IR - Chapter 16: Infrared Spectroscopy Homework: 16-1,...

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Chapter 16: Infrared Spectroscopy Homework: 16-1, 16-2, 16-6, 16-7
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Infrared Spectral Region Infrared: Near, Mid, Far E
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IR Measures Molecular Vibrations and Rotations + + e- + + e- + + e- r 0 - + Hooke’s Law: PE = 1/2k(r –r 0 ) 2 Anharmic Oscillator-Accounts for nuclear-nuclear repulsion Harmonic Oscillator
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IR Measures Molecular Vibrations and Rotations n = 0 n = 1 n = 2 n = 3
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Infrared Spectral Region Rotational transitions are often superimposed on vibrational transitions for molecules in the gas phase. These transitions are typically “blurred” for molecules in the liquid phase.
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Infrared Selection Rules Recall that vibrational “degrees of freedom” = 3N-6 or 3N-5 for a linear molecule where N is the # of atoms. So for CO 2 , we would expect 4 “normal vibrational modes”. Key Concept: Not all possible vibrational modes for a molecule are IR active.
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Infrared Selection Rules Key Concept*: Not all possible vibrational modes for a molecule are IR active. Selection Rule: A vibration must result in a change in dipole moment to IR active.
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Infrared Selection Rules Concept Te$t Which of these CO 2 vibrations is not IR active?
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Molecular Potential Energy The macroscopic (classical mechanical) version of Hooke’s Law gives: ν vib = 1/2 π (k/m) 1/2 where k is a spring constant that accounts for the stiffness of the spring and m is mass. Notice that the frequency (or energy) of the vibration can take on any value.
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Molecular Potential Energy Ε vib = (n + ½ )h/2 π (k/m) 1/2 n = 0, 1, 2…. . h is Planck’s constant But the vibrations of molecules are quantized! The quantum mechanical solution to the energies of a vibrating molecule reflects this.
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Infrared Selection Rules n = 0 n = 1 n = 2 n = 3 Selection Rule: Δ n = ±1 Exceptions: Because of anharmonicity, Δ n = ±2, ±3 may be observed. These are called overtones.
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Molecular Vibrations For a vibration at 4111 cm -1 , how many vibrations occur in 1 second? (A) <One thousand (B) Between One thousand and One million (C) Between One Million and One trillion (D) >100 trillion 120 trillion vibrations per second! One vibration every 8 x 10 -15 seconds. Concept Te$t
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Molecular Vibrations How much movement occurs in the vibration of a C-C bond?
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IR - Chapter 16: Infrared Spectroscopy Homework: 16-1,...

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