13. Structure Determination: Nuclear Magnetic Resonance Spectroscopy Based on McMurry’s Organic Chemistry, 7th edition

Analyzing organic compounds Proof of identity

Kcal/mol 1.7 X 103 5.7 X 105 9.5 X 103 4.8 X 102 72 1.2 9.5 X 10-3 10-4 EIMS NMR

The Nobel Prize in Physics 1952 "for their development of new methods for nuclear magnetic precision measurements and discoveries in connection therewith " Felix Bloch Edward Mills Purcell

Figure 13.1: (a) Nuclear spins are oriented randomly in the absence of an external magnetic field but (b) have a specific orientation in the presence of an external field, B0. Some of the spins (red) are aligned parallel to the external field while others (blue) are antiparallel. The parallel spin state is slightly lower in energy and therefore favored. Fig. 13-1, p. 441

Figure 13.2: Dependence of the difference in energy between lower and higher nuclear spin levels of the hydrogen atom Nuclei in different environments (i.e. with different amounts of electron density around them) will require different amounts of energy to “flip” to higher energy different spin state

Table 13-1, p. 442

Figure 13. 4: Schematic operation of an NMR spectrometer Figure 13.4: Schematic operation of an NMR spectrometer. A thin glass tube containing the sample solution is placed between the poles of a strong magnet and irradiated with rf energy. Fig. 13-4, p. 444

Old School: Continuous wave (CW) 40 MHz NMR spectrometer 1960

Continuous wave (CW) 60 MHz NMR spectrum 1964 Old School: Continuous wave (CW) 60 MHz NMR spectrum

Not so old: 1980’s 60 MHz

Yokohama City University 500 MHz NMR spectrometer Northern Kentucky University

The Nobel Prize in Chemistry 1991 "for his contributions to the development of the methodology of high resolution nuclear magnetic resonance (NMR) spectroscopy" Richard R. Ernst

Free-induction decay data and proton-decoupled 13C nuclear magnetic resonance spectra

13C NMR spectrum 1-pentanol : 1 scan 1-pentanol : 200 scans Fig. 13-6, p. 447 13C NMR spectrum 1-pentanol : 1 scan 1-pentanol : 200 scans Figure 13.6: Carbon-13 NMR spectra of 1-pentanol, CH3CH2CH2CH2CH2OH. Spectrum (a) is a single run, showing the large amount of background noise. Spectrum (b) is an average of 200 runs.

13.3 The Nature of NMR Absorptions 1H NMR spectrum 13C NMR spectrum

The Nobel Prize in Chemistry 2002 "for his development of nuclear magnetic resonance spectroscopy for determining the three-dimensional structure of biological macromolecules in solution" Kurt Wüthrich

The Nobel Prize in Medicine 2003 "for their discoveries concerning magnetic resonance imaging " Paul C. Lauterbur Sir Peter Mansfield

More energy to flip nucleus Less energy to flip nucleus d, ppm chemical shift

Information in a 13C NMR spectrum

sp3 Figure 13.7: Chemical shift correlations for 13C NMR. 77 ppm CDCl3 Fig. 13-7, p. 448

Figure 13.10: DEPT–NMR spectra for 6-methyl-5-hepten-2-ol. Part (a) is an ordinary broadband-decoupled spectrum, which shows signals for all eight carbons. Fig. 13-10a, p. 451

Information in a 1H NMR spectrum 13C NMR spectrum

Table 13-2, p. 457

6.5 – 8.0 Table 13-3, p. 458

C6H12O2

Figure 13. 13: The 1H NMR spectrum of bromoethane, CH3CH2Br Figure 13.13: The 1H NMR spectrum of bromoethane, CH3CH2Br. The –CH2Br protons appear as a quartet at 3.42δ, and the –CH3 protons appear as a triplet at 1.68δ. Fig. 13-13, p. 460

Figure 13. 13: The 1H NMR spectrum of bromoethane, CH3CH2Br Figure 13.13: The 1H NMR spectrum of bromoethane, CH3CH2Br. The –CH2Br protons appear as a quartet at 3.42δ, and the –CH3 protons appear as a triplet at 1.68δ. Fig. 13-13, p. 460

C3H7Br

Figure 13. 19: The 1H NMR spectrum of trans-cinnamaldehyde Figure 13.19: The 1H NMR spectrum of trans-cinnamaldehyde. The signal of the proton at C2 (blue) is split into four peaks—a doublet of doublets—by the two nonequivalent neighboring protons. Fig. 13-19, p. 466

Figure 13. 19: The 1H NMR spectrum of trans-cinnamaldehyde Figure 13.19: The 1H NMR spectrum of trans-cinnamaldehyde. The signal of the proton at C2 (blue) is split into four peaks—a doublet of doublets—by the two nonequivalent neighboring protons. Fig. 13-19, p. 466

C10H12O

3H 2H C10H12O2 3H 2H 2H