Chapter 14. Nuclear Magnetic Resonance Spectroscopy. 14.1. Introduction to NMR Spectroscopy. NMR spectroscopy is a powerful analytical technique used to characterize structure of organic molecules by identifying connectivity of elements in molecules.
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Nuclear Magnetic Resonance Spectroscopy
14.1. Introduction to NMR Spectroscopy
14.1A. The Basis of NMR Spectroscopy
cf. refrigerator magnet: 0.005Tesla
14.1B. 1H NMR Spectrum
14.2. 1H NMR : Number of Signals
14.2A. General Principles
14.2B. Determining Equivalent Protons in Alkenes and Cycloalkanes
14.2C. Enantiotopic and Diastereotopic Protons
14.3. 1H NMR: Position of Signals
14.3B. Chemical Shift Values
14.4. The Chemical Shift of Protons on sp2 and sp Hybridized Carbons
- Protons on Benzene Rings
- Protons on Carbon-Carbon Double Bonds
- Protons on Carbon-Carbon Triple Bonds
Regions in the1H NMR
14.5. 1H NMR: Intensity of Signals
14.6. 1H NMR: Spin-Spin Splitting (coupling)
14.6A. Splitting: How a Doublet Arises
Let us consider how the doublet due to the CH2 group on BrCH2CHBr2 occurs:
14.6B. Splitting: How a Triplet Arises
14.6C. Splitting: The Rules and Examples
Three general rules describe the splitting patterns commonly seen in the 1H NMR spectra of organic compounds.
If Ha and Hb are not equivalent, splitting is observed when:
Splitting is not generally observed between protons separated by more than three bonds.
14.7. More Complex Examples of Splitting
Whenever two (or three) different sets of adjacent protons are equivalent to each other, use the n + 1 rule to determine the splitting pattern.
Now consider the spectrum of 1-bromopropane. Since Ha and Hc are not equivalent to each other, we cannot merely add them together and use the n + 1 rule.
When two sets of adjacent protons are different from each other (n protons on one adjacent carbon and m protons on the other), the number of peaks in an NMR signal = (n + 1)(m + 1).
14.8. Spin-Spin Splitting in Alkenes
Splitting diagrams for the alkenyl protons in vinyl acetate are shown below. Note that each pattern is different in appearance because the magnitude of the coupling constants forming them is different.
14.9. Other Facts About 1H NMR Spectroscopy
14.9A. OH Protons
14.9B. Cyclohexane Conformations
14.9C. Protons on Benzene Rings
14.10. Using 1H NMR to Identify an Unknown
14.11. 13C NMR Spectroscopy
13C Spectra are easier to analyze than 1H spectra because the signals are not split. Each type of carbon atom appears as a single peak.
Fully decoupled spectrum!
14.11A. 13C NMR: Number of Signals
14.11B. 13C NMR: Position of Signals
14.33, 43, 45, 47, 48, 49, 50, 51, 60