Outline

1. Outline • Start Chapter 18 • Spectroscopy and Quantitative Analysis

2. Electronic SpectroscopyUltraviolet and visible

3. Where in the spectrum are these transitions?

4. Review of properties of EM! • c=ln • Where • c= speed of light = 3.00 x 108 m/s • l= wavelength in meters • n = frequency in sec-1 • E=hn • or E=hc/l • h=Planks Constant = 6.62606 x 1034 J.s

5. Where in the spectrum are these transitions?

6. Beer-Lambert Law AKA - Beer’s Law

7. The Quantitative Picture • Transmittance: T = P/P0 P0 (power in) P (power out) • Absorbance: A = -log10 T = log10 P0/P How do “we” select the wavelength to measure the absorbance? b(path through sample) • The Beer-Lambert Law (a.k.a. Beer’s Law): A =ebc Where the absorbance A has no units, since A = log10 P0 / P e is the molar absorbtivity with units of L mol-1 cm-1 b is the path length of the sample in cm c is the concentration of the compound in solution, expressed in mol L-1 (or M, molarity)

8. Absorbance vs. Wavelength Why? • Maximum Response for a given concentration • Small changes in Wavelength, result in small errors in Absorbance A 380 400 420 460 440 Wavelength, nm

9. Red 700 nm Violet 350nm Orange 610 nm Yellow 570 nm Blue 450 nm Green 510 nm

11. Limitations to Beer’s Law “Non-linear behavior” “Fundamental” “Experimental” • Not Using Peak wavelength • Colorimetric Reagent is limiting • Concentration/Molecular Interactions • Changes in Refractive Index

12. Interaction of Light and Matter Start with Atoms Finish with Molecules

13. Consider Atoms - hydrogen Very simple view of Energy states n=6 n=5 Energy n=4 A n=3 n=2 Wavelength, nm n=1

14. Molecular Spectroscopy

16. Consider molecules • With molecules, many energy levels. Interactions between other molecules and with the solvent result in an increase in the width of the spectra. s4 Rotational States s3 Vibrational States s2 Electronic States s1 s0

17. maxwith certain extinction  UV Visible Electronic Spectrum 1.0 Make solution of concentration low enough that A≤ 1 (Helps to Ensure Linear Beer’s law behavior) Absorbance 0.0 200 400 800 Wavelength, , generally in nanometers (nm)

18. UV/Vis and Molecular Structure

19. The UV Absorption process •   * transitions: high-energy, accessible in vacuum UV (max <150 nm). Not usually observed in molecular UV-Vis. • n  * transitions: non-bonding electrons (lone pairs), wavelength (max) in the 150-250 nm region. • n  * and   * transitions: most common transitions observed in organic molecular UV-Vis, observed in compounds with lone pairs and multiple bonds with max = 200-600 nm. Any of these require that incoming photons match in energy the gap corresponding to a transition from ground to excited state.

20. What are the nature of these absorptions? Example:   * transitions responsible for ethylene UV absorption at ~170 nm calculated with semi-empirical excited-states methods (Gaussian 03W): h 170nm photon  antibonding molecular orbital  bonding molecular orbital

21. Examples Napthalene Absorbs in the UV

22. Experimental details • What compounds show UV spectra? • Generally think of any unsaturated compounds as good candidates. Conjugated double bonds are strong absorbers. • The NIST databases have UV spectra for many compoundsYou will find molar absorbtivities  in L•cm/mol, tabulated. • Transition metal complexes, inorganics

23. Notes on UV/Vis • Qualitatively • Not too useful • Band broadening • Quantitatively • Quite Useful • Beer’s Law is obeyed through long range of concentrations • Thousands of methods • Most commonly used • Detection Limits ~ 10-4 – 10-6 M

24. Notes on UV/Vis (cont’d) • Quant (cont’d) • Cheap, inexpensive, can be relatively fast • Reasonably selective • Can find colorimetric method or use color of solution • Good accuracy ~1-5%

25. What happens to the absorbed energy?