IR - Basic Structural Concepts DISCRETE STRUCTURE OF MAIN...

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- molecules/polyatomic ions STRUCTURE OF MAIN GROUP COMPOUNDS Basic Structural Concepts DISCRETE uestions to be answered Applications – INFRARED SPECTROSCOPY Molecular Symmetry Questions to be answered : -What is it? - What can it tell you about main group molecules/ions? ou will learn how to You will learn how to : - Use symmetry considerations to predict IR-active vibrations - Interpret IR spectra of simple species repare an IR sample in the lab - Prepare an IR sample in the lab - Find literature sources for IR spectroscopy
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- molecules/polyatomic ions STRUCTURE OF MAIN GROUP COMPOUNDS Basic Structural Concepts DISCRETE What is it? Applications – INFRARED SPECTROSCOPY Molecular Symmetry - A physical method for probing vibrational modes of molecules/ions - ample is exposed to Infrared radiation Sample is exposed to Infrared radiation -Infrared radiation causes bonds to vibrate - the vibrations are quantized Æ at certain resonant frequencies, absorption occurs - Fundamental vibrations are most important (ground state Æ 1 st excited state)
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- molecules/polyatomic ions STRUCTURE OF MAIN GROUP COMPOUNDS Basic Structural Concepts DISCRETE ypical spectrum Applications – INFRARED SPECTROSCOPY Molecular Symmetry Typical spectrum
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- molecules/polyatomic ions STRUCTURE OF MAIN GROUP COMPOUNDS Basic Structural Concepts DISCRETE hat can an IR spectrum tell us? Applications – INFRARED SPECTROSCOPY Molecular Symmetry - Bond vibrations provide “chemical fingerprints” Æ used to identify reaction products What can an IR spectrum tell us? Æ absorptions of IR radiation can be characteristic of functional groups e.g. C–H stretching: n = 2900 – 3300 cm -1 - The frequency of bond vibrations related to bond strengths ooke’s Law Æ Hooke s Law k = spring constant m = mass of weight k = stretching force constant for bond
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- molecules/polyatomic ions STRUCTURE OF MAIN GROUP COMPOUNDS Basic Structural Concepts DISCRETE ow can we predict # of IR Active Modes? Applications – INFRARED SPECTROSCOPY Molecular Symmetry - First we determine the total number of vibrations - Requires knowledge of how molecules move…. .their egrees of freedom (DOF) How can we predict # of IR Active Modes? degrees of freedom (DOF) Æ DOF = translational, rotational and vibrational • Total DOF = 3 n ( n = number of atoms) Æ 3 DOF represent translation along x y and z Æ number of rotational modes vary ibrational modes = 3 ibrational modes = 3 For non-linear molecules : 3 For linear molecules : 2 along x y and z only along axes orthogonal to bond axis
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IR - Basic Structural Concepts DISCRETE STRUCTURE OF MAIN...

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