421-821-chapter-16-17

421-821-chapter-16-17 - Infrared Spectroscopy Despite the...

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Infrared Spectroscopy Despite the Typical Graphical Display of Molecular Structures, Molecules are Highly Flexible and Undergo Multiple Modes Of Motion Over a Range of Time- Frames Motions involve rotations, translations, and changes in bond lengths, bond angles, dihedral angles, ring flips, methyl bond rotations.
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Infrared Spectroscopy A) Introduction 1.) Infrared (IR) spectroscopy : based on IR absorption by molecules as undergo vibrational and rotational transitions. Potential Energy (E) Interatomic Distance (r) o rotational transitions o Vibrational transitions Potential energy resembles classic Harmonic Oscillator
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2.) IR radiation is in the range of 12,800 – 10 cm -1 or l = 0.78 – 1000 μ m - rotational transitions have small energy differences ≤ 100 cm , λ > 100 μ m - vibrational transitions occur at higher energies - rotational and vibrational transitions often occur together 3.) Typical IR spectrum for Organic Molecule Wavenumber (cm -1 ) % Transmittance
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Wide Range of Types of Electromagnetic Radiation in nature. 1. Only a small fraction (350-780 nM is visible light). 2. The complete variety of electromagnetic radiation is used throughout spectroscopy. 3. Different energies allow monitoring of different types of interactions with matter. E=h ν = hc/ λ
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3.) Typical IR spectrum for Organic Molecule - many more bands then in UV-vis, fluorescence or phosphorescence - bands are also much sharper - pattern is distinct for given molecule except for optical isomers - good qualitative tool can be used for compound identification functional group analysis - also quantitative tool intensity of bands related to amount of compound present - spectra usually shown as percent transmittance (instead of absorbance) vs. wavenumber (instead of λ ) for convenience Hexane Hexene Hexyne
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B) Theory of IR Absorption 1.) Molecular Vibrations i.) Harmonic Oscillator Model : - approximate representation of atomic stretching - two masses attached by a spring E = ½ ky 2 where: y is spring displacement k is spring constant
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Vibrational frequency given by: ν = 1/2 π n k / μ where: ν : frequency k : force constant (measure of bond stiffness) μ : reduced mass – m 1 m 2 /m 1 +m 2 If know ν and atoms in bond, can get k: Single bonds :
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ii.) Anharmonic oscillation : - harmonic oscillation model good at low energy levels ( ν 0 , ν 1 , ν 2 , …) as atoms approach, coulombic repulsion force adds to the bond force making energy increase greater then harmonic as atoms separate, approach dissociation energy and the harmonic function rises quicker Harmonic oscillation Anharmonic oscillation Because of anharmonics: at low E, ∆ν = ) 2, 3 are observed which cause the appearance of overtone lines at frequencies at ~ 2-3 times the fundamental frequency. Normally = 1
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iii.) Types of Molecular Vibrations
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421-821-chapter-16-17 - Infrared Spectroscopy Despite the...

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