7 Infrared Spectroscopy

Organic Chemistry: Structure and Function

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Unformatted text preview: 1 Infrared Spectroscopy Part 1 Lecture Supplement: Take one handout from the stage 2 Midterm Exam 1 Date: Monday April 30 Time: 5:00-6:50 PM Topics: Molecular Structure: Introduction and Review Carbohydrates Location: last name A-H in Franz 1178 last name I-Sh in Haines 39 last name Si-Z in PAB 1425 Question and Answer Session Lecture time, Monday April 30 Submit questions to harding@chem.ucla.edu Label as Question for Q&A Deadline for possible inclusion: noon Sunday April 29 Extra Office Hours (Steve J): Sunday 2-4 PM, Young Hall 3077F 3 Infrared Spectroscopy (IR) Molecular Vibrations Fundamental principle Absorption of photons causes changes in molecular vibrations Molecular Vibrations Bonded atoms move around in space Very fast: one vibration cycle ~10-15 seconds Stretching (H-Cl) Atoms move along bond axis Bending (H-O-H) Motion not along bond axis Less important than stretching 4 Molecular Vibrations Vibration energy vibration energy average bond length Ground state lower energy add energy Excited state higher energy 5 Molecular Vibrations Vibration energy Vibrational energy is quantized (only certain energy values are possible) Excited vibrational state Ground vibrational state E = h For bond vibrations: = dependent on bond = ~5 kcal mol-1 = lower energy than red light photons = infrared photons = stretching frequency Vibrational state energy 6 The Infrared Spectrum Many photons absorbed Spectrum = plot of photon energy versus photon quantity Number of photons absorbed Stretching frequency Proportional to photon energy Typical infrared spectrum: Few photons absorbed 7 Molecular Structure from IR Spectrum How does spectrum give information about molecular structure? Structure controls number of photons absorbed Structure controls stretching frequency 8 Structure versus Photon Quantity Chance of photon absorption controlled by change in dipole moment ( ) Intensity of IR peak Vector sum of bond dipoles + X Y - Useful approximation Consider only one bond From quantum mechanics: 9 Absorption Intensity versus Bond Dipoles...
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7 Infrared Spectroscopy - 1 Infrared Spectroscopy Part 1...

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