AY 2024 Sem 2 A364 L08 Lecture Notes (Student)

Lesson Overview

• Topic: Nuclear Magnetic Resonance (NMR) Spectroscopy

• Instructor: Lee Heng Wuan

• Approved by: Wu Jing Yi

NMR Basics

• Definition: Analytical technique used to characterize organic molecules, particularly carbon-hydrogen frameworks.

• Types of NMR:

  • 1H NMR: Identifies hydrogen atoms.

  • 13C NMR: Identifies carbon atoms.

• Energy Source: Low-energy radio waves.

Key Concepts

• Nuclear Spins: Nuclei act like little bar magnets, can align with or against an external magnetic field.

• Magnetic Field Impact:

  • Spin states change energy levels depending on alignment with the external magnetic field (B0).

  • Greater B0 causes larger energy differences (ΔE) between spin states.

Mechanism of NMR

• Resonance: Achieved when radiofrequency (RF) matches the energy difference (ΔE) between spin states, inducing a "spin flip" and generating an NMR signal.

• Relation: RF needed for resonance is proportional to the applied magnetic field strength.

Chemical Shifts

• Chemical Environment: Different hydrogen environments yield different chemical shifts; measured in parts per million (ppm) against tetramethylsilane (TMS) as a standard.

• Equivalence: Protons in identical environments are chemically equivalent and produce single signals.

• Non-equivalent Protons: Unique environments yield multiple signals in the spectrum, indicating different types of protons.

Spectrum Analysis

• Key Features:

  • Chemical Shift: Indicates shielding or de-shielding; dependent on functional groups.

  • Multiplicity: Indicates neighboring hydrogen atoms; pattern derived from adjacent hydrogens (n+1 rule).

  • Peak Integration: Area under the peak corresponds to the relative number of protons.

Applications and Case Studies

• Vanillin Production: Explore the use of NMR to analyze purity and structure of vanillin from biosynthesis.

  • Concerns include potential impurities in biosynthetically produced vanillin.

  • NMR can help confirm structure and assess purity levels.