C Triple-Resonance NMR Methods to th - American Chemical Society

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Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 11, 2017 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0598.ch013

Applications of H- F- C Triple-Resonance NMR Methods to the Characterization of Fluoropolymers 1

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Peter L. Rinaldi , Lan Li , Dale G. Ray III , Gerard S. Hatvany , Hsin-Ta Wang , and H . James Harwood 2

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Department of Chemistry, Knight Chemical Laboratory, University of Akron, Akron, OH 44325-3601 Department of Polymer Science, Maurice Morton Institute of Polymer Science and Engineering, University of Akron, Akron, OH 44325

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In this paper we illustrate some applications of 1D- and 2D- H/ F/ C triple resonance NMR techniques for characterizing fluoropolymers. These methods can be used to achieve spectral simplification, to disperse resonances permitting resolution of clearly spaced peaks due to nuclei in various stereo- and monomer sequences, and to establish one-bond and multiple-bond connectivities in order to identify structure fragments. Various permutations of INEPT, HMQC, and HMBC NMR experiments are used to obtain illustrative data from poly(1-chloro-1-fluoro-ethylene-co-isobutylene) (PCFE-IB).

The NMR analysis of polymers is very often difficult as a consequence of the numerous structures which result from the presence of variations in stereosequence and monomer distribution. The variety of structures present leads to complex spectra with numerous overlapping resonances. Very often, a particular structure or defect fragment in the molecule is of interest, such as the unique structures formed at the chain ends by initiation or termination reactions, the unique repeat units which occur at the junctions oi two dissimilar blocks, or the structures which result from chain branching or grafting of functional groups onto the polymer chain. Our efforts to characterize fluoropolymers led us to exploit multi-dimensional (1) and triple resonance (2) NMR techniques which have been used effectively by biochemists over the past 5-10 years. In employing these techniques, a third NMR active nucleus (other than *H or C), is incorporated in a substance, either through introduction of functionality or by artificial enrichment with an isotope that is normally present at low abundance levels, and then rf pulses are applied at the resonancefrequenciesof these nuclei. This permits filtering of most of the resonances from the normal NMR spectrum and enables the selective detection of resonances near the site of label incorporation. Recently developed 2D-NMR methods have provided a plethora of NMR experiments for establishing the identity of various structurefragments(3). For example, l3

0097~6156/95/0598-0215$14.00/0 © 1995 American Chemical Society Urban and Provder; Multidimensional Spectroscopy of Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

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a one-bond heteronuclear shift correlation experiment (4) provides a means of identifying all of the C-Hfragmentsin a molecule. Similar experiments such as long range HETCOR (5), COLOC (6), and XCORFE (7) exploit two- and three- bond J couplings permitting the detection of C-C-H and ^C-C-C-H fragments. These components of a molecule's structure, which are obtainedfromone or more 2D-NMR experiments, can be put together like the pieces of a puzzle to obtain a complete picture of a molecule's structure. Additionally, 2D-NMR experiments provide a means of dispersing the resonances from the 1D-NMR spectrum into a second dimension, thus achieving dispersion that is not possible in ID-spectra even at the highest resonance frequency currently available with any NMR instruments. Triple resonance 2D-NMR techniques combined with isotopic labeling provide an infinite number of experimental methods for structure identification with an even higher level of spectral dispersion and simplification. In studies on fluoropolymers, F, which is naturally present in 100% abundance can be employed as the third nucleus in the triple resonance methods discussed above. This provides the opportunity to use a countless variety of pulse sequences to selectively detect different structural features of polymers. In this paper we will provide a few examples of H/ F/ C triple resonance techniques which can be used to achieve spectral simplification and structure identification. These methods are not necessarily the ones which provide optimum sensitivity or spectral dispersion; they do serve as good examples of how these 2D-NMR techniques can be used in polymer chemistry. 13

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1D-NMR Experiments The 1D-NMR spectra of a relatively simple polymer, poly(l-chloro-lfluoroethylene-co-isobutylene) (PCFE-IB) are shown in Figure 1. These spectra illustrate the difficulties usually encountered in basic 1D-NMR analyses. The *H NMR spectrum (Figure la) consists of two groups of resonancesfromthe methylene (2.0-3.5 ppm) and methyl (1.0-1.5 ppm) groups of this polymer. Both resonance areas are complex due to the presence of resonances of a large number of monomer sequences and associated stereosequences present in the copolymer. The C-NMR spectrum also contains a large number of resonances. The shifts of C atoms are generally more sensitive to their local environments and their resonances usually occur as singlets, making it easier to resolve distinct peaks for carbons in different environments. However, some of these occur as multiplets in the spectrum due to C-F coupling. Three groups of C-F doublets are observed in the 105120 ppm region. The methylene, quaternary, and methyl carbon resonances are observed in the 50-60, 34-38, and 30 ppm regions of the spectrum, respectively. Although there is only a single F atoms per CFE repeat unit, the F shift is most sensitive to changes in local environment Consequently, the F spectrum (Figure lc) is the most complex of the three; a large number of resonances are detected and the spectrum is too complicated to be easily interpreted. We now show how these spectra can be simplified by employing new NMR experiments to study polymers. U

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Urban and Provder; Multidimensional Spectroscopy of Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1995.


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Triple-Resonance NMR To Characterize Fluoropolymers 217

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Figure 1. NMR spectra of poly(l-chloro-l-fluoioethylene-co-iso-butylene): a) *H spectrum, b) C spectrum, and c) F NMR spectrum. W

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{"FI^C Polarization Transfer. Polarization transfer experiments such as INEPT (Insensitive Nuclei Enhanced by Polarization Transfer) (8) and DEPT (Distortionless Enhancement by Polarization Transfer) (9) were originally devised to improve the sensitivity for detection of nuclei such as °C, which have small magnetic moments, by transferring magnetizationfromΉ which has a much higher magnetic moment Polarization transfer occurs from Ή (which has a much larger proportion of its spins in the ground state) to C, providing a factor of 4 (proportional to n /nc) increase in signal-to-noise for detection of ^C. Furthermore, it is the ground state population of *H atoms, which have shorter T 's than C, that must recover before the pulse sequence can be repeated in signal averaging. Consequently, more transients can be averaged in a given period of time, providing an additional increase in sensitivity. One drawback of this method of signal detection is that only proton-bearing carbons are detected, since polarization transfer is accomplished using one-bond J-couplings between the observed and decoupled nuclei. An outline of the INEPT sequence is shown in Figure 2. A combination of pulses is applied to the observed °C, and decoupled {X} nuclei (note that by convention, the decoupled nucleus is generally surrounded by brackets when the two nuclei are listed in the name of an experiment). In this way X nucleus magnetization is transferred to C. Figure 3 shows plots of how the final C signal intensity varies as a function of the number of attached protons, and the lengths of the d2 and d3 delays. Usually, the experiments are performed with d2 = 1/(2*1^). The behavior of CH, CH , and CH signal intensities as a function of d3 are shown by the plot in Figure 3b. Three different spectra are usually collected with d3 values of 1/(2* JOJ), 1/(3*JCH) and 3/(4*1^ in order to provide three spectra containing only CH resonances, C H / C H 2 / C H 3 resonances all positive, and CH/CH resonances positive and CH resonances inverted, respectively. Shortly after the report of the INEPT experiment, it was realized that its only apparent shortcoming (i.e. the inability to observe quaternary carbon resonances) could be used to advantage infilteringundesired signalsfromcomplex NMR spectra by strategically placing labels such as *H in a structure and performing polarization transfer from the label to the observed nucleus (10). The C spectra would then contain only C resonances from carbons directly bound to H . This strategy is ideally suited for selective detection of C resonances influoropolymersbecause F has nuclear properties and natural abundance similar to those of H . Consequently, all of the sensitivity advantages achieved in {Ή^ΟΙΝΕΡΤ are also obtained in the {"F^C-INEPT experiment. Figure 4 contains the C NMR spectra of l-fluorohexane. Figure 4a is the normal C spectrum with H decoupling; doublets are observed for C - l (83.9 ppm, J = 164 Hz), C-2 (30.4 ppm, J = 19.3 Hz), and C-3 (22.5 ppm, J = 5.9 Hz). When both *H and F decoupling are performed (Figure 4b) the coupling to fluorine is eliminated and the spectrum is greatly simplified. Figure 4c is a { F} C-INEPT spectrum with both H and F broadband decoupling during the acquisition time. Delays were optimized for J = 164 Hz and only C-l is detected as a singlet. The spectrum is greatly simplified by filtering all of the C resonances of carbons not directly bound to F. Only a trace of the C-2 resonance is detected in Figure 4c because although the delays are optimized for ί they are set to only about 13

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Figure 3. Plot of CH. signal intensity variations in the INEPT experiment: a) as a function of the polarization transfer, d2, delay; and b) as afonctionof the refocussing, d3, delay.


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Figure 4. C spectra of 1-fluorohexane: a) standard 1D- C NMR spectrum with *H decoupling during the acquisition time, b) 1D- C NMR spectrum with both H and F decoupling during the acquisition time, and c) { ^ F } - ^ INEPT NMR spectrum with both H and F decoupling during the acquisition time. 13

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1/(16* J ) and significant time for buildup of C magnetization has not been achieved for carbons with couplings smaller than 100 Hz. Figure 5 shows the results from similar experiments which were performed on PCFE-IB. In Figure 5a, the normal C ID-spectrum displays resonancesfromCF, C D C 1 solvent, methylene, quaternary and methyl carbons. The C ID-spectrum with both *H and F decoupling during data acquisition (Figure 5b) is greatly simplified; note the three groups of C-F doublets between 105 and 120 ppm have been collapsed to groups of single lines. The methylene region of the spectrum (5060 ppm) is also greatly simplified. The { F} C-INEPT spectrum in Figure 5c was also acquired with both *H and F decoupling during data acquisition and contains only the three groups of resonances from the three nonequivalent permutations of triads. The { F} C-INEPT spectrum of PCFE-IB in Figure 5d was acquired with delays optimized for multiple-bond C-F couplings and with both *H and F decoupling during data acquisition. Only methylene carbons a to C-F carbons are observed. The C-F resonances are greatly attenuated, and the resonances from aliphatic carbons which are not with two- or three-bondsfromfluorine are completely eliminated. CT

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{ F} H Polarization Transfer. INEPT can be use advantageously for the detection of *H resonances as well as C resonances if polarization transfer is performed from F to *H. However, because the ratio of the F to *H resonancefrequenciesis close to one, the sensitivity gains achieved in detection of C via INEPT experiments are not achieved. Ordinarily, H- F J-couplings fall in the range of 20-50 Hz, 5-20 Hz and 0-5 Hz for JHP, J and JHF, respectively. The relatively well defined ranges for two-, three-, and four-bond couplings in acyclic hydrocarbons makes it possible to perform polarization transfer experiments which are edited based on the number of bonds between the fluorine and proton nuclei. Although the sensitivity gains achieved in detection of C via INEPT experiments are not achieved, sensitivity approaching that obtained for direct detection of H via a standard one-pulse experiment can usually be achieved. Fortunately, sensitivity in the detection of *H resonances is not as critical as in the detection of C resonances. The spectral simplification achieved can be very useful since the H chemical shift dispersion in not large, and *H resonances usually occur as multiplets. The normal one-pulse and {^F^H-INEPT spectra of 1-fluorohexane are shown in Figure 6. The Ή spectrum in Figure 6a exhibits doublet splittings (from 13

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ppm, respectively. These couplings are removed if F decoupling is performed during the acquisition time, as shown in Figure 6b. Although it likely exists, fourbond H-F coupling could not be discerned in the complex multiplet from H-3. The spectrum in Figure 6c was obtained with { F} H-INEPT using delays which were optimized for ^ = 50 Hz; only the multiplet from H - l is seen in the spectrum. If the INEPT delays are optimized for J = 25 Hz, then the resonances of both H - l and H-2 become apparent (Figure 6d). When using INEPT in this manner, the multiplicities of the resonances must be kept in mind. In { H} C-INEPT experiments, polarization transfer is always from η equivalent Η atoms to a single C atom in a C resonance from a C H group. 19

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However, in the {^F^H-INEPT experiment such as the one performed on 1fluorohexane, polarization is from an isolated F to η equivalent Ή atoms (i.e. from F to CHj). One important consequence of this difference is that the signal intensity variation as a function of d2 in {"ÎyH-INEPT follows the behavior shown for d3 in the { H} C-INEPT experiment. The line for C H signals from the plot in Figure 3b must be used to determine the optimum polarization transfer (d2) delay rather than the plot shown in Figure 3a; the optimum d2 is 1/(4*1^) rather than the expected 1/(2*^). The signal variation as a function of d3 in the {^F^H-INEPT experiment follows the behavior shown in Figure 3a. If polarization transfer is from two chemically equivalent F atoms to two *H atoms, as would be the case in INEPT transfer from the geminal fluorines to the H-2 methylene protons in 1,1difluorohexane, then curves analogous to the CH line in Figure 3b must be used to determine the optimum values for both d2 and d3. An illustration of the utility of the { F} H-INEPT experiment in analyzing polymers can be seen in Figures 7a, 7b, and 7c, which depict H NMR spectra of PCFE-IB obtained from a one-pulse H experiment, a one-pulse H experiment with F decoupling, and an { F} H-INEPT experiment with F decoupling during data acquisition, respectively. F decoupling alone does not provide significant new information; nor does it provide substantial simplification of the *H spectrum (compare Figures 7a and 7b). However, the {^FJ^H-INEPT spectrum obtained with delays optimized for J„p (Figure 7c) is considerably simplified. The resonances from methyl protons (1.3 ppm) and methylenes protons flanked by two IB repeat units (2.6 ppm) which are present in Figures 7a and 7b, have been filtered from the spectrum in Figure 7c. The detection of the resonances at ca. 4.5 ppm are especially facilitated by INEPT F decoupling as seen in the spectrum in Figure 7c. By performing "filtered" experiments in this way, an additional advantage is realized. Since only the resonances of interest (i.e. those which are in the vicinity of the labeled site or third NMR active nucleus) are detected, dynamic range problems are minimized. This can be especially advantageous if the fluorine containing sites represent a smallfractionof the repeat units in the polymer, as would be the case if a fluorine-containing initiator species were used. 19

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2D-NMR Experiments 2D-NMR experiments provide a tremendously useful means of dispersing resonances of complex molecules. { F} C HETCOR experiments are particularly useful since they provide shift correlation maps relating the shifts of F to the shifts of directly bound C; the shifts of both nuclei are extremely sensitive to their chemical environments and therefore provide phenomenal dispersion of resonances when these parameters are used as the basis for correlation in a 2D-NMR experiment. 19

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{"F^C Heteronuclear Shift Correlation Spectroscopy (HETCOR). Figure 8 shows the { F} C HETCOR spectrum of PCFE-IB obtained with both H and F decoupling during the acquisition time. Resonances from approximately one dozen different stereosequences are clearly resolved despite the fact that the ID- F and C spectra reveal very little information. This experiment involved direct detection 19

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of C, and indirect measurement of the F chemical shifts through the influence of the F spins on the C intensities during the evolution time (tj in a 2D-NMR experiment. While the information content is high in this type of 2D-NMR experiment, there is a better method of obtaining the same information with much higher sensitivity. 19

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f C } F Heteronuclear Multiple Quantum Coherence Spectroscopy (HMQQ. The {^C^H-HMQC indirect detection experiment, depicted in Figure 9a, was first described in 1983 (11) as a means of providing the same information obtained from the HETCOR experiments, but with the sensitivity advantages associated with detection of the *H nucleus. In theory, sensitivity gains proportional to ( υ / υ ) = 64 are achieved compared to an experiment where C resonances are measured directly in a one-pulse experiment. This corresponds to a 64-fold lower detection limit for small concentration species, or a 4000-fold reduction in the time to achieve the same signal-to-noise level (since signal-to-noise is proportional to the square root of the number of transient acquired). Similar ideas work just as well when F is substituted for H; the resonance frequency of F is almost as high as that of *H and its natural abundance is 100% (12). Little is lost in sensitivity and a tremendous increase in spectral dispersion is gained because the F chemical shift range is so much larger than that of *H. Figure 10 shows a plot of the { C} F HMQC spectrum of PCFE-IB obtained with *H decoupling throughout the experiment and C decoupling during the acquisition time; C decoupling in the F l dimension was accomplished with the appropriate use of 180° refocussing pulses during the evolution times. Note that only the downfield region of the C (Fl) dimension was contained within the spectral window as this region alone contains C-F resonances which will exhibit crosspeaks. Resonances from at least 40 separate C-F species are resolved. Many of these resonances were not observed in the C-detected HETCOR spectrum in Figure 8 because they are present in low concentrations, and are below the C detection limit Additionally, this experiment provides useful structure information about direct attachments of C-F atoms. Three groups of resonances associated with the three groups of peaks in the C spectrum (arising from IEI, EEI/IEE, and EEE monomer sequences) are resolved (13). If we consider the group of resonances from EEE sequences, these can be further divided into three subgroups arising from EEE-centered pentad sequences with two Ε end-units, one I and one Ε end-unit, or two I end-units as illustrated in the Figure 10. Within each of these subgroups, additional fine structure is resolved which arises from the different permutations of stereosequences which are possible. 1

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{ C} F Heteronuclear Multiple Bond Correlation Spectroscopy (HMBQ. HMBC (14) is an *H detected experiment related to HMQC, which was originally devised for obtaining correlations between *H and C when there is two- or three-bond coupling between nuclei. A diagram of the HMBC pulse sequence is shown in Figure 9b; the experiment is performed with Δ = 1/(2*^) in order to suppress HMQCtype crosspeaks, and τ = l/(2* J ) to optimize crosspeak intensities resulting from η-bond couplings. When {^C^H-HMBC experiments are used, the ranges of l3

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Figure 9. Pulse sequence diagrams for a) HMQC and b) HMBC 2D-NMR experiments; Δ is set to 1/(2*^CH) and τ is normally set to 1/(2**1^ where •JQ, is the coupling range for which cross-peaks are desired.


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geminal and vicinal couplings overlap substantially, and are relatively small. It is therefore difficult in many instances to distinguish between cross peaks which arise from two-bond couplings and those which arise from three-bond couplings. Furthermore, when the experimental linewidths are comparable or larger than the magnitude of the coupling, rapid relaxation eliminates crosspeak intensities. Due to the nature of multiple bond couplings between F and C these issues are less of a problem in { C} F-HMBC: J ^ is usually in the range of 30-50 Hz; J is usually in the range of 5-20 Hz; and Jcp is usually in the range of 0-5 Hz (usually, J is not resolvable). This provides some interesting possibilities in terms of experiments which can be performed to gain additional structural information. The first consequence of the* larger couplings between F and C is that HMBC can be useful for studying the structures of high molecular weight polymers, even when the {"C^H HMBC are not effective. Ordinarily, the utility of { C } H HMBC experiments for studying high molecular weight species is limited because the resonance linewidths are larger than the multiple bond J-couplings. Under these circumstances, relaxation occurs before information can transfer between spins. The much larger couplings make it possible to study larger molecules with broader resonance linewidths. The relatively well defined ranges of two-, three-, and four-bond F- C couplings also means that it is possible to perform several HMBC experiments, each with delays optimized to produce spectra with crosspeaks arising from a specific range of couplings. In this manner, a higher level of spectral simplification can be achieved, and better discrimination between correlations from two- and three-bond connectivities can be achieved. A schematic illustration of the information attainable from HMQC and HMBC spectra is depicted in Scheme 1. When the individual bits of informationfromthese experiments are fit together like the pieces of a puzzle, a total picture of a fluoropolymer's structure can be obtained. It is important not to have the mistaken notion that the peak intensity can be directly related to the magnitude of J-coupling in any given spectrum. The overall behavior of the peak intensity over a series of spectra provides a better indication of the approximate range of coupling. The HMBC crosspeak intensities follow a sinusoidal dependence on (τ/Jcp); a maximum signal is achieved when τ = 1/(2* J ), however when longer t delays are used to detect crosspeaks from weaker couplings, crosspeaks arisingfromlarger couplings may also reappear as t achieves values of n/(2*J ) where η is an odd integer. The { C} F-HMBC spectra obtained from 1-fluorohexane obtained with delays optimized for 20, 5, and 1 Hz J values are shown in Figures 11a, l i b , and 11c, respectively. In Figure 11a, an intense crosspeak is observed at F l = 31 ppm from C-2 which is two bonds from F. A weaker crosspeak at F l = 25 ppm is also observed from C-3, but in this spectrum, the delays are clearly optimized for detection of correlations which arise from two-bond couplings. The spectrum in Figure l i b was obtained with delays optimized for detection of correlations arisingfromthree-bond (5 Hz) couplings. The crosspeak at F l = 25 ppm which arisesfromthree-bond C-F coupling to C-3 is considerably more intense than it is in Figure 11a. The crosspeak at F l = 35 ppm from two-bond coupling is much weeker than it is in Figure 11a. In this molecule, the four-bond C-F couplings 19

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Triple-Resonance NMR To Characterize Fluoropolymers 237 19

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Combined Use of ID-Filtering and 2D-NMR Experiments The 1D-NMR methods described above provide a useful method of obtaining spectral simplification; and the 2D-techniques provide spectral simplification as well as connectivity information. The combination of the two techniques can be an extremely powerful tool for obtaining spectral simplification while providing additional information about thefragmentspresent in a polymeric structure. While there are many permutations of ID- and 2D-NMR experiments which can provide spectral simplification and structural information, we have found combination of {^F^HINEPT and HMQC or HMBC to be extremely useful. 19

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{"F^H-INEPT/^C^H-HMQC and { F} H-INEPT/{ C} H-HMBC. The { Ρ}ΉINEPT/^C^H-HMQC experiment (pulse sequence shown in Figure 14a) is similar to the ^HpSi-INEFT/^Q^Si-HMQC experiment reported by Berger (15). The {^F^H-INEPT part of the experiment provides spectral simplification as well as some information about atomic connectivities. The spectral simplification can be extremely useful if the fluorine content of the polymer is low, since much more intense interfering resonances from the non fluorine-containing parts of the molecule are removed from the spectrum. The latter advantage can provide supplementary structural information about fluorine-proton proximities. The HMQC/HMBC part of the experiment provides valuable information regarding proton-carbon connectivities. The structural fragments which can be identified from each series of experiments are outlined in Scheme 2. The {^F^H-INEPT/^C^H-HMQC experiment can be performed in several different ways. In all cases, the HMQC Δ delay is optimized for ca. 125-150 Hz one-bond C-H coupling; if the A delay is optimized for a 50 Hz two-bond H-F coupling, then correlations are observed which identify all of the H-C-Ffragmentsin the structure. If the Δ delay is optimized for a 5-20 Hz three-bond H-F coupling, then cross peaks are observed in the 2Dspectrum which identify all of the C-Hfragmentswhich are separated by one bond from the C-F fragments. Similarly, if four-bond H-F coupling is resolved, the experiment could be repeated with a longer A delay to identify the C-H fragments which are separated from the C-Ffragmentby two bonds. A sample of the data obtained is shown in Figure 15. The standard { C}*HHMQC of 1-fluorohexane is shown in Figure 15a. Crosspeaks are observed which correlate the resonances from six nonequivalent carbon atoms with the resonances of directly bound protons. The {^F^H-INEPT/^C^H-HMQC spectrum, obtained with A delays set to perform INEPT polarization transfer from F to Η separated by two bonds, is shown in Figure 15b. Only correlations between resonances of Η and C atoms on the terminal methylene carbon are observed; the remaining resonances which were detected in Figure 15a arefilteredfromthe spectrum in Figure 15b. In a similar manner, { ^ H - I N E P T / ^ C ^ H - H M B C can be performed with a number of permutations of delays. Some of these permutations provide redundant information which is availablefromthe {^F^H-INEPT/^C^H-HMQC experiments 3

t

χ

t

l3

l



238


Figure 14. Pulse sequence diagrams for a) { ^ ^ - I N E P T / i ^ C I ^ - H M Q C and b) { " F l ^ - I N E P T / i ^ Q ^ - H M B C ; Δ and Δ are set based on " V as discussed in the section on { F} H-INEPT above; Δ = 1/(2* ^Œ), and τ = 1/(2**3^ as described in the text. 1

19

2

1

3



13.

RINALDI

ET AL.


Unknown Structure

INEPT-HMQC AI-1/(4-ZJHF)

Ai - 1/(4 · SJHF)

At-1/(4 ·4α„,:)

c — c — c — c —c—

t

c-c- c-c-cΓ Ϊ c—c— c— c— c—

τ

1

c-c- c- c-cINEPT-HMBC

-F I

ç-c-c-c-c

Δι -1/(4* 3JHF)

Δ , - 1 / ( 4 ·

C Triple-Resonance NMR Methods to th - American Chemical Society

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