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Spectrometry Lecture Notes
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SpectrometrySpectroscopy
Quantisation of Energy
All energy levels in molecule are QUANTISED
Occupancy of energy levels
Molecules distributed over energy according to Boltzmann distribution:
What is
electromagnetic radiation (EMR)?
What is electromagnetic radiation absorbed?
Electrons and nuclei occupy quantised energy levels
For electromagnetic radiation to be absorbed
o Must be two or more energy levels
o Charge displacement must occur:
Linear displacement- electric transition dipole
Rotation of charge- magnetic transition dipole
o ΔE=hu
Electromagnetic Spectrum
Electronic Spectroscopy Absorption/
Fluorescence Spectroscopy
, Interaction of EMR with electrons in molecule Basics of
Spectroscopy
Characteristics of Spectrum Frequency/ Wavelength
Wavelength
Types of electronic
transitions
Transition
involving p, s and
n electrons
o Organic
compounds
Saturated (ss* (<150nm), n s* (<250nm)
Unsaturated (p-p*, np* (200-700 nm)
Transitions involving charge transfer
o Electrons move from ligand donor to metal receptor
Transitions involving d and f electrons
o Transitions between d-orbitals split by presence of ligand
Electronic transitions in organic molecules
Transition involving p, s and n electrons
o Organic compounds
Saturated (ss* (<150nm), n s* (<250nm)
Unsaturated (p-p*, np* (200-700 nm)
n- non-bonding electrons
p- p bond
s- s bond
So what determines lmax
It depends on types of molecular orbitals present in molecule
No easy rule for assignment, however:
o Presence of resonance >300nm suggests presence of conjugates polyene chain/polycyclic aromatic
o Conjugated systems typically have larger extinction coefficients
Refer to databases
Intensity- population difference
Intesity depends number of molecules that can be excited from ground state
If ΔE ~ kT- population difference is small- sensitivity low
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