Introduction to spectroscopy
Introduction to spectroscopy:
The field of spectroscopy is divided into two main classes:
(i) Emission spectroscopy
(ii) Absorption spectroscopy
An emission spectrum is obtained by some light source such as a flame or an electric arc. This spectrum is due to the excitation of atoms by thermal or electrical means. In case of absorption spectroscopy energy absorbed causes electrons in a ground state to be promoted to a higher excited state. The life-time of electrons in this excited state is short and they return to either a lower excited state or to the ground state. The absorbed energy is released as light. Fluorescent lights and colours obtained by heating salts of certain elements in a flame are very common examples of emissions spectra. In some cases the excited states may have appreciable life-times. In these cases the excited states usually have appreciable life times and emission of light starts after excitation has ceased. Such a phenomenon is called phosphorescence.
An absorption spectrum is obtained by placing the substance between the spectrometer and some source of energy, usually it is an electromagnetic radiation which is applied. The spectrometer analyses the transmitted energy related to the incident energy for a given frequency of the electromagnetic radiation. The regions of electromagnetic radiation of greatest interests to the organic chemists are 200-400 mji (ultraviolet), 400-800 mji (visible), and 2-16 u (infrared).
The mechanisms of absorptions of energy are different in the ultraviolet, infrared and nuclear magnetic resonance regions, but the fundamental phenomenon is the absorption of a certain amount of energy. The energy absorbed is given by
E = hv where h is Planck’s constant and v is the frequency of incident light (in cycles per second, cps). v is related to the wavelength λ as follows,
where c is the velocity of light, X is^the wavelength in cm.
The wave number is also used in the description of spectra. The wave number k is related to X by
is the wave number in cm–1.
Interpretations of molecular spectra by the organic chemists are based largely on empirical correlations with extensive compilations of data. At the present time, the various spectral methods are the more commonly used physical methods. Absorption of ultraviolet and visible light is chiefly caused by electronic excitation; the spectrum provides limited information about the structure of the molecule. Absorption in the infrared region is due to molecular vibrations of one kind or another; the spectrum is generally very complicated and contains many absorption peaks, relatively few of which can be interpreted with a high degree of assurance. On the other hand, the proton magnetic resonance (pmr) of a compound owing to nuclear spin transitions can usually be completely interpreted, and it provides information about the number, nature, and environment of all the protons in the molecule.
A region of electromagnetic radiation whose interaction with a molecule gives rise to electronic transition exists at 100-8000 A (10-800 mµ).
Visible Light and Electromagnetic Spectrum:
Fig. 1 Wavelengths of electromagnetic radiation
The total energy (ET) of a diatomic molecule is the sum of electronic energy (εe), vibrational energy (εv) and rotational energy (εr), i.e
If the electromagnetic radiation in the region of 10-800 mµinteracts with a molecule, a change in the energy of the molecule from the ground state to a higher level i.e., excited state occurs. The transition of energy, due to the displacement of a valence electron accompanied by the electronic excitation, is a change in εv and εr of the molecule. The energy requirements for the excitation of the latter two modes is comparatively less than that for electronic excitation.