Diamagnetic Anisotropy

Introduction

• Concept: Diamagnetic anisotropy involves the creation of magnetic fields by moving charges.
• Analogy: Current in a loop of wire creates a magnetic field.

Magnetic Fields in a Loop

• Current Flow: Current (I) moves, magnetic fields created.
  • Center: Magnetic field points down.
  • Edge: Magnetic field points down inside, up outside the loop.
• Electrons Role: Electrons move opposite to defined current, creating magnetic fields.

Benzene Example

• Pi Electrons: Benzene has 6 pi electrons affecting magnetic fields.
  • Applied magnetic field (B0) pointing up.
  • Pi electrons circulate, inducing a magnetic field downward at the center.
  • Induced field opposes external magnetic field in some areas.

Magnetic Field Effects on Protons

• Protons in Benzene:
  • Experience both applied and induced magnetic fields.
  • Effective magnetic field is the sum of applied and induced fields.
  • Larger effective field leads to larger differences in energy states (alpha and beta) and higher chemical shifts.
  • Example: Proton on benzene has a chemical shift of ~7.27 ppm.

Larger Rings

• Comparison with Bigger Rings: More pi electrons, similar induction effects.
  • Inner Protons: Experience reduced effective magnetic field, resulting in lower chemical shifts (~-2 ppm).
  • Outer Protons: Experience increased effective magnetic field, resulting in higher shifts (~9 ppm).

Triple Bonds (Acetylene)

• Hybridization Effect: SP hybridized carbon increases electron density.
  • Expected to deshield proton, but shift observed is lower.
  • Proton chemical shift is ~2.5 ppm due to opposing induced magnetic field.

General Conclusion

• Effect: Observed for any molecule with circulating pi electrons in a magnetic field.
  • Influences chemical shift significantly.
• Other Examples: Can apply to protons near double bonds, carbonyls, etc.
  • Demonstrates significant impact on chemical shift due to pi electron circulation.