Chem. 133 – 3/13 Lecture

1. Chem. 133 – 3/13 Lecture

2. Announcements I • Exam 1 Average (75) + Distribution • Today’s Lecture • Spectroscopy • Introduction • Properties of Light • Relating light to molecular scale changes • Alternative transitions between ground and excited states • Interpreting spectra • Beer’s Law

3. Spectroscopy A. Introduction 1. One of the main branches of analytical chemistry 2. The interaction of light and matter (for purposes of quantitative and qualitative analysis) 3. Topics covered: - Theory (Ch. 17) - General Instruments and Components (Ch. 19) - Atomic Spectroscopy (Ch. 20) - NMR (Rubinson and Rubinson)

4. Spectroscopy Fundamental Properties of Light Wave-like properties: λ λ = wavelength = distance between wave crests n = frequency = # wave crests/s = wave number = # wave crests/length unit In other media, v = c/n where n = index of refraction Note: when n > 1, v < c v = speed of light Note in vacuum v = c = 3.00 x 108 m/s Even if light travels through other media, wavelength often is defined by value in vacuum Relationships: v = λ·n and = 1/λ

5. SpectroscopyFundamental Properties of Light 1. Wave-like properties - other phenomena: diffraction, interference (covered in Ch.19) 2. Particle-like properties a) Idea of photons (individual entities of light) b) Energy of photons E = hn = hv/l E = hc/l (if l is defined for a vacuum)

6. SpectroscopyAbsorption vs. Emission Absorption - Associated with a transition of matter from lower energy to higher energy Emission - Associated with a transition from high energy to low energy A + hn→ A* A* → A + hn Excited State Energy M* Photon out Ground State Photon in M0

7. SpectroscopyRegions of the Electromagnetic Spectrum Many regions are defined as much by the mechanism of the transitions (e.g. outer shell electron) as by the frequency or energy of the transitions Outer shell electrons Bond vibration Nuclear spin Short wavelengths Long wavelengths Gamma rays X-rays UV + visible Microwaves Radio waves Infrared High Energies Nuclear transitions Inner shell electrons Molecular rotations Low Energies Electron spin

8. SpectroscopyRegions of the Electromagnetic Spectrum Note: Higher energy transitions are more complex because of the possibility of multiple ground and excited energy levels Excited electronic state Rotational levels Vibrational levels Ground electronic state

9. Spectroscopy Alternative Ground – Excited State Transitions These can be used for various types of emission spectroscopy

10. Collisional Deactivation (A* + M → A + M + kinetic energy) Photolysis (A* → B∙ + C∙) Photoionization (A* → A+ + e-) Transition to lower excited state (as in fluorescence or phosphorescence) Some of the above deactivation methods are used in spectroscopy (e.g. photoaccustic spectroscopy and photoionization detector) SpectroscopyAlternative Excited State – Ground State Transitions

11. SpectroscopyQuestions Light observed in an experiment is found to have a wave number of 18,321 cm-1. What is the wavelength (in nm), frequency (in Hz), and energy (in J) of this light? What region of the EM spectrum does it belong to? What type of transition could have caused it? If the above wave number was in a vacuum, how will the wave number, the wavelength, the frequency and the speed change if that light enters water (which has a higher refractive index)? Is a lamp needed for chemiluminescence spectroscopy? Explain. Light associated with wavelengths in the 0.1 to 1.0 Å region may be either X-rays or g-rays. What determines this? What type of transducers could be used with photoionization to make a detector?

12. SpectroscopyTransitions in Fluorescence and Phosphorescence Absorption of light leads to transition to excited electronic state Decay to lowest vibrational state (fluorescence) Transition to ground electronic state or Intersystem crossing (phosphorescence) and then transition to ground state Phosphorescence is usually at lower energy (due to lower paired spin energy levels) and less probable higher vibrational states Excited Electronic State Triplet State (paired spin) Ground Electronic State

13. SpectroscopyInterpreting Spectra Major Components wavelength (of maximum absorption) – related to energy of transition width of peak – related to energy range of states complexity of spectrum – related to number of possible transition states absorptivity – related to probability of transition (beyond scope of class) A* DE dE Ao A dl l (nm)

14. Beer’s Law Transmittance = T = P/Po Absorbance = A = -logT sample in cuvette Light source Absorbance used because it is proportional to concentration A = εbC Where ε = molar absorptivity and b = path length (usually in cm) and C = concentration (M) Light intensity in = Po Light intensity out = P b ε = constant for given compound at specific λ value

15. Beer’s Law – Specific Example A compound has a molar absorptivity of 320 M-1 cm-1 and a cell with path length of 0.5 cm is used. If the maximum observable transmittance is 0.995, what is the minimum detectable concentration for the compound?

16. Beer’s Law–Best Region for Absorption Measurements Determine the Best Region for Most Precise Quantitative Absorption Measurements if Uncertainty in Transmittance is constant High A values - Poor precision due to little light reaching detector % uncertainty Low A values – poor precision due to small change in light 0 2 A