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587 08 wk05b spectroscopy std

587 08 wk05b spectroscopy std - Spectroscopy Why...

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1 Spectroscopy Basics Chemistry 587 •Spectroscopy, Why… absorption of radiation –Atomic –Molecular » Molecular vibrations, … » Molecular Degrees of Freedom » Beer’s Law » •Electromagnetic Radiation • Blackbody radiation – blackbodies – radiation sources used in experiments • wave properties – amplitude – wavelength – propagation – polarization c = ν λ wavelength regions and associated energies » gamma, X-ray, vacuum UV, UV, IR, microwave … • diffraction • coherent radiation • refraction – Dispersion = refractive index variation of a medium with the wavelength or frequency – Snell’s Law • reflection • scattering – Rayleigh – Raman – Bandwidth and the Uncertainty Principle – Relaxation processes (after absorption) 3200 3100 3000 2900 2800 2700 Wavenumber (cm -1 ) 2939 2971 υ a (CH 3 ) 3052 υ (=CH) 2900 υ s (CH 3 ) 2922 3004 2 υ (C=C) 2850 2733 2 δ s (CH 3 ) http://www.wag.caltech.edu/home/jang/genchem/infrared.htm The energy diagram to the right: Longer lines = higher energy transitions Why should we learn about light waves? - Molecules absorb specific frequencies of light. The spectrum is the molecule’s signature.
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2 Photon Absorption by the Molecule http://www.wag.caltech.edu/home/jang/genchem/infrared.htm For vibrational (IR) spectroscopy, Beer’s Law applies A = ε bc T = P/P 0 , so, Group frequency correlation chart for Infrared spectrum of 2-propenenitrile
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