10_Spectroscopy_IR

10_Spectroscopy_IR - Chem 267 Basic Organic Chemistry II...

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Prof. Dr. Xiangdong Fang University of Waterloo Chem 267 Basic Organic Chemistry II 1
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Light is an electromagnetic phenomenon Electric Field Magnetic Field w a v e le n g t h Electric wave Magnetic wave Direction of motion of the light beam “a beam of light consists of two mutually oscillating fields: 1) electric field; 2) Magnetic field” 2
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This electromagnetic field travels at the speed of light νλ = c C : speed of light; 2.999 x 10 8 m · s -1 in vacuum; ν : frequency of light; in Hz (number of cycles per second); λ : wavelength of light; in meter (distance spanned by one cycle); ν c = 1 = c = ν : wavenumber; number of cycles in one meter; 3
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Acceleration of a Charged Particle λ hc E = ν h E = “when a charged particle accelerates in space, it must radiate energy at the speed of light (related to the electromagnetic field) as packets of energy called photons; the energy of a photon is inversely proportional to the wavelength of light” lower energy, smaller frequency, larger wavelength, smaller wavenumber higher energy, greater frequency, smaller wavelength, larger wavenumber 4
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Spectroscopy 1) Definition: “Spectroscopy detects transitions between the quantized energy levels of an atom or molecule, where the energies are due to some internal motion of the atom or molecule.” a) in an atom: these motions are due to the changes in the electronic configuration ; b) in a molecule: these motions are due to the changes in: rotations (microwave spectroscopy) vibrations (infrared spectroscopy) electronic excitations (UV-Visible spectroscopy) nuclear spin orientation (nulcear magnetic resonance spectroscopy) 2) The transitions are recorded by measuring discrete frequencies of electromagnetic radiation that are absorbed or emitted by the atom or molecule; 3) By characterizing the energy levels of a molecule, we can describe the geometry of that molecule, including bond lengths and strengths, the identity and position of its bonding atoms, the energy levels of the molecular orbitals, and even the overall framework and shape of the molecule; 5
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100 cm (cm-1) 108 1 cm 10 -2 10 -1 10 -3 10 -3 104 106 1 100 100 m 1 m 10 nm wavenumber 10 m 100 pm wavelength 3 x 10 6 3 x 10 8 3 x 10 10 3 x 10 12 3 x 10 12 3 x 10 14 3 x 10 18 10 10 3 10 5 10 7 10 9 frequency (Hz) energy (J/mol) low energy high energy STD broad cast TV microwaves IR
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This note was uploaded on 07/07/2010 for the course CHEM 267 taught by Professor Fang during the Winter '09 term at Waterloo.

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10_Spectroscopy_IR - Chem 267 Basic Organic Chemistry II...

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