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IR Spectroscopy CHM 411 Suroviec
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I. Theory of IR Spectroscopy Frequency of absorbed radiation is the molecular vibration frequency responsible for absorption process – which is directly proportional to wavenumber
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A. Dipole Moment HCl Charge distribution around molecule is not symmetrical and therefore has a dipole moment
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B. Rotation and Vibrational Transitions Rotational Vibrational
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C. Types of Molecular Vibrations The relative positions of atoms in a molecule are not fixed but fluctuate continuously as a consequence of a multitude of different types of vibrational and rotational allowed states around the bonds in the molecule
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II. Vibrational Modes It is possible to deduce the number and kinds of vibrations in simple diatomic and triatomic molecules
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A. Motion of a molecule Need to consider: 1.Motion of a molecule through space (translational) 2.Rotation around center of gravity 3.Individual vibrations
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B. Vibrational Coupling Energy of vibrations and wavelength of absorption is influenced by several factors: 1.Strong coupling between stretching vibrations only occurs when there is an atom in common to the 2 vibrations 2.Interaction between bending vibrations requires a common bond between vibrational groups 3.Coupling between stretching and bending vibrations can occur if stretching bond forms one side of an angle that varies 4.No interaction between groups separated by 2 or more bonds
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Example: CO 2
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Example: H 2 O
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III. Instrumentation A. ATR
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B. FTIR
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