Spectral Shifts: Bathochromic vs. Hypsochromic

Bathochromic Shift (Red Shift)

It is the shift of absorption maximum (λ_max) towards a longer wavelength because of the presence of groups like -OH and -NH₂ (auxochrome) or by a change of solvent.

• Decreasing the polarity of the solvent causes a red shift in carbonyl compounds (n → π* transitions).
• When two or more chromophores are present in conjugation within a molecule, a red shift is observed.

Examples:

• Ethylene (π → π* transition): λ_max = 170 nm

• 1,3-Butadiene (2 conjugated double bonds): λ_max = 217 nm

Hypsochromic Shift (Blue Shift)

It is the shift of absorption maximum (λ_max) towards a shorter wavelength. This may be caused due to the removal of conjugation in a system or by a change of solvent.

Example:

• Aniline: Absorption maximum is at 280 nm because the lone pair of electrons on the nitrogen atom is in conjugation with the π-bond system of the benzene ring.

• In Acidic Solution: The absorption shifts to a shorter wavelength (blue shift to 200 nm). An anilinium ion is formed in acidic solution where the electron pair is no longer available, removing the conjugation.