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Chemistry 341

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.

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Chemistry 341

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  1. Chemistry 341 Spectroscopy of Organic Compounds

  2. 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.

  3. Interaction of Light and MatterThe 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.

  4. The Electromagnetic Spectrum • Continuous • Covers a wide range of wavelengths of “light” from radio waves to gamma rays. • Wavelengths (l) range from more than ten meters to less than 10-12 meter.

  5. The Electromagnetic Spectrum

  6. Relationship Between Wavelength, Frequency and Energy • Speed of light (c) is the same for all wavelengths. • Frequency (n), the number of wavelengths per second, is inversely proportional to wavelength: n = c/l • Energy of a photon is directly proportional to frequency and inversely proportional to wavelength: E = hn = hc/l (where h = Plank’s constant)

  7. Wavelength/Spectroscopy Relationships

  8. Spin of Atomic Nuclei • Spin 1/2 atoms: mass number is odd. examples: 1H and 13C. • Spin 1 atoms: mass number is even. examples: 2H and 14N. • Spin 0 atoms: mass number is even. examples: 12C and 16O.

  9. Magnetic Properties of the Proton Related to Spin

  10. D l E = a b s o r b e d l i g h t A p p l i e d M a g n e t i c F i e l d H e x t Energy States of Protons in a Magnetic Field

  11. Nuclear Magnetic Resonance (NMR) • Nuclear – spin ½ nuclei (e.g. protons) behave as tiny bar magnets. • Magnetic – a strong magnetic field causes a small energy difference between + ½ and – ½ spin states. • Resonance – photons of radio waves can match the exact energy difference between the + ½ and – ½ spin states resulting in absorption of photons as the protons change spin states.

  12. The NMR Experiment • The sample, dissolved in a suitable NMR solvent (e.g. CDCl3 or CCl4) is placed in the strong magnetic field of the NMR. • The sample is bombarded with a series of radio frequency (Rf) pulses and absorption of the radio waves is monitored. • The data is collected and manipulated on a computer to obtain an NMR spectrum.

  13. The NMR Spectrometer

  14. The NMR Spectrometer

  15. The NMR Spectrum • The vertical axis shows the intensity of Rf absorption. • The horizontal axis shows relative energy at which the absorption occurs (in units of parts per million = ppm) • Tetramethylsilane (TMS) in included as a standard zero point reference (0.00 ppm) • The area under any peak corresponds to the number of hydrogens represented by that peak.

  16. The NMR Spectrum

  17. Chemical Shift (d) • The chemical shift (d) in units of ppm is defined as: d =distance from TMS (in hz) radio frequency (in Mhz) • A standard notation is used to summarize NMR spectral data. For example p-xylene: d 2.3 (6H, singlet) d 7.0 (4H, singlet) • Hydrogens in identical chemical environments (equivalent hydrogens) have identical chemical shifts.

  18. Shielding – The Reason for Chemical Shift Differences • Circulation of electrons within molecular orbitals results in local magnetic fields that oppose the applied magnetic field. • The greater this “shielding” effect, the greater the applied field needed to achieve resonance, and the further to the right (“upfield”) the NMR signal.

  19. Structure Effects on Shielding • Electron donating groups increase the electron density around nearby hydrogen atoms resulting in increased shielding, shifting peaks to the right. • Electron withdrawing groups decrease the electron density around nearby hydrogen atoms resulting in decreased shielding, (deshielding) shifting peaks to the left.

  20. Structure Effects on Shielding The Deshielding effect of an electronegative substituent can be seen in the NMR spectrum of 1-Bromobutane Br – CH2-CH2-CH2-CH3 d (ppm): 3.4 1.8 1.5 0.9 No. of H’s: 2 2 2 3

  21. Some Specific Structural Effects on NMR Chemical Shift

  22. Spin-Spin Splitting • Non-equivalent hydrogens will almost always have different chemical shifts. • When non-equivalent hydrogens are on adjacent carbon atoms spin-spin splitting will occur due to the hydrogens on one carbon feeling the magnetic field from hydrogens on the adjacent carbon. • The magnitude of the splitting between two hydrogens (measured in Hz) is the coupling constant, J.

  23. Spin-Spin SplittingOrigin of the Doublet

  24. Spin-Spin SplittingOrigin of the Triplet

  25. Spin-Spin SplittingOrigin of the Quartet

  26. Pascal’s Triangle Pattern Relative Peak Height Singlet 1 Doublet 1 : 1 Triplet 1 : 2 : 1 Quartet 1 : 3 : 3 : 1 Quintet 1 :4 : 6 : 4 : 1

  27. The n + 1 Rule If Ha is a set of equivalent hydrogens and Hx is an adjacent set of equivalent hydrogens which are not equivalent to Ha: • The NMR signal of Ha will be split into n+1 peaks by Hx. (where n = # of hydrogens in the Hx set.) • The NMR signal of Hx will be split into n+1 peaks by Ha. (where n = # of hydrogens in the Ha set.)

  28. 1H NMR Spectrum of Bromoethane

  29. 1H NMR Spectrum of 1-Nitropropane

  30. Unknown A(Figure 9.20 Solomons 7th ed.) • Formula: C3H7I • IHD = 0 • No IR data provided • 1HNMR d: 1.90 (d, 6H), 4.33 (sept., 1H)

  31. Unknown B (Figure 9.20 Solomons 7th ed.) • Formula C2H4Cl2 • IHD = 0 • No IR data given • 1HNMR d: 2.03 (d, 3H), 4.32 (quartet, 1H)

  32. Unknown C (Figure 9.20 Solomons 7th ed.) • Formula: C3H6Cl2 • IHD = 0 • No IR data given • 1HNMR d: 2.20 (pent., 2H), 3.62 (t, 4H)

  33. Exceptions to the n+1 Rule • The n+1 rule does not apply when a set of equivalent H’s is split by two or more other non-equivalent sets with different coupling constants. • The n+1 rule does not apply to second order spectra in which the chemical shift difference between two sets of H’s is not much larger than the coupling constant.

  34. NMR: Some Specific Functional Group Characteristics • O-H and N-H will often show broad peaks with no resolved splitting, and the chemical shift can vary greatly. • Aldehyde C-H is strongly deshielded. (d = 9-10 ppm) and coupling to alkyl H’s on adjacent carbon is small. • Carboxylic Acid O-H is very strongly deshielded. (d = 10-12 ppm)

  35. NMR: Some Specific Functional Group Characteristics • Ortho splitting on aromatic rings is often resolved, but meta and para splitting is not. • Cis and trans H’s on alkenes usually show strong coupling, but geminal H’s on alkenes show little or no resolved coupling.

  36. Infrared Spectroscopy • Energy of photons in the IR region corresponds to differences in vibrational energy levels within molecules. • Vibrational energy levels are dependent on bond types and bond strengths. • IR is highly useful to determine if certain types of bonds (functional groups) are present in the molecule.

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