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Electronic Spectroscopy Notes

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CHEM 205
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Electronic Spectroscopy • UV/Vis spectroscopy (200-700nm) • Looks at transitions between different electronic states of molecules (excitement of electrons) o Electrons absorb or emit photons to change energy levels between different molecular orbitals o Usually give broad spectra due to electronic transitions accompanied by vibrational/rotational transitionsnot very useful as a structural determination technique o Absorbance plotted against wavelength • When absorption occurs, our eyes see the colors/wavelengths that aren’t absorbed • Molecular orbitals: o Atomic orbitals overlap to form bonds=linear combination of atomic orbitals (LCAO)create bonding/antibonding molecular orbital pairs o σ bonds o π bonds (double bonds) o n (nonbonding=lone pairs) o antibonding MO: σ*, π* o HOMO=highest occupied MO (σ, π, n) o LUMO=lowest unoccupied MO (σ*, π*) o We mostly look at HOMOLUMO transitions (orbital types help determine energy required for the transition) • Most of what we consider: o n σ*, π* (npi* for carbonyl group) o σσ* (not really since this occurs for most air particles at less than 200nm) o π π*----aromatic ring, conjugated pi system • relative energies are: sigma, pi, n, pi*, sigma* • conjugated pi system=resonance of double bonds that are separated by single bonds • • **the larger the conjugated system, the closer the MOs are to each other, the smaller the energy change, thus the larger the wavelength. the more conjugated, the redder the color, while less conjugated you get a nice yellow color 2 • E~1/L where L is the length of the molecule • Fluorescence + Phospho
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