Chapter 7 - Spectroscopy

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10 Terms

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Rotational spectroscopy

  • Very little energy is needed to change the rotational state of a molecule and the electromagnetic radiation absorbed or emitted lies in the microwave region.

  • Rotational spectroscopy is therefore also known as microwave spectroscopy. Pure rotation spectra are discrete line spectra.

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Vibrational spectroscopy

  • All molecules are capable of vibrating.

  • For complicated molecules the number of vibration modes is very large

  • They typical distance between vibration levels is ont he order of 10-20 to 10-19 J, which corresponds to frequencies in the infrared region of the electromagnetic spectrum.

  • Therefore vibration spectroscopy is also called infrared spectroscopy.

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Electronic transitions

  • The energy needed to change the occupation of the orbitals in a molecules is on the order of 10-19 to 10-18 J

  • Consequently, the photons absorbed or emitted when such transitions occur lie in the ultraviolet (UV) visible region of the electromagnetic spectrum.

  • The technique to probe this is called UV/Vis spectroscopy

  • These transitions are always accompanied by a wide range of transitions in vibrational and rotational states. Therefore the spectrum consists of many lines.

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Fluorescence

  • Some molecules discard their excess energy by emitting radiation.

  • When an electron relaxes back from the excited state to the ground state, it emits a photon. This process is called fluorescence.

  • When a photon is absorbed, it takes the molecule into an excited electronic state

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Production of a photon

  • Photon is absorbed, part of the energy is quickly given to the surroundings.

  • The molecule gives up vibrational energy as it collides with other molecules.

  • However the surrounding molecules might be unable to accept the larger energy needed to lower the molecule to the ground electronic state.

  • In this case the molecule can generate a photon and emits this as radiation as it falls back to the ground state.

  • This photon is fluorescent light.

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Lambert-Beer law

  • E is the molar absorption coefficient.

  • C is expressed in mol/L (concentration)

  • and b in cm (width of cuvette)

  • So the dimension of e is L mol-1 cm-1

  • I is the light intensity, and I0 is the initial light intensity before its shined through the sample.

<ul><li><p>E is the molar absorption coefficient. </p></li><li><p>C is expressed in mol/L (concentration)</p></li><li><p>and b in cm (width of cuvette)</p></li><li><p>So the dimension of e is L mol<sup>-1</sup> cm<sup>-1</sup></p></li><li><p>I is the light intensity, and I0 is the initial light intensity before its shined through the sample.</p></li></ul><p></p>
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Lambert-Beer’s law for mixtures

  • Where ei and ci are the extinction coefficient and the concentration of species i.

<ul><li><p>Where e<sub>i</sub> and c<sub>i</sub> are the extinction coefficient and the concentration of species i. </p></li></ul><p></p>
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What is transmittance?

Fraction (or %) of photons not absorbed

  • Calculated by I/I0

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What is the fingerprint region

The vibration of the whole molecule

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What is phosphorescence

Same as fluorescence but slower.