chromophores thus hold the key to confirming the presence of specific components
in a specimen.
All absorbed energy is quantized, and because electron transfer is influenced by
other transitions within the molecule (vibration and rotation transitions), light
absorption bands in the UV/visible light range appear broad, a phenomenon that
poses some constraints on observing the presence of functional groups. In the case
of quantitative analysis however, this electron transfer plays an important role, as it
is closely linked to molecular structure. This means that while UV/VIS photometry
is not a good approach for quantitatively analyzing unknown substances, it is
excellent for precise quantitative analysis of the concentrations of known
substances.
B. Infrared Spectroscopy and the Structures of Natural Substances
The infrared range is located midway between the visible and microwave ranges on
the electromagnetic wave spectrum. All molecules in this range have unique
vibrations. When a molecule is exposed to continuously changing wavelengths in
the infrared range (650–4000 cm
−1 ), rays with frequencies equal to the molecule’s
inherent vibration are absorbed, producing a characteristic spectrum. The use of this
spectrum to analyze molecular structure is known as infrared absorption
spectroscopy.
The IR spectrum can be used to identify substances by comparison with the
spectra of substances that are already known. The structure can be partly ascertained from the absorption positions of multiple linkages and characteristic functional groups. Cis and trans isomers, ring positions, hydrogen linkages and other
bonds, and breakdown reactions can also be confirmed (Fig. 10.11).
C. Nuclear Magnetic Resonance Analysis
The major component elements of natural substances are C, H, O, and N. The
atomic nucleus spins when mass number is odd, when the atomic number is odd, or
both. Nuclear magnetic resonance (NMR) focuses chiefly on hydrogen and carbon.
Hydrogen may spin when the atomic and mass numbers are both one. Carbon has
an atomic number and mass number of 12, so it typically cannot spin. Carbon
isotopes with a mass number of 13, however, are present at a rate of 1.08% in
12 C
and can be used for this analysis (Paudler 1987) (Fig. 10.12).
Resonance occurs in the nucleus when a specific magnetic field is applied to
these elements. For example, hydrogen absorbs light at a wavelength of 100 MHz
under a magnetic field intensity of 23,500 gauss, while carbon has been found to
absorb 10.7 MHz of energy at 10,000 gauss. In these cases, it is only the hydrogen
and carbon nuclei generating resonance, which does not occur in other nuclei. This
resonance phenomenon is sometimes likened to a top, where the nuclear undergoes
precession with respect to the earth’s gravitational field. When an outside magnetic
field is applied, the nucleus undergoes precession around its own spin according to
its own oscillation frequency; the greater the magnetic field strength, the faster the
364
10 Marine Natural Substances
in a specimen.
All absorbed energy is quantized, and because electron transfer is influenced by
other transitions within the molecule (vibration and rotation transitions), light
absorption bands in the UV/visible light range appear broad, a phenomenon that
poses some constraints on observing the presence of functional groups. In the case
of quantitative analysis however, this electron transfer plays an important role, as it
is closely linked to molecular structure. This means that while UV/VIS photometry
is not a good approach for quantitatively analyzing unknown substances, it is
excellent for precise quantitative analysis of the concentrations of known
substances.
B. Infrared Spectroscopy and the Structures of Natural Substances
The infrared range is located midway between the visible and microwave ranges on
the electromagnetic wave spectrum. All molecules in this range have unique
vibrations. When a molecule is exposed to continuously changing wavelengths in
the infrared range (650–4000 cm
−1 ), rays with frequencies equal to the molecule’s
inherent vibration are absorbed, producing a characteristic spectrum. The use of this
spectrum to analyze molecular structure is known as infrared absorption
spectroscopy.
The IR spectrum can be used to identify substances by comparison with the
spectra of substances that are already known. The structure can be partly ascertained from the absorption positions of multiple linkages and characteristic functional groups. Cis and trans isomers, ring positions, hydrogen linkages and other
bonds, and breakdown reactions can also be confirmed (Fig. 10.11).
C. Nuclear Magnetic Resonance Analysis
The major component elements of natural substances are C, H, O, and N. The
atomic nucleus spins when mass number is odd, when the atomic number is odd, or
both. Nuclear magnetic resonance (NMR) focuses chiefly on hydrogen and carbon.
Hydrogen may spin when the atomic and mass numbers are both one. Carbon has
an atomic number and mass number of 12, so it typically cannot spin. Carbon
isotopes with a mass number of 13, however, are present at a rate of 1.08% in
12 C
and can be used for this analysis (Paudler 1987) (Fig. 10.12).
Resonance occurs in the nucleus when a specific magnetic field is applied to
these elements. For example, hydrogen absorbs light at a wavelength of 100 MHz
under a magnetic field intensity of 23,500 gauss, while carbon has been found to
absorb 10.7 MHz of energy at 10,000 gauss. In these cases, it is only the hydrogen
and carbon nuclei generating resonance, which does not occur in other nuclei. This
resonance phenomenon is sometimes likened to a top, where the nuclear undergoes
precession with respect to the earth’s gravitational field. When an outside magnetic
field is applied, the nucleus undergoes precession around its own spin according to
its own oscillation frequency; the greater the magnetic field strength, the faster the
364
10 Marine Natural Substances