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CHEM 125A: FRESHMAN ORGANIC CHEMISTRY II - Lecture 21 - Professor J. Michael McBride

Functional Groups and Fingerprints in IR Spectroscopy; Precession of Magnetic Nuclei

Infrared spectroscopy provides information for analyzing molecular structure and for understanding bonding and dynamics. Although the normal modes of alkanes involve complex coordinated vibration of many atoms, the unusual strengths of multiple bonds give alkenes and alkynes distinctive stretching frequencies.  The intensity of characteristic out-of-plane C-H bending peaks allows assignment of alkene configuration. Characteristic carbonyl stretching peaks in various functional groups demonstrate the importance of pi- and sigma-conjugation.  The complex fingerprint region of IR spectra differentiates the subtle isomerism of polymorphic crystalline pharmaceuticals. A 90° phase lag between force and velocity explains the precession of tops and of magnetic nuclei in a magnetic field. Nuclear precession in the combination of a stationary magnet and a pulsed radio-frequency field can be visualized by means of the "rotating frame."
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