1. Lecture 3 INFRARED SPECTROMETRY
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2. INTRODUCTION TO INFRARED SPECTROMETRY
INFRARED SPECTRAL REGIONS
INTRODUCTION TO INFRARED SPECTROMETRY

3. INFRARED ABSORPTION SPECTRUM

4. INTRODUCTION TO INFRARED SPECTROMETRY
Energy of IR photon insufficient to cause electronic excitation but can cause vibrational or rotational excitation Molecule electric field (dipole moment) interacts with IR photon electric field (both dynamic) Magnitude of dipole moment determined by (i) charge (ii) separation of charge
INTRODUCTION TO INFRARED SPECTROMETRY

5. INTRODUCTION TO INFRARED SPECTROMETRY
Vibration or rotation causes varying separation
INTRODUCTION TO INFRARED SPECTROMETRY

6. INTRODUCTION TO INFRARED SPECTROMETRY
Molecule must have change in dipole moment due to vibration or rotation to absorb IR radiation! Absorption causes increase in vibration amplitude/rotation frequency
INTRODUCTION TO INFRARED SPECTROMETRY

7. Molecules with permanent dipole moments (μ) are IR active!
INTRODUCTION TO INFRARED SPECTROMETRY

8. INTRODUCTION TO INFRARED SPECTROMETRY

9. INTRODUCTION TO INFRARED SPECTROMETRY
Types of Molecular Vibrations: Stretch - change in bond length symmetric assymetric Bend – change in bond angle scissoring wagging rocking twisting/tortion
INTRODUCTION TO INFRARED SPECTROMETRY

10. INTRODUCTION TO INFRARED SPECTROMETRY
STRETCH
INTRODUCTION TO INFRARED SPECTROMETRY

11. GENERAL DESIGN OF OPTICAL INSTRUMENTS
BEND
GENERAL DESIGN OF OPTICAL INSTRUMENTS

13. GENERAL DESIGN OF OPTICAL INSTRUMENTS
Only some modes may be IR active: Example CO2 O=C=O linear
GENERAL DESIGN OF OPTICAL INSTRUMENTS

14. GENERAL DESIGN OF OPTICAL INSTRUMENTS

15. GENERAL DESIGN OF OPTICAL INSTRUMENTS

16. GENERAL DESIGN OF OPTICAL INSTRUMENTS
Classical vibrational motion:
GENERAL DESIGN OF OPTICAL INSTRUMENTS

18. GENERAL DESIGN OF OPTICAL INSTRUMENTS

19. CLASSICAL VIBRATIONAL FREQUENCY

20. INFRARED SPECTROSCOPY
What about quantum mechanics? 
INFRARED SPECTROSCOPY

21. SAMPLE PROBLEM
The force constant for a typical triple bond is 1.91 x 103 N/m. Calculate the approximate frequency of the main absorption peak due to vibration of CO.

22. INFRARED SPECTROSCOPY

23.  = ±1 Vibrational Selection Rule Since levels equally spaced - should see one absorption frequency.
SELECTION RULE

24. GENERAL DESIGN OF OPTICAL INSTRUMENTS
Anharmonic oscillator: Must modify harmonic oscillator potential for (i) electron repulsion (steeper at small distances) (ii) dissociation (bond breaks at large distances)
GENERAL DESIGN OF OPTICAL INSTRUMENTS

25. New E-y curve:
IR SPECTROSCOPY

26. IR SPECTROSCOPY

27. VIBRATIONAL MODES

28. How many vibrational mode? HCN, H2CO3

29. Coupling of different vibrations shifts frequencies Coupling likely when: (1) common atom in stretching modes (2) common bond in bending modes (3) common bond in bending + stretching modes (4) similar vibrational frequencies Coupling not likely when (1) atoms separated by two or more bonds (2) symmetry inappropriate
IR SPECTROSCOPY

30. IR SPECTROSCOPY

31. IR SPECTROSCOPY - INSTRUMENTATION
SOURCES
IR SPECTROSCOPY - INSTRUMENTATION

32. IR SPECTROSCOPY - INSTRUMENTATION
TRANSDUCERS
IR SPECTROSCOPY - INSTRUMENTATION

33. IR SPECTROSCOPY - INSTRUMENTATION

34. IR SPECTROSCOPY - INSTRUMENTATION

35. FOURIER TRANSFORM INFRARED SPECTROMETER A photodiode array can measure an entire spectrum at once. The spectrum is spread into its component wavelength and each wavelength is directed onto one detector element. Fourier Analysis is a procedure in which a spectrum is decomposed into a sum of sine and cosine terms called a Fourier series.

36. IR SPECTROSCOPY - INSTRUMENTATION
Fourier Transform Instruments have two advantages:
IR SPECTROSCOPY - INSTRUMENTATION

37. FOURIER TRANSFORM

38. FOURIER TRANSFORM

39. GENERAL DESIGN OF OPTICAL INSTRUMENTS
Unfortunately, no detector can respond on s time scale Use Michelson interferometer to measure signal proportional to time varying signal
GENERAL DESIGN OF OPTICAL INSTRUMENTS

40. IR SPECTROSCOPY - INSTRUMENTATION

41. GENERAL DESIGN OF OPTICAL INSTRUMENTS
If moving mirror moves 1/4  (1/2  round-trip) waves are out of phase at beam-splitting mirror - no signal
If moving mirror moves 1/2  (1  round-trip) waves are in phase at beam-splitting mirror - signal
GENERAL DESIGN OF OPTICAL INSTRUMENTS

42. GENERAL DESIGN OF OPTICAL INSTRUMENTS
Difference in pathlength called retardation 
Plot  vs. signal - cosine wave with frequency proportional to light frequency but signal varies at much lower frequency
One full cycle when mirror moves distance  /2 (round-trip = ) velocity of moving mirror
GENERAL DESIGN OF OPTICAL INSTRUMENTS

43. GENERAL DESIGN OF OPTICAL INSTRUMENTS

44. GENERAL DESIGN OF OPTICAL INSTRUMENTS

45. GENERAL DESIGN OF OPTICAL INSTRUMENTS

46. GENERAL DESIGN OF OPTICAL INSTRUMENTS

47. GENERAL DESIGN OF OPTICAL INSTRUMENTS

48. GENERAL DESIGN OF OPTICAL INSTRUMENTS
APPLICATIONS: FTIR single-beam, dispersive IR double-beam, but FTIR advantages include • High S/N ratios - high throughput • Rapid (<10 s) • Reproducible • High resolution (<0.1 cm-1) • Inexpensive
GENERAL DESIGN OF OPTICAL INSTRUMENTS

49. GENERAL DESIGN OF OPTICAL INSTRUMENTS
IR (especially FTIR) very widely used for • qualitative • quantitative analysis of • gases • liquids • solids
GENERAL DESIGN OF OPTICAL INSTRUMENTS

50. GENERAL DESIGN OF OPTICAL INSTRUMENTS
Most time-consuming part is sample preparation Gases fill gas cell (a) transparent windows (NaCl/KBr) (b) long pathlength (10 cm) - few molecules Liquids fill liquid cell (a) solute in transparent solvent - not water (attacks windows) (b) short pathlength ( mm) - solvents absorb
GENERAL DESIGN OF OPTICAL INSTRUMENTS

51. GENERAL DESIGN OF OPTICAL INSTRUMENTS

52. GENERAL DESIGN OF OPTICAL INSTRUMENTS

53. TRANSFORMATION OF ALKENES AND ALKYNES

55. SAMPLE

56. SAMPLE

57. Group Frequencies: • Approximately calculated from masses and spring constants • Variations due to coupling • Compared to correlation charts/databases

59. Quantitative Analysis: IR more difficult than UV-Vis because • narrow bands (variation in e) • complex spectra • weak incident beam • low transducer sensitivity • solvent absorption IR mostly used for rapid qualitative but not quantitative analysis