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
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