7 Infrared Spectroscopy

Organic Chemistry: Structure and Function

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1 Infrared Spectroscopy Part 1 Lecture Supplement: Take one handout from the stage
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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 [email protected] 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
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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
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4 Molecular Vibrations Vibration energy vibration energy average bond length Ground state lower energy add energy Excited state higher energy
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5 Molecular Vibrations Vibration energy Vibrational energy is quantized (only certain energy values are possible) Excited vibrational state Ground vibrational state E = h ν For bond vibrations: ∆Ε = dependent on bond = ~5 kcal mol -1 = lower energy than red light photons = infrared photons ν = stretching frequency Vibrational state energy
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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
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7 Molecular Structure from IR Spectrum How does spectrum give information about molecular structure? Structure controls number of photons absorbed Structure controls stretching frequency
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8 Structure versus Photon Quantity Chance of photon absorption controlled by change in dipole moment ( μ ) Intensity of IR peak Vector sum of bond dipoles δ + X Y δ - Useful approximation Consider only one bond From quantum mechanics:
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