Bathochromic Shift

Definition

A bathochromic shift, also known as a red shift, is a phenomenon in which the absorption or emission spectrum of a molecule is shifted to longer wavelengths (lower energy) compared to a reference compound. This shift is typically observed in ultraviolet and visible light spectroscopy and is closely related to the concepts of conjugation, aromaticity, and the chemistry of vision.

5 Must Know Facts For Your Next Test

1. Bathochromic shifts are often observed in molecules with extended conjugation, as the delocalization of electrons lowers the energy gap between the ground and excited states.
2. Aromatic compounds, such as benzene and its derivatives, commonly exhibit bathochromic shifts due to their highly conjugated and stabilized pi-electron systems.
3. In the context of vision, the bathochromic shift of the visual pigments in the human eye's photoreceptors is responsible for the perception of color.
4. Substituents that can donate electrons or increase the degree of conjugation in a molecule tend to cause bathochromic shifts, while electron-withdrawing groups or reduced conjugation can lead to hypsochromic shifts.
5. Bathochromic shifts are important in the interpretation of ultraviolet and visible light spectra, as they provide information about the electronic structure and degree of conjugation in organic compounds.

Review Questions

• Explain how the concept of conjugation is related to the bathochromic shift observed in ultraviolet spectroscopy.
  • The bathochromic shift observed in ultraviolet spectroscopy is closely linked to the concept of conjugation. Conjugation refers to the presence of alternating single and double bonds in a molecule, which allows for the delocalization of electrons. This delocalization of electrons lowers the energy gap between the ground and excited states of the molecule, resulting in a shift of the absorption spectrum to longer wavelengths, or a bathochromic shift. Molecules with extended conjugation, such as aromatic compounds, commonly exhibit more pronounced bathochromic shifts due to their highly stabilized and delocalized pi-electron systems.
• Describe how the bathochromic shift is involved in the chemistry of vision and the perception of color.
  • The bathochromic shift plays a crucial role in the chemistry of vision and the perception of color. The visual pigments in the photoreceptors of the human eye, known as rhodopsin and the cone opsins, undergo a bathochromic shift when they absorb light. This shift in the absorption spectrum is caused by the delocalization of electrons in the conjugated systems of the visual pigments. The specific bathochromic shift observed in each type of photoreceptor, due to the unique structure of its visual pigment, allows the eye to perceive a wide range of colors. The ability to detect and differentiate colors is a direct result of the bathochromic shifts experienced by the visual pigments in the retina.
• Analyze how the presence of substituents on an aromatic compound can influence the bathochromic shift observed in its ultraviolet spectrum.
  • The presence and nature of substituents on an aromatic compound can significantly influence the bathochromic shift observed in its ultraviolet spectrum. Substituents that are able to donate electrons or increase the degree of conjugation in the molecule tend to cause a bathochromic shift, as they lower the energy gap between the ground and excited states. For example, the addition of an electron-donating group, such as a hydroxyl (-OH) or an amino (-NH2) group, to an aromatic ring can result in a bathochromic shift. Conversely, substituents that are electron-withdrawing or reduce the overall conjugation in the molecule can lead to a hypsochromic shift, where the absorption spectrum is shifted to shorter wavelengths. Understanding the effects of substituents on the bathochromic shift is crucial for interpreting the ultraviolet spectra of aromatic compounds and gaining insights into their electronic structure and degree of conjugation.

Related terms

Hypsochromic Shift: A hypsochromic shift, or blue shift, is the opposite of a bathochromic shift, where the absorption or emission spectrum is shifted to shorter wavelengths (higher energy).

Conjugation:

Conjugation refers to the presence of alternating single and double bonds in a molecule, which can lead to the delocalization of electrons and affect the molecule's absorption and emission properties.

Aromaticity:

Aromaticity is a property of certain cyclic compounds, particularly benzene and its derivatives, which exhibit enhanced stability and unique electronic properties due to the delocalization of pi electrons.