lecture 12 - Infrared Spectroscopy Chapter 12 Figure 12.1...

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1 Infrared Infrared Spectroscopy Spectroscopy Chapter 12 Figure 12.1, p.473 Electromagnetic Radiation ± Electromagnetic radiation: Electromagnetic radiation: light and other forms of radiant energy ± Wavelength ( Wavelength ( λ ): the distance between consecutive peaks on a wave Electromagnetic Radiation Electromagnetic Radiation ± Frequency ( ν ): the number of full cycles of a wave that pass a given point in a second ± Hertz (Hz): the unit in which radiation frequency is reported; s -1 (read “per second”) Table 12.1, p.472 Molecular Vibrations Molecular Vibrations atoms joined by covalent bonds undergo continual vibrations relative to each other the energies associated with these vibrations are quantized; within a molecule, only specific vibrational energy levels are allowed the energies associated with transitions between vibrational energy levels correspond to frequencies in the infrared region, 4000 to 400 cm -1
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2 Molecular Spectroscopy ± Molecular spectroscopy: Molecular spectroscopy: the study of which frequencies of electromagnetic radiation are absorbed or emitted by a particular substance and the correlation of these frequencies with details of molecular structure Molecular Spectroscopy ± THREE TYPES OF MOLECULAR SPECTROSCOPY Vibrational energy levels Nuclear spin states Electronic energy levels Infrared Radio fequency Ultraviolet- visible Absorption of Electromagnetic Radiation Results in Transition Between Region of the Electromagnetic Spectrum Nuclear magnetic re so nan ce Infrared Ultraviolet- visible Type of Spectroscopy Infrared Spectroscopy ± The vibrational IR extends from 2.5 x 10 -6 m (2.5 μ m) to 2.5 x 10 -5 m (25 μ m) the frequency of IR radiation is commonly expressed in wavenumbers wavenumber : the number of waves per centimeter, with units cm -1 (read reciprocal centimeters) or the frequency in hertz divided by c (speed of light) ( ν ) - Infrared Spectroscopy Infrared Spectroscopy ± The vibrational IR extends from 2.5 x 10 -6 m (2.5 μ m) to 2.5 x 10 -5 m (25 μ m) expressed in wavenumbers, the vibrational IR extends from 4000 cm -1 to 400 cm -1 ν = = 400 cm -1 = 4000 cm -1 ν = 10 -2 m•cm -1 2.5 x 10 -6 m 10 -2 m•cm -1 2.5 x 10 -5 m Molecular Vibrations ± For a molecule to absorb IR radiation the bond undergoing vibration must be polar and its vibration must cause a periodic change in the bond dipole moment Molecular Vibrations Molecular Vibrations ± Covalent bonds which do not meet these criteria are said to be IR inactive the C-C double and triple bonds of symmetrically
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lecture 12 - Infrared Spectroscopy Chapter 12 Figure 12.1...

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