13.14 13C NMR Spectroscopy 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 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) 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. 1H Figure 13.23(a) (page 572) ClCH2CH2CH2CH2CH3 10.0 9.0 8.0 7.0 6.0 CH3 ClCH2 5.0 4.0 3.0 Chemical shift (, ppm) 2.0 1.0 0 13C Figure 13.23(b) (page 572) ClCH2CH2CH2CH2CH3 a separate, distinct peak appears for each of the 5 carbons 200 180 160 140 120 CDCl3 100 80 60 Chemical shift (, ppm) 40 20 0 13.15 13C Chemical Shifts are measured in ppm () from the carbons of TMS 13C Chemical shifts are most affected by: • electronegativity of groups attached to carbon • hybridization state of carbon Electronegativity Effects Electronegativity has an even greater effect on 13C chemical shifts than it does on 1H chemical shifts. Types of Carbons Classification CH4 Chemical shift, 1H 13C 0.2 -2 CH3CH3 primary 0.9 8 CH3CH2CH3 secondary 1.3 16 (CH3)3CH tertiary 1.7 25 (CH3)4C quaternary 28 Replacing H by C (more electronegative) deshields C to which it is attached. Electronegativity effects on CH3 Chemical shift, 1H 13C CH4 0.2 -2 CH3NH2 2.5 27 CH3OH 3.4 50 CH3F 4.3 75 Electronegativity effects and chain length Cl Chemical shift, CH2 CH2 CH2 CH2 CH3 45 33 29 22 14 Deshielding effect of Cl decreases as number of bonds between Cl and C increases. 13C Chemical shifts are most affected by: • electronegativity of groups attached to carbon • hybridization state of carbon Hybridization effects sp3 hybridized carbon is more shielded than sp2 sp hybridized carbon is more shielded than sp2, but less shielded than sp3 H 36 114 138 36 126-142 C C 68 84 CH2 22 CH2 20 CH3 13 Carbonyl carbons are especially deshielded O 127-134 CH2 C 41 171 O CH2 CH3 61 14 Table 13.3 (p 573) Type of carbon Chemical shift (), Type of carbon ppm Chemical shift (), ppm RCH3 0-35 RC CR 65-90 R2CH2 15-40 R2C CR2 100-150 R3CH 25-50 110-175 R4C 30-40 Table 13.3 (p 573) Type of carbon Chemical shift (), Type of carbon ppm RCH2Br RCH2Cl 20-40 25-50 RC Chemical shift (), ppm N 110-125 RCOR 160-185 O RCH2NH2 35-50 RCH2OH 50-65 O RCH2OR 50-65 RCR 190-220 13.16 13C NMR and Peak Intensities Pulse-FT NMR distorts intensities of signals. Therefore, peak heights and areas can be deceptive. Figure 13.24 (page 576) CH3 7 carbons give 7 signals, but intensities are not equal OH 200 180 160 140 120 100 80 60 Chemical shift (, ppm) 40 20 0 13.17 13C—H Coupling Peaks in a 13C NMR spectrum are typically singlets 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). 13.18 Using DEPT to Count the Hydrogens Attached to 13C Distortionless Enhancement of Polarization Transfer 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 Measuring a 13C NMR spectrum involves Steps 2 and 3 can be repeated hundreds of times to enhance the signal-noise ratio 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 Measuring a 13C NMR spectrum involves In DEPT, a second transmitter irradiates 1H during the sequence, which affects the appearance of the 13C spectrum. some 13C signals stay the same some 13C signals disappear some 13C signals are inverted Figure 13.26 (a) (page 578) O CCH2CH2CH2CH3 CH CH CH2 CH2 CH O CH3 CH2 C C 200 180 160 140 120 100 80 60 Chemical shift (, ppm) 40 20 0 Figure 13.23 (b) (page 578) O CCH2CH2CH2CH3 CH CH CH3 CH CH and CH3 unaffected C and C=O nulled CH2 inverted 200 180 160 140 120 100 CH2 80 60 Chemical shift (, ppm) 40 CH2 CH2 20 0 13.19 2D NMR: COSY AND HETCOR 2D NMR Terminology 1D NMR = 1 frequency axis 2D NMR = 2 frequency axes COSY = Correlated Spectroscopy 1H-1H COSY provides connectivity information by allowing one to identify spin-coupled protons. x,y-coordinates of cross peaks are spin-coupled protons 1H-1H COSY O 1H CH3CCH2CH2CH2CH3 1H HETCOR 1H and 13C spectra plotted separately on two frequency axes Coordinates of cross peak connect signal of carbon to protons that are bonded to it. 1H-13C HETCOR O 13C CH3CCH2CH2CH2CH3 1H