Ch. 18 Fundamentals of Spectrophotometry (spectral lines, orbitals, luminescence)

spectral lines are extremely sharp for atoms

gas phase photon absorptions by single atoms cause transitions among quantized electronic states with NO VIBRATIONS and NO ROTATIONS

18-8. absorption spectrum

graph of absorbance vs wavelength

spectrometer

light source -> wavelength selector (monochromator) -> sample -> light detector

Molar absorptivity

the constant of proportionality between absorbance at a particular wavelength and the product cb, where c is concentration and b is pathlength

Absorbance

logarithmically related to transmittance: A = -logT when all light is transmitted, absorbance is zero when no light is transmitted, absorbance is infinite Absorbance is proportional to concentration: LINEAR IN PATH LENGTH AND CONCENTRATION

spectral lines are very broad for molecules

molecules are more complicated because each electronic state has multitude of VIBRATIONAL ENERGY LEVELS and ROTATIONAL ENERGY LEVELS among which transitions can also occur

EXAM QUESTION: Arrange in decreasing order the wavelength of radiation for phosphorescence, absorption, and fluorescence.

phosphorescence > fluorescence > absorption - if it's asking for energy, absorption would be the greatest

What happens when molecule absorbs light?

- absorb photon, it is promoted to excited state - emit photon: energy of molecule falls by equal amount to energy of photon

Luminescence

- fluorescence and phosphorescence are examples of luminescence - emission of light from excited state molecule - luminescence more sensitive than absorption and can observe single molecules - mirror image of absorption spectrum

EXAM QUESTION: Why do the calibration curves for analysis of solutions by spectrophotometry use absorbance instead of transmittance?

Absorbance is linearly proportional to concentration and path length. Transmittance is not linearly proportional to either path length or concentration

18-32 What is the difference between fluorescence and phosphorescence?

Fluorescence is emission of light with no change in the electronic spin state of the molecule. Phosphorescence is when the electronic spin does not change during emission. It is less probable, so molecules spend more time in excited state prior to phosphorescence than to fluorescence. Phosphorescence has a longer lifetime than fluorescence. Phosphorescence also comes at lower energy than fluorescence because the triplet excited state is lower energy than the singlet excited state

Photon is absorbed when energy matches the gap

Gap is E=hv and conservation of spin selection rule says that electronic excitation occurs with NO CHANGE IN SPIN

18-36 What is the difference between a fluorescence excitation spectrum and a fluorescence emission spectrum? Which one resembles an absorption spectrum?

In an excitation spectrum, the exciting wavelength is varied while detector wavelength is fixed. In emission spectrum, exciting wavelength is held constant and detector wavelength is varied. Excitation spectrum resembles an absorption spectrum because emission intensity is proportional to absorption of the exciting radiation

18-33 What is the difference between luminescence and chemiluminescence?

Luminescence is light given off after a molecule absorbs light Chemiluminescence is light given off by a molecule created in an excited state in a chemical reaction

18-35 Consider a molecule that can fluoresce from the S1 state and phosphoresce from the T1 state. Which is emitted at longer wavelength, fluorescence or phosphorescence?

Phosphorescence is emitted at longer wavelength than fluorescence. Absorption is at shortest wavelength

18-26 a)Why does the slope of the absorbance vs. volume graph change abruptly at equivalence point during spectrophotometric titrations?

Prior to equivalence point, all added Fe(III) binds to the protein to form a red complex whose absorbance is measured in the figure. After equivalence point, there are no more binding sites on protein. The slight increase in absorbance arises from the color of the iron titrant

18-9. Why does a compound whose visible absorption maximum is at 480 nm (blue-green) appear to be red?

color of transmitted light is the complement of the color that is absorbed. If a blue-green light is absorbed, red light is transmitted

18-11. Why is it most accurate to measure absorbances in the range A = 0.3-2?

if absorbance is too high, too little light reaches the detector for accurate measurement. if absorbance is too low, there is too little difference between sample and reference for accurate measurement

EXAM QUESTION:

microwave: molecular rotation energies infrared: vibrational energies ultraviolet: electronic transitions, promote electrons and break chemical bonds visible light: promote electrons to excited state

transmittance

the fraction of incident light that is transmitted by substance T=P/Po where Po is incident irradiance and P is transmitted irradiance NOT LINEARLY PROPORTIONAL TO EITHER PATH LENGTH OR CONCENTRATION

chromophore

the functional group in the molecule that absorbs the light