Spectroscopy 2: Electronic Transitions CHAPTER 14

Simple analytical expressions for electronic transitions cannot be given Consider qualitative features of electronic transitions Spontaneous decay processes: Fluorescence and Phosphorescence Dissociation and predissociation Stimulated radiative decay Laser action

Fig 14.1 Visible absorption spectrum of chlorophyll blue red green

Fig Parity of a σ-orbital is determined by the sign of its ψ upon the inversion process Odd (u) Even (g)

Fig Parity of an π-orbital is determined by the sign of its ψ upon the inversion process Odd (u) Even (g)

Fig 14.2 For Σ states the + or – refers to the overall symmetry of a configuration under reflection in the plane

Fig 17.4 The electronic states of O 2 πu2 πu2 πg1 πg1πu2 πu2 πg1 πg1 Ground state configuration:

Fig 14.4 Coupling of spin and orbital angular momentum in a linear molecule

Selection Rules Changes in angular momentum ΔΛ = 0, ±1 ΔS = 0ΔΩ = 0, ±1 Laporte selection rule: parity must change g → u and u → g are allowed

Fig 14.5 A d-d transition is parity (g → g) forbidden unless a vibration destroys inversion symmetry

Fig 14.6 UV absorption spectrum of SO 2 vibrational structure occurring during the electronic transition S 0 → S 1

Vibrational structure within an electronic transition Often called: vibronic transitions To account for this, apply the Franck-Condon principle: During an electronic transition, the nuclei are effectively stationary As a result of the transition, e-density changes rapidly The nuclei respond to the new force field by vibrating

Fig 14.7 According to the Franck-Condon principle, the most intense transition is “vertical” Most intense vibronic transition is from ground vib level to vib level directly above it

Fig 14.7 Quantum mechanical version Franck-Condon principle: Wavefunctions with the greatest overlap will give the greatest intensity Intensity ~ |S(v f, v i )| 2 where S(v f, v i ) is the overlap integral |S(v f, v i )| 2 is the Franck-Condon factor

Fig 14.9 Typical value range of the Franck-Condon factor R e ≡ ground state bond length R e ’ ≡ excited state bond length

Rotational structure within a vibrational transition of an electronic transition Often called: rovibronic transitions P, Q, and R branches appear for each vibronic transition Because bond length changes significantly, rovibronic branches have more complex structure than in simple vibronic branches

Electronic spectra of polyatomic molecules Absorption can be traced to specific types of electrons Groups called chromophores Transitions involving d-d transitions charge-transfer transitions π * ← π and π * ← n transitions

Absorption Involving d − d Transitions Most transition metal ions are colored (absorb in UV-vis) due to d → d electronic transitions

Colors of Visible Light

Why are transition metal ions colored? Rationalized by Crystal-Field Theory: Normally, d-orbitals are degenerate When ligands bond to the metal ion, they cause different interactions with d electrons Result is splitting of the d-orbitals: ligand field splitting

Effect of ligand field on d-orbital energies

Fig Classification of d orbitals in an octahedral field

Effect of octahedral field on d-orbital energies in [Ti(H 2 O) 6 ] 3+

Fig Electronic absorption spectrum of [Ti(H 2 O)] 3+ (aq) absorbs ~500 nm ∴ appears purple

Effect of ligand field on d-orbital energies

Fig A C=C double bond as a chromophore

Fig A C=O double bond as a chromophore