UV Spectra 2ND Online Class PDF

Origin of UV/Visible spectra Let us try to understand how the UV/visible spectra arise. It arises due to absorption of electromagnetic radiations in UV/visible region of electromagnetic spectrum. The absorption of electromagnetic radiations in the UV/visible region results in a transition betwe...

Origin of UV/Visible spectra Let us try to understand how the UV/visible spectra arise. It arises due to absorption of electromagnetic radiations in UV/visible region of electromagnetic spectrum. The absorption of electromagnetic radiations in the UV/visible region results in a transition between electronic energy levels. In these electronic level transitions, electron from the ground state of molecule gets excited to higher electronic state in molecule. The most probable or common electronic transition is from highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) in a molecule (Figure 2). A UV/visible spectrometer records the degree of absorption by a sample as a function of wavelength (λ) of light absorbed and resulting plot between absorbance versus wavelength (λ) is known as UV/visible spectrum. The significant features of UV/Visible absorption spectra are λmax at which maximum absorption occurs i.e. the intensity of absorption is maximum. The absorption at this wavelength is represented by εmax which represents maximum molar absorptivity. These parameters are characteristics of a particular molecule. Wavelength at which absorption occurs depends upon the energy difference between ground electronic state i.e. HOMO and excited electronic state i.e. LUMO. Larger the energy difference between ground and excited electronic state lesser will be the value of absorption maxima i.e. λmax. UV/visible spectra generally consist of broad band of absorption rather than single sharp line for each such transition. The band like structures of UV/visible spectra arise due to the transitions taking place over a wide range of wavelengths rather than a single wavelength. This can be explained on the basis that, each electronic energy level in a molecule consists of closely separated vibrational and rotational energy levels. As a result, molecule can undergo electronic transitions involving vibrational as well as rotational excitations at the same time. Important terminologies in UV-Visible Spectroscopy 3.1 Chromophore When a molecule absorbs electromagnetic radiation in the ultraviolet/visible range, a transition between different electronic energy levels occurs. Since the wavelength of absorption is a measure of the separation of the energy levels of the orbital concerned. The nucleus holding the electrons together in a bond determine the wavelength of radiation to be absorbed. Thus, the nuclei, with which the concerned electrons are bound, affect the energy between the ground and excited states. Therefore, we can say that the energy of transition and the wavelength of radiation absorbed are properties of atoms not the electron themselves. The group of atoms due to which absorption occurs is called chromophore. A chromophore is defined as an isolated covalently bonded group that shows a characteristic absorption in UV/Visible region. For example C=C, C=C, C=O, C=N,N=N, R-NO2 etc. If a compound absorbs light in the visible region (400-800 nm), only then it appears coloured to our eyes. Therefore, a chromophore may or may not impart colour to compound depending on whether the chromophore absorbs radiation in the visible orUV region. There are no standard criteria for the identification of a chromophore because the wavelength and intensity of absorption depend on many factors such as the molecular environment of the chromophore and on the solvent in which the sample is dissolved. Other parameters, such as pH and temperature, also may cause changes in both the intensity and the wavelength of the absorbance maxima. All the compounds having the same functional group will absorb at almost the same wavelength if the other factors such as conjugation, substituents etc are absent. 3.2 Auxochrome(details) The substituents covalently attached to a chromophore which themselves do not absorb ultraviolet/ visible radiation, but their presence changes both the intensity as well as wavelength of the absorption maximum are known as auxochromes. The substituents like methyl, hydroxyl, alkoxy, halogen, amino group etc. are some examples of auxochromes. These are also called colour enhancing groups. The actual effect of an auxochrome on a chromophore depends on the polarity of the auxochrome, e.g. groups like CH3, CH3CH2 and Cl have very little effect, usually a small red shift of 5-10 nm. Other groups such as NH2 and NO2 show a strong effect and completely alter the spectrum. Auxochrome generally increases the value of absorption maxima by extending the conjugation through resonance. The extended conjugation brings the lowest excited state (LUMO) closer to the highest ground state (HOMO) and thus permits a lower energy (longer wavelength) transition. Actually, the combination of chromophore and auxochrome behaves as a new chromophore having different value of absorption maxima. For example, benzene shows λmax at 256 nm, whereas aniline shows λmax at280 nm. Hence the NH2 group acts as an auxochrome and causes the shifting of λmax to a larger value. The shift of an absorption maximum towards longer wavelength or lower energy is called as bathochromic shift. It may be produced due to presence of an auxochrome or change in solvent polarity. Because the red color has a longer wavelength than the other colours in the visible spectrum, therefore this effect is also known as red shift. Eg: 3.4 Hypsochromic Shift or Blue Shift The shift of an absorption maximum towards the shorter wavelength or higher energy is called hypsochromic shift. It may be caused due to presence of an auxochrome or change in solvent polarity. Because the blue color has a lower wavelength than the other colours in the visible spectrum and hence this effect is also known as blue shift. 3.5 Hyperchromic Effect It is an effect that results in increased absorption intensity. The introduction of an auxochrome usually causes hyperchromic shift. For example benzene shows B band(the secondary band in UV-Vis spectra) at 256 nm and Ɛmax 200, whereas aniline shows B-band at 280 nm and Ɛmax at 1430. The increase in the value of Ɛmax is due to the hyperchromic effect of auxochrome NH2. 3.6 Hypochromic Effect An effect that results in decreased absorption intensity is called hypochromic effect. This is caused by a group which distorts the geometry of the molecule. For example, biphenyl shows λmax at 250 nm and Ɛmax at 19,000, whereas 2-methyl biphenyl absorbs at λmax 237 nm, Ɛmax 10250. The decrease in the value of absorbance is due to hypochromic effect of methyl group which distorts the chromophore by forcing the rings out of coplanarity resulting in the loss of conjugation.