Page 88 - Org 3 theoritical book 2024-25

Clinical Pharmacy PharmD - 2024/2025 Level 2 Pharmaceutical Organic Chemistry-3 (PC 305)

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❖ NMR chemical shift: It is the difference between the
resonance frequency of certain spinning nuclei and that of

the reference molecule signal (TMS: tetramethyl-silane

used as a reference molecule at

zero (0) ppm chemical shift).

❖ NMR chemical shift of certain atom (nucleus-proton) is
influenced by the surrounding electronic density that is

controlled by the electronegativity of neighboring groups.

➢ As the electron density around nucleus  → oppose the applied magnetic field →
nucleus feel  applied field → signal is more shielded (has lower chemical shift

near 0 ppm).
➢ While, as the electron density around nucleus  → add to the applied magnetic field

→ nucleus feel  applied field → signal is more deshielded (has greater chemical
shift toward higher ppm).

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➢ Shielded means it is protected by its electron density, so appears at lower frequency
near 0 ppm.
➢ Deshielded means it is not-protected by its electron density, so appears at higher
frequency at higher ppm value.