Q1(o). Formaldehyde electronic transitions

Answer in brief

The characteristic carbonyl transitions are weak n→π* and stronger π→π*; transitions into σ* require higher energy.

Key points

• The C–H and C–O framework has occupied σ orbitals.
• The lowest characteristic absorption is n→π*, generally weak because it is electric-dipole forbidden in the ideal molecular symmetry and gains vibronic intensity.
• The allowed π→π* band is stronger and at shorter wavelength. Higher-energy n→σ* and σ→σ* excitations lie in the far-UV region.