Klein+Ch+16+NMR

16.1 Introduction to NMR Spectroscopy

  • Nuclear Magnetic Resonance (NMR) Spectroscopy

    • A powerful tool used by organic chemists.

    • Provides structural information about molecules.

  • Involves interaction between electromagnetic radiation and atomic nuclei.

  • Commonly studied nuclei: 1H, 13C, 15N, 19F, 31P.

  • Structures of complex molecules are determined using NMR combined with IR spectroscopy and mass spectrometry.

16.2 Acquiring a 1H NMR Spectrum

  • Magnetic Field Strength

    • Determines the energy gap between spin states.

    • Higher magnetic field strength leads to a larger energy gap and a broader frequency range resonated by protons.

  • NMR Spectrometers

    • Continuous-wave (CW) spectrometers are less common; replaced by pulsed Fourier-transform NMR (FT-NMR).

    • FT-NMR excites all protons simultaneously and records free induction decay (FID), which is converted into a spectrum.

  • Sample Preparation

    • Compounds dissolved in deuterated solvents to avoid interference from solvent protons.

16.3 Characteristics of a 1H NMR Spectrum

  • Three Important Characteristics

    • Location: indicates the electronic environment of protons.

    • Area: corresponds to the number of protons.

    • Shape: indicates the number of neighboring protons (multiplicity).

16.4 Number of Signals

  • Chemical Equivalence

    • Protons in identical electronic environments produce a single signal.

    • Homotopic protons can be interchanged by rotation. Enantiotopic protons can be interchanged by reflection.

    • Diastereotopic protons are not chemically equivalent.

  • Determining Equivalence

    • Use symmetry operations and replacement tests to assess relationships between protons.

16.5 Chemical Shift

  • Definition

    • Chemical shift (δ) = (observed shift from TMS in Hz) / (operating frequency in Hz).

    • Range for common organic compounds: 0-12 ppm.

  • Displacement

    • Downfield (left): deshielded protons.

    • Upfield (right): shielded protons.

  • Benchmark Values

    • Methyl (CH3): ~0.9 ppm

    • Methylene (CH2): ~1.2 ppm

    • Methine (CH): ~1.7 ppm.

16.6 Integration

  • Area Under the Signal

    • Indicates how many protons contribute to that signal.

    • Relative integrations can be represented by step curves indicating the area ratio of each signal.

16.7 Multiplicity

  • Multiplicity Patterns

    • Singlet (1), Doublet (2), Triplet (3), Quartet (4), etc.

    • Multiplicity indicates the number of neighboring protons using the n+1 rule.

  • Coupling Constant (J value)

    • Distance between peaks in a split signal measured in Hz.

  • Factors Affecting Splitting

    • Equivalent protons do not split each other; only different neighboring protons do.

16.8 Analyzing and Drawing 1H NMR Spectra

  • Steps for Structure Proposal

    1. Calculate HDI based on molecular formula.

    2. Analyze number of signals and their integrations.

    3. Analyze each signal (chemical shift, multiplicity, integration).

    4. Assemble fragments into a molecular structure.

16.11 Acquiring a 13C NMR Spectrum

  • Differences from 1H NMR

    • 13C is a minor isotope, thus lower abundance leads to simpler spectra.

    • Usually reports only chemical shift values.

  • Broadband Decoupling

    • Suppresses 13C-1H coupling to yield singlets for carbon signals.

16.12 Chemical Shifts in 13C NMR Spectroscopy

  • Chemical Shift Values

    • Range: 0-220 ppm, depending on electronic environment and hybridization of carbon.

  • Number of signals reflects the number of chemically distinct carbon atoms.

16.13 DEPT 13C NMR Spectroscopy

  • DEPT Technique

    • Differentiates signals based on the number of protons attached to carbons (positive/negative signals).

    • Results in clearer distinction of CH, CH2, and CH3 environments.