Overview of Spectroscopy in Chemistry

Spectroscopy is a branch of chemistry that uses emission and absorption of electromagnetic radiation in order to obtain information about the structure of atoms and molecules.  In addition, electromagnetic radiation can trigger changes in chemical bonds and can induce chemical reactions such as those which occur  in the branch of chemistry that is called photochemistry. 

Spectroscopy is used in all branches of chemistry such as organic and inorganic chemistries.  Mostly spectroscopy is used in the area of physical chemistry such as its use in the area of photophysics and in LASER spectroscopy.  Spectroscopic science is a relatively new area of chemistry that was developed recently with the advent of instrumentation and after the understanding of the nature of light and the electromagnetic radiation in general.

Spectroscopy is such a powerful technique that using it in chemical research can give a lot of useful information.  In most cases, spectroscopy is used to investigate electronic transitions in atoms and molecules.  Chemical changes in atoms and molecules are triggered partly through the administration of electromagnetic radiation.  

Nuclear changes are in most cases not affected by electromagnetic radiation.  Electromagnetic radiation even in its highest frequency can rarely trigger changes in the nucleus of atoms.  Therefore, the nucleus is usually stable to the effect of electromagnetic radiation or at least not so affected as the electrons of the atom and molecule. 

Electromagnetic radiation that is able to trigger electronic transitions in atoms and molecules is located in the ultra-violet region of the light spectrum or UV.  Also shorter wavelengths can trigger electronic transitions in atoms and molecules such as X-ray radiation. Electronic transitions in the UV region are observed for example in the organometallic complexes of transition metals.

UV light is used in photochemistry to induce chemical reactions in organic molecules.  Shorter wavelength than UV can also induce the breaking of chemical bonds in addition to its ability to trigger electronic transitions in atoms and molecules.  For this reason, UV radiation is considered harmful because it can trigger cancer through altering the structure of important molecules in the body such as the DNA molecule. 

X-ray spectroscopy is another powerful spectroscopic technique that is used in chemistry to obtain data about molecules such as distance between atoms and angles between them.  Diffraction of x-ray from atoms and molecules surface can give information about its structure through the application of Braggs equation.  X-ray is used to induce electronic transitions in the inner core of atoms.  By shining an x-ray light on an atom, electron in the inner core of the atom is expelled from the atom.  As a result, an electron in the upper energy levels of the atom decays to that lowered vacant orbital with concomitant release of a photon.  Investigation of this photon spectroscopically can give valuable information about the inner structure of the atom. 

The higher wavelengths of the light spectrum are also used in spectroscopy such as the infra-red region of the spectrum or the microwave region of the spectrum.  Molecules are characterized by vibrations of the atoms in each bond of the molecule.  Vibrational data can be obtained from spectroscopic data in the infra-red region of the spectrum.  Spectrum in the Microwave region can give information about the distance between atoms that form a chemical bond.  An example is the rigid rotator.  Using this model and solving the appropriate Schroedinger equation one can get information of about rotational energy levels of the molecule in addition to the distance between the atoms that form the bond.  

The harmonic oscillator is an important concept of chemistry and spectroscopy.  Vibrational spectra of molecules can give information such as whether the molecule will absorb light in the infra-red or not and if the molecule will absorb light in the Raman region of the light. 

Spectroscopy is exceedingly important especially in organic chemistry due to the exceedingly complicated organic structure of the molecules involved such as alkaloids and other natural products.  Nuclear magnetic resonance or NMR is a powerful spectroscopic technique that is used in organic chemistry.  It uses both H1 and C13 spectra to deduce the structure of organic molecules.  Also 2-D NMR or two dimensional NMR is often used in certain cases.  COSY and NOE are other NMR techniques that are also used with certain molecules.