13.14 13 C NMR Spectroscopy

1. 13.1413C NMR Spectroscopy

2. 1H and 13C NMR compared: both give us information about the number of chemically nonequivalent nuclei (nonequivalent hydrogens or nonequivalent carbons) both give us information about the environment of the nuclei (hybridization state, attached atoms, etc.) it is convenient to use FT-NMR techniques for 1H; it is standard practice for 13C NMR

3. 1H and 13C NMR compared: 13C requires FT-NMR because the signal for a carbon atom is 10-4 times weaker than the signal for a hydrogen atom a signal for a 13C nucleus is only about 1% as intense as that for 1H because of the magnetic properties of the nuclei, and at the "natural abundance" level only 1.1% of all the C atoms in a sample are 13C (most are 12C)

4. 1H and 13C NMR compared: 13C signals are spread over a much wider range than 1H signals making it easier to identify and count individual nuclei Figure 13.23 (a) shows the 1H NMR spectrum of 1-chloropentane; Figure 13.23 (b) shows the 13C spectrum. It is much easier to identify the compound as 1-chloropentane by its 13C spectrum than by its 1H spectrum.

5. 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0 1H Figure 13.23(a) (page 572) CH3 ClCH2 ClCH2CH2CH2CH2CH3 Chemical shift (, ppm)

6. 200 180 160 140 120 100 80 60 40 20 0 13C Figure 13.23(b) (page 572) a separate, distinct peak appears for each of the 5 carbons ClCH2CH2CH2CH2CH3 CDCl3 Chemical shift (, ppm)

7. 13.1513C Chemical Shifts are measured in ppm ()from the carbons of TMS

8. 13C Chemical shifts are most affected by: • electronegativity of groups attached to carbon • hybridization state of carbon

9. Electronegativity Effects Electronegativity has an even greater effect on 13C chemical shifts than it does on 1H chemical shifts.

10. Classification Chemical shift, d 1H 13C -2 primary 8 CH4 16 secondary CH3CH3 tertiary 25 CH3CH2CH3 quaternary 28 (CH3)3CH (CH3)4C Types of Carbons 0.2 0.9 1.3 1.7 Replacing H by C (more electronegative) deshieldsC to which it is attached.

11. 1H 13C 0.2 -2 2.5 27 3.4 50 4.3 75 Electronegativity effects on CH3 Chemical shift, d CH4 CH3NH2 CH3OH CH3F

12. Cl CH2 CH2 CH2 CH2 CH3 Electronegativity effects and chain length Chemical shift, d 45 33 29 22 14 Deshielding effect of Cl decreases as number of bonds between Cl and C increases.

13. 13C Chemical shifts are most affected by: • electronegativity of groups attached to carbon • hybridization state of carbon

14. 36 114 138 36 126-142 sp hybridized carbon is more shielded than sp2, but less shielded than sp3 H C CH2 C 68 84 22 20 13 CH3 CH2 Hybridization effects sp3 hybridized carbon is more shielded than sp2

15. O CH2 CH3 Carbonyl carbons are especially deshielded O CH2 C 41 171 61 14 127-134

16. RC CR R2C CR2 Table 13.3 (p 573) Type of carbon Chemical shift (),ppm Type of carbon Chemical shift (),ppm RCH3 0-35 65-90 R2CH2 15-40 100-150 R3CH 25-50 110-175 R4C 30-40

17. O O RC N Table 13.3 (p 573) Type of carbon Chemical shift (),ppm Type of carbon Chemical shift (),ppm RCH2Br 20-40 110-125 RCH2Cl 25-50 RCOR 160-185 35-50 RCH2NH2 50-65 RCH2OH RCR 190-220 RCH2OR 50-65

18. 13.1613C NMR and Peak Intensities Pulse-FT NMR distorts intensities of signals. Therefore, peak heights and areas can be deceptive.

19. CH3 OH 200 180 160 140 120 100 80 60 40 20 0 Figure 13.24 (page 576) 7 carbons give 7 signals, but intensities are not equal Chemical shift (, ppm)

20. 13.1713C—H Coupling

21. Peaks in a 13C NMR spectrum are typicallysinglets 13C—13C splitting is not seen because the probability of two 13C nuclei being in the same molecule is very small. 13C—1H splitting is not seen because spectrum is measured under conditions that suppress this splitting (broadband decoupling).

22. 13.18Using DEPT to Count the HydrogensAttached to 13C Distortionless Enhancement of Polarization Transfer

23. Measuring a 13C NMR spectrum involves 1. Equilibration of the nuclei between the lower and higher spin states under the influence of a magnetic field 2. Application of a radiofrequency pulse to give an excess of nuclei in the higher spin state 3. Acquisition of free-induction decay data during the time interval in which the equilibrium distribution of nuclear spins is restored 4. Mathematical manipulation (Fourier transform) of the data to plot a spectrum

24. Measuring a 13C NMR spectrum involves Steps 2 and 3 can be repeated hundreds of timesto enhance the signal-noise ratio2. Application of a radiofrequency pulse to give an excess of nuclei in the higher spin state 3. Acquisition of free-induction decay data during the time interval in which the equilibrium distribution of nuclear spins is restored

25. Measuring a 13C NMR spectrum involves In DEPT, a second transmitter irradiates 1H during the sequence, which affects the appearanceof the 13C spectrum. some 13C signals stay the same some 13C signals disappear some 13C signals are inverted

26. O CCH2CH2CH2CH3 O 200 180 160 140 120 100 80 60 40 20 0 Figure 13.26 (a) (page 578) CH CH CH2 CH CH2 CH3 CH2 C C Chemical shift (, ppm)

27. O CCH2CH2CH2CH3 200 180 160 140 120 100 80 60 40 20 0 Figure 13.23 (b) (page 578) CH CH CH3 CH CH and CH3 unaffected C and C=O nulled CH2 inverted CH2 CH2 CH2 Chemical shift (, ppm)

28. 13.192D NMR:COSY AND HETCOR

29. 2D NMR Terminology 1D NMR = 1 frequency axis2D NMR = 2 frequency axes COSY = Correlated Spectroscopy 1H-1H COSY provides connectivity informationby allowing one to identify spin-coupled protons. x,y-coordinates of cross peaks are spin-coupledprotons

30. O CH3CCH2CH2CH2CH3 1H-1H COSY 1H 1H

31. HETCOR 1H and 13C spectra plotted separately on twofrequency axes Coordinates of cross peak connect signal of carbonto protons that are bonded to it.

32. 13C O CH3CCH2CH2CH2CH3 1H 1H-13C HETCOR