Electromagnetic Radiation in Spectroscopy

Presented ByMr. N. P. Badgujar

Electromagnetic Radiation

• Spectroscopy is defined as the interaction of light with matter.

Light, in this context, is the broad spectrum of continuous energy

called the electromagnetic spectrum.

In a magnetic field, exposure to the low-energy radio frequency

radiation only reorients nuclei.

Exposure to the slightly higher-energy microwave region changes

electron spin states of molecules with unpaired electrons.

Microwave radiation can also change the rotational energy of

molecules; this effect is used to heat food quickly in a microwave

oven.

In the middle regions of the electromagnetic spectrum,

absorption of IR radiation causes changes in the vibrational

energy of molecules.

Visible (Vis) and ultraviolet (UV) radiations alter the electron

energies of loosely held outer electrons of atoms and

molecules.

Higher-energy X rays can cause electron transitions between

inner electron levels, and gamma radiation produces changes

within atomic nuclei.

Wave theory:

• All energies of the electromagnetic spectrum can be considered to

be waves that move at the speed of light, with the types of radiation

differing only in amplitude, frequency, and wavelength .

• Electromagnetic radiation can also be characterized by the number of waves per unit length. This is termed wave number, v.

• Frequency and wave number units have two advantages over wavelength units.

• The first is that they remain constant, regardless of the media traversed by the radiation, whereas the wavelength is reduced.

• When radiation passes through a medium with a refractive index greater than that of a vacuum.

• This change in wavelength due to refractive index is ignored except for high-accuracy experiments.

• The second advantage of the use of frequency and wave number units over wavelength is that they are directly proportional to energy.

• Thus, a transition that requires greater energy will occur at a higher wave number so it is used in IR spectroscopy.

Absorption Theory:

• The electrons of an atom occupy specific energy states or levels that are defined by the radius of the orbit of the electron around the nucleus.

• He further suggested that to move between different energy states, atoms must absorb or emit energy and that the amount of energy absorbed or emitted must be equal to the difference in energy between the two levels.

Molecular absorptions

• To understanding elemental bonding, molecular structures, and

chemical reactions.

• Specific wavelengths of energy correspond to all molecular

transitions or motions: electronic, translational, rotational, and

vibrational.

The electrons change energy levels or directions of spin.

The entire molecule shifts to a new position.

The molecule rotates around its centre of mass.

Individual atoms within a molecule to change position relative

to one another without moving or rotating the molecule.

• The molecule's movements are unique to its structure, the

measurement of these transitions makes IR a powerful tool

for compound characterization.

Degrees of freedom

• In a molecule, the atoms are constrained by molecular bonds to

move together in certain specified ways, called degrees of

freedom.

All molecules have three translational degrees of freedom that is, the

center of mass of the molecule can move in three directions (x, y, and z).

Nonlinear molecule: Have three rotational degrees of freedom as it spins

around each of the three axes (x, y, and z).

Linear molecules: Have only two rotational degrees of freedom, since

two rotation directions are equivalent.

Thus, the total of translational and rotational degrees of freedom is 6 ( 5

for linear molecules).

• All remaining motions, 3N - 6, are vibrational degrees of freedom (3N - 5

in the case of linear molecules}. This is an important number, since

vibrational transitions are the strongest and most important in IR

spectroscopy.

Selection rules:

• The degrees of freedom specify the maximum number of fundamental

vibrations for a molecule.

• A fundamental vibration-also called a first order vibration-corresponds to

a change from the molecular ground state to the first energy level.

• A first-order vibration produces the strongest energy absorptions, but

only those that are active in the IR region will be seen as an absorption

band in the IR spectrum.

• Selection rules, based on the symmetry of the molecule, determine

whether a given vibration will be seen in the spectrum.

• The primary selection rule, or requirement, for active IR absorptions is

that the vibration must change the dipole moment of the molecule.

• Factors affecting a reduction in the number of absorption bands:

Degeneracy, where two vibrational modes may occur at identical

frequencies.

Over lapping or weak absorptions.

Vibrational modes outside the instrument analysis range.

• Alternatively, the number of observed bands may be increased by

the detection of weak, or so-called forbidden, absorptions, in

which the change in the vibrational level is greater than one. These

are known as overtones.

Infrared Spectra:

• Two types of interactions-absorption and transmission-are

important in the typical IR experiment.

• For a given wavelength or frequency of IR radiation striking a

sample, these two interactions are inversely related through the

following equation:

Infrared Regions:

• The IR spectral region of the electromagnetic spectrum extends

from the red end of the visible spectrum to the microwave region; it

includes radiation with wave numbers ranging from about 14,000 to

20 cm- I, or wavelengths from 0.7 to 500 micron.

Near-infrared region:

• The near-IR (NIR, NIRS) region extends from the visible region at

14,000 cm-1 (0.7 /-Lm) to the mid-IR region at 4000 cm-1 (2.5

/-Lm). Because it is accessible with quartz optics, near-IR

instrumentation is often combined with UV-Vis spectrometers

Mid-infrared region:

• The spectral range of greatest use for chemical analysis is the mid-

IR (MIR) region. It covers the frequency range from 4000 to 500

cm-1 (2.5-20 /-Lm).

• This region can be subdivided into the group frequency region,

4000-1 300 cm- 1 (2.5-8.0 /-Lm) and the fingerprint region,

1300-500 cm -1 (8.0-20 /-Lm).

Far-infrared region:

• The far-IR ( FIR) region is generally designated as 500-20 cm-I

(20-500 /-Lm) .

• In this region, the entire molecule is involved in low frequency

bending and torsional motions, such as lattice vibrations in

crystals