13C NMR

13C NMR• 12C : no nuclear spin• 13C : spin = + 1/2– Low abundance of 13C (1.11%) = weak signal (low

intensity)

• Fourier Transform allows rapid multiple scans to magnify signals and cancel out electronic noise

13C NMR• Same principle as 1H NMR– # Signals = # nonequivalent C in compound– Chemical shift is dependent on e- density• High (shielded) = upfield• Low (deshielded) = downfield

• 13C NMR vs. 1H-NMR– No integration of carbon spectra– Wide range (0-200 ppm) of resonances for

common carbon atoms (protons 1-10 ppm)

Multiplicity

• Proton-coupled 13C NMR– Splitting is due to H atoms bonded to C atom that

is causing the signal

• N + 1 rule (N = # of H atoms bonded to C atom)– i.e. (methylene carbon, N=2)• 2 + 1 = 3, signal split into a triplet

Proton –decoupled 13C NMR• Eliminate carbon-proton interactions – Signals appear as singlets

• Decoupling is achieved with the aid of a saturation pulse.– a sample is irradiated with two different radio

frequencies • One is to excite all 13C nuclei• Second is a broad spectrum of frequencies that causes

all H in the molecule to undergo rapid transitions between their nuclear spin states

Interpreting 13C NMR Spectrum• Number of peaks shows the number of types

of carbon in the sample.• Chemical shifts show types of carbon in the

sample.• Peak size shows up quaternary carbons in the

sample.

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2-Butanol

13C NMR DEPT

• DEPT = Distortionless Enhancement by Polarization Transfer

• Developed to distinguished among CH3, CH2, CH– does not show a signal for quaternary C

• More widely used than proton coupling to determine # of H attached to a C.

13C NMR DEPT

• uses a complex series of pulses in both the 1H and 13C ranges: – 45° all H-bearing peaks are positive– 90° pulse: only CH carbons are seen– 135° pulse:• signals for CH3 and CH carbons give (+) signals

• signals for CH2 carbons give (-) signals

ETHYLBENZENE