Principles of Spectroscopy

1. HL Chemistry - Option A : Modern Analytical Chemistry Principles of Spectroscopy

2. Definition • Spectroscopy - The study of the interaction of electromagnetic radiation with matter

3. Introduction • Spectroscopy is an analytical technique which helps determine structure. • It destroys little or no sample. • The amount of radiation absorbed by the sample is measured as wavelength is varied.

4. Major Types of Spectroscopy • Infrared (IR) spectroscopy measures the bond vibration frequencies in a molecule and is used to determine the functional group. • Mass spectrometry (MS) fragments the molecule and measures the masses. • Nuclear magnetic resonance (NMR) spectroscopy detects signals from hydrogen atoms and can be used to distinguish isomers. • Ultraviolet (UV) spectroscopy uses electron transitions to determine bonding patterns.

5. Electromagnetic Spectrum

6. Electromagnetic Spectrum

7. Properties ofElectromagnetic Radiation Electromagnetic Radiation • energy radiated in the form of a WAVE caused by an electric field interacting with a magnetic field • result of the acceleration of a charged particle • does not require a material medium and can travel through a vacuum

8. Transmission of Radiation Transmission • rate at which radiation passes through a transparent material is less than through a vacuum • depends upon the kinds and concentrations of atoms, ions, and molecules in the medium • radiation must interact with material • interaction must not undergo permanent energy transfer

9. Reflection of Radiation • Reflection always occurs when radiation passes from one medium to another • Reflection is greatest when two materials have large differences in their refractive indecies

10. Scattering of Radiation Types of Scattering • Scattering by Large Molecules • can be measured • a function of the size and shape of molecule • Raman Scattering • part of the radiation undergoes quantized frequency changes • scattering involving molecules which are considerably smaller than the wavelength of radiation • blue sky results from greater scattering of shorter wavelength visible light

11. Summary of the Type of EM Interactions • Absorption - EM energy transferred to absorbing molecule (transition from low energy to high energy state) • Emission - EM energy transferred from emitting molecule to space (transition from high energy to low energy state) • Scattering - redirection of light with no energy transfer

12. Atomic Absorption • Absorption occurs with only a few well-defined frequencies • Electronic excitation • Several branches of spectroscopy are based on this concept, a few of which are discussed here

13. Absorption vs. Emission hn En En hn hn Eo Eo Absorption Emission

14. Absorption

15. Absorption of Radiation The energy of an exciting photon must equal the energy difference between the ground state and one excited state for absorption to occur… “Quantum Leap”

16. The Spectrum and Molecular Effects =>

17. Fluorescence (Emission)

18. Emission of Radiation • Radiation results from the relaxation of electrons from higher (excited) states to lower energy states • “Emission Spectroscopy” and “Fluorescence Spectroscopy” are based on this concept

19. Equation Definitions • E = energy (Joules, ergs) • c = speed of light (constant) • l = wavelength • h = Planck’s constant • n = “nu” = frequency (Hz) • nm = 10-9 m • Å = angstrom = 10-10 m

20. Key Formulas • E = hn • h = 6.626 x 10-34 J-s • n = frequency in Hz, E = energy • l = c/n • c = 3.0 x 108 m/s • l = wavelength, n = frequency in Hz

21. Visible Light

22. Visible Light Red Orange Yellow Green Blue Indigo Violet R O Y G B I V 700 nm 650 nm 600 nm 550 nm 500 nm 450 nm 400 nm

23. Complementary Colors Absorbed Observed

24. Major Types of Light Spectroscopy • Absorption spectroscopy • Measures amount of light absorbed • Most common, non-destructive • Concentration, pH measures, purity, ID • Atomic emission spectroscopy • Measures light emitted from burned sample • Elemental analysis

25. Types of Light Spectroscopy(continued) • Fluorescence spectroscopy • Samples fluoresce when they emit at higher l than what they absorb • Measures solvent interactions, distances, molecular shape, and motion • Circular Dichroism spectroscopy • Absorption of circular polarized light • Chiral compound identification • Transmission spect. (colorimetry)

26. UV vs. IR vs. NMR • UV has broad peaks relative to IR & NMR • UV has less information than IR & NMR • UV spectra are easier to collect • UV spectra are faster to collect • UV spectrometers are cheaper • UV spectra require only nanograms of material or chemicals

27. Single Beam Spectrophotometer

28. Dual Beam Spectrophotometer

29. Cuvettes (sample holder) • Polystyrene • 340-800 nm • Methacrylate • 280-800 nm • Glass • 350-1000 nm • Suprasil Quartz • 160-2500 nm

30. Definitions • Io = intensity of light through blank • IT = intensity of light through sample • Absorption = Io - IT • Transmittance = IT/Io • Absorbance = log(Io/IT) Io IT

31. Absorbance & Beer’s Law Increasing absorbance

32. Beer’s Law Io IT Io IT pathlength b pathlength b

33. Beer’s Law Absorbance = ebC

34. Absorption Methods “Readout for an inexpensive photometer.” transmittance scale is linear…… absorbance scale is exponential…….. thus, one usually reads transmittance, then calculates absorbance

35. UV Activity • Needs chromophores • C=C, C=O, N=N, NO2, • p -> p* and n -> p* transitions • napthoquinones, anthocyanins • Non-absorbers… • long chain aliphatics • alcohols, ethers, non-conjugated mols.

36. UV Activity hn p p*

37. EM Radiation E M

38. Circular Polarization

39. Circular Dichroism

40. CD can distinguish chirality L-isomer D-isomer

41. pKa Measurement with UV n i Titration of Phenylephrine Ai - A pKa = pH + log A - An