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CHEM 344

CHEM 344. Spectroscopy of Organic Compounds Lecture 1 18 June 2007. Modern Spectroscopic Methods. Revolutionized the study of organic chemistry Can determine the exact structure of small to medium size molecules in a few minutes

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CHEM 344

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  1. CHEM 344 Spectroscopy of Organic Compounds Lecture 1 18 June 2007

  2. Modern Spectroscopic Methods • Revolutionized the study of organic chemistry • Can 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 and use in this course

  3. Interaction of Light and MatterThe Physical Basis of Spectroscopy • Spectroscopy: the study of molecular structure by the interaction of electromagnetic radiation with matter • Electromagnetic spectrum is continuous and covers a very wide range of wavelengths • Wavelengths (l) range from 103to 10-15 meters

  4. The Electromagnetic Spectrum

  5. The Electromagnetic Spectrum

  6. Relationship Between Wavelength, Frequency and Energy • Speed of light (c) is constant 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 E = hc/l = hn (where h = Plank’s constant)

  7. Energy Levels in Molecules • Energy levels within a molecule are discrete (quantized) • Transitions between various energy levels occur only at discrete energies • Transition caused by subjecting the molecule to radiation of an energy that exactly matches the difference in energy between the two levels Eupper – Elower = ΔE = hn

  8. Wavelength/Spectroscopy Relationships

  9. Nuclear Spins • Spin ½ atoms: mass number is odd 1H, 13C, 19F, 29Si, 31P • Spin 1 atoms: mass number is even 2H, 14N • Spin 0 atoms: mass number is even 12C, 16O, 32S NO NMR SIGNAL

  10. Magnetic Properties of the Proton Related to Spin

  11. Energy States of Protons in a Magnetic Field No External Mag. Field External Mag. Field Bo Spin states degenerate Random orientations Two allowed orientations (2I+1) = 2 Aligned with or against direction of Bo

  12. 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

  13. The NMR Experiment • The sample, dissolved in a suitable NMR solvent (e.g. CDCl3, CCl4, C6D6), is placed in the strong magnetic field of the NMR spectrometer • The sample is bombarded with a series of radio frequency (Rf) pulses and absorption of the radio waves is monitored • The data are collected and manipulated on a computer to obtain an NMR spectrum

  14. The NMR Spectrometer

  15. The NMR Spectrometer

  16. The NMR Spectrometer

  17. The NMR Spectrum • The vertical axis shows the intensity of Rf absorption • The horizontal axis shows relative energy at which the absorption occurs (parts per million, ppm) • Tetramethylsilane (TMS, SiMe4) is 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

  18. The NMR Spectrum

  19. Chemical Shift (d) • The chemical shift (d) in units of ppm is defined as: d =shift 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) • Hydrogen atoms in identical chemical environments have identical chemical shifts

  20. 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

  21. Structural 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 (downfield).

  22. Structural Effects on Shielding The deshielding effect of an electronegative substituent can be seen in the 1H-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

  23. Some Specific Structural Effects on NMR Chemical Shift

  24. CHEM 344 Spectroscopy of Organic Compounds Lecture 2 19 June 2007

  25. Review of Lecture 1 • Spectroscopy:thestudy of molecular structure by the interaction of electromagnetic radiation with matter • Energy levels in molecules quantized (ΔE = hv) • NMR uses magnetic fields and radio-waves to flip the spin-state of a nucleus (e.g. 1H, 13C) • Different local magnetic fields within the molecule give rise to different signals in the NMR spectrum • Local magnetic field influenced by local structure of molecule (e.g. electron withdrawing groups) • Equivalent hydrogen atoms = same chemical shift

  26. Spin-Spin Splitting • Non-equivalent hydrogen atoms 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. • This is the origin of signal multiplicity • The size of the splitting between two hydrogen atoms (measured in Hz) is the coupling constant, J.

  27. Spin-Spin Splitting - Origin of the Doublet

  28. Spin-Spin Splitting - Origin of the Triplet

  29. Spin-Spin Splitting - Origin of the Quartet

  30. Pascal’s Triangle # eq. protons Multiplicity Relative Intensity 0 Singlet 1 1 Doublet 1:1 2 Triplet 1:2:1 3 Quartet 1:3:3:1 4 Quintet 1:4:6:4:1 5 Sextet 1:5:10:10:5:1 6 Septet 1:6:15:20:15:6:1

  31. The n + 1 Rule If Ha is a set of equivalent hydrogen atoms and Hx is an adjacent set of equivalent hydrogen atoms which are not equivalent to Ha: (i.e. Ha ≠ Hx) • The NMR signal of Ha will be split into n+1 peaks by Hx. (where n = # of hydrogen atoms in the Hx set.) • The NMR signal of Hx will be split into n+1 peaks by Ha. (where n = # of hydrogen atoms in the Ha set.) • If there are n equivalent protons on an adjacent atom(s), they will split a signal into n+1 peaks.

  32. 1H-NMR Spectrum of Bromoethane

  33. Formula: C3H7I 1H-NMR δ: 1.90 (d, 6H), 4.33 (sept., 1H)

  34. Formula: C2H4Cl2 1H-NMR δ: 2.03 (d, 3H), 4.32 (quartet, 1H)

  35. Formula: C3H6Cl2 1H-NMR δ: 2.20 (pent., 2H), 3.62 (triplet, 4H)

  36. Infrared Spectroscopy • Energy of photons in the IR region corresponds to differences in vibrational energy levels within molecules (~10 kcal/mol = ~40 kJ/mol). • Vibrational energy levels are dependent on bond types and bond strengths, and are quantized. • IR is useful to determine if certain types of bonds (functional groups) are present in the molecule.

  37. IR Spectrum of Ethanol

  38. IR Correlation Table

  39. Key Functional Groups by Region of the IR Spectrum

  40. IR Spectrum of Benzaldehyde

  41. IR Spectrum of Cyclohexanone

  42. IR Spectrum of Propanoic Acid

  43. CHEM 344 Spectroscopy of Organic Compounds Lecture 3 20 June 2007

  44. Review of Lecture 2 • Spin-spin splitting leads to multiplicity in NMR spectra • The size of the splitting between two hydrogen atoms is the coupling constant, J. • n+1 rule - doublet, triplet, quartet….Pascal’s triangle • Infrared radiation excites molecular vibrations • IR bands depend on bond type, strength etc. • IR spectroscopy good for functional group assignment

  45. NMR: 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. • Usually have to simulate 2nd order spectra

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