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Infrared Spectroscopy

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CHEM 205
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Infrared Spectroscopy • Relies on energy of molecular vibrations (high energy) • Frequency of radiation about 10^14Hz, making them 1000000x more energetic than NMR spin transitions • Unit used=wavenumber (in 1/cm)=1/lambda • Gives info about molecular structure, inter and intramolecular interactions, chemical composition • Oscillation of molecule (diatomics)=harmonic oscillation where wavenumber=SEE EQUATIOn. Slide 5. • Selection rules apply. The difference in energy between the resonance levels give rise to the photon energies required • Potential energy=V(x)=.5kx^2 where x is bond length. Is a parabola and the energy needed to move up the parabola is e=hf of photon • Wavenumber=E/hc • The larger the mass, the smaller the wavenumber (less vibration) • When using the wavenumber/harmonic oscillation equation, masses must be given in kg not g/mol. Speed of light should be given in cm/s. k is in N/m. • Selection rules: o 1. Electric dipole moment of molecule must change during vibration: must be polar. Homonuclear diatomics don’t absorb IR o 2. Delta N=+1,-1 •
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