Question

Bathochromic shift depends on

• A. isolated double bonds
• B. conjugated double bond
• C. thermal conductivity
• D. absorption of light

Hint:

UV-Vis Shift terminology checklist: - Bathochromic Shift: Shift to longer \(\lambda\) (Red shift) \(\rightarrow\) caused by increased conjugation. - Hypsochromic Shift: Shift to shorter \(\lambda\) (Blue shift). - Hyperchromic Effect: Increase in absorption intensity (\(\epsilon\)). - Hypochromic Effect: Decrease in absorption intensity.

Answer 1

The Correct Option is B

Concept: In UV-Visible spectroscopy, structural changes within an organic molecule can alter the energy spacing between molecular orbitals (such as the \(\pi \rightarrow \pi^*\) transition gap), resulting in distinct shifts in the wavelength of maximum absorption (\(\lambda_{\max}\)). A bathochromic shift (commonly called a red shift) is an effect that shifts the absorption maximum toward a longer wavelength (lower energy frequency).

Step 1: Understand the effect of Conjugation When a molecule contains conjugated double bonds (alternating double and single bonds), the \(p\)-orbitals overlap continuously across the carbon framework. This extensive orbital mixing creates a delocalized system of \(\pi\) molecular orbitals. As the degree of conjugation increases:
• The energy level of the Highest Occupied Molecular Orbital (HOMO) increases.
• The energy level of the Lowest Unoccupied Molecular Orbital (LUMO) decreases. This narrowing of the energy gap (\(\Delta E\)) means that less energy is required to promote an electron during excitation. According to Planck's equation: \[ \Delta E = \frac{h \cdot c}{\lambda} \] Because energy and wavelength are inversely proportional, a smaller energy gap (\(\Delta E\)) directly translates to a shift toward a longer wavelength (\(\lambda_{\max}\)), producing a bathochromic shift. Isolated double bonds lack this orbital delocalization and do not produce this red shift.