Chemistry 341 spectroscopy lecture

Chemistry 341 Spectroscopy of Organic Compounds Modern Spectroscopic Methods  Revolutionized the study of Organic Chemistry  Determine the exact structure of small to medium size molecules in a few minutes  Nuclear Magnetic Resonance (NMR) and Infrared Spectroscopy (IR) are particularly powerful techniques which we will focus on Interaction of Light and Matter The Physical Basis of Spectroscopy  Quantum properties of light (photons)  Quantum properties of matter (quantized energy states)  Photons of light act as our “quantum probes” at the molecular level giving us back precise information about the energy levels within molecules The Electromagnetic Spectrum  Continuous  Covers a wide range of wavelengths of “light” from radio waves to gamma rays  Wavelengths (λ) range from more than ten meters to less than 10-12 meter The Electromagnetic Spectrum Relationship Between Wavelength, Frequency and Energy  Speed of light (c) is the same for all wavelengths  Frequency (ν), the number of wavelengths per second, is inversely proportional to wavelength: ν = c/λ  Energy of a photon is directly proportional to frequency and inversely proportional to wavelength: E = hν = hc/λ (where h = Plank’s constant) Wavelength/Spectroscopy Relationships Spectral Region Photon Energy Molecular Energy Changes UV-Visible ~ 100 kcal/mole Electronic Infrared (IR) ~ 10 kcal/mole Bond vibrations Radio < 0.1 kcal/mol Nuclear Spin states in a magnetic field Spin of Atomic Nuclei  Spin 1/2 atoms: mass number is odd examples: 1H and 13C  Spin atoms: mass number is even examples: 2H and 14N  Spin atoms: mass number is even examples: 12C and 16O Magnetic Properties of the Proton Related to Spin Energy States of Protons in a Magnetic Field ∆ E = λ absorbed light Applied Magnetic Field H ext IR Correlation Table Key Functional Groups by Region of the IR Spectrum IR Spectrum of Benzaldehyde IR Spectrum of Cyclohexanone IR Spectrum of Propanoic Acid Unknown A (Figure 14.27 Solomons 7th ed.)  Formula = C9H12  IHD =  IR shows no medium or strong bands above 1650 cm-1 except C-H stretching bands around 3,000 cm-1  HNMR δ: 1.26 (d, 6H), 2.90 (sept., 1H), 7.1-7.5 (m, 5H) Unknown B (Figure 14.27 Solomons 7th ed.)  Formula = C8H11N  IHD =  IR shows two medium peaks between 3300 and 3500 cm-1 No other medium or strong bands above 1650 cm-1 except C-H stretching bands around 3,000 cm-1  HNMR δ: 1.4 (d, 3H), 1.7 (s, br, 2H), 4.1(quart., 1H), 7.2-7.4 (m, 5H) Unknown C (Figure 14.27 Solomons 7th ed.)  Formula = C9 H10  IHD =  IR shows no medium or strong bands above 1650 cm-1 except C-H stretching bands around 3,000 cm-1  H NMR δ: 2.05 (pent., 2H), 2.90 (trip., 4H), 7.1-7.3 (m, 4H) Unknown H (Figure 9.48 Solomons 7th ed.)  Formula = C3H4Br2  IHD =  No IR data given  1HNMR δ: 4.20 (2H), 5.63 (1H), 6.03 (1H) Unknown Y (Figure 14.34 Solomons 7th ed.)  Formula = C9H12O  IHD =  IR shows a strong, broad, absorbance centered at 3400 cm-1  HNMR δ: 0.85 (t, 3H), 1.75 (m, 2H), 4.38 (s, br, 1H), 4.52 (t, 1H), 7.2-7.4 (m, 5H)