Spectroscopic Methods

1. Spectroscopic Methods • PART 1

2. Spectroscopic Techniques for Sequence Characterization Highly Useful Spectroscopic Techniques High Resolution NMR of Polymer Solutions (Samples are dissolved) Mass Spectrometry (Samples are vaporized) Useful Spectroscopic Techniques FT-IR spectroscopy Raman Spectroscopy High Resolution Solid State NMR Spectroscopic Techniques Which are Sometimes Useful UV and Visible Spectroscopy (insufficient resolution) Useful Web site for fundamentals: www.organicworldwide.net

3. Selection of Spectroscopic Technique • Each technique is based upon a unique phenomenon: • Infrared spectroscopy; vibrational energy absorption • Raman spectroscopy: inelastic scattering from vibrational levels • NMR: nuclear energy absorption while the sample is located in a magnetic field • Mass spectrometry: ionization • One technique may be better suited than another for a particular problem • It is important to know the limitations of each technique i.e., sample preparation, etc.

4. IR Spectroscopy

5. Interaction of light with matter

11. Characteristics of waves • Amplitude Maximum height of the oscillating stuff

28. Absorption and emission of light from matter

29. The Beer_Lambert Law

30. Types of energies in a molecules • E (Molecules or Atoms)= Transition + Electronic + Vibration + Rotation Quantized Energy levels Microwave frequencies (1 – 10-3 m) IR frequencies (2.5 -15 m, 400 – 4000 (cm-1) Uv-Visb frequencies (200-400 nm)

31. Electronic and Vibration energy levels

32. Molecular Spectroscopy • Energy possessed by molecules is quantised. • When a molecule interacts with radiation there can be changes in electronic, vibrational or rotational energy. • These changes depend on the frequency of the radiation. • Analysis of the energy needed to change from one energy level to another forms basis of molecular spectroscopy.

33. Infrared Spectroscopy • Substances exposed to radiation from frequency range 1014 Hz to 1013 Hz (wavelengths 2.5μm -15μm) • Causing vibrational energy changes in the molecule • These absorb infrared radiation of specific frequencies. • Point is to identify functional groups in the molecule

34. Vibrations in Molecules and Bond Deformations

35. Bond deformation • SIMPLE diatomic molecules can only vibrate one way, by stretching. Br H For these molecules there is only one vibrational infrared absorption.

36. Bond deformation • More complex molecules have more possible deformations O C O symmetric stretch

37. Bond deformation O C O O C O asymmetric stretch

38. Bond deformation O C O

39. Bond deformation O O C

40. Bond deformation O O C

41. Bond deformation O O C

42. Bond deformation O C O

43. Bond deformation C O O

44. Bond deformation C O O

45. Bond deformation C O O bending

46. Wavenumber (cm-1) c = λ f from this equation we can get the reciprocal of the wavelength (1/λ) this is a direct measure of the frequency

47. wavenumber (1/λ) / cm-1 4000 3000 2000 1000 wavelength (λ) / μm 2.5 10 frequency (v) / Hz 1.0 x 1014 2.5 x 1013 the reciprocal is described as the wavenumberit is the wavenumber, measured in cm-1 that is recorded on an infrared spectrum

48. Simple version • Sample placed in ir spectrometer • Subjected to ir radiation • Molecule absorbs energy • Molecule bonds starts to undergo different types of vibration (stretching, bending etc.) • This produces different signals that the detector records as ‘peaks’ on the spectrum.

49. In an IR Spectroscopic chart • Frequencies are different for each molecule • Energy required for vibration depends on strength of bond • Weaker bonds requiring less energy.

50. Important … When an ir spectrum is obtained we do not try to explain the whole thing, simply look for one or two signals that are characteristic of different bonds.