interacts with the spin energies either of atomic nucleus or electrons forming the base
for NMR and ESR spectroscopy. Radiation with somewhat higher energy provokes
changes in the rotation and vibration of molecules. In particular the later one is
related to the wave length area of infrared radiation and, therefore, this area is used
for infrared or IR spectroscopy. In the energy range of visible and ultraviolet light,
molecules are able to absorb the photons by changing the energy state of their
valence electrons leading to the application of UV/Vis spectroscopy. At higher
energies in the X-ray range the inner electrons become excited to release the atom
representing the base for atom absorption spectroscopy AAS.
However, in contrast to NMR/ESR, IR and UV/Vis spectroscopy applied (not
exclusively but intensively) for the analysis of organic molecules, AAS is a method
for elemental analysis and not common in organic analyses. Further on, NMR
spectroscopy as an extraordinary powerful tool for structure elucidation has only a
very limited application in Organic Environmental and Geochemistry due to some
aspects: (i) this method needs a relative high amount of material or analytes in the
range of mg, (ii) information on chemical structures of substances are only available
for isolated material (no complex mixtures), and (iii) the time of measurement is high
(minutes to hours) and, consequently NMR is not successfully linkable to chromatographic techniques such as gas or liquid chromatography. These restrictions avoid
the analyses of environmental or fossil samples that are characterized by complex
mixtures at partly very low concentrations, that normally need an intensive separation by chromatography. However, some special applications of NMR in Organic
Geochemistry exist, e.g. the characterization of humic substances in soil by solid
state NMR.
Consequently, the following two chapters focus on the UV/Vis and IR
spectroscopy.
4.3.2 UV/Vis Spectroscopy
As the basic physical principle of UV/Vis spectroscopy, the energy of UV/Vis
radiation fits the energy differences of the valence orbitals in organic molecules. In
simple words, the absorption of a photon in the UV/Vis range provokes a valence
electron to jump from its ground state orbital to an orbital with higher energy.
Energies of molecule orbitals are certainly quantized and, therefore, only selected
differences are possible leading to clear restricted transition. A general differentiation of orbitals available for the transition are given in Fig. 4.35.
In ground state the valence electrons are either in bonding σ- or π-orbitals as well
as non-bonding orbitals. From these orbitals they can jump into anti-bonding σ
à - or
π
à -orbitals. It is obvious, that transitions from σ-orbitals to higher energy orbitals
need more energy as compared to π-orbitals or non-bonding orbitals. Hence, electrons in double bounds (π-orbitals) or from free electron pairs (e.g. nitrogen, oxygen)
can be used for absorption at lower energy, whereas electrons in single bonds need
higher energy for getting excited. Further on, the chemical setting influences the
energy levels of the individual molecular orbitals and, thus, the resulting energy
4.3 Spectroscopy
75
for NMR and ESR spectroscopy. Radiation with somewhat higher energy provokes
changes in the rotation and vibration of molecules. In particular the later one is
related to the wave length area of infrared radiation and, therefore, this area is used
for infrared or IR spectroscopy. In the energy range of visible and ultraviolet light,
molecules are able to absorb the photons by changing the energy state of their
valence electrons leading to the application of UV/Vis spectroscopy. At higher
energies in the X-ray range the inner electrons become excited to release the atom
representing the base for atom absorption spectroscopy AAS.
However, in contrast to NMR/ESR, IR and UV/Vis spectroscopy applied (not
exclusively but intensively) for the analysis of organic molecules, AAS is a method
for elemental analysis and not common in organic analyses. Further on, NMR
spectroscopy as an extraordinary powerful tool for structure elucidation has only a
very limited application in Organic Environmental and Geochemistry due to some
aspects: (i) this method needs a relative high amount of material or analytes in the
range of mg, (ii) information on chemical structures of substances are only available
for isolated material (no complex mixtures), and (iii) the time of measurement is high
(minutes to hours) and, consequently NMR is not successfully linkable to chromatographic techniques such as gas or liquid chromatography. These restrictions avoid
the analyses of environmental or fossil samples that are characterized by complex
mixtures at partly very low concentrations, that normally need an intensive separation by chromatography. However, some special applications of NMR in Organic
Geochemistry exist, e.g. the characterization of humic substances in soil by solid
state NMR.
Consequently, the following two chapters focus on the UV/Vis and IR
spectroscopy.
4.3.2 UV/Vis Spectroscopy
As the basic physical principle of UV/Vis spectroscopy, the energy of UV/Vis
radiation fits the energy differences of the valence orbitals in organic molecules. In
simple words, the absorption of a photon in the UV/Vis range provokes a valence
electron to jump from its ground state orbital to an orbital with higher energy.
Energies of molecule orbitals are certainly quantized and, therefore, only selected
differences are possible leading to clear restricted transition. A general differentiation of orbitals available for the transition are given in Fig. 4.35.
In ground state the valence electrons are either in bonding σ- or π-orbitals as well
as non-bonding orbitals. From these orbitals they can jump into anti-bonding σ
à - or
π
à -orbitals. It is obvious, that transitions from σ-orbitals to higher energy orbitals
need more energy as compared to π-orbitals or non-bonding orbitals. Hence, electrons in double bounds (π-orbitals) or from free electron pairs (e.g. nitrogen, oxygen)
can be used for absorption at lower energy, whereas electrons in single bonds need
higher energy for getting excited. Further on, the chemical setting influences the
energy levels of the individual molecular orbitals and, thus, the resulting energy
4.3 Spectroscopy
75