Outline

Start Chapter 18 Spectroscopy and Quantitative Analysis

Electronic SpectroscopyUltraviolet and visible

Where in the spectrum are these transitions?

Review of properties of EM!

c= Where

c= speed of light = 3.00 x 108 m/s = wavelength in meters = frequency in sec-1

E=h or E=hc/

h=Planks Constant = 6.62606 x 1034 J.s

Where in the spectrum are these transitions?

Beer-Lambert Law

AKA - Beer’s LawAKA - Beer’s Law

The Quantitative Picture Transmittance:

T = P/P0

b(path through sample)

P0

(power in)P

(power out) Absorbance:

A = -log10 T = log10 P0/P

The Beer-Lambert Law (a.k.a. Beer’s Law):A = bc

Where the absorbance A has no units, since A = log10 P0 / P

is the molar absorbtivity with units of L mol-1 cm-1

b is the path length of the sample in cmc is the concentration of the compound in solution, expressed in mol L-1 (or

M, molarity)

How do “we” select the How do “we” select the

wavelengthwavelength

to measure the absorbance?to measure the absorbance?

Absorbance vs. Wavelength

A

420 440 460400380

Wavelength, nm

Why?Why?

1. Maximum Response for a given concentration

2. Small changes in Wavelength, result in small errors in Absorbance

Red700 nm

Orange610 nm

Yellow570 nm

Green510 nm

Blue450 nm

Violet350nm

KMnO4

400 500 600 700 800

Abs

Limitations to Beer’s LawLimitations to Beer’s Law““Non-linear behavior”Non-linear behavior”

“Fundamental” “Experimental”

1. Concentration/Molecular Interactions2. Changes in Refractive Index

1. Not Using Peak wavelength

2. Colorimetric Reagent is limiting

Interaction of Light and Interaction of Light and MatterMatter

Start with Atoms Finish with Molecules

Consider Atoms - hydrogen

Energy

Very simple view of Energy states

n=1

n=2

n=3n=4n=5n=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

s1

s0

s2

s3

s4

Electronic SpectrumA

bsor

banc

e

Wavelength, , generally in nanometers (nm)

0.0400 800

1.0

200

UV Visiblemaxwith certain extinction

Make solution of concentration low enough that A≤ 1

(Helps to Ensure Linear Beer’s law behavior)

UV/Vis and UV/Vis and MolecularMolecular StructureStructure

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.

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 The NIST databases have UV spectra for many compoundscompounds You will find molar absorbtivities You will find molar absorbtivities in L•cm/mol, tabulated.in L•cm/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 ~ 10-4 – 10-6 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?