l14 - CH 203 O R G A N I C C H E M I S T R Y I Infrared...

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Infrared spectroscopy © Bruno I. Rubio 1 CH 203 O R G A N I C C H E M I S T R Y I Infrared spectroscopy The presence of the various functional groups in a molecule is experimentally demonstrated by infrared (IR) spectroscopy. Vibrational motion A molecule consisting of N atoms can vibrate 3 N – 5 ways if the molecule is linear or 3 N – 6 ways if the molecule is non-linear. These vibrations are called normal modes. Vibrational motions are classified as either (1) stretches or (2) bends. In a stretch, bond lengths increase and decrease; in a bend, bond angles increase and decrease: stretch bend For reasons that will become clear later, we will be most concerned with stretches. Problem How many normal modes of vibration does HCl posses? Answer HCl is linear so it possesses 3 N – 5 = 6 – 5 = 1 normal mode, namely, H–Cl stretching: H Cl H Cl Problem How many normal modes of vibration does FNO posses? Draw the molecule as it executes these vibrations. Answer FNO is non-linear so it possesses 3 N – 6 = 9 – 6 = 3 normal modes: (1) F–N stretching, (2) N–O stretching, and (3) F–N–O “scissoring”:
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Infrared spectroscopy © Bruno I. Rubio 2 N O F N O F N O F N O F The physical basis of IR spectroscopy Atoms are always executing some kind of vibrational motion, even at absolute zero (0 K). Low-energy vibrations are characterized by a low frequency of vi- bration, whereas high-energy vibrations are characterized by a high frequency of vibration. IR spectroscopy measures the energy difference between a lower- energy and a higher-energy vibrational state of a molecule. A molecule under- goes promotion from a lower-energy to a high-energy vibrational state by ab- sorbing electromagnetic radiation (i.e., photons or light) of the appropriate energy. The energy of electromagnetic radiation is given by the Planck rela- tion E = h ! = hc / " where h is Planck’s constant (6.626 # 10 –34 J·s), ! is the frequency of the electromagnetic radiation, c is the speed of light (3.000 # 10 8 m/s), and " is the wavelength of the electromagnetic radiation. If the energy E 1 of the lower vibrational state corresponds to an energy E 1 = h ! 1 = hc / " 1 and the energy E 2 of the upper vibrational state corresponds to an energy E 2 = h ! 2 = hc / " 2 , then the energy E of electromagnetic radiation that must be absorbed by the molecule to bring about promotion from the lower to the upper vibrational state is E = E 2 ! E 1 = h " 2 ! h " 1 = hc 1 # 2 ! 1 # 1 $ % & ' ( ) = hc * where $ is called the wavenumber and is expressed in units of reciprocal cen- timeter (cm –1 ). Photons whose wavenumber is less than about 14,000 cm –1 but greater than about 5 cm –1 lie in the infrared (IR) portion of the electromagnetic spec- trum. These are the photons that stimulate molecular vibrations. The range of
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Infrared spectroscopy © Bruno I. Rubio 3 wavenumbers that most concerns IR spectroscopy lies between 4000 cm –1 and 400 cm –1 .
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This note was uploaded on 02/27/2012 for the course CH 203 taught by Professor Rubio during the Fall '07 term at BU.

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l14 - CH 203 O R G A N I C C H E M I S T R Y I Infrared...

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