Factors affecting UV-Visible absorption spectra|Effect of conjugation,solvent, pH & steric hindrance

In this video, factors affecting UV-Visible absorption spectra is discussed Effect of conjugation, solvent, pH & steric hindrance. The various possible transitions in UV-Visible spectroscopy is also discussed. The spectral shifts or chromic shifts of UV-Visible spectra is also explained, Bathochromic shift, hypsochromic shift, hyperchromic effect, hypochromic effect.
Factors affecting absorption spectra, Effect of conjugation of alkenes (Bathochromic shift) Extension of conjugation leads to both Bathochromic shift and hyperchromic effect
Effect of steric hindrance - trans- stilbene – Coplanar p system is achieved easily cis- stilbene – Bulky phenyl groups on one side of the double bond.
planar structure is distorted. lower lmax and emax hypsochromic shift and hypochromic effect
Effect of solvent
Polar solvents such as water and alcohol forms hydrogen bond with polar molecules (shifts in absorption bands).
Polar solvents stabilises/destabilises the molecular orbital and changes the energy level of ground state or excited state of the molecule (alters the energy gap).
High pure non-polar solvents do not interact with solute molecules.
Effect of polar solvent
p to p* transition - dipole-dipole interaction reduces the energy of excited state more than the ground state.
Decreases the energy gap
Bathochromic shift (shifts to longer wavelength) and hyperchromic effect
n to p* transition – polar solvents form hydrogen bond with ground state molecular orbital better than excited state and reduces the energy of ground state more than the excited state.
Increases the energy gap
Hypsochromic shift (shifts to shorter wavelength) and hypochromic effect.
Effect of pH of sample solution
Acidic/Neutral – phenol
Basic – in phenoxide ion, lone pair on oxygen delocalized the p system of the aromatic ring
Conjugation increases and leads to decrease in energy gap
Bathochromic shift and increase in intensity (hyperchromic effect)