CHEM 125A: FRESHMAN ORGANIC CHEMISTRY II - Lecture 20 - Professor J. Michael McBride
Electronic and Vibrational Spectroscopy
Time-dependent quantum mechanics shows how mixing orbitals of different energy causes electrons to vibrate. Mixing 1s with 2p causes a vibration that can absorb or generate light, while mixing 1s with 2s
causes "breathing" that does not interact with light. Many natural
organic chromophores involve mixing an unshared electron pair with a
vacant pi orbital, whose conjugation determines color. Infrared spectra
reveal atomic vibration frequencies, which are related by Hooke’s law to
bond strengths and "reduced" masses. Infrared spectra are complicated
by the coupling of local oscillators of similar frequency to give
"normal" modes. Alkane chains possess characteristic stretching and
bending modes, with descriptive names, that may, or may not, absorb
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