SL13101 L39 - Interpreting NMR data

Learning Objectives

  • After studying this topic you should be able to:

    • Predict splitting patterns arising from multiple adjacent chemical environments.

    • Identify (E)- and (Z)-alkenes by their coupling constants.

    • Predict the splitting patterns of aromatic 1H NMR spectra.

    • Rationalise 1H NMR spectra of aromatic compounds.

  • Suggested additional reading:

    • "Spectroscopic Methods in Organic Chemistry"

    • "Pharmaceutical Analysis: A Textbook for Pharmacy Students and Pharmaceutical Health"

  • Contact for further information: Dr Robin Groleau (rg598@bath.ac.uk)

Interpreting 1H NMR Spectra

Nuclear Magnetic Resonance Basics

  • Nuclear Magnetic Resonance (NMR) is a widely used technique for determining the structure of organic compounds.

  • In ¹H NMR, the application of an external magnetic field results in the flipping of the spin of nuclei.

  • The environment surrounding the proton in the molecule influences the location of signals on the spectrum.

Chemical Shift Values

Typical Values

  • Different functional groups demonstrate typical chemical shift values which are impacted by various factors including solvent, temperature, and additional functional groups:

    • Carboxylic Acid: Sharp signals

    • Aldehyde: Typically around 9-10 ppm

    • Aromatic (Ar) and Vinylic: Signal behavior varies, often appearing in complex formations.

    • Alkyl Groups:

      • 1° Alkyl Alcohols and Hydroxyls (0.5-5.0 ppm)

      • 2° Alkyl (3-4 ppm) and 3° Alkyl (3.5-4.5 ppm)

Spin-Spin Coupling

  • Hydrogen nuclei generate a magnetic field which affects signals for adjacent hydrogens.

  • This phenomenon is known as spin-spin coupling, whereby:

    • The number of split signals equals the number of adjacent non-equivalent hydrogen atoms plus one.

  • Multiplet Analysis:

    • Singlet: 0 adjacent H

    • Doublet: 1 adjacent H

    • Triplet: 2 adjacent H

    • Quartet, Quintet, etc.: As the number of adjacent protons increases, the respective pattern follows a defined distribution (e.g., for triplet 1:3:3:1).

Complex Coupling Patterns

  • Two Adjacent Environments:

    • Coupling is straightforward for protons next to n non-equivalent protons, forming simple multiplets (n + 1 peaks).

  • Multiple Protons:

    • In complex molecules, protons couple to several groups, leading to more complicated patterns such as doublets of doublets (dd).

Karplus Relationship

  • Coupling Constants:

    • Coupling strengths are influenced by relative orientations of nuclei, articulated in the Karplus relationship:

    • Nuclei in the same plane exhibit strong coupling; orthogonal nuclei do not couple.

  • Application in alkenes: distinct coupling among E- and Z- forms due to their unique spatial arrangements.

Aromatic Systems in 1H NMR

Chemical and Magnetic Equivalence

  • Chemical Equivalence:

    • Refers to protons that are in identical environments. In simple systems, this is straightforward; in aromatic systems, symmetry must be accounted for.

  • Magnetic Equivalence:

    • An essential factor distinct from chemical equivalence. Protons may be chemically equivalent but magnetically distinct, influencing their coupling behavior during spectroscopy.

Coupling Across Bonds

  • In aromatic and conjugated systems, protons are capable of coupling over larger distances, often more than 3-bonds away.

  • This capability is beneficial for determining the relative positions of protons and contributes to structural elucidation.

  • As previously noted, coupling constants have overlaps requiring further characterization for accuracy.

  • Example Cases:

  • Analysis can include how many chemical environments are expected in compounds like nitrobenzene, and the multiplicity for each signal.