Outline Start Chapter 18 Spectroscopy and Quantitative Analysis

Electronic Spectroscopy Ultraviolet and visible

Where in the spectrum are these transitions?

Review of properties of EM! c= Where c= speed of light = 3.00 x 10 8 m/s = wavelength in meters = frequency in sec -1 E=h or E=hc/ h=Planks Constant = x J. s

Where in the spectrum are these transitions?

Beer-Lambert Law AKA - Beer’s Law

The Quantitative Picture Transmittance: T = P/P 0 b(path through sample) P 0 (power in) P (power out) Absorbance: A = -log 10 T = log 10 P 0 /P The Beer-Lambert Law (a.k.a. Beer’s Law): A =  bc Where the absorbance A has no units, since A = log 10 P 0 / P  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) How do “we” select the wavelength to measure the absorbance? to measure the absorbance?

Absorbance vs. Wavelength A Wavelength, nm Why? 1.Maximum Response for a given concentration 2.Small changes in Wavelength, result in small errors in Absorbance

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

Limitations to Beer’s Law “Non-linear behavior” “Fundamental”“Experimental” 1.Concentration/Molecular Interactions 2.Changes in Refractive Index 1.Not Using Peak wavelength 2.Colorimetric Reagent is limiting

Interaction of Light and Matter Start with Atoms Finish with Molecules

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

Molecular Spectroscopy

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. Electronic States Vibrational States Rotational States s1s1 s0s0 s2s2 s3s3 s4s4

Electronic Spectrum Absorbance Wavelength,, generally in nanometers (nm) UV Visible max with certain extinction  Make solution of concentration low enough that A≤ 1 (Helps to Ensure Linear Beer’s law behavior)

UV/Vis and Molecular Structure

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 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 = nm. Any of these require that incoming photons match in energy the gap corresponding to a transition from ground to excited state.

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):  bonding molecular orbital  antibonding molecular orbital h 170nm photon

Examples Napthalene Absorbs in the UV

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 Lcm/mol, tabulated.The NIST databases have UV spectra for many compounds You will find molar absorbtivities  in Lcm/mol, tabulated. Transition metal complexes, inorganics

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 ~ – M

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%

What happens to the absorbed energy?