Hypsochromic Shift
Definition and meaning of Hypsochromic Shift in chemistry.
Hypsochromic shift is the shift of an absorption band in a molecule's electronic spectrum to a shorter wavelength, that is, to higher photon energy. It is commonly called a blue shift and is the opposite of a bathochromic (red) shift.
In more detail
It occurs whenever a structural or environmental change widens the energy gap between the ground state and the excited state responsible for the absorption, for example loss of conjugation, protonation of an auxochrome's lone pair, or a solvent-polarity change that destabilizes an n→π* transition. The shift is always described relative to a reference spectrum of the same chromophore under different conditions, since "shorter" only has meaning by comparison. Along with its counterpart, the bathochromic shift, it is used qualitatively in UV-Vis spectroscopy to track changes in conjugation length, protonation state, and solvent environment.
Key facts
| Field | Analytical Chemistry |
|---|---|
| Also called | Blue shift |
| Opposite effect | Bathochromic shift (red shift) |
| Common causes | Loss of conjugation, auxochrome protonation, or solvent-polarity change affecting n→π* transitions |
Aniline shows UV absorption bands near 230 nm and 280 nm because the nitrogen lone pair conjugates with the ring. Protonating the amino group to the anilinium ion removes that conjugation, so the spectrum comes to resemble benzene's, with the longer-wavelength band shifting to about 254 nm, a hypsochromic shift.
Frequently asked questions
What causes a hypsochromic shift?
Anything that increases the energy gap between ground and excited electronic states, such as removing conjugation from a chromophore, protonating an auxochrome's lone pair, or a solvent change that destabilizes the excited state of an n→π* transition.
Is a hypsochromic shift the same as a hypochromic effect?
No. A hypsochromic shift is a change in wavelength (position of the band), while a hypochromic effect is a decrease in absorption intensity; the two often occur together but describe different aspects of the spectrum.