Ibuprofen. Example of benchtop NMR on small organic molecules

Transcription

1 R Ibuprofen Example of benchtop NMR on small organic molecules Ibuprofen (C 13 H 18 O 2 ) is a non-steroidal antiinflammatory drug (NSAID) and is commonly used for pain relief, fever reduction and against inflammation. H 3 C H 3 C CH 3 O OH The 1H NMR spectrum of 200 mm ibuprofen in CDCl 3 is shown in Figure 1. The spectrum was recorded in a single scan, taking 7 seconds to acquire. All peaks and 1H-1H couplings are well resolved, and can be assigned to the molecular structure. Figure 1: Proton NMR spectrum of 200 mm ibuprofen in CDCl 3.

2 1H NMR RELAXATION 1 H NMR relaxation The relaxation time measurements are shown in Figures 2-4. Note the long T 1 and short T 2 The relaxation time value measurements of the exchanging are hydroxyl proton. The amplitude of the first data point cales s with shown in Figures the 2 number - 4. Note of protons the long for the corresponding 1 H NMR peak. relaxation 2.1 s 2.5 s 1.4 s 2.3 s T 1 and short T 2 value of the exchanging hydroxyl proton. The relaxation amplitude time of measurements the are shown 3.5 sin Figures 2-4. Note the long T and 0.13 short s T s 2 value of the exchanging hydroxyl proton. The amplitude of the first data point scales with first data point scales with the 2.5 number the 2.1 number s of protons s for the corresponding peak. 1.4 s 2.3 s of protons for the corresponding peak. Proton T 1 (left) and T 2 (right) relaxation time for each proton 3.5 s position of the 0.9 molecule s 0.13 s 2.1 s 2.5 s 1.4 s 2.3 s Figure 2: Proton T 1 (left) and T1.3 2 (right) s relaxation time for each proton position 1.3 of the s molecule 3.5 s 0.13 s Figure 2: Proton T 1 (left) and T 2 (right) relaxation time for each proton position of the molecule Figure 3: Proton T 1 relaxation time measurement of 200 mm ibuprofen in CDCl 3. Figure 2: Proton T 1 relaxation time measurement of 200 mm ibuprofen in CDCl 3. Figure 3: Proton T 1 relaxation time measurement of 200 mm ibuprofen in CDCl 3. Figure 4: Proton T 2 relaxation time measurement of 200 mm ibuprofen in CDCl 3. Figure 4: Proton T 2 relaxation time measurement of 200 mm ibuprofen in CDCl 3. Figure 3: Proton T 2 relaxation time measurement of 200 mm ibuprofen in CDCl 3.

3 2D COSY 2D COSY The 2D COSY spectrum is shown in Figure 4. It clearly shows two spin systems (2,13) and (9,10,11,12). For example, the methyl group at position 13 only couples to the CH group at position 2D 2, COSY whilst the methyl groups at positions 11 and 12 couple to CH and CH 2 groups at positions 10 and 9. There is no coupling of positions (9,10,11,12) to either 2 or 13. The 2D COSY spectrum The 2D COSY is shown spectrum in Figure is shown 4. in Figure 4. It clearly shows two spin systems (2,13) and It clearly shows two (9,10,11,12). spin systems For example, (2,13) the and methyl group at position 13 only couples to the CH group at position 2, whilst the methyl groups at positions 11 and 12 couple to CH and CH (9,10,11,12). For example, the methyl group at 2 groups at positions 10 and 9. There is no coupling of positions (9,10,11,12) to either 2 or 13. position 13 only couples to the CH group at position 2, whilst the methyl groups at positions 11 and 12 couple to CH and CH 2 groups at positions 10 and 9. There is no coupling of positions (9,10,11,12) to either 2 or 13. Figure 4: COSY spectrum of 200 mm ibuprofen in CDCl 3. The cross-peaks and corresponding exchanging protons are marked by colour-coded ellipses and arrows. Figure 4: COSY spectrum of 200 mm ibuprofen in CDCl 3. The cross-peaks and corresponding exchanging protons are marked by colour-coded ellipses and arrows. Figure 4: COSY spectrum of 200 mm ibuprofen in CDCl 3. The cross-peaks and corresponding exchanging protons are marked by colour-coded ellipses and arrows.

4 2D HOMONUCLEAR J-RESOLVED SPECTROSCOPY In the 2D homonuclear j-resolved spectrum the chemical shift is along the direct (f2) direction and the effects of proton-proton coupling along the indirect (f1) dimension. This allows the full assignment of chemical shifts of overlapping multiplets, and can allow otherwise unresolved couplings to be measured. The projection along the f1 dimension yields a decoupled 1D proton spectrum. Figure 5 shows the 2D homonuclear j-resolved spectrum of ibuprofen, along with the 1D proton spectrum as blue line. The vertical projection shows how the multiplets collapse into a single peak, which greatly simplifies the 1D spectrum. Vertical traces through the peaks in the 2D spectrum yield the peak multiplicities, as shown by the green lines in Figure 5, and enables the measurement of proton-proton coupling frequencies. By comparing the coupling frequencies between different peaks, it is possible to extract information about which peaks are coupled to each other. For example, both the multiplet at 1.90 ppm and the doublet at 3.78 ppm have a splitting of 6.5 Hz, suggesting that these groups are coupled to each other. These couplings confirm the findings of the COSY experiment in Figure Hz 6.5 Hz Figure 5: Homonuclear j-resolved spectrum of 200 mm ibuprofen in CDCl 3. The multiplet splitting frequencies for different couplings are colour-coded as in Figure 4.

5 2D homonuclear j-resolved spectroscopy 2D homonuclear j-resolved spectroscopy 2D HOMONUCLEAR J-RESOLVED SPECTROSCOPY One unusual and often neglected feature of this experiment is that second order coupling effects show up in the indirect (f1) direction as extra peaks equidistant from the couplin One unusual and often neglected feature partners of this well experiment removed from is that the zero second frequency order in coupling the indirect dimension. These peaks a often neglected as artefacts, but provide direct evidence of second order coupling partne effects show up in the indirect (f1) direction as extra peaks equidistant from the coupling These extra peaks and coupling partners are marked by colour-coded ellipses and arrow One unusual partners and often well neglected removed from feature the of zero this frequency Note in the that indirect this dimension. spectrum These is based peaks on are the same data Figure 6. Note that this spectrum is based on the same data as Figure 5, only the scaling experiment is often that neglected second order as artefacts, coupling but provide effects changed. direct evidence as Figure of 5, second only the order scaling coupling has partners. changed. show up in the These indirect extra (f1) peaks direction and coupling as extra partners are marked by colour-coded ellipses and arrows in peaks equidistant Figure from 6. Note the that coupling this spectrum partners is based well on the same data as Figure 5, only the scaling has removed from changed. the zero frequency in the indirect dimension. These peaks are often neglected as artefacts, but provide direct evidence of second order coupling partners. These extra peaks and coupling partners are marked by colour-coded ellipses and arrows in Figure 6. Figure 6: Homonuclear j-resolved spectrum of 200 mm ibuprofen in CDCl 3 showing the extra peaks due to strong couplings. Figure 6: Homonuclear j-resolved spectrum of 200 mm ibuprofen in CDCl 3 showing the extra peaks due to strong couplings. Figure 6: Homonuclear j-resolved spectrum of 200 mm ibuprofen in CDCl 3 showing the extra peaks due to strong couplings.

6 1D 13C SPECTRA The 13C NMR spectra of 2 M ibuprofen in CDCl 3 are shown in Figure 7. The 1D Carbon experiment is sensitive to all 13C nuclei in the sample. It clearly resolves 9 resonances; the small triplet between 70 and 80 ppm is from the solvent. The 13C DEPT experiment uses polarisation transfer between proton and carbon nuclei and can be used for spectral editing. Only carbons directly attached to protons are visible in these experiments. Since the peaks at 181, 140 and 137 ppm do not show in the DEPT spectra they must belong to quaternary carbons. The DEPT-90 experiment gives only signal of CH groups, whilst the DEPT-45 and DEPT-135 give signals of CH, CH 2 and CH 3 groups, but the CH 2 groups appear as negative peaks in the DEPT-135. Through linear combination of the three DEPT spectra one can produce subspectra of the CH 3, CH 2 and CH groups alone, as shown in Figure 7. An interesting feature evident in these subspectra is that the peak at 45 ppm is made up of a CH and a CH 2 resonance with identical chemical shifts. Figure 7: Carbon spectra of 2 M ibuprofen in CDCl 3.

7 HETCOR Similar to the 2D COSY experiment, which detects proton-proton coupling partners, a series of heteronuclear 2D NMR experiments have been devised to detect coupling partners of different nuclei. The Heteronuclear Correlation (HETCOR) experiment is used to correlate proton resonances to the carbons directly bonded to those protons. The HETCOR experiment detects the carbon signal along the direct dimension and the proton signal along the indirect dimension. The HETCOR spectrum of 2 M ibuprofen in CDCl 3 is shown in Figure 8, with the 1D proton and carbon spectra from Figures 1 and 7 as vertical and horizontal traces. The peaks in the 2D spectrum show which proton is bonded to which carbon. Figure 8: HETCOR spectrum of 2 M ibuprofen in CDCl 3.

8 HMQC Another heteronuclear 2D correlation experiment is the Heteronuclear Multiple Quantum Coherence (HMQC) experiment. Similar to HETCOR, it is used to correlate proton resonances to the carbons directly bonded to those protons. However, in the HMQC experiment the carbon signal appears along the indirect dimension, and the proton signal along the direct dimension. The HMQC spectrum of 2 M ibuprofen in CDC l3 is shown in Figure 9, with the 1D proton and carbon spectra from Figures 1 and 7 as horizontal and vertical traces. The peaks in the 2D spectrum show which proton is bonded to which carbon. A similar analysis as with the HETCOR spectrum can be performed for conclusive peak assignment. Figure 9: HMQC spectrum of 2 M ibuprofen in CDCl 3.

9 HMBC The HMQC experiment shown on the previous page was designed to correlate pro carbons which are connected through a one bond coupling. To obtain long-range p carbon correlations through two or three bond couplings, the Heteronuclear Multip HMBC Correlation (HMBC) experiment can be used. Like in the HMQC experiment the c signal appears along the indirect dimension, and the proton signal along the direct dimension. HMBC The HMBC spectrum of 1 M ibuprofen in CDCl 3 is shown in Figure 10, with the 1 The HMQC experiment shown on the The HMQC experiment shown and previous carbon the previous spectra the from HMQC page was Figures additionally designed 1 and to 7 correlate as shows horizontal couplings protons and vertical to traces. The pea page was designed carbons to which correlate are connected protons 2D and through spectrum a one show bond quaternary which coupling. protons carbons, To obtain are connected long-range which are to protoncarbon connected correlations through coupling. a two one or bond three The bond couplings COSY couplings, between or HMQC. the Heteronuclear molecular For example, positions Multiple there Bond look are similar clear to the ones foun not which visible carbons in the via a long-range carbons which are coupling. To obtain Correlation long-range (HMBC) protoncarbon experiment the COSY can be spectrum, used. multibond Like but in the HMQC couplings experiment additionally from the the shows protons carbon couplings at positions to quaternary ca correlations through signal two appears or three along bond the indirect which couplings, dimension, are not visible and 2 and the in the proton 13 COSY signal the carbon or along HMQC. the at position direct For example, 1, as marked there are inclear multibo the Heteronuclear dimension. Multiple Bond Correlation couplings from the Figure protons 10. at It positions is also interesting 2 and 13 to to the note carbon that at there position is 1, as ma (HMBC) experiment The HMBC can be spectrum used. Like of 1 M in ibuprofen the in CDCl 3 is shown in Figure 10, with the 1D proton Figure 10. a correlation between carbons (11,12) to protons HMQC experiment and carbon the carbon spectra signal from Figures appears 1 and along 7 as horizontal and vertical traces. The peaks the It is also interesting (11,12). to note This that is there due is to a three-bond correlation between coupling carbons from 11 (11,12) to p the indirect dimension, 2D spectrum and show the proton which protons signal are along connected to to 12 which and vice carbons versa via (light long-range green). coupling. The couplings between (11,12). molecular This is due positions to three-bond look similar coupling to the ones from found 11 to from 12 and vice versa (light gree the direct dimension. The HMBC spectrum of 1 M the COSY spectrum, but the HMQC additionally shows couplings to quaternary carbons, ibuprofen in CDCl which 3 is are shown not visible in Figure in the 10, COSY with or the HMQC. For example, there are clear multibond 1D proton and carbon couplings spectra from the from protons Figures at positions 1 and 2 and 13 to the carbon at position 1, as marked in 7 as horizontal and Figure vertical 10. traces. The peaks in the 2D spectrum show It is also which interesting protons to are note connected that there is a correlation between carbons (11,12) to protons to which carbons (11,12). via a This long-range is due to coupling. three-bond The coupling from 11 to 12 and vice versa (light green). couplings between molecular positions look similar to the ones found from the COSY spectrum, but Figure 10: HMBC spectrum of 2 M ibuprofen in CDCl 3, with some of the long-ran couplings marked. Figure 10: HMBC spectrum of 2 M ibuprofen in CDCl 3, with some of the long-range Figure couplings 10: HMBC marked. spectrum of 2M ibuprofen in CDCl 3.

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