UV SPECTROSCOPY Absorption spectra

Introduction UV spectroscopy involves the measurement of absorption of light in the visible and ultraviolet regions (visible region 400-800 nm ; uv region 200-400nm) by the substance under investigation. Since the absorption of light involves the transition from one electronic energy level to another within a molecule, UV spectroscopy is also known as electronic spectroscopy. This technique is complementary to fluorescence spectroscopy, in that fluorescence deals with transitions from the excited state to the ground state, while absorption measures transitions from the ground state to the excited state.

Principle of absorption spectroscopy Molecules containing π-electrons or non-bonding electrons (n-electrons) can absorb the energy in the form of ultraviolet or visible light to excite these electrons to higher anti-bonding molecular orbitals. The more easily excited the electrons (i.e. lower energy gap between the HOMO and the LUMO), the longer the wavelength of light it can absorb. A very important condition for a molecule to absorb electromagnetic radiation is that the energy of photon of radiation must be equal to the energy difference between two vibrational or rotational or electronic energy states of the molecule. A record of the amount of radiation absorbed or transmitted by a given sample as function of wavelength of radiation is called absorption spectrum.

Beer’s Lambert law When a beam of monochromatic light is passed through a substance dispersed in a non-absorbing solvent, absorption of light is directly proportional to molar concentration of the absorbing substance as well as path length of the sample substance. Using the Beer-Lambert law:                              where A is the measured absorbance, in Absorbance Units (AU),      is the intensity of the incident light at a given wavelength,   is the transmitted intensity, L the path length through the sample, and c the concentration of the absorbing species. For each species and wavelength, ε is a constant known as the molar absorptivity or extinction coefficient.

Electronic excitations sigma to sigma* transition : very high energy required consequently occur at short wavelength. Eg: methane. n to sigma* transition: occur at long wavelength than sigma to sigma*. Eg: methyl chloride . n to pi*transition: require small amount of energy and take place within the range of ordinary uv spectrophotometer. Eg : carbonyl group of saturated aldehydes and ketones. pi to pi* transitions : relatively high energy requirement than n to pi * transitions and absorption generally takes place outside the ordinary uv region. Eg : aldehydes and ketones.

Effect of conjugation conjugation of double bonds lowers the energy required for electronic transition, molecules containing conjugated systems absorb radiations of longer wavelength than in case of non conjugated systems. Eg: 1,3-butadiene shows max at 217 nm in contrast to ethylene which shows at 175nm. energy gap between HOMO and LUMO decreases. as the gap decreases position of energy moves to longer wavelength which falls within the ordinary uv region.

Chromophore A chromophore is the part of a molecule responsible for its color. The color arises when a molecule absorbs certain wavelengths of visible light and transmits or reflects others. The chromophore is a region in the molecule where the energy difference between two different molecular orbitals falls within the range of the visible spectrum. Visible light that hits the chromophore can thus be absorbed by exciting an electron from its ground state into an excited state.

Auxochrome An auxochrome is a functional group of atoms with nonbonded electrons which, when attached to a chromophore, alters both the wavelength and intensity of absorption. If these groups are in direct conjugation with the pi-system of the chromophore, they may increase the wavelength at which the light is absorbed and as a result intensify the absorption. A feature of these auxochromes is the presence of at least one lone pair of electrons which can be viewed as extending the conjugated system by resonance.

Changes in position and intensity of absorption Bathochromic(red) shift: it involves the shift of absorption maximum towards longer wavelength. It can be brought about by- attachment of auxochrome to chromophore, conjugation of two or more chromophoric groups, using solvent of lower polarity. Hypsochromic(blue) shift: it involves the shift of absorption maximum towards shorter wavelength. It may be brought about by- removal of conjugation, using solvent of higher polarity. Hyperchromic effect: this effect involves an increase in the intensity of absorption. It is brought about by introduction of auxochrome. Hypochromic effect: it involves decrease in the intensity of absorption. It is brought about by introduction groups which distort the geometry of the molecule.

Solvent effects n to pi* transition: increase in polarity of solvent shifts the transition to shorter wavelength(blue shift). Pi to pi* transition: increase in polarity of solvent shifts the transitions to longer wavelength(red shift).

Woodward fieser rules for calculating absorption maxima

Assignment-I Define the term spectroscopy. Why are spectroscopic methods being increasing used for structure determination? Why are spectroscopic techniques based upon absorption spectroscopy and not emission spectroscopy? What is the range of uv-visible spectroscopy? How can you distinguish between cis- and trans- stilbenes? When p-aminophenol is dissolved in water, wavelength maximum is at longer wavelength than in acid solution. Explain. Discuss electronic energy levels and electronic transitions in UV? What is Beer’s Lambert law of absorption?

Explain the principle of absorption spectroscopy? Explain the following terms: 1. Bathochromic shift 2. Hypsochromic shift 3. auxochrome 4. Chromophore 5. molar absorptivity Explain application of UV spectroscopy in determination of configuration of geometrical isomers by taking examples. Explain how does the UV spectrum of butadiene differ from that of ethylene. By taking examples, show the factors which cause a Bathochromic shift in UV spectrum. Discuss absorption laws in UV spectroscopy. Explain effect of solvent on n to pi* and pi to pi* transitions.