Download Chapter 14 - Lecture 1

Spectroscopy 2:
Electronic Transitions
CHAPTER 14
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Simple analytical expressions for electronic
transitions cannot be given
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Consider qualitative features of electronic transitions
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Spontaneous decay processes:
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Fluorescence and Phosphorescence
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Dissociation and predissociation
Stimulated radiative decay
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Laser action
Fig 14.1 Visible absorption spectrum of chlorophyll
blue
red
green
Fig 11.22 Parity of a σ-orbital is determined by the sign
of its ψ upon the inversion process
Even (g)
Odd (u)
Fig 11.27 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 O2
Ground state
configuration:
πu2 πu2 πg1 πg1
Fig 14.4 Coupling of spin and orbital angular momentum
in a linear molecule
Selection Rules
• Changes in angular momentum
ΔΛ = 0, ±1
ΔS = 0
ΔΩ = 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 SO2
vibrational structure
occurring during the
electronic transition
S0 → S1
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(vf, vi)|2
where S(vf, vi) is the
overlap integral
• |S(vf, vi)|2 is the
Franck-Condon factor
Fig 14.9 Typical value range of the Franck-Condon factor
Re ≡ ground state bond length
Re’ ≡ 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 14.12 Classification of d orbitals in an octahedral field
Effect of octahedral field on d-orbital energies in [Ti(H2O)6]3+
Fig 14.13 Electronic absorption spectrum of [Ti(H2O)]3+(aq)
absorbs ~500 nm
∴ appears purple
Effect of ligand field on d-orbital energies
Fig 17.13 A C=C double bond as a chromophore
Fig 17.13 A C=O double bond as a chromophore