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NMR

NMR. Obtaining a 13 C NMR Spectrum. 1 H Broadband decoupling Gives singlet 13 C peaks, provided no F, P, or D present in the molecule) Continuous sequence of pulses at the 1 H frequency causes a rapid reversal of spin orientation relative to the B 0 , causing coupling to 13 C to disappear.

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NMR

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  1. NMR

  2. Obtaining a 13C NMR Spectrum • 1H Broadband decoupling • Gives singlet 13C peaks, provided no F, P, or D present in the molecule) • Continuous sequence of pulses at the 1H frequency causes a rapid reversal of spin orientation relative to the B0, causing coupling to 13C to disappear

  3. Broadband Decoupling 1H channel 13C channel

  4. H3C4-C3H=C2H-C1OOH solvent C-4 C-2 C-3 C-1 10 180

  5. 13C Chemical Shifts • Reference is TMS, sets 0 ppm • A range of 200 ppm • Chemical shifts can be predicted • Empirical correlations • Ex. Alkanes di = -2.3 + 9.1na + 9.4nb – 2.5ng + 0.3nd + 0.1ne + Sij 2-methylbutane di = -2.3 + 9.1*1 + 9.4*2 – 2.5*1 - 1.1 = 22.0 (22.3)

  6. Signal averaging • 13C experiment generally take longer than 1H experiments because many more FIDs need to be acquired and averaged to obtain adequate sensitivity. • NOE effect (enhancement/reduction in signal as a result of decoupling) N4 N4 13C 1H W2 1H 13C N2 N2 N3 N3 W1 13C 1H 1H 13C N1 N1

  7. NOE effect • W2 (Enhancement) dominates in small molecules • Relevant for all decoupling experiments

  8. Other more complex 1D Experiments • 1H NOE experiment • Inversion Recovery Experiment; Determination of T1 • J modulated Spin Echo • INEPT Experiment • DEPT Experiment

  9. Targeted 1H Spin Decoupling • Continuous irradiation at a frequency (n2) that corresponds to a specific proton in the molecule during the 1H NMR experiment • All coupling associated with the protons corresponding to n2 disappears from the spectrum

  10. 1H targeted decoupling (NOE) n2channel 1H channel

  11. 1 3 2 TMS n2

  12. Inversion Recovery

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