,, = 7 Hz, a typical 3 5 for long-chainhydrocarbons.% Examination of the one- and twostep lH,lH-COSYRCT 2-D spectra illustrates incomplete coherence transfer throughout the entire molecular spin systems (Figure 5). This is determined through the examination of all slices associated with a given component. Although some rows in the one- and two-step cases appear complete, other coherencepathways or slices are not complete. In contrast, all slices of the three-step COSYRCT show complete 1-D subspectra of the individual n-butanol and 2-propanol components. One can concludethat it is necessary to perform the full three-step relay on hydrocarbon samples to ensure complete crosspeak detection. The success of coherence transfer to the full spin system or molecule can be checked by examining slices at several frequencies corresponding to peaks of a given molecule. If full coherence transfer has occurred, the same crosspeaks will appear in all slices. The crosspeak intensities are likely to be different when compared from slice to slice due to their creation by different coherence pathways.14 Disadvantages of the full three-step coherence transfer experiment are an increased minimum number of scans for phase cycling, and loss of crosspeak intensity due to T2(spin-spin relaxation) process. The small magnitude of 1HJH coupling constants leads to a long coherence period, D2, due to ita inverse relationship to Jmm (refer to Experimental Section). Loss of crosspeak intensity to T2 processeslogically occurs. Also, TZlosses grow in significance with the addition of more pulses and delays to a sequence. Keeping in mind the need to extract full individualcomponent 1-Dspectra,the COSYRCT experiment succeeds with limitations in this area when applied to hydrocarbons. Extension of the COSYRCT experiment to fluorinated materials, however, was successful. Figure 6 shows a comparison between a routine l9F,'9F-COSY and a two-step lgF,lgF-COSYRCT2-D spectrum of 1,3-DMC. Examination of slices taken at the two major component methines from the lgF,19F-COSYRCTspectrum (Figure 2) shows that all 1-D spectral features in the mixture can be accounted for in the two extraded rows. Reasons for the success of COSYRCT to achieve spectroscopic separation of complex perfluorinated liquid mixtures are several-fold and unique to 19F NMR. Unlike hydrocarbons, typical perfluorinated cyclic systems have Jm, values as high aa 300 Hz for geminal couplings or 26 Hz for 4J, (Table I). This results in significantlylonger D2 mixing times (D2 0.5/J-) for fluorinated materials when compared to hydrocarbons, reducing the negative impact on crosspeak intensity due to T2. A short TI(spin-lattice relaxation time) for 19Fnuclei permits short relaxation delays, a positive aspect when pulse sequences involving full 90 and 180° pulses are applied. The high sensitivity of 19F,in conjunction with the short T1,allow rapid data acquisition employing a minimum
-
(26) Silverstein, R. M.; Baseler, G. C.; Morrill, T. C. Spectrometric Identification of Organic Compounds; John Wiley & Sone;New York, 1981.
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number of scans. Importantly, this technique is readily implemented on any modern NMR spectrometer with '9F , * oDserve capaoimies. IYO speciai naraware moairications are necessary. Although COSYRCT appears to be the technique of choice to achieve spectral separation in perfluorocarbon mixtures, two minor disadvantages exist. In light of the large 19F chemical shift range, 512-1024 slices are often necessary in complicated spectra to achieve adequate slice resolution in a 2-D NMR spectrum. Even with this requirement, total ,.---4--l .- LL, ,.E ..-l.. 1- I n 1. ...--uyeurru t x q u i u i u w i w u c u uii w e uiucr UL uiiiy --IU ii w c i c routine with perfluorinated materiala at concentrations -7
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ANALYTICAL CHEMISTRY, VOL. 85, NO. 8, MARCH 15, lQQ3 757
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Figwe 5. One, twth and three-step (A-C, respectively), 'H,"-COSYRCT spectra (500.13 MHz) and sllces (E and F) taken at peaks associated wlth 2-propanol and +butanol In a 50150 (by volume) mlxture. 128 Increments wlth 32 translents each were recorded. Two-dlmensbnal Fourler transformation resulted In data matrices of 2048 X 1024 polnts. Nonshlfted slne-bell apodlzatlon was applied prior to transformation. Spectrum D 1s the hlgh-resolutlon 'H 1-D NMR spectrum of the alcohol mixture.
substituted cyclohexanestructures (molecular weight Of 400) were confirmed for the two major components in the 1,3DMC sample. The alkyl substitution patterns, however, could not be determined and, due to incompletephysical separation using GC, the assignment of specific chromatogram peaks to different isomer configurations could not be done. The 1,3cis-DMC isomer, formed by equatorial substitution on the ring of both CFBgroups, was expected to be the major sample component. However, since GUMS detected two major components, we logically examined the possibility of other configurationalisomers and their expected '9F NMR spectra. In the case of the rigid 1,3& isomer, the l9F NMR 1-D spectrum would contain single CF and CF3 peaks. All methylene fluorine8 would appear as doublets (300-Hz splitting) due to geminal coupling with their equatorial or axial partners. Features in the 19F 1-D NMR spectrum of 1,3-DMC support the presence of the 1,348 isomer but the appropriate chemical shift values cannot be assigned unam-
biguously. After subtraction of the cis component peaks from the 1-D mixture spectrum, unexpected residual features become more apparent. CF/CF3 single peaks and CF2 doublets are observed, indicating structural similarity to the l,3& structure; however, two singlets,whose chemical shifts suggest perfluorinated methylene functionalities, are also present. Singlets originating from a CF2 functionality are usually associated with an averaged structure resulting from very rapid ring-flipping or the presence of a long-chain aliphatic. Our expectation that additional spectral features could be due to a nonrigid 1,34rans isomer did not immediately support the spectral componentsdetected. Only after examination of the COSYRCT slices were we able to positively determine that the remaining NMR peaks could be attributed entirely to the 1,3-transstructure (oneequatorial and one axial CF3 substitution on the ring). This structure is not conformationally rigid; however, it is unlike the perfluorinated decalin case where broad line widths are
758
ANALYTICAL CHEMISTRY, VOL. 85, NO. 8, MARCH 15, 1993
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-100
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singlets. Fluorine5 c and t, however, are not equivalent due to their different configurations relative to the CFs groups, thereby producing the two doublets seen in the l,&trans slice. With access to 1-D 'spectra" of neat isomers using COSYRCT, fluorine chemical shifts of difficult to resolve isomers can be determined and used to expand current 19F chemical shift additivity rule databases (refer to Table 11). With this valuable information, 19F 1-D NMR spectra of compounds unknown as neat materials can be simulated and further used as an identification tool in other complex mixtures.
CONCLUSIONS -140
-120
-100
-80
' -b0 ' - I b O ' -1120 ' ' -1140 ' -1160' ' -1100 ' PPM Flgurr 6. (A) leF,lsFCOSYand (B) two-step lsF,leFCOSYRCT2-D spectra. For the leF,lsFCOSY,258 slices with 16 transisnts each were recorded, while 5 12 slices with 32 transients each were recorded for the lsF,lsF-COSYRCT.Two-dimensionalFourier transformation on each resulted in data matrices of 1024 X 512 polnts and 2048 X 1024 points, respedvaly. Nonshiftedsine-beliapodbtbn was applied prlor to transformation.
observed due to exchange effects. Instead, the molecule undergoes rapid ring-flippingon the NMR time scale, to the extent that an averaged flat 1,3-CF3-substitutedring structure (producing sharp singlet methylene features) is observed at room temperature. This conclusionis supported by a variabletemperature study performed on 1,3-DMC. This results in a trana configuration of the CF3 groups which support the observation of singlets for both CF3 and CF functionalities. Under these conditions,methylene fluorines bare equivalent as are methylene fluorinesa (Figure2) and, therefore, produce
On the basis of our exploration of available 2-D NMR experiments, COSYRCT is the most effective technique for perfluorinated mixture component identification from 2-D NMR spectroscopic results. Although HOHAHA has been widely used to achieve a similar result in large molecular systems such as proteins and polysaccharides, current standard hardware limitations make its extension to perfluorocarbons unavailable to most NMR laboratories. The ease of COSYRCT implementation with only 19F observe capabilities makes its impact on structure identificationJseparationof wide-spread applicability. Future efforts in our laboratory are designed to eliminate the need for full 2-D spectral acquisition for productivity reasons. These efforts involve exploration of shaped pulses for selective 19F 1-DCOSYRCT. Selective excitation of a single peak associated with a perfluorinated mixture component would cut the acquisition time to that required for a simple 1-D 19F NMR experiment. Further examination of 1-D and 2-D lgF,lgF-HOHAHA in light of recent and pending hardware advances is also underway.
ACKNOWLEDGMENT We would like to acknowledge and thank M. R. Seger for his helpful discussions and D. F. H. Swijter for his aid in acquisition of the GCJMS data.
RECEIVED for review July 17, 1992. Accepted December 3, 1992.
