Ultraviolet Spectroscopy

Ultraviolet Spectroscopy Ultraviolet radiation is electromagnetic radiation which has a wave length in the region of 200 to 400 nm. The UV light is invisible to our eyes and the of UV light region lies next to violet color in visible light spectrum. It has higher energy than IR region. If you pass white light through a colored substance, some of the light gets absorbed. Some colorless substances also absorb light - but in the ultra-violet region. Since we can't see UV light, we don't notice this absorption. Different substances absorb different wavelengths of light, and this can be used to help to identify the substance - the presence of particular metal ions, for example, or of particular functional groups in organic compounds. A beam of light is incident on the sample to be tested, allowed to pass through the sample and then received by the instrument which compares this beam with the beam received directly from the source of light. The energies in UV region are sufficient to promote or excite a molecular electron to a higher energy orbital. When light passes through the compound, energy from the light is used to promote an electron from a bonding or non-bonding orbital into one of the empty anti-bonding orbital.

In each possible case, an electron is excited from a full orbital into an empty anti-bonding orbital. Each jump takes energy from the light. Each wavelength of light has a particular energy associated with it. If that particular amount of energy is just right for making one of these energy jumps, then that wavelength will be absorbed - its energy will have been used in promoting an electron. A high energy jump will need to absorb light of a higher frequency. The greater the frequency, the greater the energy and greater the frequency lower is the wave length. So to absorb higher energy lower wave length will be absorbed Compound whose molecule contain dienes usually have absorption of energy below 200 nm. This wave length is beyond the UV range. Hence UV spectroscopy cannot be employed to detect dienes bonds. Compound whose molecule contain conjugated multiple bond have absorption wavelength longer than 200 nm comes well within the region of UV spectroscopy. Therefore UV spectroscopy is used to study the nature of conjugated Β electron system. The energy in UV spectrum is sufficient to move electron from non-bonding orbitals to pi anti-bonding orbitals from pi bonding orbitals to pi anti-bonding orbital

from non-bonding orbitals to sigma anti-bonding orbitals. Conjugated system with Β bond has vacant orbital at energy level close to their highest occupied molecular orbital (HOMO) in these cases it is a pi bonding orbital. The vacant orbital are called lowest unoccupied molecular orbital (LUMO) and it is a pi anti-bonding orbital. UV radiations are capable to momentarily displace electron to LUMO and energy is absorbed. In practice an UV spectrum is recorded by irradiating the sample with UV light of continuously changing wavelength. When the wavelength corresponding to the energy level required to excite an electron to a higher level energy is absorbed. This absorption is detected and displayed in the form a graph of wavelength versus absorbance. Spectrum of 1,3,butadiene is shown below

Absorbance ‘A’ is defined as A = log I0/I where I0 is the intensity of the incident light and I is the intensity of light transmitted through the sample The exact amount of UV light absorbed is expressed as molar absorptivity ‘,’ defined as , = A/C.l where A is absorbance, C is concentration, l is sample path length.

AN important factor effecting the wave length is the extent of conjugation Energy difference between HOMO and LUMO decrease as extent conjugation increases. Thus 1,3 butadiene absorb wavelength of 217 nm while 1,3,5 hextriene absorb 258nm and 1,3,5,7octatetracene absorb290 nm. UV spectrum does not give much detail. Sample must be extremely poor otherwise spectrum will not be clear. If a compound has eight or more double bond its absorbance will show in the visible region Carrots having ∃-carotene absorb blue light giving them complementary color of red-orange.

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