# 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