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 atoma signal for a 13C nucleus is only about 1% as intense as that for 1H because of the magnetic properties of the nuclei, andat 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 nucleiFigure #1 shows the 1H NMR spectrum of 1-chloropentane; Figure #2 shows the 13C spectrum. It is much easier to identify the compound as 1-chloropentane by its 13C spectrum than by its 1H spectrum.

Chemical shift (d, ppm)ClCH2Figure #1CH3ClCH2CH2CH2CH2CH31H

Chemical shift (d, ppm)Figure #2ClCH2CH2CH2CH2CH313CCDCl3a separate, distinct peak appears for each of the 5 carbons

13C Chemical Shiftsare measured in ppm (d) from the carbons of TMS

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

Examples (chemical shifts in ppm from TMS)23138sp3 hybridized carbon is more shielded than sp2

Examples (chemical shifts in ppm from TMS)sp3 hybridized carbon is more shielded than sp261202

Examples (chemical shifts in ppm from TMS)23 an electronegative atom deshields the carbon to which it is attached61

Examples (chemical shifts in ppm from TMS)138an electronegative atom deshields the carbon to which it is attached202

Table 1Type of carbonChemical shift (d), ppmRCH30-35R2CH215-40R3CH25-50R4C30-40

Table 2Type of carbonChemical shift (d), ppmType of carbonChemical shift (d), ppmRCH30-3565-90R2CH215-40R3CH25-50R4C30-40100-150110-175

Table 3Type of carbonChemical shift (d), ppmRCH2Br20-40RCH2Cl25-5035-50RCH2NH250-65RCH2OHRCH2OR50-65

Table 4Type of carbonChemical shift (d), ppmType of carbonChemical shift (d), ppmRCH2Br20-40RCH2Cl25-5035-50RCH2NH250-65RCH2OHRCH2OR50-65RCOR160-185RCR190-220

13C NMR and Peak IntensitiesPulse-FT NMR distorts intensities of signals. Therefore, peak heights and areas can be deceptive.

FigureChemical shift (d, ppm)7 carbons give 7 signals, but intensities are not equal

13CH Coupling

13C13C splitting is not seen because the probability of two 13C nuclei being in the same molecule is very small.13C1H splitting is not seen because spectrum is measured under conditions that suppress this splitting (broadband decoupling).Peaks in a 13C NMR spectrum are typicallysinglets

Using DEPT to Count the HydrogensAttached to 13CDistortionless Enhancement of Polarization Transfer

1. Equilibration of the nuclei between the lower and higher spin states under the influence of a magnetic field2. Application of a radiofrequency pulse to give an excess of nuclei in the higher spin state3. Acquisition of free-induction decay data during the time interval in which the equilibrium distribution of nuclear spins is restored4. Mathematical manipulation (Fourier transform) of the data to plot a spectrumMeasuring 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 state3. Acquisition of free-induction decay data during the time interval in which the equilibrium distribution of nuclear spins is restoredMeasuring 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 samesome 13C signals disappearsome 13C signals are inverted Measuring a 13C NMR spectrum involves

Chemical shift (d, ppm)Figure #3CCCHCHCHCH2CH2CH2CH3

Chemical shift (d, ppm)Figure #4CHCHCHCH2CH2CH2CH3

Chemical shift (d, ppm)Figure #5CHCHCHCH2CH2CH2CH3CH and CH3 unaffectedC and C=O nulledCH2 inverted

Proton-Decoupled 13C NMR of 2-Butanol

Proton-Coupled 13C NMR of 2-Butanol

DEPT 13C NMR distinguish among CH3, CH2, and CHgroups

The COSY spectrum identifies protons that are coupledCross peaks indicate pairs of protons that are coupled

COSY Spectrum of 1-Nitropropane

The HETCOR spectrum of 2-methyl-3-pentanoneindicates coupling between protons and the carbon to which they are attached

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