How the
measurements of radiation frequency are made experimentally and the energy
levels deduced from these comprise the practice of spectroscopy.
All techniques are mostly dependent on the emission or
absorption of electromagnetic radiation characteristic of certain energy within
an atomic or molecular system. Spectroscopy is one of the powerful tools for
the study of atomic and molecular structure and is used in the analysis of a
wide range of samples.
Atomic spectroscopy deals with the interaction of
electromagnetic radiation with atoms which are most commonly in their lowest
energy state called the ground state. Molecular spectroscopy deals with the
interaction of electromagnetic radiation with molecules.
Rotational spectra
These spectra arise due to the transition between the
rotational energy level of a gaseous molecule on the absorption of radiations
falling in the microwave region. These spectra are shown by molecules that
have a permanent dipole moment. For instance
HCl, CO, H2O, etc. Homonuclear diatomic molecules such as H2,
Cl2 etc and linear polyatomic molecules such as CO2,
which do not have a dipole moment, do not show microwave spectra. Usually, microwave spectra appear in the spectra range of 1-100cm-1.
Vibrational spectra
These spectra arise due to transitions induced between the
vibrational energy levels of a molecule on the absorption of radiation
belonging to the infrared region. IR spectra are shown by the molecule when
vibrational motion is accompanied by a change in the dipole moment of the
molecule. Vibrational spectra appear in the spectral range of 500-4000 cm-1.
Raman spectra
Raman spectra relate to vibrational or rotation transitions
in molecules but in a different manner. In this case, only the scattering is
measured but not the absorption of radiation. An intense beam of monochromatic
radiation in the visible region is allowed to fall on a sample and the intensity
of scattered light is observed at right angles to the incident beam.
The classical theory of the Raman
Effect:
When an electric field is applied to a molecule, its
electrons and nuclei are displaced. Thus an induced dipole moment is produced in
the molecule due to the displacement of the electrons and nuclei, and the
molecule is said to be polarized.
µ = ᾳE
Where,
E- Strength of electric field
µ- magnitude of induced dipole moment
ᾳ-
Polarizability of the
molecule
v- Radiation frequency
E = E0sin
2πvt
µ = ᾳ E0sin
2πvt
Electronic spectra
Electronic spectra result from electronic transitions in a
molecule by absorption of radiations falling in the visible and ultraviolet regions.
While spectra in the visible region span 12,500-25,000 cm-1, those
in the ultraviolet region span 25,000-70,000 cm-1. As electronic
transitions in a molecule are invariably accompanied by vibrational and
rotational transitions, the electronic spectra of the molecule are highly complex.
Nuclear magnetic
resonance and nuclear quadrupole resonance spectra
NMR spectra arise due to transitions induced between the nuclear
spin energy levels of a molecule in an applied magnetic field. NQR spectra
arise due to the transitions between the nuclear spin energy levels of a
molecule arising from the interaction of the unsymmetrical charge distribution
in nuclei with the electric field gradients which arise from the bonding and
non-bonding electrons in the molecule. NMR and NQR’s radio frequency regions
5-100MHz.
Electron spin resonance
ESR spectra arise due to the transitions induced between the
electron spin energy levels of a molecule in an applied magnetic field. These spectra
are exhibited by systems that contain odd electrons such as free radicals and
transition metal ions. ESR spectra region is 2-9.6 GHz.
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