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UV-Vis Spectra

  • UV-Vis excitations are between occupied and virtual orbitals
  • In the gas phase, individual excitations can be seen with vibrational intermediates
    • Solvent effects smooth this into a broad convoluted curve

Allowed transitions

  • There must be a change in the dipole for the transition to occur

Spin selection rule:

  • Allowed transitions cannot change the spin of the electron
  • Specifically, they cannot change electron spin direction (up to down, down to up)
    • Singlet to singlet
    • Triplet to triplet

Orbital Selection rule

  • A photon can only (and must) change the electron’s angular momentum by 1
    • \(s\ce{->}p\hskip{2cm}p\ce{->}d\hskip{2cm}d\ce{->}f\)
    • No chance or too large of a change is forbidden
  • Can be relaxed when orbital mixing occurs, however absorbance will be small when not enough of a character change occurs

Laporte selection rule:

  • In molecules that have symmetry - dyes tent to be asymmetric so that all excitations are allowed
  • Transitions are forbidden if the orbitals have symmetry around the same point of inversion
    • Forbidden - different symmetry
    • Allowed - same symmetry
    • Forbidden - different symmetry
    • Forbidden - different symmetry