NMR

NMR Spectrometry Overview

  • NMR (Nuclear Magnetic Resonance) determines molecular structure by analyzing the response of nuclear spins to magnetic fields.

Instrumentation

  • Uses AVANCE 700 ULTRASHIELD™ for signal detection.

  • The sample is placed in a superconducting magnet, emitting and receiving RF (radio frequency) radiation.

NMR Workflow

  • Steps: Sample preparation, data collection, and obtaining NMR spectrum.

  • Output is an NMR spectrum displaying chemical shifts (in ppm).

Basic Principles of NMR

  • NMR identifies organic compounds via the behavior of nuclei (1H and 13C) in a magnetic field.

  • Nuclear Spin: Related to the magnetic dipole generated by spinning charge; can align with or against an external magnetic field.

  • Energy difference between aligned and opposed states affects RF absorption.

  • Resonance occurs when RF energy is applied, causing nuclear spin transitions.

NMR Spectrum Information

  • Four key types of information from an NMR spectrum:

    • Number of signals

    • Chemical shifts (X-axis positions)

    • Integration (area under peaks)

    • Multiplicity (signal splitting)

Chemical Equivalence

  • Chemically equivalent protons show the same chemical shift.

  • Protons are equivalent if they can be interconverted by bond rotation.

  • Diastereotopic protons in the presence of a stereocenter behave differently.

Chemical Shift Dynamics

  • Chemical shifts are affected by electron density:

    • More electron density = shielded (upfield)

    • Less electron density = deshielded (downfield)

  • Typical range for 1H shifts: from d = 0 to 12 ppm.

Signal Splitting and Multiplicity

  • Signal splitting occurs due to neighboring non-equivalent protons:

    • n+1 rule: If there are n adjacent hydrogens, observe n + 1 signals.

    • Example: A doublet indicates one neighboring hydrogen.

Coupling Constants (J)

  • Coupling constant indicates the interaction between hydrogen nuclei, measured in Hz.

  • If two sets of hydrogens are coupled, J is the same for both sets.

Practical Applications

  • Use integration to count hydrogen atoms in each environment.

  • Analyze multiplicities for detailed structural information.

Assignment Instructions

  • Prepare NMR peak assignment tables as part of the assignment for unknown compounds.

  • No wet-lab activities for this experiment; focus on theoretical understanding and data analysis.