Ethyl crotonate. Example of benchtop NMR on small organic molecules

Transcription

1 R Ethyl crotonate Example of benchtop NMR on small organic molecules Ethyl crotonate (C 6 H 10 O 2 ) is a colourless liquid at room temperature. It is soluble in water, and is used as a solvent for cellulose esters as well as a plasticizer for acrylic resins. It is known for its pungent odour. O H 3 C O CH 3 The 1H NMR spectrum of 25% ethyl crotonate 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 25% ethyl crotonate in CDCl 3.

2 1 H NMR RELAXATION 1 H NMR relaxation The relaxation time The measurements relaxation time measurements are are shown in Figures 2-4. Note that the relaxation shown in Figures times 2-4. are Note longest that for the the CH protons and shortest 5.2 s for the CH 3 protons. 3.9 s The amplitude 3.0 of sthe first data point scales with the number s 3.3 s 2.5 s relaxation times are longest for the H NMR of protons relaxation for the corresponding peak. CH protons and shortest for the CH s 2.9 s The relaxation time measurements are shown in Figures 2-4. Note that the relaxation protons. The amplitude of the first data times are point scales with the number 5.2 longest s for the CH protons and shortest for the CH of protons 3.9 s s protons. The amplitude of the Proton T 2.7 s first data point scales with the number of protons 1 (left) and T for the corresponding 2 (right) relaxation time for each proton peak. 3.1 s for the corresponding peak. position 3.3 s of the 2.5 smolecule. 2.6 s Figure 2: Proton T 1 (left) and T 2 (right) relaxation time for each proton position of the molecule 5.1 s 2.9 s Figure 2: 5.2 Proton s T 1 (left) and T (right) s relaxation time for each 3.0 s proton position of the 2.7 molecule s 3.1 s 3.3 s 2.5 s 2.6 s 5.1 s 2.9 s Figure 2: Proton T 1 (left) and T 2 (right) relaxation time for each proton position of the molecule 2.7 s 2.6 s Figure 3: Proton T 1 relaxation time measurement of 25% ethyl crotonate in CDCl 3. Figure 3: Proton T 1 relaxation time measurement of 25% ethyl crotonate in CDCl 3. Figure 2: Proton T 1 relaxation time measurement of 25% ethyl crotonate in CDCl 3. Figure 3: Proton T 1 relaxation time measurement of 25% ethyl crotonate in CDCl 3. Figure 4: Proton T 2 relaxation time measurement of 25% ethyl crotonate in CDCl 3. Figure 4: Proton T 2 relaxation time measurement of 25% ethyl crotonate in CDCl 3. Figure 4: Proton T 2 relaxation time measurement of 25% ethyl crotonate in CDCl 3. Figure 3: Proton T 2 relaxation time measurement of 25% ethyl crotonate in CDCl 3.

3 2D COSY 2D COSY The 2D COSY spectrum is shown in Figure 4. It clearly shows two spin systems ( (6,7,8). For example, the methyl group at position 1 only couples to the ethylene g position 2, whilst the methyl group at position 8 couples to the two CH groups at p 6 and 7. There is no coupling of positions (6,7,8) to either 1 or 2. The 2D COSY spectrum is shown in Figure 4. It clearly shows two spin systems (1,2) and (6,7,8). For example, the methyl group at position 1 only couples to the ethylene group at position 2, whilst the methyl group at position 8 couples to the two CH groups at positions 6 and 7. There is no coupling of positions (6,7,8) to either 1 or 2. Figure 4: COSY spectrum of 25% ethyl crotonate in CDCl 3. The cross-peaks and Figure 4: COSY spectrum of 25% corresponding ethyl crotonate exchanging CDCl 3 protons. The cross-peaks are marked and by corresponding colour-coded ellipses and arrows. exchanging protons are marked by colour-coded ellipses and arrows.

4 2D HOMONUCLEAR J-RESOLVED SPECTROSCOPY 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 In the 2D homonuclear j-resolved spectrum otherwise unresolved different couplings peaks, to it be is measured. possible The to projection extract along information the f1 dimension chemical shift is along the direct (f2) direction yields a decoupled about 1D which proton peaks spectrum. are Figure coupled 5 shows to each the 2D other. homonuclear j-resolved and the effects of proton-proton coupling along spectrum of ethyl For crotonate, example, along both with the the triplet 1D proton at 1.06 spectrum ppm as blue and line. The vertical projection shows how the multiplets collapse into a single peak, which greatly simplifies the indirect (f1) dimension. This allows the full the quartet at 3.96 ppm have a splitting of 7.10 the 1D spectrum. assignment of chemical shifts of overlapping Vertical traces Hz, through suggesting the peaks in that the these 2D spectrum groups yield are the coupled peak multiplicities, to as show multiplets, and can allow otherwise unresolved by the green lines each in other. Figure 5, The and size enables of the measurement coupling frequency of proton-proton coupling couplings to be measured. The projection along frequencies. By provides comparing information the coupling about frequencies the between coupling different strength. peaks, it is possible to extract information about which peaks are coupled to each other. For example, both the the f1 dimension yields a decoupled 1D proton For example, the splitting of 1.53 Hz is due to the triplet at 1.06 ppm and the quartet at 3.96 ppm have a splitting of 7.10 Hz, suggesting that spectrum. Figure 5 shows the 2D homonuclear these groups are long coupled range to coupling each other. The between size of the positions coupling 6 frequency and 8. provides All j-resolved spectrum of ethyl crotonate, along information with about these the couplings strength. confirm For example, the findings the splitting of the of COSY 1.53 Hz is due to th the 1D proton spectrum as blue line. The vertical long range coupling experiment between in positions Figure 64. and 8. All these couplings confirm the findings of the COSY experiment in Figure 4. 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 easurement of proton-proton coupling frequencies. By comparing the coupling frequencies between Hz 6.65 Hz Hz 1.53 Hz 7.10 Hz 6.65 Hz 1.53 Hz 7.10 Hz Hz 6.65 Hz Hz 1.53 Hz 7.10 Hz 6.65 Hz 1.53 Hz 7.10 Hz Figure 5: Homonuclear j-resolved spectrum of 25% ethyl crotonate in CDCl 3. The multiplet splitting frequencies for different couplings are colour-coded as in Figure 4. Figure 5: Homonuclear j-resolved spectrum of 25% ethyl crotonate in CDCl 3. The multiplet splitting frequencies for different couplings are colour-coded as in Figure 4. Figure 5: Homonuclear j-resolved spectrum of 25% ethyl crotonate in CDCl 3. The multiplet splitting frequencies for different couplings are colour-coded as in Figure 4.

5 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 coupling partners well removed from the zero frequency in the indirect dimension. These peaks are often neglected as artefacts, but provide direct evidence of second order coupling partners. 2D homonuclear j-resolved spectroscopy One unusual and often neglected feature of this experiment is that second order coupling These extra peaks and coupling partners are effects show up in the indirect (f1) direction as extra peaks equidistant from the coupling partners well marked removed by from colour-coded the zero frequency ellipses in the indirect and dimension. arrows in These peaks are often neglected Figure as artefacts, 6. Note but that provide this direct spectrum evidence of is second based order on coupling partners. These extra peaks and coupling partners are marked by colour-coded ellipses and arrows in the same data as Figure 5, only the scaling has Figure 6. Note that this spectrum is based on the same data as Figure 5, only the scaling has changed. changed. Figure 6: Homonuclear j-resolved spectrum of 25% ethyl crotonate in CDCl 3 showing the extra peaks due to strong couplings. Figure 6: Homonuclear j-resolved spectrum of 25% ethyl crotonate in CDCl 3 showing the extra peaks due to strong couplings.

6 1D 13 C SPECTRA The 13C NMR spectra of 25% ethyl crotonate in CDCl 3 are shown in Figure 7. The 1DCarbon experiment is sensitive to all 13C nuclei in the sample. It clearly resolves 6 resonances, as well as a weak solvent triplet between 70 and 80 ppm. 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 peak at 167 ppm does not show in the DEPT spectra it must belong to the quaternary carbon at position 4. 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. This is the case for the peak at 60 ppm, which must therefore belong to the carbon at position 2. By combining the three DEPT spectra, it is possible to confirm the peak assignment shown in Figure 7. The peaks at 144 and 123 ppm belong to the methyne groups at positions 7 and 6, and the peaks at 18 and 14 ppm belong to the methyl groups at positions 8 and Figure 7: Carbon spectra of 25% ethyl crotonate 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 25% ethyl crotonate 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 25% ethyl crotonate 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 25% ethyl crotonate in CDCl 3 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. Figure 9: HMQC spectrum of 25% ethyl crotonate in CDCl 3.

9 HMBC HMBC The HMQC experiment shown on the previous page was designed to correlate protons and carbons which are connected through a one bond coupling. To obtain long-range protoncarbon correlations are through connected two or to three which bond carbons couplings, via the Heteronuclear a long-range Multiple Bond The HMQC experiment shown on the previous Correlation (HMBC) experiment can be used.like in the HMQC experiment the carbon page was designed to correlate protons and coupling. The couplings between molecular signal appears along the indirect dimension, and the proton signal along the direct carbons which are connected through a one dimension. bond positions look similar to the ones found from the coupling. To obtain long-range protoncarbon The HMBC spectrum COSY of spectrum, 25% ethyl crotonate but the in HMQC CDCl 3 (v/v) additionally is shown in shows Figure 10, with correlations through two or three bond couplings, the 1D proton and couplings carbon spectra to quaternary from Figures carbons, 1 and 7 as which horizontal are and not vertical traces. the Heteronuclear Multiple Bond Correlation The peaks in the visible 2D spectrum in the show COSY which or protons HMQC. are For connected example, to which there carbons via a long-range coupling. The couplings between molecular positions look similar to the ones (HMBC) experiment can be used. Like in the are clear multibond couplings from the carbon at found from the COSY spectrum, but the HMQC additionally shows couplings to quaterna HMQC experiment the carbon signal appears carbons, which position are not visible 4 to in the the protons COSY or HMQC. at positions For example, 7 and there 2, are clear along the indirect dimension, and the proton multiband signal couplings marked from as the dark carbon blue at position in Figure 4 to the 10. protons at positions 7 and 2, along the direct dimension. The HMBC spectrum marked as dark blue in Figure 10. of 25% ethyl crotonate in CDCl 3 is shown in Figure 10, 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 protons Figure 10: HMBC spectrum of 25% ethyl crotonate in CDCl 3 (v/v). Figure 10: HMBC spectrum of 25% ethyl crotonate in CDCl 3.

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