Page 10 - Instrumental Analysis - Pharm D Clinical- 07-PA403

UV-visible Spectrophotometry

• UV and visible radiations promote electrons to higher energy
excited states.

• In excited state, the σ-electrons occupy an anti-bonding energy
level (σ*) and the transition is termed: σ - σ* transition.

• In excited state, -electrons occupy an anti-bonding energy level
* and the transition is termed:  - * transition.

• While the n-electrons may occupy σ* or * levels to give n - σ* or n
- * transitions.

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• In each possible case, an electron is excited from a bonding orbital into an
empty anti-bonding orbital. Each jump takes energy from light, and a big jump
obviously needs more energy than a small one.

• Each wavelength of light has a particular amount of energy. If that amount of
energy is just right for making one of these energy jumps, then that wavelength
will be absorbed.

• Most applications of absorption spectroscopy are based upon the transitions
 - * and n - * corresponding to absorption into the UV-visible spectral
region (200 – 800 nm).
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