Ch_322b_13.07

Ch_322b_13.07 - Chapter 13.7 lecture note

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Chromophores The absorption bands in uv-visible spectroscopy arise from electronic transitions from filled to unfilled molecular orbitals. Usually, these transitions can be assigned to specific structural components within the molecule called chromophores . Examples of chromophores are carbonyl , alkene and aromatic functional groups. These chromophores show characteristic wavelength ranges, absorption patterns, and intensities ( ! max ). Examples of Organic Chromophores chromophore molecule solvent ! max (nm) " max ( M -1 cm -1 ) C=C C C C=O 1-hexene heptane 180 12,500 1-butyne vapor 172 4,500 benzene water 254 203.5 205 7,400 acetone cyclohexane 275 190 22 1,000
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Typical Energies of Electronic Transitions Many organic molecules contain both ! - and "# electrons , and sometimes n-electrons . The energy changes associated with promotion of these different types of electrons varies greatly as shown in the generalized molecular orbital diagram below. electronic energy n (nonbonding) ! (bonding) " (bonding) filled molecular orbitals !# (antibonding) "# (antibonding) Relative Transition Energies n-> !" !# > !" $# > $"
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alkanes , the only electronic transitions are !" > !# that occur in the vacuum uv region. Alkenes show absorptions due to $" > $# transitions just below 200 nm. Polyenes
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This note was uploaded on 10/20/2009 for the course CHEM 322BL at USC.

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Ch_322b_13.07 - Chapter 13.7 lecture note

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